The operation physics group has started the study of optimization scenario for the operation conditions of JT-60U after the divertor modification. In the initial three weeks in June, the plasma current will be carefully increased from ~1MA to ~2MA , where the halo current distribution and direction of plasma displacement at disruptions should be systematically investigated for the stress analyses of the new divertor structure.
Through the series of February experiments, the NNB injection has progressed, where 2-3.2 MW/source at 350 keV was successfully injected with a pulse length of ~1-2 sec. Systematic study of NNB current drive was carried out. The profile of beam driven current was well reconstructed using EFIT code (JT-60U/DIII-D collaboration) and MSE (motional Stark effect spectroscopy) measurements by contribution of Dr. C.B. Forest from General Atomics. The measurements show total driven current and current density profiles consistent with the theoretical predictions.
Density dependence of shine through fraction of NNB was measured. A rough agreement was found with the numerical calculation with Janev's ionization cross section. The effect of NNB injection on stabilization of ICRF-driven TAE modes was investigated for the first time. The mode structure was changed with NNB injection. The mechanism is under investigation.
BOUNDARY AND STEADY STATE OPERATION PHYSICS
Two papers were submitted to EPS conference to be held in June, entitled ; "Behavior of the Radiation Loss in Radiative Divertor Plasmas of JT-60U" and "Behavior of Neutral Deuterium and Helium Atoms in the Divertor Region in JT-60U". The former discusses radiation profiles around the X-point in ELMy H-mode discharges and an improved confinement mode of reversed shear discharge with neon puff. The latter studies basic process of Franck-Condon atoms and generation mechanism of energetic (1-2 eV) helium atoms in the divertor region by spectroscopic measurement.
The divertor modification of JT-60U started on Feb. 12 is in progress. The present open divertor will be modified to a W-shaped pumped divertor consisting of inclined targets, a dome in the private region and baffles. The construction work in the vacuum vessel will be completed at the end of April.