Plasma–wall interaction issues in ITER

In 1998 the four ITER parties decided to investigate a smaller and cheaper (50% cost) machine design with reduced technical objective but without jeopardizing the programmatic objective. Based on various design studies a machine with 6.2 m major radius and aspect ratio 3.1 has been selected. Two main regimes of operation are foreseen namely ELMy H-mode and steady-state operation, the latter in discharges with internal transport barriers (ITB). In ELMy H-mode, an important issue is the maximum tolerable energy for repetitive pulsed energy loads (ELMs) of 0.4 and 0.64 MJ m−2 for CFC and W, respectively, in order to achieve adequate divertor lifetimes (ca. a few 1000 discharges). Based on present predictions there is only a marginal compatibility of the expected ELM energy loads with these thresholds. Reducing the ELM size, by gas puffing, tends to reduce also the energy confinement. The regimes with ITB may pose a problem for the divertor operation due to the expected low plasma edge density and the large power crossing the separatrix (up to 130 MW). Using the present knowledge about plasma–wall interaction (erosion in steady and pulsed loads, re-deposition as well as of tritium co-deposition), the majority of the plasma facing surface has to be clad with Be and W. At present, a small area of CFC at the divertor strike zones is foreseen in the reference design. However, due to the relatively high T co-deposition rate predicted, the use of CFC may not be possible in a D–T machine.