skip to main content

Graduate Seminars

Graduate Student Seminars

September 3, 2015

Kyle Swanson

A New Perspective for Cosmology?

Thursday, September 3, 2015

Early this year, January 6th 2015, two physicists Das and Ali published work that predicts that our universe is ageless. Through the use of the quantum corrected Raychaudhuri equation (QRE) Das and Ali derive the Friedmann equation of second order which results in two quantum correction terms. One of the corrections is interpreted as the cosmological constant. While the other correction they view as a radiation term for the early universe which holds profound implications. This presentation will first review some issues that cosmology currently faces: the smallness problem, the coincidence problem, the flatness problem, dark matter, and the big-bang. Following with an exploration of the work by Das and Ali in order to understand how and why these correction terms are “generic and unavoidable”⍏. Concluding with how these findings affect the current problems in the field of cosmology.

April 23, 2015

Marco Giordano

On the Basic Physics of Surface Wind Analysis: An Overview of Data Collection, Analysis, and an Application

Thursday, April 23, 2015

The focus of the talk will be to present some of the physics of measuring, and analyzing surface wind data. We will discuss some of the reasons why the collection of surface wind is important, and how the data is analyzed. The simplest case will be presented – that of a fluid system measured by a single, fixed-position instrument at a point location. Such an instrument/fluid system can serve as a point source of time-series information about the fluid moving with respect to the instrument. This Eulerian approach can yield useful information about the Atmospheric Surface Layer (ASL), the Atmospheric Boundary Layer (ABL), and near surfaces fluxes of energy and momentum. Specifically we will present the basic functioning of 3-Dimensional sonic anemometers, and the data they collect. We will examine a specific experimental configuration for surface layer testing - what is in the literature referred to as “the smooth wall” test. A preliminary portion of data from a location in UTAH at the Surface Layer Turbulence and Environmental Science Test (SLTEST) site will be discussed. Some simple analysis of this data will be presented. We will close by projecting ideas about what remains to be analyzed, and what can be extracted from such types of experiments.

April 16, 2015

Vijay Khanal

Photo-Catalysis in generation of Hydrogen from water

Thursday, April 16, 2015

The development of a clean and renewable energy carrier that does not utilize fossil fuels is a great technological challenge. In this regard, a potential solution is harvesting solar energy efficiently to separate Hydrogen and Oxygen from water. That would lead to infinite source of renewable and clean energy. Since the seminal work of Honda and Fujishima, 1972, where they first showed that the solar energy can be utilized for splitting water using TiO2 as electrode, many researchers have tried to develop more efficient photo-catalysts. While the goal is to use entire solar spectrum to split water, only a small fraction has been utilized so far. I will cover some of the experimental works that I am involved in synthesizing TaON as a photocatalyst and carrying out the reaction in our lab.

April 9, 2015

Kyle Carpenter

An Introduction to Genetic Algorithms

Thursday, April 9, 2015

Optimization techniques are frequently needed to analyze data from physics experiments. Often times the data is complex and analysis is computationally intensive. Much time has been devoted to the development of many different optimization techniques. Genetic algorithms (GAs) are types of search algorithms that are based on the mechanics of natural genetics and natural selection. They use the idea of survival of the fittest among string structures along with structured yet randomized information exchange. In this talk I will introduce GAs and provide a simple discussion about how they work. I will compare them to other optimization techniques and point out their advantages and shortcomings. Lastly, I will present some examples of how GAs have been applied to the field of physics.

February 26, 2015

Marco Giordano

On Aspects of Opto-physically Characterizing the Terrestrial Aerosol: Strategies, Methods and Experiments

Thursday, February 26, 2015

In this presentation we discuss the strategies and methodology invoked to develop an aerosol classification scheme – a “typology” based upon optical properties of the aerosol. NASA’s AERONET data product provides the source data for the classification scheme. We used level 2.0 almucantar retrieval products to define several reference aerosol clusters which are characteristic of the following aerosol types: Urban Industrial, Biomass Burning, Mixed Aerosol, Dust and Maritime. The preliminary classification of a particular aerosol event is made based upon its opto-physical similarity with respect to each reference cluster. We also discuss the method of variable selection and reduction to construct the model optical phase-space. Also discussed are the results of preliminary studies using the degree of linear polarization as a basis for a classification scheme. The discussion includes a mention of directions for further analysis and modeling experiments.

February 12, 2015

Kyle Carpenter

Magnetized Liner Inertial Fusion

Thursday, February 12, 2015

The natural geometry of magnetically imploded systems is cylindrical. This increases the radial convergence required to reach fusion conditions. This is due to the fact that volume scales with r2 instead of r3, which is the case for spherically imploded systems. Magnetized Liner Inertial Fusion, or MAGLIF, aims to lower the requirements needed to obtain the plasma conditions needed for significant fusion yields [S. A. Slutz et al Phys. Plasma 17, 056303 (2010)]. Magnetizing (B>10T) and preheating (~100eV) the fuel before implosion can reduce the required radial convergence. During this talk I will discuss the MAGLIF concept, and provide simulation results[S. A. Slutz and R. A. Vesey PRL 108, 025003 (2012)]. Lastly, I will present the results of the first experiments that were conducted using the Z-machine and Z-beamlet laser at Sandia National Laboratories [M. R. Gomez et al PRL 113, 155003 (2014)].

February 5, 2015

Tyler Daykin

Fast Impact Ignition, a Promising Approach for achieving ICF

Thursday, February 5, 2015

‘Fast Impact Ignition’ is a relatively new alternative method than the ‘Central Hot Spot’ approach in achieving thermonuclear ignition and burn for Inertial Confinement Fusion. Fast Impact ignition has the potential to be a more feasible approach by needing a lower laser driver energy and higher energy gain than competing methods. A few different and innovative schemes of ‘Fast Impact Ignition’ will be addressed in this presentation as well as the future for this exciting approach.

January 29, 2015

Amin Nourmohammadi

Nano-physics and its application in different areas

Thursday, January 29, 2015

Over the last decades, scientists have realized the importance of nano-scale structures and nano-materials in different fields. Having very unique properties such as thermal, mechanical, electrical and chemical; nano-structures have demonstrated to be very promising in the future of technology. From energy generation to biological applications, nano-science is playing an undeniable role in different aspect of modern developments. In this presentation we will quickly review the importance of nano-physics and we will highlight its role in three different fields: Hydrogen generation, super hydrophobicity and anti-bacterial application.

December 4, 2014

Marco Giordano

On Aspects of Opto-physically Characterizing the Terrestrial Aerosol: Strategies, Methods and Experiments

Thursday, December, 2014

In this presentation we discuss the strategies and methodology invoked to develop an aerosol classification scheme – a “typology” based upon optical properties of the aerosol. NASA’s AERONET data product provides the source data for the classification scheme. We used level 2.0 almucantar retrieval products to define several reference aerosol clusters which are characteristic of the following aerosol types: Urban Industrial, Biomass Burning, Mixed Aerosol, Dust and Maritime. The classification of a particular aerosol is determined by its multi-dimensional Mahalanobis distance (MD) to each cluster. The observed aerosol is assumed to be of the same type as the reference cluster nearest to it in Mahalanobis distance. We have calculated the fractional aerosol type distribution at various AERONET sites, as well as the monthly variations in aerosol type at those locations. From this typology and seasonal variation, we present a spatio-temporal averaged aerosol climatology. We also discuss the method of variable selection and reduction to construct the model optical phase-space. Also discussed are the distribution histograms of significant variables classified by aerosol type and location; with an attempt to determine their suitability as a different mechanism for constructing an aerosol typology. The discussion includes a mention of directions for further analysis and modeling experiments.

November 20, 2014

Matt Wallace

Spectrum recorded from two internal planes – towards a single crystal Spectropolarimeter

Thursday, November 20, 2014

The polarization of radiation from a plasma can be used to reveal information about the electrons and anisotropy of their velocity distribution. The polarization of emitted x-rays, typically measured with two crystals at a 45 degrees Bragg angle, can now be determined in a new way using a single crystal. This crystal utilizes two sets of planes that diffract the incident x-rays in two directions that are perpendicular to each other and to the incident beam. These two diffracted sets of x-rays are then linearly polarized perpendicularly to each other. The polarization splitting properties of quartz crystals were confirmed with linearly polarized x-rays from the Advanced Photon Source. A crystal with surface cut [10-10] that splits polarization with [11-20] planes at 3.5 keV was among those tested. Aluminum spectra was then obtained with x-rays from wire array z-pinches using reflections from two internal planes of a crystal that splits polarization at around 2.0 keV. The results of these measurements, characteristics of a single crystal polarimeter, and limitations to fielding such a diagnostic will be discussed.

November 13, 2014

Emil Petkov

Spectroscopic Analysis of K- and L-Shell Radiation from Gas-Puff Jet Experiments on the UNR Leopard Laser

Thursday, November 13, 2014

The study of cluster formation in gas-puff jet experiments and the x-ray emission from them under fs and ns laser impulses is of interest as there are many applications in physics (x-ray backlighting, lithography, high-harmonic generation, etc.) We present spectroscopic analysis of K- and L-shell radiation from Ar and Kr gas-puff jet experiments performed on the high-power Leopard laser at UNR. The diagnostics used include x-ray spectrometers along with a set of filtered Si diodes with various cutoff energies. To enhance our understanding of this radiation, non-local thermodynamic equilibrium (non-LTE) kinetic models of Ar and Kr have been developed. Approximate plasma parameters have been calculated for experiments with Ar and Kr jets performed under different conditions indicating a broad value of electron temperatures. In particular, modeling of hotter plasma with He-like lines as well as much cooler plasma with “cold” characteristic lines, which are indicative of difference processes in plasmas, will be highlighted.

October 30, 2014

Rishi Pandit

Gamma-ray transport via radiative damping in ultra-fast heated high Z matter

Thursday, October 22, 2014

Radiation transport code coupled with fully relativistic collisional Particle-in-Cell (PIC) code, PICLS, has been developed to study the transport of X-ray photons produced in laser solid interaction. We have implemented the spectral and angular distribution of radiative damping to simulate gamma-ray transport in ultrafast heated high Z matter by an intense short pulse laser. We discuss the laser energy dependence of the emission energy and the intensity dependence of the angular distribution of gamma-rays. By solving the transport of hard X-rays we find that high energy photons emitted by relativistic electrons are co-moving with the electrons and they are intensified continuously. As a result the gamma-rays have the signature of the fast electrons’ temporal and spatial distribution. Comparing the details of gamma-rays via radiative damping with varying laser intensities in simulations, we will discuss the laser parameters and the target conditions (material) to produce the higher yield.

October 22, 2014

Kim Schultz

Analysis and comparison of x-ray image and x-ray burst features produced in laser-gas cluster interactions on the Leopard laser at UNR*

Thursday, October 22, 2014

Results of Ar and Kr gas puff experiments performed on the high-power Leopard laser at NTF are presented. The Leopard laser operated in two regimes: 350 fs, 40 TW pulse or 0.8 ns, 25 GW pulse at a wavelength of 1.057 μm. Flux density of laser radiation in focal spot was from 3x1016 W/cm2 (ns pulse) up to 2x1019 W/cm2 (fs pulse). Additional gas jet characterization was completed in the Radiation Plasma Laboratory at UNR using interferometry and Rayleigh scattering approaches. A comparison is made between the x-ray output of a supersonic and subsonic nozzle. Additionally, two different gases, Ar and Kr, are used as laser targets. Diagnostics included two sets of filtered Si-diodes, x-ray pinhole cameras, and PCDs. Specifically, x-ray images and structure of x-ray bursts are investigated and compared as a function of the nozzle type and gas used. The importance of analysis and comparison of x-ray image and x-ray burst features for a better understanding the mechanisms of the laser energy to x-ray conversion efficiency and future research directions are discussed.

