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

A large discharge plasma is generated whose electron pressure exceeds that of an external magnetic field. A magnetic cavity exists in the plasma inter ior. The ions are unmagnetized while the electron magnetization varies from complete to none. In the region of pressure and field gradients a strong i nstability is observed. It is a cross-field instability driven by the elect ron diamagnetic drift through the unmagnetized ions, creating large density and magnetic field fluctuations near the lower hybrid frequency that propa gate at the sound speed in the diamagnetic drift direction. The basic plasm a parameters leading to the instability have been presented in Part I of th ree companion papers. Spectra, correlations, amplitude distributions, and m agnetic hodograms derived from single-point fluctuation measurements have b een presented in Part II. These led to the discovery of density cavities an d current sheet formation by nonlinear wave steepening. The present Part II I deals with multipoint fluctuation measurements using on-line conditional averaging, which resolves the structure of typical fluctuations in space an d time. Propagation velocity and coherence of the flutelike density perturb ations is measured. The topology of the magnetic fluctuations and associate d current density is investigated and shown to consist of flux ropes of neg ative self-helicities. Superposition of the nonuniform mean field and fluct uating 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 demagnetiza tion of electrons in large positive density fluctuations provides a new sat uration mechanism for the instability. (C) 2000 American Institute of Physi cs. [S1070-664X(00)03311-5].