Laser-triggered water switching

We are experimentally exploring the use of lasers as a method of command triggering high-voltage water switches, by focusing the laser light to a breakdown arc in pure water. We are using Nd-YAG laser radiation at 1064 nm and its harmonics at 532, 355, and 266 nm. We report a measured breakdown threshold of 1.4 /spl times/ 10/sup 10/W/cm/sup 2/ at 532 nm, a power level that is easy to achieve with commercially-available lasers. The breakdown threshold is an order of magnitude higher for 1064 nm or 355 nm light. Scattering of the laser beam by laser-induced index of refraction gradients is noticeable at 532 nm and becomes an increasing problem at shorter wavelengths, preventing us from breaking down water at all at 266 nm. We produced breakdown arcs more than 4-cm in length by focusing 75 mJ of 532 nm laser energy in water with a 29-cm-focal-length lens. In addition to the breakdown arcs, we note that focusing 532-nm light into pure water vibrationally excites the water, causing the generation of coherent beams of red Raman-shifted radiation. The vibrationally-excited columns of water are readily visible as they emit red light in all directions. Some of the vibrationally-excited water columns we observed were over 30 cm long.