J.R. Blickem

Low-Inductance Gas Switches for Linear Transformer Drivers

Summary form only given. We are investigating several alternate gas-switch designs for use in linear transformer drivers. To meet linear-transformer-driver requirements, these switches must be DC charged to 200 kV, be triggerable with a jitter of 5 ns or less, have very low pre-fire and no-fire rates, and have a lifetime of at least several thousand shots. Since the switch inductance plays a significant role in limiting the rise time and peak current of the LTD circuit, the inductance needs to be as low as possible. The switches will be required to conduct current pulses with ~100-ns rise times and 20-80 kA peak currents, depending on the application. Our baseline switch, designed by the High Current Electronics Institute in Tomsk, Russia is a robust six- stage switch with an inductance on the order of 120 nH that is insulated with 30-40 PSIG of air. We are also testing two smaller two-stage switches that have inductances on the order of 60 - 70 nH. The smaller switches are insulated with 100-200 PSIG of air. We will discuss details of the operation of each of the switches, including voltage hold-off, triggerability, and lifetime testing.

170-kV laser-triggered water switch experiments

We report the results of experiments using a small Q-switched Nd:YAG laser at 532 and 1064 nm to trigger a 170-kV pulse-charged water switch. 1-/spl sigma/ jitters as low as /spl plusmn/1.7 ns were demonstrated; an order of magnitude improvement over the /spl plusmn/25-ns jitter of the switch in its self-breaking mode. At the optimum observed triggering wavelength of 532 nm, a 7-ns laser pulse gave better results than a 0.15-ns laser pulse. Time resolved optical diagnostics suggest a multistage triggering process in which the laser forms a string of point plasmas between the switch electrodes. These point plasmas expand, cool and merge, forming a vapor column between the electrodes that breaks down rapidly with low jitter.

Laser-triggered water switching

Summary form only given. We are studying the use of lasers to command trigger high-voltage water switches. The jitter in self-breaking water switches currently; limits the peak power and timing accuracy in a number of high-voltage accelerators. We have previously demonstrated water breakdown using focused laser beams at 1064, 532, and 355 nm (1). The lowest breakdown threshold of 110 J/cm/sup 2/ or 14 GW/cm/sup 2/ occurred at 532 nm. In initial experiments, we demonstrated laser triggering of a 3.5-mm water gap that was pulse-charged to 170 kV in 300 ns using 70 mJ of focused laser energy at 532 nm. The laser was focused through a hole in one electrode to break down the water in the middle of the gap. The, repeatability of these initial experiments appeared to be degraded by small bubbles on the electrodes and by material in the water that was blasted off the electrodes by the laser. New experiments using a debubbled, flowing water system are in progress.

Green-Laser-Triggered Water Switching at 1.6 MegaVolts

Multiple water switches are used in the self-breaking mode in many large pulsed power systems. We are studying laser-triggering of water switches at voltages of up to 1.6 MV to see whether we can lower the command jitter of water switches. We have previously reported studies of 170-kV water switching with command jitters as low as plusmn2 ns. Our experiments are performed on a water switch in the middle of a 1.8-meter long 7.8-ohm coaxial water line that is directly charged by a 65-kJ Marx generator. The zero-to-peak risetime of the sinusoidal pulse impressed across the water switch is ~350 ns. To trigger the switch, we transport a green laser beam (0.4 J, 7-ns pulsewidth) radially inward through the water of the coaxial line to a box inside the inner coax line. There, the laser beam is turned 90 degrees and focused through a hole in one electrode to a breakdown arc in the water between the switch electrodes. Best results, of plusmn8.3 ns jitter and 100 ns delay at 60% of the self-break voltage, have been achieved using an axicon lens to focus the beam to a long narrow chain of point breakdowns between the switch electrodes