Ion energy distribution in extremely high electric field discharges

Abstract

Singly charged ion energy distribution functions (IEDFs) have been investigated by the particle-in-cell/Monte Carlo method in practical discharges at extremely high reduced electric field E/n, i.e. the ratio of electric field and gas density, up to 700 kTd in helium. It is found in high E/n regime that IEDFs deviate from Maxwellian form due to complex processes: (a) ions undergo a transition to runaway regime, (b) the copious energetic fast neutrals generated by charge exchange produce slow ions via ionization collisions, which load in the low-energy part of IEDF, as well as commonly assumed energy-dependent charge exchange cross section. For E/n > 100 kTd, the ratio of the volumetric ionization rate and charge exchange rate exceeds 10% due to heavy particles impact ionization. The ion mean energy close to the cathode is approximately reduced by in comparison with the value obtained by the reduced model with only the charge exchange collision considered. Charge production attributed to fast-neutral impact ionization not only causes the decrease of ion mean energy, but also enhances ion non-equilibrium.