* The work was supported by the Defense Threat Reduction Agency, Basic Research Award # HDTRA1-13-1-0033, to University of Nevada, Reno, and in part by the DOE/NNSA Cooperative agreement DE-NA0001984 and DE-NA0002075.

May 8, 2014

Naima Tariq

Laser Spectroscopy of LiHe, a van der Waals Molecule

Thursday, May 8, 2014

Van der Waals molecules are extremely long-range, extremely weakly-bound molecules. Lithium helium (LiHe) is an interesting van der Waals molecule due to theoretical interest in its molecular structure and properties. We report the first observation of ground state LiHe molecules. We use cryogenic helium buffer gas cooling to produce high densities of atomic lithium at temperatures ranging from 1-5 Kelvin. LiHe molecules are formed by three body recombination:
Li + He + He → LiHe + He (1)
The Li density is continuously monitored via laser absorption spectroscopy. LiHe is detected spectroscopically using both laser induced fluorescence and laser absorption spectroscopy. The LiHe spectrum shows good agreement with a theoretical model of the molecular structure, with only a single bound rovibrational state. In thermal equilibrium, the expected density of LiHe is given by
where is the density of the given species and is the binding energy of the single LiHe bound state. Our data shows good agreement with this model, and we use it to determine the binding energy of the LiHe ground  state. The measured binding energy is consistent with the calculated value. Our work is in progress to measure the rate coefficients for the reactions of Eq. 1.

May 1, 2014

Ben Hammel

Demonstration of Two-Dimensional High-Resolution Velocimetry

Thursday, May 1, 2014

The 500 J (527 nm) laser at the Jupiter Laser Facility (JLF) was used to generate pressures in excess of 13 GPa in single-crystal ([111], [110], and [100]) silicon via direct-drive laser ablation. The time-history profile of shock-velocity was recorded using two Line-VISARs (Velocity Interferometer System for any Reflector) with differing sensitivities. A newly developed two-dimensional high-resolution VISAR was used to study the corresponding elastic-plastic transitions - providing information on the instantaneous velocity of the sample with a spatial resolution of 2.5 μm. These results demonstrate the detector's feasibility for measuring microscopic perturbations on shock fronts in materials being considered as ablators for inertial confinement fusion.

April 24, 2014

Dambar Air

Aerosol Optical and Physical Properties during the California Wildfires

Thursday, April 24, 2014

Multifilter Rotating Shadowband Radiometer (MFRSR) and Cemel sun photometer are used for the measurement of the aerosol optical depth (AOD) and Ångström exponent (AE). The bimodal lognormal distribution and Mie theory are used for column-averaged size distribution of aerosol in the atmosphere. The aerosol optical and physical properties retrieved with the MFRSR show excellent agreement with those obtained with a co-located Cimel sun photometer. The values of the aerosol optical depth (AOD) and Ångström exponent (AE) during the California wildfires event remained higher than clear sky days due to the presence of smoke particles in the atmosphere. The AOD at 500 nm remained larger than 1 during the most intense period of wildfire event and reached an extremely unusual maximum value of 3.2 on August 23 during the wildfires. Back trajectory analysis during the California wildfire indicates that the southerly wind brought the plumes of smoke which originated at California Rim fire, toward Reno. The aerosol fine mode (particle having diameter less than 1 micron) contribution is higher than the coarse mode (particle having diameter greater than 1 micron) and fine mode dominates the coarse mode by about 10:1 during the California wild fire.

April 17, 2014

Erik McKee

Neutron Detection, Production, and Detector Modeling on Zebra

Thursday, April 17, 2014

An intense source of 2.45 MeV neutrons has been produced on the Zebra pulsed-power generator (1-MA/100ns) using wire-array loads that have been treated to hold deuterium. Neutron emission from wire/fiber loads that contain deuterium have been investigated [1]. The fibers are traditionally composed of cryogenically frozen, solid deuterium fibers [1], deuterited polyethylene strands [2], or deuterited palladium wires [3]. Our approach was to use palladium wires as the constraining matrix to physically put deuterium between electrode gap of Zebra and produce neutrons through the reaction branch:In preparation for measuring neutrons generated in experiments on Zebra, we use the Monte Carlo N-Particles (MCNP) radiation transport code to characterize neutron activation detectors. For reasonable assumptions regarding the emitted neutron pulse characteristics, the MCNP code is used to predict the expected neutron signal taking into account scattering events and thermalization effects from the neutron scatter. These effects can bolster neutron yield derivations via unique room geometries and, in general, higher absorption cross-section values for lower neutron energies. We report on neutron yield detector characterization, calibrated against NSTec's dense plasma focus for a known neutron yield, and the build-up factors calculated with MCNP necessary for a measurement of a correct neutron yield at NTF.

[1] J. D. Sethian, A. E. Robson, K. A. Gerber, and A. W. DeSilva, "Enhanced stability and neutron production in dense Z pinch plasma formed from a frozen deuterium fiber," Phys. Rev. Lett., vol. 59, pp. 892-895, 1790 (1987).[2] S. J. Stephanakis, L. S. Levine, D. Mosher, I. M. Vitkovitsky, and F. Young, "Neutron production in exploding wire discharges," Phys. Rev. Lett., vol. 29, pp. 568-569 (1972).[3] Anurag Shyam and R. K. Rout, "Observation of neutrons from palladium deuteride X pinches using a simple low energy capacitor bank," IEEE Trans. Plasma Sci., vol. 27, no. 4, pp. 1210-1213 (1999).

April 10, 2014

Pawan Pathak

AbsorptionAbstract: TBA Spectrum of Rubidium in Solid Neon Matrix

Thursday, April 10, 2014

We have grown solid neon crystals doped with rubidium atoms. Experiments were performed by depositing mixtures of rubidium with neon onto a sapphire window at 4.5 K. Once the crystal was grown, an absorption spectrum was acquired with white light spectroscopy. We applied narrow band Ti:Saph laser light tuned to different peaks and observed the modification of the absorption spectrum due to application of the laser light. We also observed annealing behavior in the rubidium neon absorption spectrum. Annealing experiments were performed by warming to 5-10 K and refreezing to 3.2 K. Annealing the crystal at 8 K and lower temperatures did not produce any significant change in the spectrum; annealing at temperatures higher than 8 K results an absorption spectrum with a high background level.

April 3, 2014

Tirtha Joshi

Hydrodynamic Stability and Tracer Spatial Distribution in Ti-Doped OMEGA Implosions

Thursday, April 3, 2014

We discuss the observation and analysis of implosion-core spectrally-resolved image data from titanium-doped, deuterium-filled, low-adiabat OMEGA direct-drive implosions. The targets were spherical plastic shells of diameter 840μm, the wall thickness varied from 15μm to 27μm, and the filling pressures of deuterium were in the range from 5 atm to 20 atm. In addition, a half micron thick titanium-doped plastic layer (6% atomic concentration) was included and initially located on the inner surface of the shell. The spectral features from the titanium tracer are primarily observed at the collapse of the implosion and recorded with a streaked spectrometer and three identical gated, multi-monochromatic x-ray imager (MMI) instruments fielded along quasi orthogonal lines-of-sight. Both streaked and gated data show simultaneous emission and absorption features associated with titanium K-shell line transitions. The spectrally-resolved images recorded with MMI were processed to obtain narrow-band images1 and spatially-resolved spectra characteristics of contour regions on the image2. An Abel inversion of the image's intensity profiles reveals the spatial distribution of the titanium tracer in the core and thus provides critical information on the symmetry and hydrodynamic stability of the implosion. A multi-layer spectroscopic model was used for the analysis of space-resolved spectra to extract electron temperature and density of the plasma in the core. A new analysis method of the spatially-resolved titanium x-ray lines provides information about the mixing of Ti into the core. In addition, space-resolved spectra were also used to extract the size of the hot-spot in the implosion core.
Work supported by DOE/NLUF Grant DE-NA0000859, and LLNL
1T. Nagayama, et al, J. App. Phys. 109, 093303 (2011).
2T. Nagayama, et al, Phys. Plasmas 19, 082705 (2012).

February 20, 2014

Austin Anderson

X-Ray Radiography and UV Laser Diagnostics at NTF

Thursday, February 20, 2014

1-MA wire array Z-pinches were studied using x-ray imaging at 6.65Å. X-ray imaging allows for the viewing of the dense solid-liquid core of plasma column during the ablation stages. Backlighting was provided using x-ray emissions from a laser-plasma generated by hitting a Si target with the 50 TW Leopard laser. Using a laser based backlighting system offers an advantage of flexible timing and material choices when compared to backlighting provided by x-pinch radiation. UV laser diagnostics were also fielded as a supplemental diagnostic. UV interferometry and Faraday rotation diagnostics allows one to map electron density in excess of 1020 electrons per cm3 and magnetic field on the order of a MG. Fielding both diagnostics allows one to study both the plasma corona and dense core of the wire array during ablation stage of the z-pinch.

February 13, 2014

Tom Lockard

Improvements to a Laboratory Photoionized Plasma Experiment at Z Relevant to Astrophysics

Thursday, February 13, 2014

Due to the observations of x-ray orbiting telescopes like Chandra and XMM-Newton astrophysical photoionized plasmas have been observed with high spectral resolution. A large scale effort has been made to understand and explain these observations in part by the ZAPP (Z Astrophysical Plasma Properties) collaboration. Observations made by astronomers of astronomical objects provide a wealth of detailed information about the atomic kinetics and radiation physics in photoionized plasmas. These observations have also motivated new efforts in understanding these plasmas. Creating a photoionized plasma in a controlled laboratory environment is not a trivial matter due to the large flux of high energy photons needed to drive the photoionization process. This is overcome by the intense source of x-rays produced by the pulsed power Z-machine at Sandia National Laboratory. This powerful driver allows us to produce and study photoionized plasmas in a laboratory environment under well characterized conditions. To understand the complex environment where a collapsing wire array is used to create the intense source of x-rays that are used to produce the photoionized plasma, radiation hydrodynamic simulations help to give insight into the x-ray environment and plasma hydrodynamics of the system. We will also discuss a series of atomic kinetics calculations that were made using a collection of sophisticated codes. This study is an effort to deepen our understanding of the competing processes in laboratory photoionized plasmas.

December 16, 2013

Erik McKee, Bradley Allured, Benjamin Hammel

Applications of the BeagleBone Microprocessor

Monday, December 16, 2013

The BeagleBone is one of a growing array of small, cheap, but versatile and powerful microprocessor boards. Using a 32 bit/ 1GHz ARM RISC CPU, a Linux operating system and enhanced i/o capabilities, these boards are becoming more common in local, real-time control and measurement applications. As part of a Special Problems course, graduate students Brad Allured, Ben Hammel and Erik McKee have developed experimental applications using the BeagleBone board and will present a short summary of the capabilities and operation of the board, and the projects they have implemented. This seminar would be of interest to any experimenter looking at using this technology.

November 21, 2013

Daniel Mayes

Laboratory Astrophysics: Neon Photoionized Plasma Experiments

Thursday, November 21, 2013

Photoionized plasmas are a special class of plasma common in astrophysical environments, but they are a relatively unexplored regime of laboratory plasmas. We discuss an experimental and modeling effort to study the atomic kinetics in plasmas of this type via K-shell line absorption spectroscopy. The experiment employs the intense x-ray flux emitted by the collapse of a z-pinch to produce and backlight a Neon photoionized plasma in a cm-scale gas cell placed at various distances from the z-pinch. High-resolution spectra show absorption in several ionization stages of Neon. Analysis yields the ion areal-densities and charge state distribution, which can be used to benchmark atomic kinetics codes. Results from our dataset are compared with results from simulations using several modeling codes.

