Characteristics of RF Hollow Cathode Discharge: Particle-In-Cell/Monte Carlo Simulation

Abstract

RF sheath heating as well as secondary electron acceleration play important role in the radio-frequency (RF) hollow cathode discharge (HCD). HCDs form in cylindrical cavities in the cathode, and one can use an array of such cavities to create large area HCDs. The plasma in the hollow cavities can become more intense due to the hollow cathode effect (HCE) under certain conditions. In this study, a single hollow cathode hole is modeled using Particle-in-Cell/Monte Carlo simulation. In this model, using charge density of particles, Poisson equation is solved for electric potential, which yields the electric field. Using this electric field, all charged particles’ velocities are updated and the particles are moved. This PIC code considers particle collisions with each other and with neutral fluid using a Monte Carlo model. Statistics of these collisions are used to determine how electron energy is dissipated in the plasma. RF hollow cathode behavior is characterized for different hole size, pressure and RF voltage. The plasma penetrates inside the hollow cathode hole with increase in pressure, leading to HCE enhancement. With increase in hole size, plasma penetrates further into the hole at a given pressure. At high RF voltage, plasma density enhancement is limited as plasma spreads over larger volume. To evaluate the effect of RF sheath heating, the hollow cathode discharge is modeled at different frequencies ranging from high to low frequency. To understand the importance of secondary electron acceleration on the discharge, we modeled the discharge for different secondary electron emission coefficients. In order to determine the collective behavior of an array of hollow cathode holes, a reduced order model based on a neural network is developed utilizing the detailed PIC modeling results. Preliminary results using the reduced order model are also presented.