Testing of low-Z-coated limiters in tokamak fusion devices

Coatings of materials with low atomic number Z are being developed for application both in experimental tokamak devices such as ISX and TFTR and for possible use on various components of large power-producing controlled thermonuclear reactors. A suitable coating has the potential of improving the plasma energy balance by lowering the atomic number of the impurities injected by plasma-wall interactions. This results in a decrease in plasma radiation losses and improved plasma stability. The coatings will be subjected to a severe environment and must be able to withstand physical erosion, chemical erosion, thermal shock, thermal fatigue and arcing without rapid degradation or loss of adhesion. Extensive testing on a laboratory scale was used to select the coatings most suitable for this environment. From this testing, which included pulsed electron beam heating, low energy ion bombardment and arcing, chemically vapor-deposited coatings of TiB2 and TiC on Poco graphite substrates were selected and tested as limiters in ISX. Both limiter materials gave clean stable reproducible tokamak discharges on the first day of operation. After an exposure of 1 week, the TiC limiter showed only superficial damage with no coating failure. The TiB2 limiter showed some small areas of coating failure. TiC-coated graphite limiters were also briefly tested in the tokamaks Alcator and PDX with favourable results. (Auth.)