Simulations of ion heating in the presheath due to ion-acoustic instabilities*

We find that ion-acoustic instabilities result in significant ion heating near the sheath edge.  The heating extends into the presheath since some of the wave power reflects from the sheath.  Particle-in-cell simulations were designed to test whether the instability was the source of heating by varying the source electron temperature across the threshold for exciting the ion-acoustic instability. The simulations confirm the instabilities cause heating and demonstrate that the electron-to-ion temperature ratio is locked to the threshold for instability in the unstable region near the sheath edge.  The instability heating effect is significant at low pressures, but is eliminated at higher pressures where the instability is damped by ion-neutral collisions. This effect is distinct from the well understood ion heating caused by inelastic collisions with neutrals. Low temperature plasma systems that utilize a presheath for ion acceleration, such as etching and ion beam sources, could experience unwanted and significant ion heating due to this effect.