Spoke and spiral arm structures in Penning discharges*

Abstract: E×B plasmas generated by the electron beam are widely used in various applications for plasma propulsion and material processing. Such plasmas are subject to various instabilities affecting the operation and performance of the devices [1,2]. We investigate the formation of azimuthal structures in a partially magnetized E×B plasma self-consistently generated by the electron beam using a 2D3V PIC/MCC model [3].  Our emphasis is on the instabilities mechanisms and mode transitions between different length scales, in particular, between the large scale m=1 mode (spoke) and m>1 spiral arms structures regimes. We perform parametric studies by varying plasma parameters and magnetic fields and investigate the role of the electric field in the mechanism of the instabilities. We conclude that the large-scale structures are generated in the regime when the conditions for the gradient driven lower hybrid type instabilities are satisfied while the spiral arms regime occurs in the regime when the electric field is weak (absent) and the  Simon-Hoh type conditions do not apply.