EX/P3-10 Counter-NBI Assisted LH Transition in Low Density Plasmas in the TUMAN-3M

Abstract. Paper reports observation of the LH transition at very low density in the experiments on countercurrent Neutral Beam Injection (NBI) in the TUMAN-3M tokamak. The transition has been found at target average density as low as 0.5⋅10 m. Low input power in these experiments should be noticed: Pinput ≈ 1.3POHM < 240 kW. Contrary, in the case of co-current NBI the LH transition is difficult at low density. No transition is possible at the above density in the co-NBI heating scheme with Pinput up to 500 kW. Model predicting generation of a negative (inward directed) radial electric field Er, which is thought to help LH transition at low density, 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. Results of direct measurements of the plasma potential by Heavy Ion Beam Probe technique (HIBP) confirming negative Er appearance in counter-NBI scenario are presented. Doppler spectroscopy of BIV impurity has shown an increase in the toroidal rotation of 15 km/s after application of the counter-NBI. The measured Vφ is in good agreement with the above model estimations.