Electron presheaths: 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 a Langmuir probe collecting electron saturation current. Generally, the formation of electron sheaths requires that the electron-collecting area be sufficiently smaller ( 2.3me/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 in a helium discharge, particle-in-cell simulations, and theory it is shown that under low temperature plasma conditions (Te Ti) electron sheaths are far from local. Instead, a long presheath region (27 mm, approximately an electron's mean free path) 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.

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

“…The electron sheath is observed to cause electrons to "funnel" into the electrode, influencing the electron flow far beyond the thin region of negative space charge. The measurements and simulations both support the prediction that the electron sheath and plasma are adjoined by a long presheath region Although the electric field is weak in this region, the pressure gradient is large enough to drive a fast electron drift [16,18]. The funneling effect causes the presheath to have a two-dimensional nature.…”

“…Figures 17 and 19 confirm that the general behavior predicted by this simple analysis is observed in PIC simulations; see Refs. [16,18,19] for details.…”

“…A number of interesting features of electron sheaths have been discovered recently. These include the presence of an electron presheath [18], which is a long region with an electron pressure gradient that acts to accelerate electrons toward the boundary. Electrons are observed to gain a drift approaching the electron thermal speed as they near the electron sheath [19].…”

Theory and simulation of electron sheaths and anode spots in low pressure laboratory plasmas

“…2D measurements are shown inFig. 2.3along side simulation results from the same work[3]. Similar simulations will be discussed in Chapter 2.3.Comparing the electron density profiles inFig.…”

“…Experimental measurements of the sheath and plasma near a small positively biased electrode at typical low temperature plasma conditions have recently been reported [3]. The measurements were taken in a GEC reference cell 1 with a plasma generated in 20 mTorr of helium by a thermionic emitter placed 10 cm above the electrode as shown in Fig This density gradient is likely due to ambipolar di↵usion.…”