Modeling of plasma generation in repetitive ultra-short DC, microwave, and laser pulses

Reduction of the power budget for sustaining a prescribed electron density in plasmas generated by DC or oscillating electric fields can be achieved if high-energy electrons are generated in these plasmas, which requires the ratio E/N in the DC case, or the ratio E/co in low-pressure microwave or laser plasmas, to be very high. Applying the strong field for only a short time, and then allowing the plasma to decay is the essence of the concept of ultra-short repetitive pulse ionization explored in this paper. Modeling of repetitive pulses show that there is an optimum electric field that minimizes the power budget. This minimum power budget is much lower than that in a continuous DC or RF plasma, but still substantially higher than in plasmas sustained by electron beams. Modeling of spatio-temporal dynamics of plasmas in nanosecond pulses shows strong coupling between non-local dynamics of high-energy electrons, ionization and electric field. At high gas pressures, cathode sheath phenomena and ionization non-uniformity are suppressed. With an appropriate choice of frequency and amplitude, the efficient repetitive-pulse ionization method can be extended to electromagnetic waves, i. e., to electrodeless systems. At low gas density, highpower nanosecond microwave pulses at frequency 10-100 GHz can accelerate electrons to ~ 1 keV energy, resulting in much more efficient ionization than that in conventional low-power systems. Moreover, most of the ionization would occur after the pulse, as the high-energy electrons produced during the pulse lose their energy on ionization. In principle, the method of plasma generation by ultrashort repetitive high-power electromagnetic pulses can be extended to high gas densities, and the requirements for the laser source are estimated in the paper.