Formation and Sustainment of Spherical Tokamak by Electron Cyclotron Heating

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Experiments on start-up and formation of spherical tokamak plasmas were carried out on the LATE device by electron cyclotron heating alone without Ohmic heating, and following results were obtained.[1] A plasma current UP to 3 kA was generated and maintained for 1 sec by injecting a 2.45 GHz, 5 kW microwave power. Magnetic measurement showed that closed flux surfaces were formed. The electron density was more than the plasma cutoff density, suggesting that electron Bernstein wave (EBW) heating occurred.[2] In order to form closed flux surfaces, it was necessary that (1) the gas pressure was 2 - 4 x 10^<-3> Pa, and that (2) the n index of the vacuum vertical magnetic field was more than 0.18.[3] The mechanism of current drive before formation of closed flux surfaces were investigated on the basis of electron density and temperature profiles measured by electrostatic probes. Th estimated value of driven current was in fairly good agreement with the observed value.[4] By injecting 2GHz/0.1s, 50 kW microwave pulse and increasing the vertical magnetic field after formation of closed flux surfaces, the plasma current increased up to 5 kA. The strength of the vertical field had to be controlled to maintain the MHD equilibrium. Additional gas puffing during the discharge increased the density more than 6 times the cutoff density.[5] We found the method to calculate the polarization of incident electromagnetic waves which maximized the mode conversion efficiency to EBW for arbitrary density gradient and injection angle, and made the calculation code. The new result was obtained that the mode conversion efficiency more than 90 % can be obtained by optimizing the polarization even in the condition where previous theory predicted bad conversion efficiency.