November 14, 2013

Jeremy Iratcabal

Experimental Characterization of C2 Molecules in DQp White Dwarfs

Thursday, November 14, 2013

The laser ablation of solid targets with laser intensities of 107-1010 W/cm2 produces plasmas with a large fraction of simple molecules in neutral and low ionization states. Temperatures reached in these plasmas are similar to those found in the outer atmosphere of stellar objects. Observational data from DQp spectral class white dwarfs shows an anomalous blue shifted C2 Swan band absorption spectrum. A leading explanation for this is pressure shifting. A discussion of the C2 Swan bands found in white dwarf atmospheres and a discussion of a laser ablation experiment attempting to reproduce this anomalous pressure shifting will be presented.

November 7, 2013

Justin Wojdula

Design Improvement and Novel Applications for Photoacoustic Aerosol Detection Instrumentation

Thursday, November 7, 2013

The aim of studying aerosol pollution is its repercussions in human health and climate forcing. Aerosol can cause damages in the cardio-respiratory system and therefore premature death. In addition, airborne particles can impact the radiative forcing through light scattering (cooling effect) and light absorption (warming effect), with possible impacts on weather patterns in the atmosphere. The study of aerosol optical properties is associated with a great uncertainty in the measurements due to localized weather patterns and relative uncertainty in the actual measurements techniques. Current filter based techniques are labor intensive and take a significant amount of time to process. Through use of a novel photoacoustic instrument design, it is possible to offer near-ground instant aerosol concentration data, and vital health and safety information. These developmental efforts are moving towards improving current designs by decreasing instrument size and noise floor, both of which have far reaching implications. The new design is going to be lightweight, compact, and inexpensive for applications in Unmanned Arial Vehicles (UAV), remote pollution detection, and industry regulations.

October 31, 2013

Nicholas Beres

Atmospheric Soot Superaggregates: Implications for Health and Radiative Forcing

Thursday, October 31, 2013

The conventional view holds that fractal dimension (Df) 1.8-2.5, mobility diameter (Df) ≤ 1 microns, and aerodynamic diameter (Da) ≤ 300 nanometers are the end-point physical characteristics of soot aggregates emitted from combustion sources. Recent observations of soot emissions from heavily sooting (10-100 parts per million) anthropogenic fires, such as open-air agricultural residue burning and oil pool fires, contradict this view. Last year, findings from three recent independent investigations showed soot superaggregates (SAs) - aggregates of soot aggregates having Df ≈ 2.6, Dm ≥ 3 microns, and Da ≤ 300 nm being emitted in quantities greater than 70% in number and mass from large-scale, open-burning emissions. Average Dm and specific surface areas of these soot SAs are ten and three times greater, respectively, than those of conventional soot particles. These unusually large Dm values render these aerosols undetectable using conventional mobility sizing instruments (such as the SMPS). Alternatively, the aerodynamic diameters - used for estimating the probability of deposition within lungs - of these aerosols are similar to those of diesel soot. These observations suggest that soot SAs could be a large component of open burning emissions with the potential to have deleterious effects on human health and the environment, and previously unaccounted-for impacts on climate forcing. In this talk, we will present our findings on the microphysical properties of soot SAs, and discuss their potential impacts on climate and human health.

October 17, 2013

Matthew Wallace

Techniques for Characterization of Electron Beams in Z-Pinch Plasmas

Thursday, October 17, 2013

A well-known feature in Z-pinch plasmas is the presence of intense beams of energetic electrons inferred by the presence of hard rays. The beams are thought to have electrons with energies exceeding the voltages applied, and contain currents comparable to pinch current. A mystery which remains is the mechanisms leading to this energetic electron component in pinches. Energy measurements of electrons in these beams can provide insight to the mechanisms producing these beams. The use of magnetic deflection coupled with Faraday cup collection to provide these energy measurements, as well as a technique to provide spectroscopic evidence of these energetic electron beams will be discussed.

October 10, 2013

Mark Cunningham

Laser Frequency Stabilization: Two Lasers, One Cavity

Thursday, October 10, 2013

In current atomic, molecular, and optical physics sensitive measurements requiring laser systems are subject to electronic and thermal noise leading to frequency instability. Since the early 1980's there have been a myriad of techniques to help stabilize laser frequency. I will briefly discuss semiconductor laser systems, external optical feedback, temperature control, and current control to help stabilize frequency. I will review saturation spectroscopy, and the Pound-Drever-Hall laser frequency stabilization method as it applies to locking a laser to an optical cavity. Finally I will review procedure and results of locking two lasers to an ultra-stable optical cavity.

October 3, 2013

Steven Keim

Neuroprosthetic Interfaces: An Overview

Thursday, October 3, 2013

The "neuroprosthetic interface" is a relatively new avenue of research, combining electrophysiology, computer science, and robotics. However, electrodes have been used to both stimulate nerve cells and acquire information from action potentials at the neuronal level since as early as the 1950s. Research into the first incarnations of brain-computer interfaces began in the 1970s at UCLA under a grant from the NSF and a contract from DARPA. During the mid-90s, digital processing power began to catch up with an already well-established understanding of individual brain structures and their functions. With this development, the field advanced significantly, allowing for real-time processing of data from multi-electrode arrays as well as the less invasive electroencephalography (EEG) arrays. This talk will focus on the status of the field today, drawing from research being pursued by groups in the neurosciences and biomedical engineering fields. As an introduction to the material, relevant neurophysiology will be discussed, focusing on the motor and somatosensory cortices.

September 26, 2013

Jordan Stutz

Short Range Force Measurement

Thursday, September 26, 2013

Short range forces have been studied for the past several years. Having good sensitivity is important due to how small these forces may be, and one way the sensitivity of these measurements can be increased is by using optically levitated microspheres. Several benefits of using levitated spheres include mechanical decoupling, thermal isolation from the environment, and high Q factors. Using a 300 nm dielectric levitated sphere, it may be possible to measure forces of ~2*10-21 N/(Hz)½. I will discuss our experimental set up, a few tools used, and future implementation that will allow us to make these measurements.

September 19, 2013

Mahmoud Ahmed

Genetic Algorithms in Control Problems

Thursday, September 19, 2013

Genetic algorithms are search algorithms based on some biological mechanisms such as natural selection, mutation and cross over. They are used in searching for optimal solutions in scientific and engineering applications. For instance, they may be used in the control problems to manipulate the parameters in order to force the output to go through a predestined trajectory. Genetic algorithms are inefficient if a global solution is known to the problem under study, or if the exhaustive search for that global optimal solution is doable. Although the global optimal solution is not guaranteed to be reached by a genetic algorithm, one of the optimal local solutions may be found in relatively short times.

September 12, 2013

Kevin Yates

Plasma Formation and Evolution on a Copper Surface Driven by Mega-Ampere Current Pulse

Thursday, September 12, 2013

The conductivity and phase state of a metal surface pulsed by a mega-ampere source is of broad interest. We have designed and conducted experiments where copper and aluminum mm-diameter rods have been driven by a mega-ampere current pulse at UNR’s Nevada Terawatt Facility. The facility’s z-pinch delivers 1 MA in ~100 ns producing megagauss surface magnetic fields that diffuse into the skin layer, ohmically heating the load and causing plasma formation. During the current rise, the metal is heated to temperatures that cause multiple phase changes. When the surface magnetic field reaches a threshold, the metal ionizes and the plasma becomes pinched against the underlying cold liquid metal. Diagnostics fielded include visible light radiometry, two-frame shadowgraphy in both 266 nm and 532 nm wavelengths, and single frame/2ns gated ICCD imaging. Surface temperature, expansion speeds, instability growth, time of plasma formation and plasma uniformity are determined from the data.

April 25, 2013

Tom Lockard

Photoionized Neon Plasma Experiment at the Z Facility

Thursday, April 25, 2013

Astrophysical environments such as x-ray binaries, active galactic nuclei, and accretion disks of compact objects contain photoionized plasmas. Developments in pulsed power sciences like those at the Z pulsed-power facility facility at Sandia National Laboratories have led to the availability of a powerful x-ray source that enables us to produce and study in the laboratory photoionized plasmas relevant for astrophysics under well characterized conditions. We discuss an experimental and modeling effort in which the intense x-ray flux emitted at the collapse of a z-pinch experiment conducted at Z is employed to produce a neon photoionized plasma. The broadband radiation flux from the z-pinch is used to both drive the photoionized plasma and provide a source of backlighting photons to study the atomic kinetics through K-shell line absorption spectroscopy. The design of the experiment involves view factor calculations to model the radiation flux driving the plasma, and radiation hydrodynamic simulations to evaluate the overall dynamics and uniformity of the plasma. The plasma is contained in a cm-scale gas cell located at several distances from the z-pinch, and the filling pressure is carefully monitored in situ all the way to shot time since it determines the particle number density of the plasma. Time-integrated and gated transmission spectra are recorded with a TREX spectrometer equipped with two elliptically-bent KAP crystals and a set of slits to record up to six spatially-resolved spectra per crystal in the same shot. The transmission data shows line absorption transitions in several ionization stages of neon including Be-, Li-, He- and H-like Ne ions. Detailed modeling calculations of the absorption spectra are used to interpret and perform the analysis of the transmission spectra with the goal of extracting the charge state distribution and the atomic population kinetics of the photoionized plasma. The data analysis is performed with the aid of a novel application of genetic algorithms to plasma spectroscopy. Plans for producing photoionized plasmas of other elements and mixtures are discussed as well.

This research was sponsored in part by the National Nuclear Security Administration under the High Energy Density Laboratory Plasmas grant program through DOE Grant DE-FG52-09NA29551, and SNL.

April 18, 2013

Austin Stafford

High Temperature Ni and Ni/Al Precursor Plasmas produced on Zebra at UNR

Thursday, April 18, 2013

Previous work based on experiments on the 1 MA Zebra generator at NTF/UNR was dedicated to studies of precursor plasmas with Cu cylindrical wire arrays (CWA) which were shown to have much higher temperatures compared to previously considered precursor plasmas of CWA. Continuing research at 1 MA on Zebra found precursors for moderate atomic number CWAs to be consistent with Cu results. Recently experiments have been performed on Zebra using mixed Ni/Al CWAs to study how the precursor will be affected by the non-uniformity of the load. Examples of the analysis of results of Ni and Ni/Al precursor plasmas being produced are presented and compared, and future work is discussed.

April 11, 2013

Austin Anderson

Intro to Chaos Theory

Thursday, April 11, 2013

Commonly referred to as the “butterfly effect”, chaos theory is the study of complex systems that feature a hypersensitivity to the initial conditions of the system. The unpredictable nature of chaotic systems arises from the mathematical instability of the deterministic equations that govern the system itself, rather than deriving from random chance. First discovered by Henry Poincaré near the end of 19th century, chaos theory became well-known in the mid-20th when Edward Lorenz accidently discovered it while working on a computer simulation. A general overview of chaos theory, Lorenz attractors, Lyapunov exponents, and some of their applications will be discussed in this talk.

April 4, 2013

Zephyr McCormick

Photoelastic Force Measurements

Thursday, April 4, 2013

The sensing of small forces is an important problem in many fields. In particular, to better understand small animalia locomotion, techniques to quantify the small forces exerted by swimming, crawling, and burrowing “critters” have been investigated. As a photoelastic material, gelatin can be used to sense micron-scale forces. Gelatin can be formed into spherical and other geometries, to mimic granular material and other interesting environments. The motivation, formation, applications, and preliminary characterization of gelatin spheres will be discussed.

November 8, 2012

Rishi Pandit

Spectral and Angular Distribution of Photons via Radiative Damping in Super Intense Laser Matter Interaction

Thursday, November 8, 2012

The spectral and angular distribution of photons produced in the interaction of an extremely intense laser (> 10^22 W/cm^2) with dense plasma are studied with the help of a collisional particle-in-cell simulation, PICLS. In ultra-intense laser-plasma interaction, electrons are accelerated by the strong laser fields and emit gamma-ray photons mainly via two processes, namely, Bremsstrahlung and radiative damping. Such relativistic gamma-rays have a wide range of frequencies and the angular distribution depends on the hot electron source. From the power loss calculation in PICLS we found that the Bremsstrahlung will get saturated at I > 10^22 W/cm^2 while the radiative damping will continuously increase. Comparing the details of gamma-rays from the Bremsstrahlung and the radiative damping in simulations, we will discuss the laser parameters and the target conditions (geometry and material) to distinguish the photons from each process and how to catch the signature of radiative damping in future experiments.

