Laser triggering of water switches in terrawatt-class pulse power accelerators.

Abstract

Focused Beams from high-power lasers have been used to command trigger gas switches in pulse power accelerators for more than two decades. This LaboratoryDirected Research and Development project was aimed at determining whether high power lasers could also command trigger water switches on high-power accelerators. In initial work, we determined that focused light from three harmonics of a small pulsed Nd:YAG laser at 1064 nm, 532 nm, and 355 nm could be used to form breakdown arcs in water, with the lowest breakdown thresholds of 110 J/cm 2 or 14 GW/cm 2 at 532 nm in the green. In laboratory-scale laser triggering experiments with a 170-kV pulse-charged water switch with a 3-mm anode-cathode gap, we demonstrated that ~90 mJ of green laser energy could trigger the gap with a 1-σ jitter of less than 2ns, a factor of 10 improvement over the jitter of the switch in its self breaking mode. In the laboratoryscale experiments we developed optical techniques utilizing polarization rotation of a probe laser beam to measure current in switch channels and electric field enhancements near streamer heads. In the final year of the project, we constructed a pulse-power facility to allow us to test laser triggering of water switches from 0.6-MV to 2.0 MV. Triggering experiments on this facility using an axicon lens for focusing the laser and a switch with a 740 kV self-break voltage produced consistent laser triggering with a + 16-ns 1-σ jitter, a significant improvement over the + 24-ns jitter in the self-breaking mode.

Two-dimensional photoacoustic imaging of femtosecond filament in water

“…Thus, the evolution of the laser pulse and the laser-induced electron density distribution in the filament are of fundamental interest. Prior estimates of such param eters have been obtained through numerical calculations and several indirect experimental methods including optical probing [2][3][4][5], plasma fluorescence [6], shadow photography [7,8], sonography [9], interferometry [10], polarimetry [11] and secondary electrical discharges [12].…”