Secondary-Electron-Emission Instability in a Plasma

Abstract

A 200-eV electron beam is incident on an electrode in a laboratory plasma. The emission of secondary electrons produces a region of negative differential resistance in the current-voltage characteristic of the electrode. Spontaneous dynatron oscillations are driven by the negative differential resistance when a resonant circuit is placed in series with the electrode. The instability is driven by the beam energy, produces large amplitudes comparable to the beam voltage, modulates beam and plasma parameters, and excites plasma eigenmodes such as ion acoustic waves. [S0031-9007(98)

Instability, collapse, and oscillation of sheaths caused by secondary electron emission

“…Nonsteady surface effects due to SEE have been observed in numerous plasma simulations 7,8,9,10,11,12,13,14 and experiments 15,16,17 . Because the plasma properties in any device are coupled to the PSI, this can affect more than just the surface.…”

“…The simplest example of this is when a high energy electron beam is incident on the surface. The emitted current induced by the beam depends on the beam impact energy, which varies with surface potential in a way that gives the current-voltage (I-V) trace an unstable branch 15 . It is natural to ask whether a similar mechanism could generate dynamic sheath behavior in any plasma-wall system with SEE, since plasma electrons can be considered a superposition of beams to an extent.…”

“…Because the plasma properties in any device are coupled to the PSI, this can affect more than just the surface. Instabilities attributed to SEE may cause sudden changes to the whole plasma 7,9,10 , enhance cross-B-field transport 9,10,13 and drive spontaneous oscillations 8,9,15 which can modulate critical parameters (wall fluxes, energy loss, plasma potential, total plasma energy, etc. ), launch plasma waves, produce electromagnetic radiation and cause interference.…”

General Cause of Sheath Instability Identified for Low Collisionality Plasma in Devices with Secondary Electron Emission

“…9 argues if dγ b /dΦ < 0, the SEE outflux increases so that Δσ e < 0 and instability occurs, similar the case of Ref. 8. But the derivation overlooked the effect of WCE's on the system when Φ decreases.…”

“…This may arise for example when a strong electron beam is incident on a target in a plasma, see Ref. 8. The sheath reduces the velocity of beam electrons as they approach the target.…”