Investigation of the electric field structure in ISX-B plasmas

Plasma potentials have been measured for the first time in neutral-beam-heated tokamak discharges. Radial potential profiles have been obtained for coinjection, counterinjection, and balanced injection discharges as well as for ohmically heated plasmas in the Impurity Study Experiment (ISX-B) tokamak. Within experimental uncertainties, the measured values of potential are consistent with calculations based on radial momentum balance using experimental values of rotation velocities, density, and ion temperature. The measurements were made using a heavy-ion beam probe, with typical plasma conditions of I/sub p/ approx. = 150 kA, B/sub T/ approx. = 12.3 kG, n-bar/sub e/ approx. = 4 x 1013 cm-3, and P/sub b/ approx. = 0.9 MW. A negative potential well depth of about 1.0 kV was observed in ohmically heated plasmas and increased somewhat for balanced injection. Counterinjection resulted in a significantly larger well depth of approximately 3 to 4 kV, while coinjection showed an outward-pointing electric field in the plasma interior. The particle confinement times of both ions and impurities were observed to improve with counterinjection as compared with coinjection