Recent progress on TRIAM-1M

A steady state plasma with high performance and high current drive efficiency is reported. In 2.45 GHz LHCD plasmas T_i is studied as a function of n_e at the edge of the high ion temperature (HIT) window. Different characteristic timescales are found for T_i and n_e to enter the HIT regime and the observed hysteresis behaviour of T_i with respect to n_e is attributed to this difference. The electromagnetic emission (<3.5 GHz) is studied in order to understand ion heating mechanisms in the HIT regime. The spectrum shows several sidebands whose peak frequencies correspond to the ion plasma frequency. The spectral narrowing of the width of the sideband shows a clear correlation with ion heating. In 8.2 GHz LHCD plasmas an enhanced current drive (ECD) regime where both current drive efficiency η_CD (= n̄_eI_CDR₀/P_LH approx. 1 × 10¹⁹ A m^-2 /W) and energy confinement time τ_E (approx. 8-10 ms) are simultaneously improved is obtained at an n̄_e of 4.3 × 10¹³ cm^-3 and B = 7 T under full current drive conditions. There exists a certain threshold power above which the ECD transition occurs. A hysteresis of η_CD is found around the threshold power, which is explained by the different characteristic time for the ECD transition in power rampup and rampdown schemes. Current profile control experiments are performed by using two opposite travelling LHWs. Current compensation (ΔI_CD/I_CD < -10%) is clearly seen when the backward (BW) travelling LHW (8.2 GHz) is added to a target plasma whose current is driven by a forward travelling LHW (8.2 GHz). As the BW wave power is increased, however, the current tends to flow in the forward direction. The mechanisms of this non-linear behaviour of the driven current with respect to the BW wave power are discussed.