Development and operation of first wall in JT-60U high-power heated discharges

In JT-60U, the highest fusion product of 1.2×10²¹ keV sm^-3 was recorded in July 1994, and the confinement performance has been also improved in the quasi-steady states (duration time ~1.5 s). The JT-60U has various wall conditioning methods. In particular, the boronization system using decaborane and B₄C-converted CFC divertor tiles was effective for reduction of the oxygen impurity and particle recycling. In the latest boronization session, gas mixture of deuterium and helium was used in glow discharge instead of helium only to reduce the hydrogen content in the boron film. On the other hand, the B₄C-converted CFC tiles caused, so called, the real-time solid target boronization (STB) by plasma impact, and then low concentration level of impurities was achieved. Water cooling of the divertor tiles, which was carried out in October 1994, was also effective for the reduction of impurity production. The amount of neutron and tritium generated by the D-D reaction have also increased with the improved plasma performance. From the measurement of the tritium in the first wall tiles, it is expected that tritium of 1.6×10¹⁰ Bq, which corresponds to approximately 50% of tritium generated until 1993, still remained in the first wall