(EX/P1-10) Controllability of Large Bootstrap Current Fraction Plasmas in JT-60U

Y. Sakamoto1), H. Takenaga, T. Fujita, S. Ide, T. Suzuki, M. Takechi, Y. Kamada, N. Oyama, A. Isayama, Y. Koide, JT-60 Team
1) Japan Atomic Energy Agency, Naka, Ibaraki-ken, 311-0193 Japan

Abstract.  A large fraction of bootstrap current driven by the high beta plasma itself is required for the steady-state tokamak concept. The profiles of pressure, current and flow should be optimized to keep stability limit and confinement as high as possible, because for example local reduction of diffusivity improves confinement while the produced steep pressure gradient sometimes causes MHD instability. The internal transport barrier (ITB) contributes to enhance the bootstrap current franction. Since the large bootstrap current fraction strengthens the linkage among these profiles, such a plasma is characterized as a self-regulating system. Active control of the ITB structure is required for reaching steady-state condition, because total pressure and current profiles are mostly determined by its structure such as the ITB radius, the ITB strength and its width. Towards the control of ITB structure, parameter linkage and controllability in the large bootstrap current fraction plasma have been investigated in JT-60U. In the case of a discharge with a large fraction of bootstrap current (75%) sustained as long as for 2.7 times longer than the current diffusion time, it is demonstrated that the evolution of the inductive field is greatly affected by the change in the structure of the ITB, indicating a strong linkage between pressure and current profiles. Real time control logic for rotation control based on real time calculation of the minimum value of safety factor using Motional Stark Effect diagnostic has been newly installed. Active control of the ITB radius is demonstrated by changing the plasma current profile, where the change in the loop voltage is found as one of the key factors for control of the radii of ITB and the minimum safety factor.

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