IMPURITY AND PARTICLE RECYCLING REDUCTION BY BORONIZATION IN JT-60U

In JT-60U boronization using decaborane was carried out. Boronization reduced the oxygen impurity in OH discharges and shortened the wall conditioning after the vacuum vessel vent and consequently enabled JT-60U to produce clean plasmas easily except for NB heated plasmas. After boronization, particle recycling was reduced drastically in OH and NB discharges. High confinement plasmas were obtained including high beta(p) mode and H-mode discharges. In the latest boronization part of divertor plates were replaced with B4C coated tiles with a B4C thickness similar to 300 mu m. After introducing B4C divertor tiles, an explosive generation of boron particles from the tiles was observed. By the combined effects of boronization with decaborane and boron generation from B4C tiles, oxygen impurity was so low that oxygen line signals were reduced to noise levels after the latest boronization. On the other hand, boron burst from the B4C tiles restricted the operation of JT-60U.