Current profile control experiments in the LHCD plasma on TRIAM-1M

Controllability of current profile in long term discharges is studied by two kinds of combinations of wave spectra. First by superposition of higher Nparallel-spectrum wave at 2.45 GHz on a target plasma driven by lower one at 8.2 GHz, a hollow j(r) has been achieved and sustained for 20 sec. The current profile can be well controlled below a threshold power of 14 kW by varying the power of 2.45 GHz. A spontaneous transition phenomenon to a peaked j(r), however, has occurred over the threshold power even after a reasonable steady state condition has been obtained. Second using two opposite travelling waves at 8.2 GHz the total current is clearly reduced and the j(r) becomes peaked when the backward travelling LHWs (BW) are superposed at low power of 20 kW. When the BW power is increased above 27 kW, further current reduction is suppressed, and then above 34 kW the direction of the current driven by BW is completely reversed and the j(r) becomes broad. Thus, current modification by BW shows a highly nonlinear behavior with respect to the BW power. (author)