Effusive Particle Losses at Probes in Electron Saturation

A theory for presheaths near probes biased at and above the plasma potential for plasmas with an electron-neutral mean free path on the order of the chamber scale is presented. The theory predicts a presheath density depression that results from the free streaming of electrons and an effusion loss process from the chamber at a small electrode. A loss-cone-like velocity distribution function for electrons is predicted where the loss angle of the depletion region corresponds to the angular size of the electrode at a specified distance. The predictions are compared with previously published PIC simulations [Scheiner et al., Phys. Plasmas 23, 083510 (2016)] and are contrast against those of a previously published collisional electron presheath theory [Scheiner et al., Phys. Plasmas 22, 123520 (2015)].