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

Abstract

The Debye sheath is shown to be unstable under general conditions. For surface materials with sufficient secondary electron emission (SEE) yields, the surface's current-voltage characteristic has an unstable branch when the bulk plasma temperature (T e ) exceeds a critical value, or when there are fast electron populations present. The plasma-surface interaction becomes dynamic where the sheath may undergo spontaneous transitions or oscillations. Using particle-in-cell simulations, we analyze sheath instabilities occurring in a high T e plasma slab bounded by walls with SEE. As the plasma evolves, whenever the sheath enters an unstable state, its amplitude rapidly collapses, allowing a large flux of previously trapped electrons to hit the wall. These hot electrons induce more than one secondary on average, causing a net loss of electrons from the wall. The sheath collapse quenches when the surface charge becomes positive because the attractive field inhibits further electrons from escaping. Sheath instabilities influence the current balance, energy loss, cross-B-field transport and even the bulk plasma properties. Implications for discharges including Hall thrusters are discussed. More generally, the results show that common theories that treat emission as a fixed (time-independent) "coefficient" do not capture the full extent of SEE effects. a) Author to whom correspondence should be addressed. Electronic

Anomalous conductivity in Hall thrusters: Effects of the non-linear coupling of the electron-cyclotron drift instability with secondary electron emission of the walls

“…The time scale of the coupling is on the order of the transit time of electrons between the walls, which is typically on the order of few 10 À8 s. In other words, the SEE can no longer be estimated in the context of a static Debye sheath and must rather be considered as part of a dynamic system in constant evolution. 5,18,21,22 To further illustrate this point, we have plotted in Figs. 11(c) and 11(d) the same quantities as in Figs.…”

Negative plasma potential relative to electron-emitting surfaces

“…5 of Ref. [36] and the discussion therein). Hence the nonmonotonic ϕ(x) is not a true SCL sheath as the corresponding charge density profiles cannot exist in steady state.…”

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

“…If this electron beam is sufficiently dense and energetic, it can be a source of electron-electron two-stream instabilities. Such instabilities have been observed in experiments [225], studied theoretically [227] and simulated using PIC methods [226,228]. Secondary electron emission has been proposed as a mechanism to cause anomalous electron transport in Hall effect thrusters [229,230].…”