Outgassing of AXF-5Q and other grades of limiter graphites

During high power tokamak discharges, graphite limiter surfaces may reach temperatures greater than 2000°C. The underlying material also experiences smaller temperature excursions. One result is that volatile compounds are introduced into plasmas from the limiters. An expensive tokamak “conditioning” period extending over weeks of discharge cleaning and hundreds of high power shots may be required to obtain acceptably clean plasmas. In general, graphite components are given a bake-out treatment prior to installation to reduce the conditioning period. In order to select appropriate graphites and determine bake-out and handling procedures for given applications, we have conducted outgassing experiments for several candidate graphites. We heat small samples of different types of graphite to >1500°C using high intensity quartz lamps and find that significant amounts of impurities are desorbed. Hydrogen, carbon, and oxygen make up the various compounds observed. Absolute levels are determined by graphite type and prior air exposure and heat treatment.