E(R) MODIFICATION BY MEANS OF COUNTER-NEUTRAL BEAM INJECTION IN A LOW-I-P PLASMA

The idea of controlling the radial electric field (E(r)) by means of a radial current resulting from ion orbit loss caused by counter neutra l beam injection has been theoretically and experimentally investigate d. A large fraction (similar or equal to 38%) of the 75 keV deuterium ions counter-injected into a low-I-p plasma (similar or equal to 0.4 M A) suffers prompt orbit loss, which forces an inward ion current to ma intain charge neutrality. Monte Carlo guiding-centre orbit calculation s predict a radial current of 80 A at the last closed Bur surface. In these discharges, E(tau) is negative everywhere, owing to the counter- going toroidal rotation, and exhibits a double-bump shape, in contrast to the usual positive parabolic shape for the co-injection case. The measured carbon impurity ion toroidal rotation profile shows a pedesta l over the outer region where fast ions are lost, possibly due to the effect of J(r)(orbit) x B torque. The momentum diffusion process tends to slow down and to spatially spread the J(r)(orbit) x B torque effec t. The LH transition did not occur more quickly in these discharges th an in similar co-injected discharges.