Length scales of the electron sheath and presheath

Provided a sufficiently small electrode, it is possible to bias it above the plasma potential. This results in an electron sheath, common to Langmuir probes in electron saturation. The traditional understanding of electron sheaths has been that they are isolated from the bulk plasma and result in a truncation of the electron velocity distribution at the sheath edge [1]. Recent results reveal that the electron sheath is adjoined by an electron presheath which drives a significant flow of electrons, excites instabilities, and extends far into the bulk plasma [2]. The extent of the electron presheath is predicted to scale with the electron collision rate which may be determined by one of several potential mechanisms. This work examines the length scales of the electron sheath and presheath in PIC simulations as a function of neutral pressure. It is found that at high gas pressures electron-neutral collisions suppress the formation of streaming instabilities and are responsible for determining the presheath length scale. Conversely, low pressures result in the electron presheath length being determined by the effective collisions of streaming instabilities.