October 25, 2012

Nada Al Taisan

Formation and Spectroscopy of LiHe Van der Waals Molecule

Thursday, October 25, 2012

Van der Waals (vdw) molecules are extremely long-range, extremely weakly-bound molecules. They are characterized by the weakest binding energies of any ground- state molecules, on the order of a wavenumber (1cm^-1 = 1.23 x 10^-4eV). This is three orders of magnitude smaller than the binding energy of a typical ionically bound molecule. Helium vdw molecules have been the subject of many theoretical studies, but have never been spectroscopically detected. Using cryogenic helium buffer-gas cooling, we formed lithium helium(LiHe) Van der Waals molecules at low temperatures ranging from 1-5 Kelvin. Spectroscopic detection and thermodynamic equilibrium properties of LiHe will be presented.

October 18, 2012

Nagendra Aryal

Confinement Resonances in Photoionization of Xenon Inside a Fullerene Ion Cage

Thursday, October 18, 2012

The first experimental evidence for confinement resonances associated with photoabsorption by a Xe atom trapped inside a C60 cage is presented. The well-known giant 4d resonance in photoionization of the Xe atom has been predicted theoretically to be split into multiple peaks when a Xe atom is encaged inside a C60 molecule due to multipath interference of photoelectron waves from the Xe atom which may be reflected or transmitted by the spherical C60 cage. The experiment was conducted at the Advanced Light Source, Lawrence Berkeley National Laboratory, using an ion-photon merged-beams endstation. Prior to the measurements, Xe@C60 endohedral fullerene molecules were synthesized by bombarding vapor-deposited C60 molecules by 160 eV Xe+ ions.
Photoionization measurements were performed in the photon energy range 60eV - 150eV. The interference phenomenon was observed in the cross sections for double photoionization of Xe@C60^+ yielding Xe@C60^3+, Xe@C58^3+ and Xe@C56^3+ product ions.

October 11, 2012

Austin Stafford

High Temperature Precursor Moderate Atomic Number Plasmas produced on Zebra at UNR

Thursday, October 11, 2012

Previous work based on experiments on the 1 MA Zebra generator at NTF/UNR was dedicated to studies of precursor plasmas with Cu cylindrical wire arrays (CWA) which were shown to have much higher temperatures compared to previously considered precursor plasmas of CWA. Continuing research at 1 MA on Zebra found precursors for moderate atomic number CWAs to be consistent with Cu results. Recent Cu CWA experiments have been performed on Zebra using a Load Current Multiplier (LCM) that raises the current up to 1.7 MA using a full set of diagnostics. Examples of the analysis of results of moderate atomic number precursor plasmas being produced, at both 1 MA and 1.7 MA on Zebra, are presented and compared and future work is discussed.

October 4, 2012

Kiattichart Chartkunchand

Photodetachment of Group 14 Negative Ions: Observation of Semi-Forbidden Transitions

Thursday, October 04, 2012

The technique of Velocity-Map Imaging (VMI) spectroscopy has been utilized to investigate photon-ion interactions in the Ge^{-}, Sn^{-}, and Pb^{-} isoelectronic sequence at 532 nm photon wavelength. Fine-structure-resolved photoelectron kinetic energy spectra and angular distributions are presented, as well as evidence of a "forbidden'' ^{4}S_{3/2} --> ^{1}D_{2} transition in both Ge^{-} and Sn^{-}. This transition is explained in terms of the breakdown of LS-coupling for these species.

September 27, 2012

Kiran Baral

Photoionization and Photofragmentation of Fullerene Molecular Ions

Thursday, September 27, 2012

Experimental results are reported for ionization and ionization with fragmentation of the fullerene molecular ions C60+ and C70+ after excitation by monochromatized vacuum ultraviolet synchrotron radiation at different photon energies: 22 eV, 35 eV, 65 eV, 105 eV and 140 eV. Since fullerenes are composed of even numbers of carbon atoms, the fragmentation occurs by the loss of differing numbers of carbon atom pairs. The energy dependences of relative cross sections for direct photoionization yielding C602+ and C702+ are compared with those for forming different doubly charged fullerene fragment ions.

September 13, 2012

Shambhu Das

Entropy of a Black Hole using Hawking's Theory and String Theory

Thursday, September 13, 2012

A black hole is a place in space where gravity is so strong that even light cannot escape out of it. The black hole has tremendous gravity because a very large amount matter has been squeezed into a tiny space. Since no light can get out of it, a black hole is invisible to the eye but a space telescope with special tools can help to find it. A black hole has entropy. Entropy is a measure of the disorderedness of a system. The entropy of a black hole was first formulated by Bekestein and Hawking. Entropy of a black hole can also be given by "string theory". In the last few decades, string theory has been emerged as the most promising theory to provide a complete, unified, and consistent description of the fundamental structure of our universe. So it is also known as the theory of everything, it describes the entire world, made of strings.

May 3, 2012

Narayan Adhikari

Ground-Based Remote Sensing of Temperature and Humidity Profiles of the Atmospheric Boundary Layer

Thursday, May 3, 2012

The atmospheric boundary layer (ABL) plays a vital role in the exchange of heat, momentum, humidity and other trace substances with the Earth's surface. The knowledge of ABL is extremely important for weather forecasting and air quality assessment. Though balloon soundings of atmospheric temperature, humidity, pressure and winds are performed in many world-wide locations, they are normally done twice a day, primarily for large scale weather forecasting applications, leaving much of the boundary layer evolution and structure without assessment. Ground-based remote sensing methods provide detailed knowledge of the ABL more frequently. The Fourier transform infrared (FTIR) spectrometer is used to measure the downwelling thermal infrared radiances emitted by the gases in the cloud-free atmosphere. The measured radiance spectra are then compared to the model spectra to infer the structure of the ABL. This presentation will describe the ABL, atmospheric infrared measurements and remote sensing of the vertical structure of the atmospheric temperature and water vapor.

April 26, 2012

T.E. Lockard

Experiments and modeling of photoionized plasmas at the Z facility

Thursday, April 26, 2012

Astrophysical environments such as x-ray binaries, active galactic nuclei, and accretion disks of compact objects contain photoionized plasmas. Developments in pulsed power sciences like those at the Z pulsed-power facility facility at Sandia National Laboratories have led to the availability of a powerful x-ray source that enables us to produce and study in the laboratory photoionized plasmas relevant for astrophysics under well characterized conditions. We discuss an experimental and modeling effort in which the intense x-ray flux emitted at the collapse of a z-pinch experiment conducted at Z is employed to produce a neon photoionized plasma. The broadband radiation flux from the z-pinch is used to both drive the photoionized plasma and provide a source of backlighting photons to study the atomic kinetics through K-shell line absorption spectroscopy. The design of the experiment involves view factor calculations to model the radiation flux driving the plasma, and radiation hydrodynamic simulations to evaluate the overall dynamics and uniformity of the plasma. The plasma is contained in a cm-scale gas cell located at several distances from the z-pinch, and the filling pressure is carefully monitored in situ all the way to shot time since it determines the particle number density of the plasma. Time-integrated and gated transmission spectra are recorded with a TREX spectrometer equipped with two elliptically-bent KAP crystals and a set of slits to record up to six spatially-resolved spectra per crystal in the same shot. The transmission data shows line absorption transitions in several ionization stages of neon including Be-, Li-, He- and H-like Ne ions. Detailed modeling calculations of the absorption spectra are used to interpret and perform the analysis of the transmission spectra with the goal of extracting the charge state distribution and the atomic population kinetics of the photoionized plasma. The data analysis is performed with the aid of a novel application of genetic algorithms to plasma spectroscopy. Plans for producing photoionized plasmas of other elements and mixtures are discussed as well.

April 19, 2012

Naima Tariq

Spectroscopic search for LiHe, a Van der Waals molecule

Thursday, April 19, 2012

LiHe is an interesting Van der Waals molecule due to theoretical interest in its molecular structure and properties. A Van der Waals molecule has very different molecular potential than a normal molecule due to its low binding energy and large internuclear separation. LiHe molecules have never been observed. I will discuss an experiment to create LiHe at cold temperatures and measure it spectroscopically using laser induced fluorescence.

April 12, 2012

Sara Altemara

High Resolution UV Laser Probing for the 1-MA Z-Pinch, Zebra

Thursday, April 12, 2012

High resolution laser probing diagnostics at 266nm were developed on the 1-MA z-pinch machine, Zebra. A spatial resolution of 5-8μm was achieved to investigate the fine structure of the dense z-pinch plasma in the late implosion and stagnation phases. The smaller absorption and refraction of the 266nm beam in the high energy density plasma allows the UV radiation to penetrate deeper into the pinch than the 532nm laser probing diagnostic. The UV laser beam penetrates the trailing mass plasma to image the dynamics of the dense pinch inside of the plasma column. Instabilities, breaks in the dense pinch, and micro-pinches were imaged. A delay channel was installed to study the dynamics of the dense pinch as they change in time.

April 5, 2012

Nancy Quiros

High Energy Density Laboratory Astrophysics: Modeling Supernova Explosions

Thursday, April 5, 2012

Near the end of a star's life, its dense core is surrounded by outer layers of hydrogen and helium. After the core collapse of a supernova, the heavier elements are moving outward and decelerated by the hydrogen and helium layers, where they mix. The purpose of a laboratory experiment performed at the Nevada Terawatt Facility (NTF), is to simulate the explosion of a supernova and learn about the mixing process after its core collapse. A current pulse of 1 MA generated by the Zebra machine at the NTF, is sent through insulator-coated metal wires to produce the Rayleigh-Taylor instability, believed to be responsible for the mixing process in supernovae.

March 29, 2012

Ryan Royle

Effects of Laser Frequency on Hot Electron Source in Fast Ignition

Thursday, March 29, 2012

Sufficient energy coupling between ignition laser and implosion core is crucial for the feasibility of the fast ignition scheme for inertial confinement fusion. The ignition laser energy is absorbed by plasma electrons at the critical density which can be more than 100?m away from the compressed core. The hot electrons must traverse the gap and deposit their energy into a small region of the core, creating a hot spot to ignite fusion burn. To achieve sufficient energy coupling, it is necessary to reduce divergence of the hot electron beam and limit electron energies to a few MeV so they do not overshoot the intended target. In this study, 2D particle-in-cell simulations are used to examine the effects of a 1?m wavelength laser and its 2nd and 3rd harmonics (2 and 3 times the initial frequency) on hot electron divergence and energy spectra. Results suggest that the number of hot electrons in the few MeV range increases linearly with frequency due to the inverse relationship with average electron energy. However, electron divergence remains unaltered.

March 8, 2012

Cris Montoya

Atomic spin coupled to a mechanical system

Thursday, March 8, 2012

Ever since the birth of quantum mechanics, the boundary between quantum and macroscopic phenomena has been studied, ultracold systems have highly contributed to the break down of this limit. A microcantilever coupled to the atomic spin of cold trapped atoms showing resonant magnetic interaction between the cantilever's frequency and the atomic spin states, would be a milestone for applications on precision measurement and quantum information systems as well as for the study of ground state mechanical systems, and quantum coherence in macroscopic systems. The experimental preparation for such a system and intermediate results toward the completion of it are discussed as well as the future second generation of the ongoing experiment.

