Influence of Neutral Pressure on Instability Enhanced Friction and Ion Velocities at the Sheath Edge of Two-Ion-Species Plasmas

Ions are well-known to enter the sheath at the Bohm velocity in single-ion plasmas. However, for two ion species plasmas, the ions' sheath edge speed has been observed to deviate from the individual Bohm velocity [1]. The Instability-Enhanced Friction (IEF) theory [2] accurately predicts this deviation by accounting for an enhanced friction between the ions that merges their velocities. The enhanced friction is caused by ion-ion two-stream instabilities in the presheath. Here we report an advancement of the IEF theory which includes the effect of neutral pressure when predicting sheath edge ion speed in a two ion species plasma [3]. The predictions for the ions' sheath edge flow were tested against Particle-in-Cell Monte-Carlo Collision (PIC-MCC) simulations for a range of neutral pressures and were shown to be accurate. The theory and simulations indicate that the two-stream instability can persist up to 10's of mTorr. This result implies ion-ion streaming instabilities can affect the ions' sheath edge flow speed in plasma based manufacturing devices, which can operate in the 10's of mTorr of pressure. [1] G. D. Severn, \textit{et. al.}, PRL.~90, 145001 (2003) [2] S. D. Baalrud, \textit{et. al.} PRL 103, 205002 (2009) [3] P. J. Adrian, \textit{et. al.}, POP 24, 123505 (2017)