Electron presheaths: the outsized influence of positive boundaries on plasmas

• Published in: Plasma sources science & technology Vol. 26; no. 2; pp. 25009 - 25015
• Main Authors: Yee, B T, Scheiner, B, Baalrud, S D, Barnat, E V, Hopkins, M M
• Format: Journal Article
• Language: English
• Published: IOP Publishing 01.02.2017
• Subjects: electron sheath; laser collision-induced fluorescence; particle-in-cell; presheath
• ISSN: 0963-0252; 1361-6595; 1361-6595
• DOI: 10.1088/1361-6595/aa56d7

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 ( 2.3 m 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 in a helium discharge, particle-in-cell simulations, and theory it is shown that under low temperature plasma conditions ( T e > T i ) 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.