Plasma Boundaries

Abstract

A plasma separates itself from a metallic or dielectric wall by forming a space-charge layer, in which the electron density is dramatically reduced. The current flow in the space-charge layer is determined by the Child–Langmuir law. It is shown why ions must enter the space-charge region with supersonic speed. Internal space charge sheaths in a plasma occur as double layers. The volt-ampere characteristic of a small electric probe can be used to measure ion density and electron temperature. Basic probe techniques are exemplified for planar probes. Advanced probe methods cover double-probes, triple probes and modulation techniques. Druyvesteyn’s method for determining the electron distribution function is described. Probe measurements in rf plasmas and rf compensation complete the chapter.

“What I tell you three times is true.”

Lewis Carroll, The Hunting of the Snark