March 1, 2012

Heman Gharibnejad

Coupled cluster and convergence

Thursday, March 1, 2012

The coupled-cluster (CC) method has been applied to solve many-body problems in quantum chemistry as well as computational atomic and nuclear physics. A relativistic linearized variant of the CC method, which is generally referred to as the all-ordered method, has been successfully used in atomic physics for atomic structure calculations. Calculations of this sort yield results such as energies, transition amplitudes, hyperfine constants, polarizabilities, isotope shifts and parity-nonconserving amplitudes. The development of the all-ordered method in monovalent systems such as Cs, Fr, Rb+ has had important consequences on the study of symmetries, the development of atomic clocks, ultra cold physics and others.
The all-order method requires iterative solutions of the linearized coupled-cluster equations, which sometimes lead to convergence issues. These problems are of two kinds:
-Non-convergence, meaning the iterations oscillate and never meet the convergence criteria.
-Convergence to nonphysical results, which is caused by the existence of more than one solution to the original nonlinear CC equations.
Furthermore, the convergence of iterations may be slow and require many iterations.
Several methods have been developed in general quantum chemistry that address the said convergence issues. Here, we consider one such method dubbed direct inversion of iterative subspace . It uses approximate solutions found through linear combinations of last few iterations and tries to minimize the errors through the use of least square technique.

February 23, 2012

Yadab Paudel

Self-proton/ion radiography in high intensity laser solid interactions

Thursday, February 23, 2012

Protons and multicharged ions generated from high-intensity laser interactions with thin foil targets have been studied at Nevada Terawatt Facility. We have observed the first experimental evidence that protons/ions accelerated from the front surface of the target, opposite to the laser propagation direction, are pulled back to the target by the magnetic field generated in the front of the target surface by the laser pre-pulse. This protons/ions beam is able to create a radiograph of the target and glass stalk holding the target itself through the RCF stack. This self-radiography phenomenon could be explained by the existence of the magnetic field in the laser pre-pulse in relativistic laser matter interaction.

February 16, 2012

Tirtha Joshi

Investigation of hydrodynamic stability in OMEGA direct-drive implosions using spectrally-resolved imaging

Thursday, February 16, 2012

We discuss the observation and analysis of implosion-core spectrally-resolved image data from low-adiabat OMEGA direct-drive implosion experiments. The targets were spherical plastic shells filled with various filling pressures of deuterium. In addition, a thin titanium-doped plastic layer (6% atomic concentration) was included and initially located on the inner surface of the shell. The x-ray signal from the titanium tracer is primarily observed at the collapse of the implosion and recorded with a streaked spectrometer and the gated, multi-monochromatic x-ray imager (MMI) instrument. Both streaked and gated data show simultaneous emission and absorption features associated with titanium K-shell line transitions. The spectrally-resolved images recorded with MMI were processed to obtain narrow-band images and spatially-resolved spectra based on the x-ray lines of the titanium tracer. On one hand, the narrow-band images reveal the spatial distribution of the titanium tracer in the implosion core and thus provide an indication of the hydrodynamic stability of the implosion. On the other hand, detailed spectroscopic analysis of the spatially-resolved line spectra yield electron temperature and density, and mixing in the implosion core.

February 2, 2012

Philippe Leblanc

Comparison between equilibrium and non-equilibrium modeling of ionization dynamics in hot dense plasmas

Thursday, February 2, 2012

An ultra-intense laser pulse is capable of creating hot dense plasma, which is an interesting testbed to study high energy density physics related to astrophysics and fusion sciences. It is not understood how the plasma ionizes in high density matter. Electron generation and transport in ultrafast heated materials play an important role in determining the evolution of the plasma. Correctly modeling electron transport in solid materials begins with understanding the hot electron generation mechanism, that is the process of ionization. To allow for the modeling of the heating of matter from its cool neutral state to the state of plasma, the Thomas-Fermi equilibrium model has been widely used to dynamically predict the ionization level based on the local properties of the plasma. However, plasmas generated by intense laser heating are inherently not equilibrated for femtosecond timescales. Using the particle-in-cell code PICLS, which is a kinetic plasma simulation code, a new ionization model based on impact cross-section which is intended to correctly model ionization for non-equilibrium plasmas has been proposed and implemented. Simulation results of high intensity short pulse laser-matter interactions using each ionization model are presented for comparison.

November 3, 2011

Heather Johns

Hydrodynamic Analysis of Direct-Drive ICF Implosions at OMEGA

Thursday, November 3, 2011

Early hydrodynamic simulations of experiments performed at LLE's OMEGA laser system will be discussed. These 1-d simulations, performed with the radiation hydrodynamics code HYDRA, represent a precursor to benchmarking attempts between 3-d HYDRA results, the experimental spectra, and spectroscopic models. The experimental diagnostics, including 3 DDMMI, allow spatially resolved spectra along 3 lines of sight. A brief overview of the physics of inertial confinement fusion and hydrodynamics will be given as part of this discussion.

October 20, 2011

Dan McEvoy

The Utility of Precipitation Indices as Indicators of Drought in Nevada

Thursday, October 20, 2011

Throughout the western United States snow-dominated watersheds prevail and are critical for water supply for both human and natural environmental demands. Therefore, understanding characteristics of water shortages, such as length and severity, is extremely important to both scientists and the general public. Drought indices provide an assessment of drought conditions using easily accessible input data such as precipitation and temperature. This study examines the effectiveness of two indices, the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI), to identify drought periods and consider the role of evapotranspiration processes in drought conditions. Whereas the SPI reflects solely precipitation deficit or surplus, the SPEI takes into account the effects of both precipitation and temperature and incorporates a simple water balance. At certain locations large differences were found between SPI and SPEI, which lead to the underlying question of this study: Which index more accurately identifies drought periods of the past? Three steps were taken to draw conclusions from this study: 1) compare SPI and SPEI and determine where and at what time scales the largest differences appear, 2) establish relationships between drought indices and hydrologic and atmospheric variables, and 3) identify historical drought periods and evaluate index performance. The largest discrepancies between indices appear in the southwest United States and more specifically southern California, western Arizona, and southern Nevada. Initial results suggest in arid locations a drought index which incorporates a water balance will prove more useful for drought monitoring applications.

October 11, 2011

Trevor J. Burris-Mog

Laser Accelerated Protons: The Transition from Fundamental Research to Applications

Thursday, October 11, 2011

Over the last decade, there has been extensive research regarding the generation of multi-MeV proton beams from the interaction of intense (I > 1018 W/cm2) short-pulse (t [ps]) laser light with solid micron-thick foils. Although this results in high (>1013) per-shot proton yields with small transverse (<0.004 mm mrad) and longitudinal (<104 eV s) emittances, the transition from fundamental research to applications will require control of the 100% energy spreads and large divergences (>20o half-angles) associated with laser acceleration. Results demonstrating efficient capture, collimation and focusing of laser accelerated proton beams from the PHELIX laser system and the ability to control their broad energy range using a pulse power solenoid are presented. Simulations using measured data for the input parameters yields information about the phase-space and transport efficiencies of the captured proton beams. To conclude, results from a feasibility study of a pulse power, compact, achromatic gantry concept for a distant-future laser-driven proton therapy system will be presented.

September 29, 2011

Phillipe Leblanc

Ionization models for hot electron transport in ultra-intense laser-solid interactions

Thursday, September 29, 2011

Having a good model of ionization is important to study the hot electron transport in ultra-intense laser-solid interactions. One of the commonly applied models to calculate ionization in a plasma is the Thomas-Fermi (TF) model. This model is derived from equations of state which in turn assumes the plasma is in an equilibrium state. However, for the timescales of interest for the study of laser-matter interactions, laser created plasmas are highly transients and thus are not in an equilibrium state. The predicted ionization levels by the TF model are based on incorrect assumptions about the plasma conditions. In an effort to improve the accuracy of predicted ionization levels, an impact ionization model is applied to particle-in-cell simulations. This model calculates ionization levels from electron-ion collisional cross-sections and should be better suited to predict ionization levels for non-equilibrium plasmas. A comparison between the TF and Impact model is made for different plasma parameters. Preliminary results indicate the discrepancies between the models to increase as the atomic number (Z) of the target increases. Additionally, the laser intensity is varied to explore how each models reacts in different hot electron energy regimes.

September 22, 2011

Daniel Papp

Non-precursor implosion in closely spaced nested wire arrays

Thursday, September 22, 2011

Wire array Z-pinches are the most intense and energetic laboratory X-ray sources. In such targets consisting of one to a few hundred wires are driven by MA-magnitude current pulses for a few hundred ns. The most commonly used targets has the wires arranged cylindrically, in a single or double arrays of wires. The latter are referred as nested cylindrical arrays. In most of wire array loads, at the beginning of the implosion, the wire material ablates and jxB forces collect it on the axis of the array. Closely spaced nested arrays were studied at the 1 MA Zebra generator at UNR Nevada Terawatt Facility. These nested cylindrical arrays have a small gap between the inner and outer wires. They were designed so that the outward jxB forces on the inner wires should prevent the formation of the precursor plasma. Closely spaced nested arrays with 6-8 wires imploded without precursor, as observed on shadowgrams and optical streak images. Precursor formation was observed in arrays with 12-16 wires, despite calculated outward ablative forces. Two-frame UV probing was used to resolve details in the ablating plasma. Implosion was found to similar to the cascading implosion in "star" arrays. The keV X-ray yield of closely spaced arrays was found to be 1.5-2.5 times the yield of cylindrical arrays of the same mass.

April 7, 2011

Dambar Air

Many-body Calculation of Energy Resonance in Be-like carbon

Thursday, April 7, 2011

The dielectronic recombination was described by applying the configuration interaction method together with the many body perturbation theory (CI+MBPT). In this work an advanced method is used in which CI+MBPT approach combine with the complex rotation method(CRM). The energy positions and widths of DR resonances of J=1 odd symmetry were calculated by using CI+MBPT+CRM method. This method found a good agreement with NIST recommended values.

March 10, 2011

Mike Weller

Radiative Properties of Mixed Nested Cylindrical Wire Arrays on Zebra at UNR

Thursday, March 10, 2011

Our existing theoretical and experimental tools were applied to study mixed nested cylindrical wire arrays (NCWA) performed on the Zebra generator at UNR. In particular, experimental results of mixed brass (70% Cu, 30% Zn) and Al (5056, 5% Mg) NCWAs are compared and analyzed. The inner and outer arrays were approximately the same mass (~30 micro g/cm for both brass and Al), such that Al was either all in the outer array or inner array, and the same with brass. Consequently, radiative properties of K-shell Al and Mg ions and L-shell Cu and Zn ions are compared as functions of the placements of the brass and Al wires on the inner and outer arrays. Results show that the placement of brass or Al, whether inside or out, dramatically changes the intensity of the x-ray emission of L-shell Cu to K-shell Al (by a factor of ~5, i.e. Cu L-shell and Al K-shell are more intense when on outside compared to when inside) and the overall energy yield (~17 kJ when brass is outside, compared to ~14 kJ when brass is inside). A full diagnostic set which included more than ten different beam-lines was implemented. Identical loads were fielded to allow the timing of time-gated pinhole and x-ray spectrometers to be shifted to get a more complete understanding of the evolution of plasma parameters over the x-ray pulse, which showcases the importance of time-gated diagnostics, in particular time-gated spectroscopy. In addition, these results are compared with mixed Mo and Al NCWAs from previous experiments. The importance of the study of NCWAs with different wire materials is discussed.

December 2, 2010

Vijay Singh

Inelastic collisions of CaH molecules at cryogenic temperatures

Thursday, December 2, 2010

We are interested in using doublet-sigma molecules to investigate the role of electron spin in cold chemical reactions. As a first step towards this goal, we are creating polarized samples of CaH molecules and studying their collisional properties. We create CaH molecules by laser ablation of a solid CaH2 target and cool them by helium buffer gas cooling. To study the polarization changing collisions of CaH molecules with He atoms, we spin polarize CaH by optical pumping. We measure the spin depolarization rate by setting the return to equilibrium. At 4 Kelvin, our measurement shows a depolarization rate coe?cient of 10-11 cm3 s-1, which is very large compared to the measured spin depolarization rate at 0.4 Kelvin. This large depolarization rate is likely due to excited rotational states. Progress towards measuring this rate coe?cient as a function of temperature will be discussed.

