2 TW Zebra Pulsed-Power Generator
The 2 TW Zebra pulsed-power generator has three stages of power amplification. The Marx bank consists of 32 1.3 μF capacitors charged in parallel up to 100 kV and discharged in series, and which can store a total energy of up to 200 kJ.
The bank is discharged into a second (intermediate) stage of power amplification, which consists of a coaxial, 28 nF, 3.5 MV capacitor. This capacitor uses water as dielectric, and is fired by an SF6-insulated Rimfire switch. When this intermediate storage capacitor is about 80% charged, the switch is electrically triggered or self-breaks, thus connecting to the third stage of the pulse compression. This third stage consists of a 50 ns, 1.9 Ω coaxial vertical transmission line and an eight-channel, self-breaking water switch.
Once this switch is closed, a voltage pulse with amplitude of about 2 MV is applied on the final feed to the load, located in an experimental vacuum chamber on top of the last, vertical, transmission line. Finally, a current pulse with maximum intensity of up to 1.2 MA, and with a 90-ns rise time is delivered to the load.
Complementary Apparatus include:
- Load Current Multiplier (LCM) system - for currents near 2 MA
- External pulsed vertically directed (z-direction) magnetic field.
Current and voltage measurements are taken in all sections of Zebra; in the Marx section, close to the load region, and at the insulator stack. The voltage is measured with V-dot (capacitive divider) detectors and resistive dividers. The current measurement is taken by B-dot monitors (pick-up coils) or by CVR's. Current through the load is measured with three differential B-dot monitors located about 15 cm from the chamber axis at three different azimuthal locations.
Current Pulse Regimes:
Typically, the Zebra Marx bank charging voltage is 85 kV and the energy stored is 150 kJ.
Two load current pulse regimes are currently available:
|Regime||Maximum Current Intensity||Current Rise Time|
|Short Pulse||1 MA||~100 ns|
|Long Pulse||0.6 MA||~200 ns|
In the short pulse regime, the current has a pedestal that increases approximately linearly for about 100 ns to about 5% of the maximum current intensity. The long-pulse regime is obtained by forgoing the last stage of power amplification (the water gap switches are closed), thereby, eliminating the >300g acceleration of the load region components associated with the water gaps firing.
The Zebra experimental chamber sits atop the Zebra vertical transmission line. It has 16 diagnostics ports, equally spaced at 22.5 degrees, with alternating diameters of 7.6 cm and 4.4 cm. In the current configuration, the chamber wall plays the role of the current return conductor, so all diagnostics have to be installed outside the wall, at a minimum distance of 30 cm from the chamber axis to avoid damage from EMP effects. The load region of Zebra is conveniently accessible, for fielding a variety of x-ray, optical and particle diagnostics, and for developing the coupling of the high intensity short pulse laser to the load chamber.
Trigger pulses with jitter lower than 5 ns rms with respect to the maximum load current intensity are available for diagnostics that require delay times shorter than 350 ns. If the delay required is longer, the available trigger pulses have jitter around 15 ns rms.
Experiments and loads
Being a high-impedance generator (1.9 Ω ), Zebra can accommodate a variety of different loads in the experimental chamber without a significant effect on the load current pulse characteristics. Examples of Zebra experiments and loads frequently used include:
- Wire-array Physics: Single and nested cylindrical wire arrays, planar, star-like, conical, x pinches, single wires
- Megagauss magnetic field generation: Helical and horse-shoe coils
- Magnetic acceleration of flyers: Short-circuit conductors
- Atmospheric Physics: Gas spark
Short-pulse, high-intensity laser pulses (from the 50 TW Leopard laser) are commonly transported to the Zebra vacuum chamber through the Optical Switching Apparatus (an evacuated beam line). Currently, the laser beam can be focused with refractive optics in a region of about 5 cm in diameter around the center of the Zebra experimental chamber.