Identification of gradient drift instability evolving into a rotating spoke in a crossed field plasma*

The rotating low frequency and large scale structures referred to as spokes are frequently observed in low temperature E × B plasma devices. Spoke can result in the electron anomalous transport across the magnetic field, prohibiting the establishment of a predictive model for the optimization and operation of E × B plasma devices. So far, fundamental aspects of the spoke are not well understood and one of the core questions under open debate is the driving mechanism behind the formation of the rotating spoke. This work provides direct evidence of gradient drift instability being the origin of a rotating spoke in a crossed field plasma using 2D radial-azimuthal fully kinetic PIC/MCC simulations. The kinetic model exhibits a pronounced rotating spoke and a clear linear-nonlinear transition and reveals the whole perturbation spectrum of the gradient drift instability in the linear stage: Simon-Hoh, lower-hybrid and ion sound modes. The two-fluid dispersion relation of the gradient drift instability was utilized to interpret the linear development of instabilities in the simulations. It was shown the simulated spectrum and growth rate agree very well with the theoretical dispersion relation over the investigated cases. The most linearly unstable mode was found to be the lower hybrid instability and the mode transition into the m=1 macroscopic rotating structure after saturation of the linear phase is accompanied by an inverse energy cascade.