Boundary-induced effect on the spoke-like activity in E × B plasma

Abstract

The spoke instability in an E × B Penning discharge is shown to be strongly affected by the boundary that is perpendicular to B field lines. The instability is the strongest when bounded by dielectric walls. With a conducting wall, biased to collect electron current from the plasma, the spoke becomes faster, less coherent and localised closer to the axis. The corresponding anomalous cross-field transport is assessed via simultaneous time-resolved measurements of plasma potential and density. This shows a dominant large-scale E×B anomalous character of the electron cross-field current for dielectric walls reaching 40-100% of the discharge current, with an effective Hall parameter β eff ∼ 10. The anomalous current is greatly reduced with the conducting boundary (characterised by β eff ∼ 10 2 ). These experimental measurements are shown to be qualitatively consistent with the decrease of the E field that triggers the collisionless Simon-Hoh instability.

Physics of E × B discharges relevant to plasma propulsion and similar technologies

“…For cylindrical devices, the spokes rotate azimuthally in the direction of the ExB drift, but with a speed that is an order of magnitude smaller than the ExB drift velocity [67], [68], [78], [80], [89], [90], [91], [92]. The mode number of these spokes is usually low, m =1 -8.…”

“…Although the mechanism for spoke formation is still debated, one candidate is the Simon-Hoh (SH)-type instability, [95], [96] driven by the combination of the applied electric field and the gradient in plasma density. A modified theory of this instability for partially magnetized collisionless plasmas was developed [74], [79], [97], [98], [107] and experimentally verified for some conditions [67], [79], [92], [93], [97].…”

The nonmodal kinetic theory for the electrostatic instabilities of a plasma with a sheared Hall current

“…The developed theory predicts that the separate spatial Fourier mode of the perturbations in the Hall plasma with the inhomogeneous electric field is determined in the frame convected with one of the plasma components. The relations (20) - (23), which are the generalization on the sheared current velocity the relations (29) - (31) of the Doppler effect for the uniform current velocity, display that due to the different shearing of the ion and electrons flows in the Hall plasma, this mode is detected by the second component as the Doppler -shifted continuously sheared mode with time -dependent wave numbers. This effect of the mode shearing grows continuously with time and the interaction of the plasma components forms the nonmodal time-dependent process which should be investigated as the initial value problem.…”

“…This specific current, which is absent in plasmas with all magnetized species, is the source of numerous current-driven instabilities which have been observed experimentally [7][8][9][10][11] and in simulations [12][13][14][15] , and were investigated analytically [16][17][18][19][20][21][22][23][24][25][26][27][28] . The discovered instabilities are very dependent on the specific conditions and regions of a particular device and develops in a large range of frequencies and wavelengths which includes large scale low frequency 'rotating spokes' 14,[29][30][31] , the modified two-stream (MTS) instability and ion sound (IS) and lower hybrid instabilities with frequencies between the ion cyclotron and electron cyclotron frequencies, and the submillimeter electron cyclotron drift instabilities 28,32 in the MHz frequency range. Important set of the Hall plasma instabilities is the gradient-drift plasma instabilities [16][17][18][22][23][24][25][26][27] which develop in spatially inhomogeneous Hall plasma due to the combined effect of the Hall current and of the gradient-drift modes formed by the plasma density and temperature inhomogeneity.…”

Fast Camera Analysis of Plasma Instabilities in Hall Effect Thrusters Using a POD Method under Different Operating Regimes

“…In this work, we combined the use of a high-speed camera that can acquire images up to 900 kfps with the Proper Orthogonal Decomposition (POD) image analysis method [31] to extract characteristics of the unstable regimes developing in the plasma. The focus was made on two main instabilities: the low-frequency breathing mode, in the kHz range, that develops axially and is related to the neutral gas dynamics [32,33] and rotating modes, with a frequency around ten times higher, that propagate in the azimuthal E × B direction [34,35] and could be related to rotating spokes [36] or gradient-induced instabilities [37]. It appears that these rotating modes are not only located near the anode but propagate in the thruster plume.…”