Photographic study of rapid events by picosecond techniques

Former studies of high-speed events, e.g., spark formation and laser produced plasmas by nanosec light pulses, revealed the necessity of shorter exposure times. This can be done using picosec light pulses, either a single one or a train of pulses, generated in the cavity of a mode-locked neodymium or a ruby laser. Schlieren photos of laser triggered open-air sparks taken by multiple exposure with a series of pulses from a neodymium laser show the advantages of these techniques compared to one-dimensional streak photographs. As another example, photos of a plasma induced by a series of ps-light-pulses on a metal surface are shown. The application of single-pulse techniques is demonstrated by the study of a laser spark, induced by one or two ps pulses in air. To get sufficient power for a breakdown, the single pulse extracted from the mode-locked laser passes a two-stage ruby amplifier. After focalization, the main part of the pulse energy produces the breakdown. The remaining part of the pulse passes a variable optical delay system and serves as a light source for a schlieren system. In this way, single-shot schlieren photos of laser sparks with life times less than one nanosec are obtained. For the study of later stages in spark development multiple exposure by a train of pulses is used. The above mentioned experimental arrangements are discussed in detail, the measurement of pulse duration by TPF and image converter is described, and some results as regards plasma behavior are presented.