Overview of the HL-1M Tokamak Experiments

Experimental progress with the HL-1M tokamak has been made in many areas including confinement improvement, auxiliary heating, plasma fueling, and wall conditionings. An H-mode induced by a biased electrode was obtained with the formation of an internal transport barrier at the region of r/a ~ 0.4 to 0.5. Confinement improvement by lower-hybrid current drive (LHCD) was extensively investigated. Confinement improvement seems to be related to the production of the radial electron field during LHCD. In off-axis electron cyclotron resonance heating (ECRH), double sawteeth in soft X-ray radiation were observed, which implies that reversed magnetic shear could be formed during ECRH. At higher ECRH power, when the resonance position is near the q = 1 surface, fishbone instability was observed and investigated. An eight-shot pellet injector was used for the experiments. The pellet ablation process was investigated with a charge-coupled device (CCD) camera and an H emission detector array. Clearly, asymmetry in the pellet cloud was observed in both the toroidal and poloidal directions. It has been found that the pellet velocity slows down clearly after the pellet enters the plasma. The density limit has been investigated on HL-1M at different wall conditionings with three kinds of fueling methods. It was found that a higher density limit could be achieved under the following conditions: (a) a strong reduction of the impurity content after siliconization and (b) a peaked density profile with pellet injection and/or supersonic molecular beam injection. With a neutral beam injection (NBI) system of 1 MW, preliminary results of NBI experiments were obtained with an increase of ion temperature from 450 to 700 eV.