Suitability of boron-doped graphite as plasma facing material in fusion devices

Boron-doped graphite and a-C/B: H film are regarded as attractive low Z plasma facing materials, since radiation loss due to oxygen and carbon impurities is largely reduced. It is also suggested that the radiation enhanced sublimation (RES) might be reduced by the boron doping. However, suitable temperature range for this material has not yet been clarified. From the oxidation experiments of the boron-doped graphite, it is observed that B2O3 formed by oxygen gettering evaporates at the temperature over 900degC. From the measurement of RES, it is also shown that the boron evaporation becomes dominant when the temperature exceeds approximately 1000degC. In addition, the reduction of the RES yield is not clearly observed in our experiment. Thus it is proposed that the suitable temperature range of boron-doped graphite should be less than about 900degC. The boron-doped graphite can be used only for walls of relatively low temperature, i.e., the first wall region. To extend this material to the divertor region of the fusion experimental reactor such as ITER, the heat flux must be greatly reduced by expanding the divertor configuration. (author)