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 neutral beam injection has been theoretically and experimentally investigated. A large fraction (≃ 38%) of the 75 keV deuterium ions counter-injected into a IOW-I_p plasma (≃ 0.4 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, E_r 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 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 effect. The L-H transition did not occur more quickly in these discharges than in similar co-injected discharges.