Measurements of gas preionization for plasma radiation sources

Azimuthally symmetric ultraviolet (UV) preionization of the outer periphery of a gas puff z pinch, prior to current initiation, may reduce the growth of magnetically driven Rayleigh-Taylor instabilities or other nonuniformiti es affecting the final implosion stage of the pinch, leading to an improvem ent in K-shell x-ray yield. We report on measurements of a flashover UV pho toionization scheme, capable of ionizing the periphery of argon gas puffs t o 1%-10% of the initial gas density. Measurements are made with a two-color , (1064 and 532 nm), high sensitivity (similar to 10(-5)lambda). Mach-Zehnd er type interferometer. Two methods of measuring preionization are investig ated. The first uses a single laser wavelength, 1064 nm, to probe a chord o f the cylindrical gas puff. The gas density is measured first without preio nization and then with the UV flashover discharge. The difference in phase shift determines the free electron density. The second technique uses both wavelengths to simultaneously probe the same line of sight, determining neu tral and free electron densities using only one gas puff event. Gas preioni zation may introduce refractive species that complicate the conversion of p hase shift to density. However, single-color interferometry with the IR and green beams indicates that it is sufficient to consider only two species, neural argon and free electrons for times of interest. [S0034-6748(99)65401 -6].