An investigation of density fluctuations and electron transport in the Madison Symmetric Torus reversed-field pinch

The origin of large scale density fluctuations and their role in electron transport has been investigated in the Madison Symmetric Torus reversed-field pinch [R. N. Dexter et al., Fusion Tech. 19, 131 (1991)]. At medium frequencies (10-30 kHz), ñ is associated with core-resonant magnetic fluctuations having poloidal mode number m=1 and toroidal mode numbers n=6-10. These chord-averaged density fluctuations are shown to be coherent with large scale radial flow fluctuations, permitting fluctuation-induced particle flux of the form 〈ñṽ_r〉 to be studied. In the edge, although amplitudes are large, ñ and ṽ_r are out of phase, consistent with advection of the mean density gradient by perturbed magnetic field lines and no electron transport. During improved confinement discharges, produced by inductive current drive, both fluctuation amplitudes and the total radial electron flux are greatly reduced over the entire plasma cross section.