November 18, 2010

Rishi Pandit

Radiation Damping

Thursday, November 18, 2010

Laser intensity is getting stronger and stronger. Usually, electron motion is influenced by electromagnetic field and it is influenced to a small extent by radiation damping. We studied the radiation damping effect in the super intense laser regime. We solve the Dirac equation of motion for a single particle interacting a strong laser field. When laser intensity is greater than 1022 w/cm2, radiation damping is become very large and important to include the particle motion.

October 21, 2010

David Martinez

X-ray yield from pinch-on-target interaction

Thursday, October 21, 2010

The wire array z-pinch is an efficient x-ray source which has been proposed for use in indirect drive inertial confinement fusion schemes. Extensive research has focused on methods to enhance the x-ray yield in a z-pinch. In recent experiments, performed at the Nevada Terawatt Facility, it was observed that a wire added on axis as a target for the implosion increased the total x-ray production of the pinch. This effect was strongly accentuated when a conical wire array was used. To study the effect the center wire had on the z-pinch x-ray emission, an experiment compared conical and cylindrical wire arrays with different size and material center targets. The x-ray emission's dependence on the target diameter will be presented.

October 14, 2010

Kiran Baral

MElectron-Impact Ionization of Multiply Charged Ions

Thursday, October 14, 2010

A crossed-beams apparatus based on the "animated beam" technique is used at the University of Nevada, Reno to measure absolute electron-impact ionization cross sections of multiply charged ions. Experimental results are presented as a function of electron energy for single ionization of Se2+ and Se3+.The data suggest that inner-shell excitation followed by autoionization dominates the single- ionization cross sections for these ions.

October 7, 2010

Kevin Yates

Magnetized Target Fusion and the FRC

Thursday, October 7, 2010

A description is given for the scientific basis for Magnetized Target Fusion (MTF). This qualitatively different approach to fusion is being explored at Los Alamos National Laboratory and Air Force Research Laboratory at Kirkland Air Force Base. MTF is in an intermediate plasma regime between magnetic and inertial fusion energy. MTF is attractive due to its low cost allowing its development to occur on a faster time scale. MTF uses a field-reversed configuration as its target plasma and is surrounded by an imploding cylindrical metal liner for compression. The kinetic energy of the imploding liner does work on the magnetized DT fuel, bringing it to a temperature relevant for fusion reactions. The magnetic field lines are compressed during the implosion increasing the magnetic field strength to approximately 500 Tesla. This magnetic field is used to suppress thermal conduction losses allowing for fusion burn.

September 30, 2010

Nagendra Aryal

Photoionization Cross - Section Measurements for Xe@C60+ Ions

Thursday, September 30, 2010

Experimental evidence for confinement resonances associated with photoabsorption by a Xe atom trapped inside a C60 cage is presented. The well-known 4d giant resonance in photoionization of Xe atom has been predicted theoretically to be split into multiple peaks when Xe atom is encaged inside a C60 cage due to the interference of photoelectron waves from Xe atom as they are reflected by the C60 cage. The experiment was conducted at the Advanced Light Source, Lawrence Berkeley National Laboratory, CA using the ion-photon merged-beams endstation. Measurements were performed in the photon energy range 60eV - 150eV and this phenomenon was observed in the cross section for double photoionization of Xe@C60+ yielding Xe@C583+ product ions.

September 23, 2010

Daniel Papp

Introduction to the physics of wire array Z-pinches

Thursday, September 23, 2010

Fast wire array Z-pinches are the most intense laboratory X-ray sources, and have the potential for inertial confinement fusion. In such an experiment, electric current of MA size is driven through a load of thin wires, with a total mass of around 1mg in a few hundred of ns. Experiments on the Z machine at Sandia National Lab achieved 350TW power and 2.7MJ energy with a 26MA drive current. The implosion of such loads have four stages. Wire initiation, ablation of plasma rom the wires, implosion of wire cores and stagnation of the pinch on the central axis. The dynamics of the ablation and implosion phase is strongly dependent on the wire array configuration. The pressure equilibrium of a stagnating pinch can be described by the Bennett equation. The collapse of the pinch is caused by the growth of the m=0 and m=1 mode magnetic Rayleigh-Taylor instabilities. Wire array Z-pinch radiation consist of atomic lines superimposed on a continuous soft X-ray Bremsstrahlung background. Kinetic energy and Spitzer resistive heating only accounts for a fraction of the total radiated energy. Several possible mechanisms were proposed in literature to explain this discrepancy.

March 26, 2010

Sandra Stein

Kelvin-Helmholtz Instability in a Sheared Flow Actuated by a Magnetic Field

Thursday, March 26, 2010

The Kelvin-Helmholtz instability can lead to plasma transport across a magnetic field; one example is the solar wind transport across the earth's magnetopause in the magnetotail. In an experiment done at the Nevada Terawatt Facility, we observed the Kelvin-Helmholtz instability in a laser produced plasma that interacted with an external magnetic field. This instability is evidenced by the presence of evenly spaced vortices on the plasma-field boundary. Due to the interaction with the external magnetic field, a velocity gradient perpendicular to the plasma velocity forms at this boundary. The presence of vortices in a region of sheared flow is characteristic of the development of the Kelvin-Helmholtz instability. The observed structure and its growth rate indicate that large ion Larmor radius effects contribute to its formation. Discussions of the mechanism producing the sheared flow and the resulting instability will be presented.
Work supported by DOE/NNSA grant DE-FC52-06NA27616

March 18, 2010

Mahmoud Ahmed

Green Lasers

Thursday, March 18, 2010

Semiconductor lasers are built as a thin layer of indium gallium nitride (InGaN) sandwiched between two layers of GaN,and. By applying a suitable voltage, the electrons and holes recombine driven by the electric field annihilating one another and generating photons. The wavelength of this light depends on the indium content of the active layer-more leads to longer wavelengths and thus greener light. But the more indium in these layers, the more that indium is likely to pool into small "islands" during manufacture. The islands can alter the light's wavelength- an unacceptable flaw in a laser. These problems are solved by starting with a thin wafer of pure, crystalline GaN that has been sliced along a larger crystal's m-plane and then polished giving rise to a new bunch of application to these multi-colored and highly efficient lasers.

September 10, 2009

Kenneth Williamson

The Life and Times of a Plasma Formation

Thursday, September 10, 2009

High energy-density (HED) plasmas have many applications as soft x-ray radiators that can be used to drive fusion reactions, to study the effects of radiation on materials, to study the physics of stellar interiors, or to study basic plasma science. The wire array z pinch is an effective method for producing and studying HED plasmas in a laboratory setting. It is essentially an array of wires strung between the anode and cathode of a pulsed-power generator. The fast-rising current heats and ionizes the wires while the Lorentz force 'pinches' the plasma on the 'Z' axis. Many types of wire materials and configurations are used to produce various plasmas of interest, but the topic of this presentation is the double-planar wire array configuration. The break from symmetry produces unexpected, long-lived, off-axis plasma formations before the bulk implosion. Analysis of the life and times of these formations will be presented along with a discussion of energy balance, radiation symmetry, and equilibrium.

September 3, 2009

Glenn Osbourne

Comparative Analysis of Implosions of Tungsten Single and Double Planar Wire Arrays Produced on the 1-MA Generator at UNR

Thursday, September 4, 2009

Analysis of the results of tungsten single and double planar wire array (PWA) experiments performed on the UNR 1MA generator "Zebra" is presented. Uniform as well as combined PWAs with Al wires arrays were investigated and compared. Previous spectroscopic work focused on analysis of time-integrated spatially resolved (TISR) stagnation phase data, while this presentation continues and expands that study to include the early implosion plasma stage and more data from recent shots. A full diagnostic set was utilized, including a pulse laser shadowgraphy system, a gated fast intensified CCD (ICCD) camera, an x-ray time-gated pinhole camera, a TISR spectrometer, and a time-gated spatially integrated (TGSI) spectrometer. Non-LTE kinetic modeling is used to describe spectroscopic data for combined PWAs in particular. Wire Ablation Dynamics Model is employed to model the implosion and interpret shadowgraphy data. Opacity effects in plasmas from uniform and combined PWAs with Al wires are discussed and compared. The advantage of using the tracer Al wires is emphasized. Applications to current fusion research are also taken into consideration.

April 30, 2009

Mei-Ju Lu

Atomic Coherence Experiments in a Pure Nuclear Spin System

Thursday, April 30, 2009 at 1:00 pm

Spin coherence is widely used in many atomic physics experiments such as atomic magnetometers and quantum memory. Having long spin coherence times and large numbers of atoms are of essential importance for those experiments. However, it is often difficult to maintain long spin coherence times while having large atomic numbers because of inelastic collisions. We propose using a pure nuclear spin system which is expected to have much better inelastic collisional behaviors than alkali atoms and less dephasing problems due to stray magnetic fields. In this seminar, I will present the details of how we use laser ablation and buffer-gas cooling to generate large atomic numbers and atomic density of ytterbium atoms at 5 Kelvin. I will also discuss the slow light experiment by using electromagnetically induced transparency (EIT) in Yb-173 atoms as well as the measurements of spin depolarization (T1) and spin decoherence (T2).

April 30, 2009

Brian Chrisman

Study of Ultra-Intense Laser Produced Plasmas via Computer Simulation

Thursday, April 30, 2009 at 1:00 pm

Recent advances in the development of intense short pulse lasers have led to exciting progress in high energy density physics (HEDP). As an example, a several _m thin foil that is irradiated by a 100 TW, sub-picosecond laser pulse reaches keV (1 keV _ 11,000,000 C) temperatures at solid density. The resultant electron distribution is temporarily far out of equilibrium, featuring two or more widely distinct temperatures. In modeling such extreme plasmas, both kinetic and collisional effects on the energy transport are essential. Of particular difficulty is the large density gradients between the critical density,(the density at which the laser is absorbed), and solid densities exceeding several hundred times the critical density. For a 1 _m wavelength laser pulse, the critical density, nc, is 1021 cm??3. This means that a numerical model needs to describe the laser-plasma interaction in the low density region, as well as fast particle transport in the extremely dense target region where Coulomb collision processes are important for energy transfer. In cone-guided fast ignition inertial confinement fusion experiments, fuel previously compressed by an ablative implosion is ignited by the injection of an intense short laser pulse via a cone embedded within the fuel target. The implosion precondition creates density scales which range over five orders of magnitude from the cone interior to the highly compressed core. A critical issue for this process is whether the hot electrons produced in the interaction are in an energy range conducive to efficient heating of the core. In this work, Particle-in-Cell simulations evaluate the entire cone- guided fast ignition experiment for the first time, including hot electron generation at the cone tip, energy transport to the compressed fuel core, and subsequent collisional core heating. The laser- plasma interaction within the cone target is particularly important, as temperatures of hot electrons generated here are found to be lower than previously expected while overall absorption is influenced .

April 23, 2009

Sandrine Gaillard


Thursday, April 23, 2009

Fast ignition fusion requires transporting a large amount of energy into the compressed ICF capsule in a short period of time. One method for efficiently transporting laser energy into a compressed fusion capsule is to use a cone situated near or in the capsule, decreasing the distance to the core, improving laser conversion efficiency and concentrating the charged particle energy flow. Results from recent experiments performed at the LANL 200 TW Trident short-pulse laser at ~1020 W/cm2 (80-100 J and ~600 fs) are presented. We have conducted laser-ion acceleration experiments comparing flat foils and new Cu micro-cone targets in three separate geometries (flat-top, funnel, and snub-nose) to elucidate the production of hot electrons and ions in these cones, which have been shown to produce higher proton energies and conversion efficiencies than flat foils at lower laser intensity and energy (~2x1019 W/cm2, 20 J). Data from a Cu K? 2D imaging crystal, an X-ray single hit CCD, proton beam images on RCF film stacks, and an electron/proton spectrometer are presented and compared, showing the importance of not just generating hot electrons, but in the case of ion acceleration, efficiently propagating these hot electrons to the accelerating "tip", where they can then be efficiently converted to ion energy.

