Green-laser-triggered Water Switching at 1.6 MV

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. The 1.6 MV triggering experiments reported here are performed on a water switch in the middle of a 1.8 m long 7.8 Omega coaxial water line that is directly charged by a 65 kJ Marx generator. The 10 to 90% rise-time of the sinusoidal pulse impressed across the water switch is 250 ns. To trigger the switch, we transport a green laser beam (0.4 J, 7 ns pulsewidth) radially inward though the water of the coaxial line to a 'dry 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.