Narrow power profiles seen at JET and their relation to ion orbit losses

Recent JET measurements of heat load profiles during NBI-heated, natural density, ELMy H-modes revealed highly peaked profiles. In this study, ion orbit loss (IOL) from the core periphery was simulated numerically on four selected JET discharges (two high power H-modes, a fuelled H-mode and an L-mode), using realistic magnetic geometry and 2-D plasma/neutral background. The simulated and measured profiles agree quite well in all discharges, suggesting that IOL is indeed the mechanism responsible for the narrow profiles. The interaction between energetic ions and scrape-off layer (SOL) ions/atoms was also investigated, resulting in SOL power stopping factors of 60–80%. The momentum transfer via IOL to background ions was found to be in the same direction as the observed SOL flows (towards the inner target) but too small to be their primary cause. Preliminary simulation of IOL on ITER indicate peak heat loads <2 MW/m2 at the outer target.