Comparison of beryllium and graphite first-walls in JET

JET has operated with beryllium as a first-wall material in 1989 and 1990. An initial period with beryllium evaporation onto the original graphite surfaces was followed by operation with beryllium belt-limiter tiles. Beryllium Faraday shields for the ICRH antennae and lower X-point target tiles were installed for experiments in 1990. The use of beryllium has increased the density limit, significantly reduced deconditioning following disruptions, allowed heavy gas fueling for impurity control, reduced the impurity influx from the ICRH antennae so that ICRH-only H modes were possible for the first time and permitted hot-ion plasmas on the outer limiters. This paper describes the primary effects of beryllium which led to these improvements in performance. 11 refs., 3 figs