Plasma Interaction with Electron-Emitting Surfaces

Abstract

Electron emission from surfaces occurs in many plasma systems. Several types including secondary, thermionic and photon-induced emissions are intense under certain conditions. Understanding the effects of emission on the sheaths that govern plasma-surface interaction is important. This dissertation predicts some emitting sheath phenomena that were not reported in past studies. For example, most previous theoretical models assumed that an emitting sheath potential is always negative and that ions always accelerate into the wall. We show when the emission is intense that the sheath potential can become positive, fundamentally changing how the plasma and wall interact. In this inverse sheath state, ions are repelled, suggesting for instance that (a) no presheath exists in the plasma interior, (b) emitting walls could be used in applications to stop sputtering. Another topic considered is the transit of emitted electrons across the plasma to other surfaces, which is possible in low collisionality plasma systems. When transit occurs, the flux balance is a complex global problem where the sheaths at opposite surfaces are coupled through their exchange of emitted electrons. We also show that secondary emission can trigger a variety of sheath instability phenomena that change the state of the plasma-wall system or cause oscillations preventing steady state. Lastly, we analyze a mechanism where emitted electrons return to the same surface and knock out secondaries, which return and knock out more secondaries, etc., feedback amplifying the emission intensity. The four phenomena will be analyzed theoretically and verified with particle-in-cell simulations: (a) inverse sheath, (b) sheath coupling via transiting electrons, (c) sheath instabilities, (d) returning electron amplification. Consequences of these processes on the sheath potentials, wall heating, loss rate of charge, and cross field transport (near-wall conductivity) are discussed throughout. Possible implications are suggested for fusion machines, plasma propulsion engines, probes, dusty plasmas, RF discharges, and surfaces in space.