(EX/P6-6) Non-inductive Plasma Current Start-up by EC and RF Power in the TST-2 Spherical Tokamak

A. Ejiri1), Y. Takase1), J. Sugiyama1), T. Oosako1), T. Yamaguchi1), Y. Adachi1), O. Watanabe1), T. Masuda1), M. Sasaki1), H. Tojo1), S. Kainaga1), Y. Nagashima1), B.I. An1), H. Kobayashi1), H. Kurashina1), H. Hayashi1), H. Matsuzawa1), K. Yamada1)
 
1) The University of Tokyo, Kashiwa, Japan

Abstract.  Key issues in spherical tokamak (ST) research are plasma current start-up and formation of the ST configuration without the use of a central solenoid (ohmic coil). Successful start-up, ST formation and sustainment have been achieved by injecting RF power (usually in the EC frequency range) to a configuration with a toroidal field and a weak vertical field. However, the current generation mechanism is still not clearly understood. Non-inductive plasma current start-up experiments were performed in the TST-2 spherical tokamak using EC (2.45 GHz, up to 5 kW) and RF (21 MHz, up to 30 kW) power. In order to determine the mechanism experimentally, dependences on such parameters as EC wave polarization and vertical field configuration are studied. It was found that the sustained current after a current jump is proportional to the vertical field strength, but dependences on other parameters are very weak. On the other hand, the initial current ramp-up rate depends on several parameters such as the filling pressure, the injected EC power, the vertical field strength, the resonance position and the mirror ratio, and a scaling law was obtained for the first time. Low frequency RF power was injected to the low current EC plasma. A current jump was induced and ST configuration was sustained after the EC power was terminated. The sustained currents were similar to those sustained by the EC power. These results demonstrate that a low frequency RF power can be used as an important and flexible tool for starting up ST plasmas.

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