Noncorrelation of ``Anomalous'' Plasma Transport, Equilibrium Plasma Density, and Low-Frequency Potential Oscillations in a Q-Device

Abstract

The minimum-B magnetic field, provided by the superposition of a tranverse quadrupole-cusp field and a uniform axial field, suppressed certain of the low-frequency oscillations spontaneously present in the thermal cesium plasma of the TOPSY Q-device with the uniform axial field alone, in conjunction with a sharp increase in the center-line equilibrium plasma density of up to two orders of magnitude. However, measurements of the transverse plasma flux immediately prior to and immediately after oscillation suppression failed to detect any change in the radial plasma transport; the ``anomalous diffusion'' observed in the TOPSY plasma in a uniform magnetic field is still present in the absence of low-frequency potential oscillations. Rather than being a function of plasma-column oscillations, the observed cross-field plasma transport is quantitatively explained in terms of E/B drifts due to azimuthal ionizer-plate temperature gradients. The sharp density increase which accompanied oscillation suppression must thus be ascribed to changes in the ionizer sheath conditions leading to an increased ``source'' flux of plasma into the column from the ionizer, rather than to a decrease in the radial plasma ``loss'' flux. These results have serious implications for the study of transport processes in plasmas and, in particular, call into question the use of Q-devices for the experimental study of ``anomalous diffusion'' in complex magnetic field geometries.