modification by means of counter-neutral beam injection in a low- plasma

The idea of controlling the radial electric field by means of a radial current resulting from ion orbit loss caused by counter neutral beam injection has been theoretically and experimentally investigated. A large fraction (%) of the 75 keV deuterium ions counter-injected into a low- plasma ( MA) suffers prompt orbit loss, which forces an inward ion current to maintain charge neutrality. Monte Carlo guiding-centre orbit calculations predict a radial current of 80 A at the last closed flux surface. In these discharges, 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 pedestal over the outer region where fast ions are lost, possibly due to the effect of torque. The momentum diffusion process tends to slow down and to spatially spread the torque effect. The L - H transition did not occur more quickly in these discharges than in similar co-injected discharges.