PROPAGATION OF A DISTURBANCE CREATED BY A PROBE IN ELECTRON COLLECTION

Experiments in the edge plasma of TEXT-U (the Texas Experimental Tokam ak-Upgrade) [K. W. Gentle, Nucl. Technol. Fusion 1, 479 (1981)] have u sed a positively biased Langmuir probe in conjunction with two triple probes to examine the ''wake'' or disturbance in the plasma caused by the electron current collected by the driven probe. Spatial measuremen ts of the disturbed volume have been made in the radial and poloidal d irections at two points along the disturbed flux tube, at 12 meters an d 28 meters away from the driven probe. Both non-time dependent and ti me-dependent experiments are used to determine the dispersion of the d riven wave. A classical collisional two-fluid model of electron collec tion by a biased surface, which accounts for electric potential pertur bations and the associated currents only, successfully predicts many o f the qualitative features of the disturbance. The magnitudes of the c ollision rates, however, have to be increased beyond the best estimate s in order to achieve quantitative agreement, indicating the possible involvement of anomalous cross-field current transport processes. (C) 1998 American Institute of Physics.