Rotating structures in low temperature magnetized plasmasâ€”insight from particle simulations

Jean-Pierre Boeuf

doi:10.3389/fphy.2014.00074

Abstract

The EXB configuration of various low temperature plasma devices is often responsible for the formation of rotating structures and instabilities leading to anomalous electron transport across the magnetic field. In these devices, electrons are strongly magnetized while ions are weakly or not magnetized and this leads to specific physical phenomena that are not present in fusion plasmas where both electrons and ions are strongly magnetized. In this paper we describe basic phenomena involving rotating plasma structures in simple configurations of low temperature EXB plasma devices on the basis of PIC-MCC (Particle-In-Cell Monte Carlo Collisions) simulations. We focus on three examples: rotating electron vortices and rotating spokes in cylindrical magnetrons, and azimuthal electron-cyclotron drift instability in Hall thrusters. The simulations are not intended to give definite answers to the many physics issues related to low temperature EXB plasma devices but are used to illustrate and discuss some of the basic questions that need further studies.

Highlights

In discharges devices operating at low pressure, the electron mean free path is longer than the discharge dimensions and plasma sustainment by electron impact ionization is possible only if the electrons are confined
We show in Section Rotating Spokes at Higher Pressure (>10−3 torr) recent PIC MCC simulation results in a cylindrical magnetron geometry, that appear to be very consistent with some theoretical interpretation of rotating spokes in homopolar discharges
CONCLUDING REMARKS We have illustrated in this paper the question of electron transport in three typical but simplified E × B configurations of low temperature plasma sources on the basis of results from PICMCC simulations

Summary

INTRODUCTION

In discharges devices operating at low pressure, the electron mean free path is longer than the discharge dimensions and plasma sustainment by electron impact ionization is possible only if the electrons are confined. Another feature which is specific to E × B configurations such as those described above is the fact that the large electron drift in the E × B direction (Hall current) can lead to instabilities and to charge separation and the formation of plasma non-uniformities and a non-zero component, EH, of the space charge field along this direction Such an azimuthal component of the electric field can in turn generate an axial EH × B current, i.e., an electron current across the magnetic barrier. For pressure below 10−4 torr and typical device dimensions (few centimeters) and magnetic field intensity (in the 0.01–0.1 T range), the electron confinement time in Penning or magnetron discharges is much larger than the ion transit time. Using the classical collisional mobility and neglecting diffusion, the electron current density at the anode is: jea

Va ds

Findings

Rotating structures in LTP B