Excitation of soliton like structures in radio-frequency capacitively coupled plasmas

In this study, we examine the kinetics of radio-frequency capacitively coupled plasma discharges using the Particle-In-Cell method. Plasma is generated by applying two radiofrequencies (60 MHz and 1MHz), which results in the formation of a plasma sheath in the entire discharge region with electron density that oscillates between the two electrodes. However, ions remain relatively stationary. By increasing the gas pressure and radio-frequency power, or replacing helium gas with argon gas, a neutral discharge region known as the plasma bulk is formed. In this neutral discharge regime, electrons oscillate only in the sheath. Prior to the formation of the plasma bulk, the discharge is referred to as a non-neutral discharge, in which soliton-like electron structures are present. An analytical model is presented to interpret the simulation results, and the Korteweg-de Vries equations predicted the existence of damped electron- and ion-soliton-like structures. In the non-neutral discharge regime, supersonic electron solitons can be observed when the driven radio-frequency is much smaller than the electron plasma frequency. Supersonic ion soliton-like structures are present in the non-neutral discharge generated via radio-frequencies comparable to or smaller than the ion plasma frequency.