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

A magnetic void is created by high electron pressure in a large nonuniform laboratory plasma. A strong instability is observed in regions of high pres sure and magnetic field gradients. It is associated with the electron diama gnetic drift through the essentially unmagnetized ions. Its spectrum is bro ad and peaks near the lower hybrid frequency. The coupled fluctuations in d ensity, electron temperature, plasma potential, and magnetic field are meas ured with probes and cross-correlated. The temporal correlation extends onl y over 1-2 oscillations. The fluctuations propagate in the direction of the electron diamagnetic drift but at the lower ion acoustic speed. In the sat urated regime of the instability, the fluctuation waveforms are highly nonl inear. Density cavities with deltan/n similar or equal to -40% are formed w ith steepened density rise at the trailing edge. The associated high pressu re gradient forms a diamagnetic current sheet. Positive density perturbatio ns are smaller (deltan/n less than or equal to 20%), broader, and produce r egions of weak magnetic fields where the electrons become nearly unmagnetiz ed. Amplitude distributions of nonlinear density, magnetic field, and curre nt waveforms are evaluated. The three-dimensional magnetic field fluctuatio ns are analyzed with hodograms. The direction of the average wave vector po ints essentially across the mean field in the direction of the diamagnetic drift. The magnetic fluctuations can be interpreted as highly oblique elect ron whistlers, the density fluctuations as sound waves, but both modes are coupled in a high-beta plasma. Fluctuations in the electric and magnetic fi elds lead to a time-averaged electron drift, i.e., anomalous transport, acr oss the mean field. (C) 2000 American Institute of Physics. [S1070-664X(00) 03211-0].