Presheath-like structures and effusive particle losses for biased probes at and near I-V electron saturation
Physics of Plasmas(2023)
摘要
A theory for presheath-like structures near probes biased at and above the
plasma potential is developed for collisionless plasmas with an
electron-neutral mean free path on the order of the chamber scale. The theory
predicts presheath-like perturbations to the plasma that result from the free
streaming of electrons and an effusion loss process from the chamber at the
electrode. For these situations, 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 angle of the loss cone becomes 180 degrees at the sheath edge. In
comparison to a previous collisional electron presheath model that required
electrons satisfy a Bohm criterion at the sheath edge [B. Scheiner et al. Phys.
Plasmas 22, 123520 (2015)], the present work suggests that no such condition is
needed for collisionless low pressure plasmas in the ≲10 mTorr range.
The theory predicts the generation of a density depletion of roughly 0.5n_e
and an electron velocity moment of 10s of percent of the electron thermal speed
by the sheath edge in a presheath with a potential drop of less than T_i/e.
The range of this presheath perturbation is determined by the electrode
geometry instead of the collisional mean free path. These predictions are
tested against previously published particle in cell simulations and are found
to be in good agreement.
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