Steady-state sustainment of high-β plasmas through stability control in Japan Atomic Energy Research Institute Tokamak-60 Upgrade

Abstract

Recent results from steady-state sustainment of high-β plasma experiments in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) tokamak [A. Kitsunezaki et al., Fusion Sci. Technol. 42, 179 (2002)] are described. Extension of discharge duration to 65s (formerly 15s) has enabled physics research with long time scale. In long-duration high-β research, the normalized beta βN=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 15s with confinement enhancement factor H89PL above 2, where the duration is about 80 times energy confinement time and ∼10 times current diffusion time (τR). In the scenario aiming at longer duration with βN∼1.9, which is comparable to that in the ITER standard operation scenario, duration has been extended to 24s (∼15τR). 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 βN≲2.5, and in fact neoclassical tearing modes (NTMs), which limit the achievable β in the stationary high-βp H-mode discharges, are suppressed throughout the discharge. In high-β research with the duration of several times τR, a high-β plasma with βN∼2.9–3 has been sustained for 5–6s 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.