Theory of sheaths near positively biased electrodes

Recent work [1] has shown that electron sheaths, such as those around Langmuir probes collecting the electron saturation current, can affect the bulk plasma through a presheath. Unlike typical ion presheaths, where ions are directly accelerated through an electric field, the electron presheath accelerates electrons to approximately an electron thermal speed through a force due to pressure gradients. Under typical low temperature plasma conditions, the electron presheath can be several times longer than an ion presheath under the same plasma conditions. This work shows that electron presheaths can contain a virtual cathode, which is a local minimum in the plasma potential in front of the sheath, when the electrode is adjacent to a dielectric surface. Physics associated with this layer, including ion pumping from the potential well, is revisited in view of the recent advances in understanding electron flow in electron presheaths [1].