February - March 1999
Operations of JT-60U in 1999 commenced with wall conditioning and calibration of diagnostics using tokamak discharges. In the 2nd week of March, boronization with 70 g decaborane (B10H14) was carried out. The coated Boron layer was estimated to be ~400 nm in thickness. After the boronization, oxygen impurity content in ohmic and low power NBI-heated (< 10 MW) discharges was reduced to ~0.5%.
CORE PLASMA
After the integrated tests of 110 GHz RF system, the RF waves in the electron cyclotron range of frequencies were first launched into the torus on February 4. Basic characteristics of the fundamental electron cyclotron resonance heating by the waves have been investigated by changing the polarization angle, the injection angle, the resonance position, etc. Throughout two-week experiments, the central ECH at 0.6 MW for 0.3 s led to a distinctive electron temperature rise of 2 keV.
Conditioning of a negative-ion-based NBI (N-NBI) has been carried out to stretch the beam pulse width, prior to plasma heating and current drive experiments planed for April. On March 19, N-NBI was injected into the plasma for the first time in this year at 350 keV with 3.3 MW for 0.8 s.
DIVERTOR/SOL PLASMA
The W-shaped divertor was modified to open the outside divertor slot and to enhance the pumping speed in November through December 1998 when JT-60U operation was halted: Only the inside leg pumping was capable before the modification. After the modification, the divertor is pumped through the outer slot with a gap of 2 cm, as well as the inside slot with a 3 cm gap. Divertor experiments with both channel pumping was started in February 1999. A test of filling a deuterium gas in the vacuum vessel confirmed that the effective pumping speed for the both channel divertor pumping was 15.9 m3/s at the vessel pressure of about 0.1 Pa, being 25% higher than the pumping speed in the previous inner pump. The pumping rate was strongly affected by the gap_in (the distance between the inner separatrix and the dome) and gap_out (the distance between the outer separatrix and the dome) in both L- and H-mode plasmas. The combination of gas puff to the main plasma and the divertor pumping was efficient to enhance impurity shielding effect and thus to suppress the impurity contamination of plasma, especially when the height of the x-point against the divertor dome was low. The divertor closure resulting from such lower x-point configuration also has the advantage of increasing the MARFE onset density by about 10%.
H-MODE POWER THRESHOLD
H-Mode Power Threshold (Pth) Working Group was organized to conduct H-mode experiments in the 3rd week in March. The mission of the group is to clarify whether or not Pth is dependent on divertor geometry. Using 85 keV NBI, Pth was determined for the W-shaped divertor with pumping at inside and outside divertor channels. The results indicate that Pth for the W-shaped divertor is reduced by 30% compared with that for the previous open divertor. The reduction in Pth is likely to be attributed to the divertor geometry rather than pumping.