Characteristics of Mode Transition of Partially Magnetized Plasmas in a E×B Penning discharge

The discharge mode transition in a E×B Penning discharge is characterized by a host of inherent instabilities, among which global oscillation and rotating spokes are key features. Traditionally, these phenomena have been viewed as independent entities during the discharge mode transition, with no reported interplay. Here, for the first time, we elucidate their interaction during this transition, unveiling unique physical phenomena that enhance our understanding of a E×B Penning discharge. Employing wave diagnostics and frequency spectrograms, we demonstrate asynchronous shifts in the energy of global oscillations and azimuthally rotating spokes throughout the discharge mode transition. Notably, it turns out that the energy phase shift between the global oscillations and spokes is accompanied by the temporal cross-excitation and extinction of both large-scale and smaller-scale spokes within a single global oscillation cycle. These experimental results suggest that different types of instabilities in plasma may not be independent, and the need for additional theoretical and experimental studies considering the coexistence of these instabilities.