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Electron sheaths form near the surface of objects biased more positive thanthe plasma potential, such as in the electron saturation region of a Langmuirprobe trace. Generally, the formation of electron sheaths requires that theelectron-collecting area be sufficiently smaller ($\sqrt{2.3m_e/M}$ times) thanthe ion-collecting area. They are commonly thought to be local phenomena thatcollect the random thermal electron current, but do not otherwise perturb aplasma. Here, using experiments on an electrode embedded in a wall,particle-in-cell simulations and theory, it is shown that under low temperatureplasma conditions ($T_e \gg T_i$) electron sheaths are far from local. Instead,a long presheath region (in excess of 70 $\lambda_D$) extends into the plasmawhere electrons are accelerated via a pressure gradient to a flow speedexceeding the electron thermal speed at the sheath edge. This fast flow isfound to excite instabilities, causing strong fluctuations near the sheathedge.

Electron Preheaths: The Outsized Influence of Positive Boundaries on Plasmas