February 1996


Experiments in 1996 were initiated from February 6 after the two month vacuum vent. The impurity and hydrogen atoms on the first wall and the divertor plate wall has been greatly reduced with ohmic and NB discharges, TDC and GDC. Conditioning with NB heating was most effective to reduce the deuterium recycling on the divertor target plates. Hydrogen/deuterium ratio from spectroscopic measurement was significantly reduced below 10% after the two week operation. The oxygen concentration was also reduced down to 2% of the electron density during the NB heating phase. Further wall conditioning to reduce the recycling on the divertor plates and the NB power-up commissioning have been continued throughout this month until the wall condition becomes sufficiently good for intense boronization planned just before the upcoming high QDT campaign.


High triangularity experiments have started with the upgraded modification of power supply systems to allow 2 MA operation for high triangularity configurations. The discharges up to 2 MA were successfully demonstrated to show the improved controllability of the configurations, in which the highest triangularity of d~0.6 was obtained at 1 MA. While the commissioning of NBI power up proceeded including port aging up to 26 MW, it was confirmed at 1.8 MA that the density limit due to a giant ELM is improved with triangularity by about 30% in comparison to low-d H-mode discharges as obtained at about 1 MA in the last year experiment. Optimization of the discharges for confinement improvement will continue for two weeks after intense boronization in March. Degassing from the ICRF launcher and the coupling commissioning have started as scheduled with the slightly reduced frequency of 102 GHz for 3.5 T discharges.


The fast reciprocating probe measurement was performed in OH and L-mode discharges with PNB=4 MW for SOL database of ITER. The SOL density, temperature and potential fluctuations were measured with triple probe. Inner and outer SOL regions with different e-folding lengths were observed both in the Te and ne profiles. The electron pressure pe at the midplane was comparable with pe at the divertor target. The frequency and cross-correlation analysis of the fluctuation data will be done. The effect of central and edge fueling on the particle confinement was investigated for NB heated discharges. The particle confinement time was increased with the ratio of NB fueling rate to wall recycling and gas-puffing fueling rate. The confinement times of edge fueled particles and centrally fueled particles were separated. It was found that the confinement time of centrally fueled particles was five times longer than that of edge fueled particles. Discharge conditions in this experiment were; the NB power was 4 MW to 16 MW, 1.8 MA/3.0T, ne=2 1019 m-3 and 3.7 1019 m-3.