Micro instabilities and rotating spokes in the near-anode region of partially magnetized plasmas

Abstract

Electron and ion transport in the near-anode region of a partially magnetized plasma under conditions typical of Hall thrusters or magnetron discharges is studied with fully kinetic, Particle-In-Cell Monte Carlo Collision (PIC-MCC) simulations assuming a uniform magnetic field and no ionization. We derive a simple relation that defines the magnetic field at the transition point between negative and positive sheath. For magnetic fields around or above this transition point, PIC-MCC simulations show the development of short wavelength azimuthal instabilities that cascade to longer wavelengths ("rotating spokes") as the magnetic field is increased. Both short-wavelength and large-wavelength fluctuations can coexist under some conditions. A detailed study of the fluid dispersion relation is used to analyze the PIC-MCC results. Small coherent structures can be associated with the destabilization of ion sound waves by density gradient and collisions. Longer wavelengths or rotating spokes are characteristic of the collisionless Simon-Hoh instability. The small structures are dominant for larger plasma density gradients while the larger structures correspond to smaller density gradients and larger magnetic fields. Anomalous transport associated with these instabilities can be significant, with effective collision frequencies larger than 2 × 10 7 s −1 in xenon for magnetic fields above the transition point.

New insights into the physics of rotating spokes in partially magnetized E×B plasmas

“…The main results can be summarized as follows. 1) For sufficiently large magnetic fields, the modified Simon-Hoh instability 17 is likely to develop 40 in the region of decaying magnetic field between cathode and anode in magnetron discharges or between channel exhaust and anode in Hall thrusters. When self-consistent ionization is taken into account in the model, the PICC MCC simulation results show that the Simon-Hoh instability evolves into an ionization instability that rotates in the azimuthal direction.…”

“…We studied in Ref. 16 anomalous electron transport across the magnetic field associated with the development of instabilities in the nearanode region, assuming a given ionization rate profile, and showed that Simon-Hoh instabilities 17,18 were likely to form for sufficiently large magnetic fields. In the present paper we present a self-consistent study of plasma transport in conditions typical to a magnetron discharge (Figure 1b) and show that the Simon-Hoh instability evolves into an ionization instability.…”

Rotating Spokes, Ionization Instability, and Electron Vortices in Partially Magnetized E×B Plasmas

“…The two-dimensional (2D) PIC-MCC model described in previous papers 15,16,26 has been used to simulate this experiment. The simulation domain is defined by the axial and azimuthal directions ̂ and ̂.…”

“…The main results can be summarized as follows. The small cross-field electron mobility is responsible for an instability (presumably of the Simon-Hoh type 3,26 in its initial phase) that leads to an azimuthal distortion of the plasma potential and density. This distortion allows some electrons to flow to the anode along an equipotential line connecting the cathode and anode regions.…”

Direct evidence of the gradient drift instability being the origin of a rotating spoke in a crossed field plasma

“…There were also attempts to identify the spoke nature by comparing the gradient drift instability theory with experiments performed in magnetron [32,33] and hybrid model (fluid electrons and kinetic ions) [34]. Further, 2D fully kinetic PIC/MCC simulations with Cartesian grid and artificial ionization were also performed to study micro instabilities and macroscopic structures in the near-anode region of a partially magnetized plasma under conditions typical of Hall thrusters or magnetron discharges [26]. In that work, a magnetic field threshold indicating the transition point between the positive sheath and the negative sheath was proposed, and the micro instability development was investigated for different magnetic fields with respect to the threshold value.…”