Study on enhancement of current drive efficiency in long RF current drive plasmas

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Steady state operation of tokamak is a key for the realization of cost-effective fusion power plant. TRIAM-1M (R=0.8m, a x b = 0.12 x 0.18m, B_t<8T) is positively tackled with the study of LH current drive The enhanced current drive (ECD) mode which accompanied with the improvement of energy confinement, could be observed.The direct loss of passing energetic electrons may play an essential role in reduction of current drive efficiency. Therefore, the measurements of the confinement time of energetic electrons are important.The absolute value of the prompt loss of energetic electrons was measured as the thermal input on the movable limiter located in the low field side of the plasma. In fact, the measurement was carried out in the plasma sustained by injected RF power of 65 kW and the estimated stored energy of energetic electron was about 500 J. The measured heat flux is about 0.5 kW and most of the thermal input to the movable limiter came from SOL plasma. In addition, ripple loss of energetic electron is enhanced in ECD mode.This clearly shows that the improvement of the loss of energetic electrons does not play a significant role in the enhancement of current drive efficiency.