Wall concepts and density control for JET

Only low-Z materials will be used for first wall components of JET. This is particularly important with regard to radiation losses in the outer region of the plasma which can lead to instabilities limiting the plasma density. To avoid implications from redistribution of wall material by erosion and redeposition, the first wall is intended to consist of one material only. Two alternative concepts are being developed. The first employs graphite tiles for limiter and wall protection, with the rest of the wall being carbonised, the second uses beryllium limiters and a thin beryllium layer on the graphite protection and the rest of the wall. Density control will be achieved by pellet injection to increase the central density and by pumping at the plasma edge to control the recycling. (author)