Effects of presheath dynamics on radio-frequency sheaths

A common approach to the study of radio frequency (rf) sheaths is to separate the description of the sheaths from that of the bulk plasma. In order to solve the resulting set of equations, an appropriate boundary condition for the sheath model has to be specified at the sheath–plasma boundary. In the existing sheath models, this boundary condition is assumed to be stationary. In present article, we investigate the ion dynamics in both presheath region and sheath region by using numerical as well as analytical methods. It is found that the presheath introduces an additional time scale ωpre=VB/l (here, ℓ is the characteristic scale of the presheath and VB is the Bohm velocity). If the rf frequency ω is low enough so that ω⩽ωpre, the boundary condition for the sheath depends on the ion dynamics in the presheath. In the very low-frequency regime ω≪VB/l, an analytical expression for the ion current is obtained which agrees well with the numerical result.