Free-electron distribution in laser-induced plasmas

Summary form only given. The second harmonic (532 nm) of a Nd:YAG laser beam was used to probe the plasma induced by the fundamental beam (1064 nm). Sufficient neutral filtering, utilizing a stack of glass slides, was required to reduce the probe beam intensity below the damage threshold of the spatial filter and schlieren optics. Phase-amplitude information, generated by the probe beam passing through the plasma region and schlieren optics, is recorded on 35-mm black-and-white film. Since the laser pulse is nominally 5 ns long, these photographs represent approximately a 5-ns time integration of the plasma behavior. Time delay for observing the evolution of the plasma is accomplished by varying the optical path length of the probe beam. The principle behind this method is based on phase-amplitude conversion involved with the schlieren technique. The versatility and the relatively low cost of the system are the primary advantages. The accuracy of the method is sufficient for many applications and should provide information on the electron density variation to within λ/2 spacing in the plasma, given λ/10 (probe beam) or better quality optical components in the schlieren system