Estimate of 3D wall heat loads due to Neutral Beam Injection in EU DEMO ramp-up phase

High energy Neutral Beam Injection (NBI) is one of the methods being considered in EU DEMO pre-conceptual design phase to provide auxiliary power to the plasma. From recent studies [1], it appears clear that auxiliary heating power is needed during the ramp-up (and ramp-down) phase to guarantee a robust access to H-mode (and to compensate for high radiation power losses in ramp-down). The use of NBI during ramp-up has to be carefully considered due to possible shine-through losses which can exceed the maximum heat load tolerated by the first wall (for DEMO the steady state peak heat flux limit is 1 MW m-2 [2]). In ITER, shine-through losses pose a lower limit on density for NBI operation at n~3x1019 m-3 [3]. This limits for ITER the operational window of the NBI system and can prevent its use during the ramp-up phase due to low plasma density. In this work the heat wall loads due to NBI shine through and orbit losses are calculated for the diverted plasma ramp-up phase of EU DEMO pulsed scenario by numerical simulations performed by BBNBI [4] and ASCOT [5] Monte Carlo codes. The simulations have been done in a complete 3D geometry considering the latest DEMO NBI design [6], which foresees NBI at 800 keV energy with respect to 1MeV beam energy for ITER. Location and power density of NBI-related heat loads at different time-steps of DEMO ramp-up are evaluated and compared with the maximum heat flux limit. Since NBI shine-through losses depends mainly on the beam energy, plasma density and volume, DEMO has a more favorable situation than ITER, enlarging NBI operational window. This increases the appeal of neutral beam injectors as auxiliary power systems for DEMO.