(EX/4-1Ra) Active Control of Neoclassical Tearing Modes toward Stationary High-Beta Plasmas in JT-60U

A. Isayama1), N. Oyama1), H. Urano1), T. Suzuki1), M. Takechi1), N. Hayashi1), K. Nagasaki2), Y. Kamada1), S. Ide1), T. Ozeki1), JT-60 Team1)
 
1) Japan Atomic Energy Agency, Naka, Ibaraki 311-0193, Japan
2) Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan

Abstract.  Results from active control of neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD) are described. In JT-60U, it was previously demonstrated that the amplitude and period of sawtooth oscillations can be controlled by optimizing the location and direction of the ECCD. In particular, ECCD inside the sawtooth inversion radius can increase the frequency and amplitude of the sawtooth oscillations. By utilizing the sawtooth destabilization, a new discharge scenario to control the evolution of an m/n = 3/2 NTM through sawtooth control has been developed (m and n are poloidal and toroidal mode numbers, respectively). For the cases of no EC wave injection and 1.3 MW of EC wave injection, a 3/2 NTM appears and grows steadily. On the other hand, for the case of 2.6 MW EC wave injection (EC-driven current is 10% of the total plasma current), the growth is suppressed and the amplitude of the NTM is kept low throughout the high-beta phase even after the turn-off of EC wave injection. Frequency spectrum of magnetic perturbations shows that the mode growth is interrupted by a sawtooth crash, which clearly shows the interaction between the sawtooth crash and the 3/2 NTM. The value of the normalized beta for the 2.6 MW EC case is higher than that for the other cases, suggesting the improvement of the beta value and confinement by the NTM control. An NTM with m/n = 2/1, which causes larger degradation of confinement than a 3/2 NTM, has been also successfully stabilized by injecting EC wave to the mode rational surface. Analysis with the ACCOME code and a Fokker-Planck code shows that EC-driven current density at the q = 2 surface, which is located at 0.5 in the normalized minor radius, is comparable to the local bootstrap current density. In addition, a transport code TOPICS has been improved to solve the modified Rutherford equation, which enables a self-consistent analysis of the evolution of island width and current profile during NTM stabilization. By using the TOPICS code, effects of ECCD width on NTM stabilization have been quantitatively evaluated. It has been found that the stabilization effect significantly increases with decreasing the ECCD width. The simulation also clarifies the importance of ECCD width: EC wave power required for complete stabilization can be reduced less than half by narrowing the ECCD with by about 30%.

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