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

摘要

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 × 10~(19) 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: P_(input) = 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 P_(input) up to 500 kW. A model that predicts the generation of a negative radial electric field E_r, which is thought to help LH transition during counter-NBI, is suggested. The model conjectures the development of the E_r and toroidal rotation V_φ 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 E_r emergence in the counter-NBI scenario are presented. Doppler spectroscopy of B~(3+) impurity ions has shown an increase in the V_φ of 16 ± 6 km s~(-1) after counter-NBI switch on. The measured V_φ agrees with the above model estimations.