High performance discharges near the operational limit in HT-7

Efforts have been made on the HT-7 tokamak to extend the stable operation b oundaries. Extensive RF boronization and siliconization have been used and a wider operational Hugill diagram has been obtained. The transit density r eached 1.3 times the Greenwald density limit in ohmic discharges. A station ary high performance discharge with q(alpha) = 2.1 has been obtained after siliconization. Confinement improvement was obtained as a result of the sig nificant reduction of electron thermal diffusivity chi (epsilon) in the out er region of the plasma. An improved confinement phase was also observed wi th LHCD in the density range of 70-120% of the Greenwald density limit. Off -axis LH wave power deposition was attributed to the weak hollow current de nsity profile. Code simulations and measurements showed good agreement with the off-axis LH wave deposition. Supersonic molecular beam injection has b een successfully used to achieve stable high density operation in the regio n of the Greenwald density limit.