Study of lower hybrid current drive efficiency and its correlation with photon temperatures in the HT-7 tokamak

Lower hybrid current drive (LHCD) efficiency is a very important parameter. The experimental current drive efficiency is defined as ? = IrfneR/PLH, where Irf is the current driven by the lower hybrid waves (LHWs), ne is the central line-average density, R is the major radius of the plasma and PLH is the injected LH wave power absorbed by the plasma through Landau damping. A study of current drive efficiency of LHWs in the HT-7 tokamak has been carried out in the parameter ranges: ne = (1.2?2.5) ? 1019?m?3, Ip = (80?200)?kA, Bt = 1.8?T, PLH = (188?532)?kW in the limiter configuration. Current drive efficiency is investigated through a simple correlation with photon temperature and normalized intensity of fast electron bremstrahlung emission, which is, in the first approximation, proportional to the averaged velocity and population of the fast electrons. The plasma current scanning experiment shows that CD efficiency increase is due to the increase in both the photon temperature and the population of the fast electrons generated by LHWs. The density scanning experiment shows that as the plasma density is increased, an increment in CD efficiency along with the increase in the population of fast electrons is observed. The slowing down through the collisions with bulk electrons is mainly responsible for the decreased photon temperature during the plasma density scan. These experiments strongly suggest the dominant role of the population of fast electrons generated by LHCD and the generation of the current carried by fast electrons.