P2.140 in 31st EPS

Bootstrap Current Effects on Stable Non-Inductive Current Buildup in Low Aspect Ratio Tokamaks

The fundamental geometry of a low aspect ratio tokamak precludes the presence of a large center solenoid normally equipped for inductive current buildup. Therefore, non-inductive techniques [1] to establish, maintain, control and modify the current distribution have been expected for promising low aspect ratio reactor concepts [2]. One important consequence clarified by way of theoretical and experimental studies might be that the natural timescale for the non-inductive current buildup with MHD stability could be even longer than one for the totally inductive buildup [3, 4]. We present a simulation whereby a newly found non-linear link between the non-inductive driven current and bootstrap (BS) current plays a substantial role in the reduction of the current buildup time to attain negative shear (NS) plasmas, avoiding a formation of current hole.

Time-evolution of the current buildup was investigated via axisymmetric MHD simulations using Tokamak Simulation Code (TSC) [5], taking the magnetic shear profile and relevant internal transport barrier (ITB) into consideration such that the ITB foot was continually adapted in accordance with the movement of the magnetic shear reversal monitored throughout the simulation [6]. Despite the intention controlling a monotonic transition from positive shear (PS) to NS, cooperative link between non-inductive driven and ITB-generated BS currents exhibited a self-organized recurrent appearance of PS and NS profiles. Profile mismatch between the non-inductive and BS currents was clarified to give rise to this recurrence, which provides a favorable effect on the reduction of the non-inductive current buildup time.

[1] S.C. Jardin, C.G. Bathke, D.A. Ehst et al., Fusion Eng. Des. 48 (2000) 281.
[2] S. Nishio et. al., Proc. 19th IAEA Fusion Energy Conf. (Lyon) FTP1/21 (2002).
[3] S.C. Jardin, Nucl. Fusion 40 (2000) 1101.
[4] Y. Takase, S. Knowlton, M. Porkolab, Phys. Fluids 30 (1987) 1169.
[5] S.C. Jardin, N. Pomphery and J. Delucia, J. Comput. Phys. 66 (1986) 481.
[6] Y. Nakamura et. al., 13th Toki Conf. (Toki) PI-14 (2003).