Application of a fluid model in the local energy approximation to numerical simulation of the ionization wave front of negative polarity in He

Abstract

The fluid equation model in the local energy approximation is applied to computer simulation of the negative ionization wave front in He stimulated by the applied electrostatic field. Numerical calculations are carried out based on 1D dimensionless representation of the equation system, in the context of streamer propagation in this gas. Pooling the known experimental and theoretical data on the ionization-transport coefficients of electrons in He enabled their approximations by the analytical functions within the range of the reduced electric field 10−3–104 Td and integration into the calculation code. The front propagation velocity and spatial thickness along the direction of its propagation, defined by the level 0.1–0.9 of the maximum electric field ahead of the front, and the electron density close on the front tail are calculated depending on the applied electric field strength and presented in the work. The obtained results allowed estimating the ratios of the electron momentum and mean energy relaxation rates to the rate of the changes induced in the gas by the front propagation. The local equilibrium condition of the fluid model validity requires the ratios to exceed the unity repeatedly. Their values are shown to decrease with an increase in the electric field, thus restricting its range from above the approved model application. For example, the admission of their excess over 10 to be sufficient for the electron relaxation within the front leads to the restriction of the application range by about 600 Td (the mean electron energy about 46 eV).