Physics basis for the ITER tungsten divertor

The first ITER tungsten (W) divertor is the largest and most complex of its kind ever to be constructed. It must survive an expected ~2000 hours of plasma exposure through the non-active to the first DT campaigns. A key parameter determining the operational range for stationary loading is the peak target power flux density, q_pk, itself fixed by the allowable surface temperature to remain below W recrystallization. There is a strong relationship between q_pk and the divertor neutral pressure, p_n, the latter crucial for adequate He exhaust during burning plasmas. The main divertor design features were established using a database generated with the SOLPS code, which found an acceptable operational range within the constraints fixed by the input assumptions. However, some of the latter require modification in the light both of recent improvements in the physics model and the need for divertor component shaping. All tend to push the operating space to higher p_n and higher concentration of extrinsic impurities with increased upstream densities. Nevertheless, there appears to be sufficient margin to avoid the deep detachment often associated with operational limits in today‘s devices. Analysis of the state-of-the-art knowledge of expected ELM transient amplitudes on the divertor targets, the computed ELM ion orbit loading profiles on the shaped component edges and material surface fatigue effects strongly indicate that ITER must achieve essentially complete ELM mitigation during baseline fusion power operation.