(EX/P1-5) Studies on impact of electron cyclotron wave injection on the internal transport barriers on JT-60U

S. Ide1), H. Takenaga1), A. Isayama1), Y. Sakamoto1), M. Yoshida1), C. Gormezano, JT-60Team
1) Japan Atomic Energy Agency, Naka, Japan

Abstract.  In ITER advanced operations and a steady-state tokamak reactor, an internal transport barrier (ITB) is expected to play an important role in confinement improvement and increasing the bootstrap current. In many tokamaks, formation of ITB has been observed and contributed to improving performance. A weak shear (WS) ITB plasma with the safety factor at the plasma center slightly above unity can be a good candidate for the ITER hybrid scenario and even for an attractive operational scenario in a reactor. However it has been observed that the ion temperature (Ti) ITB in a WS plasma can be degraded by a electron cyclotron range of frequency wave (ECRF). Since ECRF directly heats electrons, this phenomenon can be a critical issue in a burning plasma where electron heating is predominant. Even if this phenomenon is intrinsic to ECRF, this should be avoided. Because localized current drive by ECRF is one of the best ways to suppress the neo-classical tearing modes, which can be destabilized in an ITER hybrid discharge with an ITB for example. Therefore understanding of this phenomenon and identification of the operational space for this phenomenon to occur are important issues not only for plasma physics but also fusion development. Towards the understanding of this phenomenon, impact of ECRF injection on ITBs in a WS plasma has been investigated in JT-60U with ECRF of 110 GHz. It is observed that the Ti ITB in a WS plasma can be degraded by ECRF. It is clarified for the first time that the degradation depends increasingly on the EC power but decreasingly on the plasma current (Ip). Moreover it is confirmed that ECRF affects the toroidal rotation (Vt) indirectly and results in flattening of Vt(r) regardless to the direction of the target Vt(r), peaking co- (relative to the Ip direction) or counter. Furthermore, it is newly found that the central Ti and Vt are affected with almost no delay from the EC onset even with off-axis EC deposition. These results suggest that an EC injection acts on semi-global structure that characterizes Ti ITB in a WS plasma. In this paper, details of these dependences of the Ti ITB degradation by ECRF on plasma and operational parameters will be discussed.

Full paper available (PDF)