Neoclassical tearing mode control using electron cyclotron current drive and magnetic island evolution in JT-60U

摘要

The results of stabilizing neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD) in JT-60U are described with emphasis on the effectiveness of the stabilization. The range of the minimum EC wave power needed for complete stabilization of an m = 2/1 NTM was experimentally identified for two regimes using unmodulated ECCD to clarify the NTM behaviours with different plasma parameters: 0.2 < jec/jbs < 0.4 for W_(sat)/d_(EC) ～ 3 and W_(ssy)/W_(marg) ～ 2, and 0.35 < j_(EC)/j_(BS) < 0.46 for W_(sat)/d_(EC) ～ 1.5 and W_(sat)/ W_(marg) ～ 2. Here, m and n are the poloidal and toroidal mode numbers; j_(EC) and j_(BS)the EC-driven current density and bootstrap current density at the mode rational surface; W_(sat), W_(marg) and d_(ec) the full island width at saturation, marginal island width and full-width at half maximum of the ECCD deposition profile, respectively. Stabilization of a 2/1 NTM using modulated ECCD synchronized with a mode rotation of about 5 kHz was performed, in which it was found that the stabilization effect degrades when the phase of the modulation deviates from that of the ECCD at the island O-point. The decay time of the magnetic perturbation amplitude due to the ECCD increases by 50% with a phase shift of ±50° from the O-point ECCD, thus revealing the importance of the phasing of modulated ECCD. For near X-point ECCD, the NTM amplitude increases, revealing a destabilization effect. It was also found that modulated ECCD at the island O-point has a stronger stabilization effect than unmodulated ECCD by a factor of more than 2.