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April 2008

  After the operation in March, maintenance of JT-60U inside the vacuum vessel was performed. In this month, therefore, conditioning operation for first wall and heating system were mainly carried out. Boron coating of the first wall was also performed on May 7-9.

(1) Conditioning operation
Wall conditioning was carried out by the combination of the Taylor discharge cleaning (TDC) and the glow discharge cleaning (GDC) before the beginning of tokamak operation. Conditioning of NBI port, ECRF system and first wall were carried out after the tokamak operation became possible. In this month, the wall temperature was kept at 280°C to enhance degassing from the first wall. The pulse length of NBs with the maximum pulse length of 30s was extended up to 14-21 s. Port conditioning of the other perpendicular NBs, whose maximum pulse length is 10 s, was finished.
The H/D ratio, evaluated from the ratio of Hα line intensity to Dα line intensity at the plasma start up (t = 0.3 s), decreased smoothly to 0.1. The amount of oxygen content, which was evaluated from the ratio of OVIII line intensity to CVI line intensity, also decreased steadily and the value reached low level enough to perform Boron coating. Detailed analysis from a discharge to evaluate impurity contents showed that oxygen density was less than 2% of the electron density.

(2) Wall conditioning under steady toroidal magnetic field
Since TDC and GDC cannot be performed in a tokamak with super-conducting coils, a different method for wall cleaning is needed to be developed. For this purpose, effectiveness of wall cleaning using ECRF was investigated. Helium plasma discharges initiated by fundamental O-mode ECH or second harmonic X-mode ECH were compared. In the former case, uniform plasma in the vacuum vessel was obtained with horizontal magnetic field and effectiveness of wall cleaning was demonstrated. In the latter case, however, only non-uniform plasma was obtained even with horizontal magnetic field, and effective wall cleaning was not possible.

(3) Boron coating of the first wall
After two weeks of the conditioning operation, boron coating of the first wall was performed using GDC with the He gas and the decaborane (B10D14) vapor. A total amount of 50 g decaborane was used during ~44 hours GDC.