Electron magnetohydrodynamic turbulence in a high-beta plasma. III. Conditionally averaged multipoint fluctuation measurements

Abstract

A large discharge plasma is generated whose electron pressure exceeds that of an external magnetic field. A magnetic cavity exists in the plasma interior. The ions are unmagnetized while the electron magnetization varies from complete to none. In the region of pressure and field gradients a strong instability is observed. It is a cross-field instability driven by the electron diamagnetic drift through the unmagnetized ions, creating large density and magnetic field fluctuations near the lower hybrid frequency that propagate at the sound speed in the diamagnetic drift direction. The basic plasma parameters leading to the instability have been presented in Part I of three companion papers. Spectra, correlations, amplitude distributions, and magnetic hodograms derived from single-point fluctuation measurements have been presented in Part II. These led to the discovery of density cavities and current sheet formation by nonlinear wave steepening. The present Part III deals with multipoint fluctuation measurements using on-line conditional averaging, which resolves the structure of typical fluctuations in space and time. Propagation velocity and coherence of the flutelike density perturbations is measured. The topology of the magnetic fluctuations and associated current density is investigated and shown to consist of flux ropes of negative self-helicities. Superposition of the nonuniform mean field and fluctuating fields yields the net field and current density. Instantaneous field lines and magnitude distributions in three-dimensional space are presented for a turbulent high-beta plasma. The results suggest that the demagnetization of electrons in large positive density fluctuations provides a new saturation mechanism for the instability.