Enhanced electron density and plasma dynamics on nanosecond time scales in Helium plasma discharges

Enhanced electron density and plasma dynamics are investigated for Helium discharges on nanosecond timescales with Particle-In-Cell simulations. The plasma discharges are driven between planar electrodes with DC, single pulses, and dynamic frequency square waves. It is assumed that the DC and pulse discharges operate in the glow regime. It is shown that as pressure increases with narrowing gap distance, the peak transient electron density rises. This is in contrast to what is observed under a constant pressure-gap (pd) and electric field reduced by neutral density (E/N) values at saturation time. It is shown that although the pd and E/N values and therefore the breakdown voltage are the same across cases, the plasma kinetics are different due to a change in the energy relaxation lengths. The cross-points between the sheath length and energy relaxation length move to higher electron energies at higher pressure. This facilitates high-energy electrons to undergo inelastic collisions and produces different rates of increasing electron density and temperature at nanosecond timescales. Moreover, using a plasma frequency-dependent square wave, the electron density can be increased to 50 times higher over that of the DC case because of a reverse electric field. The electron kinetics on nanosecond time scales can be exploited for high electron density and fast ionization applications.