On the utilization of high Z materials as a plasma facing component

This paper summarizes recent results on high Z performance as PFM in tokamak machines and remarks upon subjects in research and development of high Z PFC referring basic experiments such as high heat load test and surface physics data base (sputtering, reflection, etc.). The dangerous behavior of high Z PFM is the possible accumulation of high Z impurities in the plasma core and accompanying plasma collapse due to their high radiation. However, recent experiments in Alcator-C Mod, TEXTOR and USDEX-U, which employ high Z materials as a divertor target or a limiter, have shown that this occurred only under special conditions. The accumulation of high Z has been observed mainly in ohmic heated TEXTOR plasma. Such accumulation is, however, not necessarily connected with the released high Z impurities from the limiter but is very likely controlled by the impurity transport in the plasma. Therefore, if the central accumulation of high Z impurities can be avoided by suitable transport control, the high Z can be used as PFM. The appearance of ‘prompt redeposition’ of high Z atoms is very promising for the utilization of high Z materials. Heat deposition is likely reduced for higher Z materials owing to its higher reflection coefficient, which must be confirmed in the future. Mo and W bulk limiters are approved to be utilized above their DBTT. However, since intergranular cracking originating from recrystallization and grain growth are unavoidable at high temperature utilization, some optimization at operating temperatures to avoid brittlement (higher is better) and recrystallization (lower is better) and control of grain structures are needed. In this respect, CVD coating with a columnar grain structure seems promising. Hydrogen effect should be taken into account for the crack formation under plasma loading conditions, though tritium retention in Mo and W at high temperatures would be small.