Electron magnetohydrodynamic turbulence in a high-beta plasma. II. Single point fluctuation measurements

Abstract

A magnetic void is created by high electron pressure in a large nonuniform laboratory plasma. A strong instability is observed in regions of high pressure and magnetic field gradients. It is associated with the electron diamagnetic drift through the essentially unmagnetized ions. Its spectrum is broad and peaks near the lower hybrid frequency. The coupled fluctuations in density, electron temperature, plasma potential, and magnetic field are measured with probes and cross-correlated. The temporal correlation extends only over 1–2 oscillations. The fluctuations propagate in the direction of the electron diamagnetic drift but at the lower ion acoustic speed. In the saturated regime of the instability, the fluctuation waveforms are highly nonlinear. Density cavities with δn/n≃−40% are formed with steepened density rise at the trailing edge. The associated high pressure gradient forms a diamagnetic current sheet. Positive density perturbations are smaller (δn/n⩽20%), broader, and produce regions of weak magnetic fields where the electrons become nearly unmagnetized. Amplitude distributions of nonlinear density, magnetic field, and current waveforms are evaluated. The three-dimensional magnetic field fluctuations are analyzed with hodograms. The direction of the average wave vector points essentially across the mean field in the direction of the diamagnetic drift. The magnetic fluctuations can be interpreted as highly oblique electron whistlers, the density fluctuations as sound waves, but both modes are coupled in a high-beta plasma. Fluctuations in the electric and magnetic fields lead to a time-averaged electron drift, i.e., anomalous transport, across the mean field.