A kinetic study of electron heating and plasma dynamics in microwave microplasmas

Microwave microplasmas ignited in argon are studied using a one-dimensional particle-in-cell with Monte Carlo collision (PIC-MCC) approach. One-dimensional PIC-MCC simulations are performed at specified input power densities to determine the influence of the applied frequency (ranging from 1 to 320 GHz), pressure, and total deposited power on the plasma dynamics. The frequency response study performed at a fixed input power density shows the presence of off-axis peaks in the electron number density profile at intermediate frequencies. These peaks are attributed to the interplay between the production of hot electrons by the oscillating sheath and their inability to diffuse sufficiently at higher operating pressures, thereby resulting in enhanced ionization at off-axis locations. This is confirmed by the pressure dependence study which shows that the electron number density peaks at the mid-point when the microplasma is ignited at lower pressures. As the excitation frequency is increased further, the sheath oscillation heating decreases and eventually vanishes, thereby requiring the bulk plasma to couple power to the electrons which in turn leads to an increase in electron temperature in the plasma bulk and the electron number density peak appearing at the mid-point. When the power coupled to the microplasma is decreased, the sheath oscillation at a given frequency decreases, thereby leading to higher contribution from heating in the bulk plasma which leads to the disappearance of off-axis peaks even at intermediate frequencies. The microplasma dynamics at all conditions considered in this work demonstrate the interplay between the electron momentum transfer collision frequency, the angular excitation frequency, and the plasma frequency.