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