Quasi-steady-state high confinement at high density by lower hybrid waves in the HT-6M tokamak

The quasi-steady-state (t(H) > 10(tau Eoh)) H mode with high plasma density (ELMy and ELM free) was routinely obtained by the injection of lower hybri d wave heating and lower hybrid current drive with a power threshold of 50 kW. The antenna spectrum was scanned over a wide range and tau(E) was about 1.5-2.0 times that of the L mode scaling. The density increases by almost a factor of 3 during the H phase by gas puffing and the particle confinemen t time increases by more than this factor even with a line averaged density of 3 x 10(13) cm(-3), which is about 60% of the Greenwald density limit. A hollow T-e profile was achieved in the high density case. The experimental results reproducibly show a good agreement with the theoretical prediction for the LH off-axis power deposition profile. When a certain fraction of t he plasma current is non-inductively sustained by the LH waves, a hollow cu rrent density profile is formed and the magnetic shear is reversed. This of f-axis hollow profile and enhanced confinement improvement are attributed t o a strong reduction of the electron thermal diffusivity in the reversed sh ear region.