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 the HT-7 tokamak. Simulations show that it is more effective for driving plasma current than an ordinary phase-controlled LHW antenna (3(rows)×12(columns)) (traditional coupler). The plasma--wave coupling experiments show that the reflection coefficient (RC) is below 10%, implying that the MJ grill can launch the wave into the plasma effectively. The effect of power spectrum launched by the MJ coupler on RC indicates that an optimal condition is requisite for a better coupling in the lower hybrid current drive (LHCD) experiments. Studies indicate that the drive efficiency of the MJ antenna is higher than that of the traditional one, which is mainly ascribed to the discrepancy in impurity concentration, plasma temperature, and spectrum directivity. An improved confinement with an electron internal transport barrier is obtained by LHCD. The analysis shows that the modified negative (low) magnetic shear and the change of radial electric field profile due to LHCD are possible factors responsible for the eITB formation.