Power and particle fluxes at the plasma edge of ITER: Specifications and Physics Basis

As a result of the ITER design review activities the specifications for power and particle fluxes onto plasma facing components (PFCs) have been revisited. The revised specifications for steady-state fluxes are based on 2-D/3-D SOL modelling code predictions with input from experiments in tokamaks, as well as on simple models and empirical extrapolations and they are provided for all phases of the discharges and scenarios in ITER. In particular, it has been identified that power fluxes along the field can be present for field lines in the far-SOL of ITER (more than 4 - 6 cm from the separatrix at the outer midplane) with parallel power flux densities of ∼ 5 MWm-2. Evaluation of the expected characteristics of power fluxes to the divertor during ELMs in ITER and experimental measurements of plasma material erosion under such loads has shown that an acceptable divertor target lifetime can only be ensured for ELMs causing an energy loss of ∼ 1 MJ or less. This is a factor of 20 smaller than that expected under uncontrolled ELMs and has determined the new requirements for ELM control in ITER. The expected range of energy and power fluxes on main chambers plasma facing components for controlled and uncontrolled ELMs in ITER is described on the basis of present models and empirical scaling of experimental data from various divertor tokamaks. Analysis of the energy outflow from the plasma to PFCs during the thermal quench of disruptions has shown that there is a significant degradation of plasma confinement for resistive-MHD disruptions expected in ITER scenario 2 at full performance. For ITER scenario 4, it is more likely that disruptions will be caused by exceeding ideal-MHD limits. In this second case, and that of VDEs, the plasma thermal energy remains close to its maximum value up to the thermal quench. As a consequence of this and the large broadening of the edge power flux profile at the thermal quench, the parallel energy flux during disruptions (mapped to the midplane) is expected to be in the range of 50 - 200 MJm-2 for scenario 2 and 200 - 400 MJm-2 for scenario 4. The specifications of the energy flux during VDEs for ITER have been refined on the basis of analysis of new experimental data that take into account the details of the plasma energy outflow up to the thermal quench of VDEs.