Overview on stationary and transient divertor heat loads

The divertor concept and the divertor materials envisaged for ITER FEAT restrict the maximum values of the stationary and transient heat fluxes. The maximum stationary heat flux is limited by the active cooling structure. The limit for transient events is given by the maximum tolerable surface temperature. This paper will review different options for divertor heat flux reduction. Proper geometric orientation of the divertor targets reduces the heat flux to the target plates at most by a factor of 60 relative to the parallel heat flux determined by upstream transport. The radiation capability of closed divertor configurations is doubled compared to open configurations. A further reduction of maximum heat load can be achieved by increasing the wetted area in double-null plasma configurations. The energy transported by transient events (type I ELMS) is up to 30% of the total energy deposited in the divertor. The heat flux profile during ELMS is not significantly broadened and is comparable to the profile between ELMS. The duration of the energy deposition varies with edge plasma parameters between 0.1 and 1 ms. The expected transient heat flux for ITER FEAT conditions is above the tolerable value. This requires the further development of operation scenarios with tolerable ELMS (e.g. type 11) preserving the high energy confinement of type I ELM scenarios.