MEASUREMENT OF VELOCITY DISTRIBUTIONS AND RECOMBINATION KINETICS IN TUNNEL-IONIZED HELIUM PLASMAS

We report the first measurements on the distribution of electron energ ies and associated recombination kinetics in tunnel-ionized plasmas of interest to recent x-ray laser proposals. Subpicosecond laser pulses, focused to an intensity of 1 x 10(16) W/cm(2), create a fully ionized helium plasma. Measurement of the resulting emission spectrum allows a determination of the time-dependent electron velocity distribution. The initial distribution is found to be strongly non-Maxwellian and su bsequent recombination kinetics proceed more slowly than predicted for a Maxwellian distribution of electrons.