Effects of runaway electrons in partially ionized plasmas

Electron dynamics plays a significant role in partially and fully ionized gases. The electric field accelerates the electrons while various collisional processes, such as electron-ion and electron-neutral collisions, lead to deceleration of electrons. When taking the particle ensemble, the competition of the energy gain and loss can lead to a non-Maxwellian velocity distribution function. In this study, a zero-dimensional (0D) set up is considered where the electric field (E) and gas density (N) are imposed. Varying the ionization degree, i.e., the ratio of plasma density and gas density, the effects of the Coulomb collisions on the discharge are accounted for. In this study, we construct a theory using the equation of motion (force balance) to show that there are particles with a particular velocity range that experiences unbounded acceleration, resulting in runaway electrons. The 0D Monte Carlo collision simulation shows that the higher-order moment (e.g., the 4^th moment) indeed does not reach steady state depending on the E/N values while the lower-order moments (e.g., bulk velocity, temperature, and heat flux) can achieve steady state, confirming the observation of the theory.