Experimental characteristics of a lower hybrid wave multi-junction coupler in the HT-7 tokamak

A phase-controlled lower hybrid wave (LHW) multi-junction (MJ) coupler(3(rows)×(columns)× (subwaveguides)) has been developed in theHT-7 tokamak. Simulations show that it is more effective for driving plasma currentthan an ordinary phase-controlled LHW antenna (3(rows)×12(columns))(traditional coupler). The plasma--wave coupling experiments show that thereflection coefficient (RC) is below 10%, implying that the MJ grill can launchthe wave into the plasma effectively. The effect of power spectrum launched by theMJ coupler on RC indicates that an optimal condition is requisite for a bettercoupling in the lower hybrid current drive (LHCD) experiments. Studies indicate thatthe drive efficiency of the MJ antenna is higher than that of the traditional one,which is mainly ascribed to the discrepancy in impurity concentration, plasmatemperature, and spectrum directivity. An improved confinement with an electroninternal transport barrier is obtained by LHCD. The analysis shows that themodified negative (low) magnetic shear and the change of radial electric fieldprofile due to LHCD are possible factors responsible for the eITB formation.