Dynamics of plasmas sustained by repetitive ultrahigh-voltage DC or subpicosecond laser pulses

Nanosecond breakdown and the so-called fast ionization wave in gases was studied by several groups. In those studies, the gas was not preionized by the beginning of the pulse. Here, we analyze dynamics of plasmas sustained by ultrahigh-voltage (with E/N above the electron runaway threshold) pulses with high repetition rate, so that each pulse begins with pre-existing plasma. In contrast with nanosecond breakdown, strong current instantaneously develops in the entire volume, forming cathode sheath. Interesting spatio-temporal evolution of the plasma is revealed with a fully coupled model where ionization kinetics is strongly coupled with ion and electron motion in self-consistent nonuniform electric field and with non-local electron energy distribution function (EEDF). EEDF is calculated in "forward-back" approximation. The modelling demonstrated that rapidly evolving redistribution of potential creates conditions for electron high-voltage pulses can reach about 100 eV, and the EEDF develops a long plateau extending to hundreds and thousands eV. High-energy electrons far from the cathode are not produced locally, but rather arrive from the sheath. New predicted phenomena include reversal of electric field in the anode half of the discharge gap and double-peak profiles of mean electron energy and ionization rate. Ionization and excitation of molecules continue and could even peak after the pulse, with no voltage, as the high-energy electrons move into the gas. We also explore a new way of plasma generation by ultrashort, high-power, repetitive laser pulses, with ionization efficiency comparable to that of e-beams. The idea is to accelerate free electrons remaining from the previous pulse to keV due to high quiver energy in laser field, before collisions and plasma polarization (this imposes upper limits on pulse duration and laser wavelength). Suppressing tunnelling ionization imposes a lower limit on the wavelength. The accelerated electrons will act like a beam and ionize the surrounding gas.