Propagation of a disturbance created by a probe in electron collection

Abstract

Experiments in the edge plasma of TEXT-U (the Texas Experimental Tokamak-Upgrade) [K. W. Gentle, Nucl. Technol. Fusion 1, 479 (1981)] have used 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 measurements of the disturbed volume have been made in the radial and poloidal directions at two points along the disturbed flux tube, at 12 meters and 28 meters away from the driven probe. Both non-time dependent and time-dependent experiments are used to determine the dispersion of the driven wave. A classical collisional two-fluid model of electron collection by a biased surface, which accounts for electric potential perturbations and the associated currents only, successfully predicts many of the qualitative features of the disturbance. The magnitudes of the collision rates, however, have to be increased beyond the best estimates in order to achieve quantitative agreement, indicating the possible involvement of anomalous cross-field current transport processes.

Biased electrodes for SOL control in NSTX

“…Thus the biasing potential has a relatively short range across the magnetic field. Similar results have been seen in previous experiments on SOL biasing [8][9][10].…”

“…In CASTOR [7] an electrode was biased +100 to +200 V in the SOL, a poloidal electric field of up to 5 kV/m was created on flux surfaces connected to the electrode, and a strong poloidal modulation of the radial particle flux was measured. In related experiments, a positive DC plate bias in DITE changed the floating potential in a probe $2.5 mm away along B [8], a positive DC and 30 kHz bias in TEXT were detected $12 m away along B [9], and a 60 KHz probe bias in W7-AS was observed at a distance up to $12 m away along B [10]. Thus several previous biasing experiments have created local poloidal electric fields in the SOL.…”

Local scrape-off layer control using biased electrodes in NSTX

“…However, this could be explained if the potential perturbation in the flux tube due to positive biasing was actually much smaller than the applied potential, such that the v rad due to this potential was less than the ambient turbulence speed. A similar result was obtained in TEXT [17], where the downstream the ion saturation current perturbation due to a positively DC biased probe was much less distinct than the downstream potential perturbation.…”

Reciprocating probe measurements in the test divertor operation phase of Wendelstein 7-X

“…However, fusion plasmas are usually large and dense enough that typical (ion saturation) currents of up to ∼100 mA drawn by the probe do not significantly affect the plasma. For DC and AC biasing experiments using probes as an active perturbation source [57,58], typically larger currents are drawn. In W7-X, particularly strong perturbations by probes on the MPM have been observed in the case of swept Langmuir probes being in the electron collection mode, i.e.…”