Electron Preheaths: The Outsized Influence of Positive Boundaries on Plasmas

Abstract: Electron sheaths form near the surface of objects biased more positive than the plasma potential, such as in the electron saturation region of a Langmuir probe trace. Generally, the formation of electron sheaths requires that the electron-collecting area be sufficiently smaller ($\sqrt{2.3m_e/M}$ times) than the ion-collecting area. They are commonly thought to be local phenomena that collect the random thermal electron current, but do not otherwise perturb a plasma. Here, using experiments on an electrode embedded in a wall, particle-in-cell simulations and theory, it is shown that under low temperature plasma 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 plasma where electrons are accelerated via a pressure gradient to a flow speed exceeding the electron thermal speed at the sheath edge. This fast flow is found to excite instabilities, causing strong fluctuations near the sheath edge.