April 16, 2009

Michael Weller

Overview of three research topics: Electron Fluid Dynamical Waves, STM Imaging of GaAs, and The Measurement of Optical Properties of Aerosols

Thursday, April 16, 2009

Students in physics often encounter a broad range of research topics. What will be presented here are three main research projects that I have worked on at three different institutions: Arkansas Tech University, The University of Arkansas, and Jet Propulsion Laboratory. The first topic covers Electron Fluid Dynamical Waves, in which a one-dimensional, steady state, electron fluid dynamical model is used to describe ionizing breakdown waves. The second topic covers an attempt to more routinely attain STM imaging for cross sectional study of III-V compound semiconductor heterostructures, in particular GaAs. The third topic covers a method of measuring optical properties of aerosols by measuring the complex forward scattering of the electromagnetic field.

April 9, 2009

Taisuke Nagayama

Overview of Spectroscopic Electron Temperature and Density Diagnostics of ICF Implosion Cores

Thursday, April 9, 2009

Inertial confinement fusion (ICF) is a process to initiate nuclear fusion reactions by heating and compressing a fuel target. The goal of the ICF research is to achieve an efficient fuel burn as known as "ignition." Thus, techniques to study plasma conditions in the core are significant to study what actually happened in ICF experiments, and also to benchmark the hydrodynamics simulation codes. Two different levels of electron temperature and density diagnostics can be achieved by analyzing data recorded by two spectrometers SSCA and MMI. Basic principles will be discussed along with the processing and analysis of the data.

April 2, 2009

Chris Plechaty

The Role of Flute Instabilities in Plasma Penetration of a Magnetic Field

Thursday, April 2, 2009

In both natural and laboratory environments, plasma-magnetic field interactions are common. Under many circumstances, a plasma can penetrate across an externally applied magnetic field. To investigate the role of flute instabilities in the penetration of a plasma across an applied magnetic field, an experiment was performed at the Nevada Terawatt Facility (NTF). In the experiment a plasma produced by laser ablation of a polyethylene target, penetrates an external magnetic field generated by passing a 0.6 MA current (in 200 ns) through a straight cylindrical electrode. The laser produced plasma penetrates due to flute instabilities which form on the interface layer between the plasma and the magnetic field. The results of two experiments, performed in different plasma regimes, will be presented.

March 26, 2009

Kiran Baral

Overview of Endohedral Fullerenes

Thursday, March 26, 2009

A general review of endohedral fullerenes is presented. Their structures, properties, synthesis and possible applications in various areas are discussed. The photoionization of endohedral fullerenes using the photon-ion merged-beams technique is presented along with a brief introduction of the ALS photon-ion end station.

March 6, 2009

David Martinez

Overview of Wire Array Z-pinch Instabilities

Thursday, March 6, 2009

Wire array Z-pinches are an effective way to generate short intense X-ray pulses for inertial confinement fusion. In order to optimize the X-ray yield and power, it is important to have knowledge of the pinch instabilities which play a profound role in plasma confinement. In the Z-pinch, instabilities form as early as the ablation of the wires and then seed Rayleigh-Taylor instabilities during the implosion of the plasma shell. Lastly, these instabilities manifest themselves as the pressure driven MHD instabilities, such as the "sausage" and "kink" instabilities, that are responsible for the disruption of the pinch plasma column. In this talk I will review the Z-pinch implosion and discuss the prominent instabilities.

February 26, 2009

Rohini Mishra

Hot electron generation at steep interface in ultra intense laser-solid interaction

Thursday, February 26, 2009

In laser plasma interaction experiments, super intense laser lights >1020 W/cm2 are able to sweep the pre-plasma over short times and compress the density gradient. Hot electron generation and absorption of ultra-intense laser light in the step-like densities, highly overdense plasma is discussed. To understand absorption physics we performed one-dimensional collisional particle simulation (PICLS1D). These simulations are able to simulate hot electrons generation in the laser plasma interaction, and energy transport thorough the cold resistive plasma. For a high intensity normal incident laser light the absorption is mainly due to JXB absorption. We observed the electrons are trapped in the interaction region by the electrostatic potential, and the JXB force drives the high energy electrons inwards from the trapped region with the frequency 2w (twice the laser frequency). Except these 2w electron jets there are also large number of diffusive electrons with lesser energy and with the frequency 3w, 6w. When we increase the target density, the JXB electron jets start to disappear resulting in to the decrease of total absorption. The physics inhibiting the production of 2w frequency electrons discussed. Together with this analytical prediction for 2w and diffusive electrons is predicted which is consistent with the simulation results.

November 20, 2008

Ken Williamson

Scaling of Radiation Yields of Planar Wire Arrays at the 1MA Zebra Generator

Thursday, November 20, 2008

Analysis is presented on scaling of radiated x-ray power, energy and implosion timing of single planar wire arrays (SPWA) and double planar wire array (DPWA) of Mo and W with respect to current peak (0.8-1.4 MA), mass and array dimensions at 100ns current pulse. Such scaling investigations are important for understanding the potential of these loads as an ICF radiation source1. These data are used to identify promising directions to pursue with regard to highest x-ray output, smallest load size, and most consistent shot-to-shot performance. It is shown that W SPWA and DPWA total energy yield and peak power increased near-quadratically with current. DPWA scans of inter-planar gaps from 1.5mm to 9mm show an output maximum at 1.5mm with decreasing output for 6mm and 9mm. A DPWA width scan shows that radiation yields decrease slowly as the width is decreased, which may allow for more compact loads without significant sacrifice to the output radiation. A mass scan of several W loads show that the implosion timing increases with mass.

November 13, 2008

Nagendra Aryal

An Introduction to Fullerenes and Their Photoionization

Thursday, November 13, 2008

A brief introduction to the Nobel Prize winning discovery of fullerenes, a new form of carbon molecules, will be discussed with their unique structures, characteristics and possible applications in various areas. Current absolute cross-section measurements for the single photoionization of various fullerene ions using the ion-photon merged-beams endstation at the Advanced Light Source in Berkeley, California will be discussed along with the photo excitation of surface and volume plasmons in fullerenes.

November 6, 2008

Vijay Singh

Increasing Lifetime of an Air Plasma Channel Using Short Laser Pulses

Thursday, November 6, 2008

An experiment in pump-probe configuration was conducted to increase the lifetime of an air plasma channel. The plasma channel was created by focusing a 50ps, 36mJ laser pulse in air. The lifetime of the plasma channel was estimated by monitoring the temporal evolution of the fluorescence of a spectral line at 504.5 nm of N+ transition 3p 3S - 3s 3p�. A four-fold increase in lifetime of the plasma channel was observed by propagating a 6ns, 12mJ laser pulse through the channel at 7ns delay after the picosecond laser pulse. In order to understand the process of increase in lifetime, we characterized our plasma channel using optical emission spectroscopy. Assuming the plasma to be in local thermodynamic equilibrium, plasma temperature of ~ 8.2 eV was calculated from Boltzmann plot of relative intensities of nitrogen lines. The electron density of ~ 1018 cm-3 was estimated using Stark broadening of 649.2 nm line of N+ transition 3d 3D� - 4p 3D. An enhancement in electron density of the plasma channel by a factor of ~ 2 was observed at a 7ns delay of the nanosecond laser pulse relative to the picosecond laser pulse.

October 30, 2008

Kyle Beloy

Microwave clock based on atoms in an engineered optical lattice

Thursday, October 30, 2008

We propose a new class of atomic microwave clocks based on the hyperfine transitions in the ground state of aluminum or gallium atoms trapped in optical lattices. For these elements magic wavelengths exist at which both levels of the hyperfine doublet are shifted at the same rate by the lattice laser field, cancelling its effect on the clock transition. The accuracy of the proposed microwave lattice clock is competitive to that of the state-of-the-art primary frequency standard.

October 23, 2008

Glenn Osborne

Diagnostic of Charge Balance in High-Temperature Tungsten Plasmas Using LLNL EBIT

Thursday, October 23, 2008

Diagnostic of high-temperature M-shell W plasmas is challenging because of contribution of numerous ionization stages in a relatively narrow x-ray spectral region. A method using LLNL EBIT data generated at different electron beam energies has been established for the identification of prominent spectral features and for the determination of charge balance in x-ray M-shell W spectra between 3.5 and 8.5 �. This diagnostic procedure was tested with results from Z-pinch plasmas produced on the 1 MA pulse power generator Zebra at UNR.

October 9, 2008

Michael Bakeman

Magnetic Characterization and Design of an Undulator-based Electron Beam Diagnostic

Thursday, October 16, 2008

The LOASIS Laser Wakefield Accelerator (LWFA) has achieved quasi-mono-energetic electron beams with energies up to 1 GeV. These beams offer the potential for use with insertion devices such as wigglers and undulators to create tabletop XUV and x-ray free-electron laser (FEL) sources. To achieve a high quality light source producing high brightness radiation requires an electron beam with low energy spread and low emittance. Here we discuss the use of an undulator to generate XUV radiation to characterize in a single shot the electron beam energy spread and emittance with high precision.

October 9, 2008

David Martinez

Analysis of Conical Wire Array Z-Pinch Stability with a Center Wire

Thursday, October 9, 2008

The presence of sheared flow has been conjectured to stabilize z-pinch instabilities. Its effect has been investigated theoretically and computationally. However, experimental testing in the high energy density regime is still lacking and, generally, dedicated experiments are rare. In a z-pinch relevant current-carrying configuration, plasma flows can be generated using conical wire arrays. Supersonic and s streams of plasma ablated from the wire surfaces are accelerated towards the array axis in a direction perpendicular to the wires. At the axis, the kinetic energy of the radial motion is thermalized. The axial momentum is preserved, leading to the formation of an axial plasma flow. The implosion of a conical wire array normally produces a current carrying, unstable, non-uniform emitting column on the axis. Placing an additional wire on the axis of the array introduces a velocity shear (radial gradient of the axial velocity) in the flow. A second important role of the central wire is to carry current early on. After it turns into plasma, the instabilities are afforded time to grow before the flow fully develops. Therefore, adding a wire on the axis of a conical wire array provides a simple way of investigating the effects of sheared flows upon the z-pinch stability. Based on this concept, an experiment was developed and performed at the Nevada Terawatt Facility on the 1 MA Zebra pulsed power generator. Results show that a wire on the axis of a conical wire array stabilizes the precursor and the pinch. To identify the stabilization mechanism, the study included equivalent cylindrical wire arrays. This comparison indicated that a thick (~50 �m) central wire is sufficient to stabilize the kink mode in both arrays. In contrast, a thin (~10 �m) central wire produced a stable pinch only in the conical configuration, indicating that the sheared flow is effective. Additional experimental tests have been designed to further verify this observation.

