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July 2004

  Modification of power supply system has been done to connect the heating systems (NB and RF) to the motor generator for toroidal field coils (T-MG) instead of the original motor generator for the heating systems, which had been damaged in the accident in February 2004. The power for the toroidal field coil is now supplied directly from a commercial line and the strength of toroidal field is limited below ~3 T and is without feedback control.

  In this month, tests of the modified power supply system are done at first. After that, JT-60U operation in FY2004 has been started. The experimental studies for (1) conditioning of heating systems, (2) long-pulse high-recycling H-mode, (3) fast plasma shutdown, and (4) tungsten (W) tiles have been carried out. The results are shown below.

Machine tests

  The interlock-system test of control systems, TDC (Taylor Discharge Cleaning) sequence check, power supply tests of toroidal and poloidal field coils, tokamak discharge test, and operation of heating systems (NB, ECRF and LHRF) with T-MG were performed successfully. Calibration of the diamagnetic measurement was done when the toroidal field was changed with time.

Experimental Results
(1) Conditioning of heating systems: Conditioning of the heating systems (positive-ion NB (P-NB), negative-ion NB (N-NB), LHRF and ECRF) has been carried out for long pulse operation.
P-NB: perpendicular beams -- #14: 11 s, others (#2,3,4,6,12,13): 10s.
  tangential beams -- #7: 26 s, #8: 19 s, overlap of #7 and #8: 16 s,
    #9: 21 s, #10: 19 s, overlap of #9 and #10: 11 s.
N-NB: just started
LHRF: 0.6 MW (40% duty) for 1.6 s.
ECRF: 1 MW (at gyrotrons) for 1-2 s for all 4 units.
(2) Long-pulse high-recycling H-mode: Particle balance and divertor plasma behavior have been investigated in a long pulse (~30s) ELMy H-mode discharge with a high recycling condition and with the wall temperatures of 150°C and 250°C. The electron density increased gradually without gas puffing in a later period of heating, which implies that the sum of wall pumping and active divertor pumping became smaller than the fuelling rate by neutral beams. At 150°C, the density was lower than at 250°C, and the ELMy H-mode was maintained for 30 s. At 250°C, the MARFE (multi-faceted asymmetric radiation from the edge) occurred without gas puffing, which indicates the saturation of wall/divertor-plates. Stable MARFE was kept for several seconds probably due to enhanced pumping efficiency with increased neutral gas pressure.
(3) Fast plasma shutdown: argon(Ar)+deuterium(D2) gas puffing and impurity pellet (neon (Ne)) injection were compared. Faster decrease in electron temperatures was observed in the impurity pellet injection case, but the current quench rate was not changed so much.
(4) W tiles: Sputtering of W installed partly as the outer divertor tiles and accumulation of W in a core plasma have been investigated by locating the outer strike point on the W tiles for a long time in NB heated plasmas. A W line was observed and the time evolution of line intensity agreed with the change in a strike-point position and NB power. The W accumulation in the core plasma was suggested for a disrupted discharge due to increase in radiation loss.