Control of plasma current during lower hybrid current drive in the JFT-2M tokamak

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Abstract
Current drive by lower hybrid waves that have a phase velocity five times the electron thermal velocity has been investigated in the JFT-2M tokamak. Two different modes of tokamak operation, the automatic current regulation (ACR) and the automatic voltage regulation (AVR) modes, were employed to study the plasma control during the current drive. In the ACR mode, which is the ordinary tokamak operation in JFT-2M, the current of the poloidal-field coils which produce the main magnetic flux outside the torus is controlled by thyristor power supplies, while in the AVR mode their voltage is controlled. The results indicate that control of the poloidal field determines the time evolution of the plasma current during current drive whereas operation in the AVR mode suggests the sustainment of a steady-state plasma current through recharging of the Ohmic transformer. A high current drive efficiency was obtained which increased with the plasma current as up to Ip = 150 kA. The dependence of the current drive efficiency on the plasma current is related to the electron temperature and to propagation effects of the lower hybrid waves. The interaction of the excited lower hybrid waves with the electrons was investigated by pulse height analysis of the soft-X-ray emission. It was shown that the measured soft-X-ray energy spectrum is consistent with the theoretical spectrum of the launcher. The characteristic times for the modification of the electron distribution function and for the drop in loop voltage are proportional to the electron density.