Discharge Mode Transition in Partially Magnetized E×B Penning Discharge

We investigate the discharge mode transition accompanied by a change in the global ‘discharge current oscillation' and the local azimuthally 'rotating spoke' in E B Penning discharge. It is observed in the experiments that large-scale (m=2 , where m is the azimuthal mode number) azimuthally rotating spoke and discharge current oscillation occur simultaneously at low discharge voltages. With increasing discharge voltages, stabilization of discharge current oscillation is correlated with attenuation of the large-scale (m=2) rotating spoke and appearance of small-scale (m=3) rotating spoke. Simultaneous measurements at a discharge voltage where spokes of multiple mode numbers coexist show that the energies of small-scale and large-scale spokes vary periodically and are strongly correlated with discharge current oscillation. Furthermore, we present a global model to identify the mechanism of discharge current oscillation in the Penning discharge. Finally, linear perturbation analysis of the global model suggests that the discharge current oscillation can be destabilized by the gradient of plasma density and stabilized by collisional damping with neutral gas.