  Particle-in-cell direct simulation Monte Carlo simulations reveal that
ion-acoustic instabilities excited in presheaths can cause significant ion
heating. Ion-acoustic instabilities are excited by the ion flow toward a sheath
when the neutral gas pressure is small enough and the electron temperature is
large enough. A series of 1D simulations were conducted in which neutral plasma
(electrons and ions) was uniformly sourced with an ion temperature of 0.026 eV
and different electron temperatures (0.1 - 50 eV). Ion heating was observed
when the electron-to-ion temperature ratio exceeded the minimum value predicted
by linear response theory to excite ion-acoustic instabilities at the sheath
edge ($T_e/T_i\approx28$). When this threshold was exceeded, the temperature
equilibriation rate between ions and electrons rapidly increased near the
sheath so that the local temperature ratio did not significantly exceed the
threshold for instability. This resulted in significant ion heating near the
sheath edge, which also extended back into the bulk plasma; presumably due to
wave reflection from the sheath. This ion-acoustic wave heating mechanism was
found to decrease for higher neutral pressures, where ion-neutral collisions
damp the ion-acoustic waves and ion heating is instead dominated by inelastic
collisions in the presheath.
