Key plasma wall interaction issues towards steady state operation

With the ITER construction approaching, fusion devices have started to tackle the technological and physical challenges associated with steady state operation. Significant progress has been achieved with pulses longer than 400 s and coupled energy larger than 1 GJ. This shed a new light on plasma wall interactions (PWI), occurring over long time scales. The main PWI limitations for long pulses are localized heat loads associated with fast particles losses, and density control linked to outgassing from heated plasma facing components (PFCs). In long pulses, particle recovery after shot is independent of the retained fuel, leading to a significant wall inventory build up in contrast with short pulses. Different retention mechanisms (codeposition, implantation, bulk diffusion) have been identified as dominant, depending on the plasma and PFCs characteristics. However, the above results have been obtained with carbon PFCs, leaving effects related to the material mix foreseen for ITER as an open issue.