Gradient transport processes in E×B plasmas

Various classes of non-quiescent magnetized plasma exhibit anomalous energy and/or momentum diffusivity. In this case, ‘anomalous’ is meant to characterize a transport regime wherein the measured property flux disagrees with classical theory. In most examples anomalous transport is substantially higher than classical expressions predict; it is no different for the Hall accelerator. It has been well documented that the electron transport in a Hall accelerator can vary dramatically along its channel, and cannot be accounted for within an accepted physical framework. A myriad of justifications have been forwarded in an attempt to understand the cause of the observed transport, but all have fallen short especially at low discharge voltage. When plasma fluctuations are considered to be the cause of anomalous transport, they are commonly suggested in the context of azimuthal fluctuations. Recent numerical calculations in an r-z hybrid code predict the existence of strong 2-5MHz axial oscillations best characterized by the dispersion relation for a beamplasma instability. This paper reports on the instability, and forwards a saturation mechanism for longitudinal electrostatic oscillations considering gradient transport processes in the presence of an applied magnetic field. It is found that the nonlinear saturation of the observed instability is sufficient to explain the anomalous transport near the exit plane of the Hall accelerator.