Steady-state sustainment of high-β plasmas through stability control in Japan Atomic Energy Research Institute Tokamak-60 Upgrade -: art. no. 056117

Recent results from steady-state sustainment of high-beta plasma experiments in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) tokamak [A. Kitsunezaki , Fusion Sci. Technol. 42, 179 (2002)] are described. Extension of discharge duration to 65 s (formerly 15 s) has enabled physics research with long time scale. In long-duration high-beta research, the normalized beta beta(N)=2.5, which is comparable to that in the steady-state operation in International Thermonuclear Experimental Reactor (ITER) [R. Aymar, P. Barabaschi, and Y. Shimomura, Plasma Phys. Controlled Fusion 44, 519 (2002)], has been sustained for about 15 s with confinement enhancement factor H-89PL above 2, where the duration is about 80 times energy confinement time and similar to 10 times current diffusion time (tau(R)). In the scenario aiming at longer duration with beta(N)similar to 1.9, which is comparable to that in the ITER standard operation scenario, duration has been extended to 24 s (similar to 15 tau(R)). Also, from the viewpoint of collisionality and Larmor radius of the plasmas, these results are obtained in the ITER-relevant regime with a few times larger than the ITER values. No serious effect of current diffusion on instabilities is observed in the region of beta(N)less than or similar to 2.5, and in fact neoclassical tearing modes (NTMs), which limit the achievable beta in the stationary high-beta(p) H-mode discharges, are suppressed throughout the discharge. In high-beta research with the duration of several times tau(R), a high-beta plasma with beta(N)similar to 2.9-3 has been sustained for 5-6 s with two scenarios for NTM suppression: (a) NTM avoidance by modification of pressure and current profiles, and (b) NTM stabilization with electron cyclotron current drive (ECCD)/electron cyclotron heating (ECH). NTM stabilization with the second harmonic X-mode ECCD/ECH has been performed, and it is found that EC current density comparable to bootstrap current density at the mode location is required for complete stabilization. Structure of a magnetic island associated with an m/n=3/2 NTM has been measured in detail (m and n are poloidal and toroidal mode numbers, respectively). By applying newly developed analysis method using motional Stark effect (MSE) diagnostic, where change in current density is directly evaluated from change in MSE pitch angle without equilibrium reconstruction, localized decrease/increase in current density at the mode rational surface is observed for NTM growth/suppression. In addition, it is found that characteristic structure of electron temperature perturbation profile is deformed during NTM stabilization. Hypothesis that temperature increase inside the magnetic island well explains the experimental observations. It is also found that the characteristic structure is not formed for the case of ECCD/ECH before the mode, while the structure is seen for the case with ECCD/ECH just after the mode onset, suggesting the stronger stabilization effect of the early EC wave injection. (c) 2005 American Institute of Physics.