Troidal current drive experiment by m = +1, -1 non-axisymmetric alfven wave

The present study deals with troidal current drive by Alfven wave with a poloidal mode number, m, of +1 or -1. These waves are expected to effectively drive a bulk current because the pointing vectors in the troidal direction are large in the central portion of the plasma. A helical antenna is employed because of its high directivity and poloidal mode selectivity. Observations are made to determine the waveforms of plasma current and loop voltage. Results indicate that the plasma current and loop voltage undergo large changes though the pulse widths of the high-frequency current supplied to the antanna are small. Investigation is then carried out on dependence of driven current on m and ω/Ωi, dependence of radiative resistance of the helical coupler on ω/Ωi, and radial distributions of the driven current density and magnetic fields of the wave. It is shown that troidal current with a rise rate of 150 A/μs and maximum of 270 A is driven in plasmas having density in the range from 1019 to 1020 m-3. The maximum loop voltage is found to be 55 V. The driven current tends to increase when ω < Ωi for m = +1 and when ω < Ωi for m = -1. It is also shown that bulk wave is driven when m = +1. The high-frequency current used in this study have very short pulse widths in the range of 2 ∼ 4 μs. Further study is required using a power source with a larger pulse width. (Nogami, K.)