Plasma-sheath instability in Hall thrusters due to periodic modulation of the energy of secondary electrons in cyclotron motion

Abstract

Particle-in-cell simulation of Hall thruster plasmas reveals a plasma-sheath instability manifesting itself as a rearrangement of the plasma sheath near the thruster channel walls accompanied by a sudden change of many discharge parameters. The instability develops when the sheath current as a function of the sheath voltage is in the negative conductivity regime. The major part of the sheath current is produced by beams of secondary electrons counter-streaming between the walls. The negative conductivity is the result of nonlinear dependence of beam-induced secondary electron emission on the plasma potential. The intensity of such emission is defined by the beam energy. The energy of the beam in crossed axial electric and radial magnetic fields is a quasi-periodical function of the phase of cyclotron rotation, which depends on the radial profile of the potential and the thruster channel width. There is a discrete set of stability intervals determined by the final phase of the cyclotron rotation of secondary electrons. As a result, a small variation of the thruster channel width may result in abrupt changes of plasma parameters if the plasma state jumps from one stability interval to another.

Observation of reduction of secondary electron emission from helium ion impact due to plasma-generated nanostructured tungsten fuzz

“…Also, in plasmas with continuous conducting walls, where local ambipolarity is not necessarily satisfied, SEE can modify current flows in the plasma. SEE plays an important role in a wide range of plasma devices and diagnostics, such as Hall thrusters [3], plasma processing devices [4], magnetic fusion experiments [5,6], and Langmuir probes [7].…”

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

“…The sheaths were observed to transition briefly to a SCL-like state in past studies of instabilities 7,8 , but it was not known why this happened. A SCL sheath theoretically arises when solving Poisson's equation with the plasma electron density, plasma ion density and secondary electron density written in terms of υ(x).…”

“…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.…”

“…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

“…Sheath instability can also occur naturally in hot plasmas that induce strong SEE from the walls. Hall thruster simulations reveal sheath instabilities that abruptly alter the state of the plasma [9] and drive oscillations [10,11]. Sheath oscillations may considerably increase near-wall conductivity in HT's [10,12] and cause interference [13].…”