Counter-NBI assisted LH transition in low density plasmas in the TUMAN-3M

Abstract

This paper reports observations of the LH transition at very low density in the experiments on counter-current neutral beam injection (NBI) in the TUMAN-3M tokamak. The transition has been found at a target average density of as low as 0.5 × 1019 m−3, which is lower by a factor of 2.5 than the LH transition density boundary in ohmic and co-current NBI heated plasmas. Relatively low input power in counter-NBI experiments is noticed: Pinput = 130–270 kW. In contrast, in the case of co-NBI the LH transition is difficult at low density. No transition is possible at the above density with co-NBI Pinput up to 500 kW. A model that predicts the generation of a negative radial electric field Er, which is thought to help LH transition during counter-NBI, is suggested. The model conjectures the development of the Er and toroidal rotation Vϕ in the presence of large ion orbit losses in the counter-NBI scheme. Measurements of the plasma potential by the heavy ion beam probe technique and edge radial electric field by electrostatic probes indicating negative Er emergence in the counter-NBI scenario are presented. Doppler spectroscopy of B3+ impurity ions has shown an increase in the Vϕ of 16 ± 6 km s−1 after counter-NBI switch on. The measured Vϕ agrees with the above model estimations.