A mode-locked laser as a light source for Schlieren photography

Second harmonic radiation from a mode-locked neodymium: glass laser has been used to carry out time-resolved Schlieren studies of sparks produced by focused ruby laser radiation. Synchronization of the two lasers was achieved by means of a spark gap triggered by the neodymium laser radiation, which switched a Pockels cell within the ruby laser cavity. The use of this technique permitted a detailed study of the plasma development during the initial stage immediately after breakdown and results were obtained in four gases (neon, argon, nitrogen, and air) at pressures between 60 and 1520 torr, using peak laser powers ranging from 35 to 300 MW and pulse risetimes between 14 and 24 ns. The results obtained indicate that, on the rising part of the laser pulse, the motion of the plasma along the laser beam axis can be explained in terms of a traveling ionization-breakdown wave.