September 25, 2008

Madhu Gyawali

Aerosol Optics of Summer 2008 California Wildfires: Comparison with a 'Normal' Month

Thursday, September 25, 2008

Hundreds of wildfires in Northern California were sparked by lightning during the summer of 2008, resulting in downwind smoke for the months of June and July associated with the flaming and smoldering stages of the fires. These fires are consistent with a growing trend towards increasing biomass burning worldwide. Climate impacts from the smoke depend critically on the smoke amount and aerosol optical properties. We report comparison of aerosol optics measurements in Reno Nevada made during the very smoky summer month of July with the relatively clean, average month of August. Photoacoustic instruments equipped with integrating nephelometers were used to measure aerosol light scattering and absorption at wavelengths of 405 nm, and 870 nm. Total aerosol optical depth was measured with a sun photometer operating at 430nm, 470nm, 530nm, 660nm, 870nm and 950nm. These measurements document the intensity of the smoke optical impacts downwind. They are processed further to reveal a strong variation of the aerosol light absorption on wavelength, indicating the presence of light absorbing organic carbon and perhaps wavelength dependent absorption caused by black carbon particles coated with organic and inorganic particulate matter. On the day with most smoke in Reno (July 10, 2008), Angstrom coefficients for absorption as high as 3.6 were found for wavelengths of 405 nm and 870 nm, with the corresponding single scattering albedo near 0.92 at 405 nm. Aerosol optical depths of 3.5 were found for 430 nm on July 10th from the sun photometer measurements. A roughly fourfold increase in aerosol optical quantities was observed between the months of July and August 2008, attesting to the large average effects of biomass aerosols from the California wildfires. The 'normal' month of August exhibits surprisingly low Angstrom exponents for aerosol light absorption at mid day when the single scattering albedo is highest, likely as a consequence of the wavelength dependence of aerosol light absorption by particles coated with non absorbing organic and inorganic matter.

September 18, 2008

Nicholas Ouart

Analysis of Compact Cylindrical Wire Arrays Implosions with Brass and also by Alternating Brass and Al wires on the 1-MA COBRA Generator

Thursday, September 18, 2008

Implosions from compact cylindrical wire arrays (CCWA) with mid-Z and low-Z wires were carried out on the 1-MA COBRA generator at Cornell University. In particular, the CCWA used either Brass 310 (70% Cu, 30% Zn) or Al 5056 (95% Al, 5% Mg) wires, as well as combination of them arranged in an alternating pattern. This study will focus mainly on the spectroscopy of the loads containing brass by applying the non-LTE kinetic models of Cu and Zn to account for the L-shell radiation. The resulting plasma parameters, electron density and temperature, will be discussed and compared. Simulations with the Wire Ablation Dynamics Model to analyze the differences in implosion dynamics of the uniform and alternating CCWA will be discussed.

September 11, 2008

Heather Johns

Inertial Confinement Fusion Implosions

Thursday, September 11, 2008

Inertial confinement fusion (ICF) is an experimental method in which the plasma is confined only by the inertia of the initial implosion, driven by either lasers or particle beams. A project involving time-resolved data from direct-drive implosions at OMEGA is given as an example of scientific ICF research. The state of the compressed Ti-doped shell is determined using a model of Ti K-shell line absorption spectra, developed to create synthetic spectra at known electron temperatures (Te) and electron number densities (Ne). Synthetic spectra which compare to the data well therefore reveal the Te, Ne state of the compressed shell at the time the spectrum was produced.

May 1, 2008

Guoxun Tian

Oxygen Measurement using Photoacoustics

Thursday, May 1, 2008

Available instrumentation includes innovative optical and acoustic instruments for the real-time quantification of particulate matter (PM) and its optical properties. Instruments have been designed to measure aerosol extinction, dots scattering and absorption components, and are being used for ambient measurements in air quality studies. We use oxygen as a sample and built a photo-acoustic detection system to measure the absorption of oxygen by using frequency and amplitude modulation of a laser at the same time. Compared with the theoretical spectrum, measurements using this system match very well.

April 24, 2008

Sudarshan Dhugana

Electrons in an External Oscillator Potential

Thursday, April 24, 2008

The problem of the Schrodinger equation for one electron in harmonic oscillator potential is exactly solvable. The same problem for two electrons (interacting with Coulomb potentials) can have an approximate analytical solution. For large separation distance between the two electrons, the corresponding potential is cast in the form of a parabolic potential and hence the problem can be solved exactly using quantum mechanics. The consequence arising due to the presence of two electrons will be discussed. A small separation distance between the two electrons leads to the anharmonic oscillator problem which may be solved by the perturbation method. The three-dimensional Schrodinger equation for three electrons in a simple harmonic confinement potential can be decoupled into three pairs of problems, provided the expectation value of the center of mass vector R is small compared with the average distance between the electrons. The solutions for different correlation limits will be discussed.

April 17, 2008

Mei-Ju Lu

Measurements of Spin-Relaxation Collisions for Cold Atoms with Orbital Angular Momentum

Thursday, April 17, 2008

Atoms with nonzero orbital angular momentum have highly anisotropic interactions resulting in strong coupling between their internal quantum state and their motion. While the collisional properties of these atoms have been studied theoretically for decades, there is little experimental data at cold temperatures. We use helium buffer gas cooling, optical pumping, and laser spectroscopy to measure spin-relaxation collisions in anisotropically interacting atoms. We also use electromagnetically-induced-transparency to measure spin-decoherence collisions. The presentation will include experimental methods and results for Ti-He inelastic collisions.

April 3, 2008

Marin Djendjinovic

Volumetric Properties of Solvated Protein: A Molecular Dynamics Study

Thursday, April 3, 2008

Proteins are large organic molecules whose building blocks, amino acids, are arranged in a linear chain and joined together by peptide bonds. A functioning protein is solvated inside of a living cell, and has its amino acids arranged in a 3D conformation, called native state. A change in environment (such as change in pressure, temperature, a solvent pH value or other) can cause protein to "unfold" to a linear chain (denatured state). In denatured state, protein cannot perform its biological function. This transition, which is a reversible thermodynamic process, is yet to be fully understood. It has been shown, however, that the understanding of volumetric properties of solvated proteins in the native state, namely the coefficient of thermal expansion and compressibility, is crucial to the understanding of the transition between the two states. In this presentation,a recent computer simulation study of the compressibility of the BPTI protein will be discussed.

March 20, 2008

Rajan K. Chakrabarty

Aerosol Classification by Morphology using Electrostatic Charge

Thursday, March 20, 2008

A novel technique to select fractal-like aerosol agglomerates based on their morphology using electrostatic classifiers will be presented. Given a population of agglomerates with similar mobility diameters, the technique distinguishes agglomerate morphology based on the net charge carried by the agglomerate. This talk will discuss the successful application of this technique to flame-soot agglomerates where singly and doubly net-charged particles corresponding to mobility diameters of 220 and 460 nm, respectively, were size selected using electrostatic classifiers and shown to have different morphologies. The flexibility and simplicity of this technique does not limit its application to aerosols, but makes it an attractive candidate for performing particle shape selection of different types of nano and micro materials.

March 13, 2008

Kiattichart Chartkunchand

Laser Photodetached Electron Spectroscopy of Negative Ion Beams

Thursday, March 13, 2008

Laser Photodetached Electron Spectroscopy (LPES) is an important technique in the study of negative ions. Here a beam of negative ions produced by a cesium-sputter source is crossed perpendicularly with monochromatic laser light, ejecting the excess electron in a process known as photodetachment. The photodetached electrons, or photoelectrons, are then measured by an electron energy spectrometer to produce a photoelectron kinetic energy spectrum. Along with providing measurements of the electron affinity of the neutral species which makes up the negative ion, the LPES technique also facilitates the measurement of photodetachment differential cross sections through angular distribution measurements of the ejected photoelectrons. Use of the LPES technique on both atomic and molecular negative ion beams, with particular emphasis on the hydrogen molecular anion, will be discussed.

March 6, 2008

Madhu Gyawali

'NASA-A TRAIN' Coordinated Satellite Measurements of The Earth's Atmosphere

Thursday, March 6, 2008

The 'NASA-A Train' constellation of seven Earth-orbiting satellites remotely sense clouds, aerosols, water vapor, and trace gases such as CO2, NO2, and O3. The satellites orbit the earth sun-synchronously at an altitude of 705 km with Aqua in the lead, followed in order by CALIPSO, CloudSat, PARASOL, and Aura (two more satellites OCO and Glory will join the constellation at the end of 2008). With Aqua in the lead and Aura at the tail, this formation has been termed the afternoon constellation or A-Train, and all satellites cross the equator within a few minutes around 1:30 pm local time (ascending node). These satellites employ revolutionary measurement methods to probe the Earth's atmosphere, and will improve our weather and climate forecasts. Data from these satellites can be used together to obtain comprehensive information about atmospheric processes. This talk will give an overview of the active and passive remote- sensing measurement techniques used on satellite as well as description of the importance of the atmospheric properties being measured. A discussion will be presented on the importance of synergistic measurements of the satellite suite.

February 28, 2008

Tasha Goodrich

A Semi-Analytic Liner Implosion Model for Flux Compression on ATLAS

Thursday, February 28, 2008

A flux compression experiment is being designed for the Atlas pulsed power facility. The purpose is to investigate generation of megagauss fields with liner technology in the geometry needed for compression of a stabilized diffuse z pinch. To survey possible parameters quickly and conveniently, a semi-analytic model has been developed that computes liner motion under the assumption that the liner remains cylindrically symmetric during the implosion and the metal of the liner is incompressible. Thus the liner thickness increases during implosion in a predictable way to conserve liner mass. Equations are derived for the time variation of liner position and circuit current including the effect of back pressure from the compressed flux. The model allows using realistic Atlas circuit parameters. The equations are integrated using the Matlab program and a standard Runge Kutta method. Recently the model has been extended to account for a shunt resistor and the resulting time-dependent current that would be generated inside the liner. The important advantage of a shunt resistor is that an auxiliary power supply is not needed to generate the seed flux which liner motion will compress. By tapping the power of Atlas to generate the seed flux, the incremental cost of a flux compression experiment is minimized. The selection of shunt material and dimensions must consider both the heating of the shunt and the amount of trapped flux, which along with the liner kinetic energy determines the final level of compressed magnetic field. Initial results suggest that readily available materials (a steel shunt and an aluminum liner) and properly chosen dimensions give a workable combination that generates magnetic field of several megagauss.

February 21, 2008

Nicholas Ouart

Measurements of plasma conditions in precursor plasmas on the 1-MA Zebra generator

Thursday, February 21, 2008

Precursor plasmas have been observed on many z-pinch experiments at various facilities, including low current (~1 MA, MAGPIE, Zebra, COBRA) and high current (>15 MA, Z) facilities. The impact of the precursor on stagnated plasmas and inertial confinement fusion targets is still under evaluation. Experiments have been performed on Zebra to study precursor plasmas with copper cylindrical wire arrays in collaboration with Sandia National Labs. Significant precursor radiation at photon energies >1 keV was observed on filtered PCDs. Precursor temperatures and densities have been obtained from modeling of the Cu L-shell emission recorded from a time-gated x-ray spectrometer. The precursor plasma temperatures are consistently >250 eV, which is higher than previously observed.

February 14, 2008

Essam Yasin

Plasma Micro and Macro Instabilities

Thursday, February 14, 2008

Plasma instabilities are of great importance, especially in controlled fusion research. This seminar will focus on the classification of plasma instabilities, how they develop and what are their roles in studying plasma. The seminar will focus on the two-stream instability and its importance, particularly in beam physics.

February 7, 2008

Brian Chrisman

Intensity Scaling of Hot Electron Coupling in Cone-Guided Fast Ignition

Thursday, February 7, 2008

A critical issue for the fast ignition of inertial fusion targets, where compressed fuel is ignited by injection of an intense short laser pulse, is whether the hot electrons produced in the interaction are in an energy range conducive to efficient heating of the core. This work presents the first comprehensive two dimensional kinetic simulation of the cone-guided approach to fast ignition. Simulation results predict the hot electron temperature to be much lower than previous expected, which indicates the possibility to use them for optimum core heating. Furthermore the roles of collisional versus kinetic processes for transport and as heating mechanisms of the core plasmas are clarified. The core heating efficiency scales linearly with intensity at ranges of 1019 - 1020 W/cm2, but falls below this range due to deflection of hot electrons in strong magnetic filaments behind the cone target.

Take the next step...