Rl. Stenzel et Jm. Urrutia, Electron magnetohydrodynamic turbulence in a high-beta plasma. III. Conditionally averaged multipoint fluctuation measurements, PHYS PLASMA, 7(11), 2000, pp. 4466-4476
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].