Operating range 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 ~2500 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 and strongly influenced by the transparency for neutral recirculation between inner and outer targets. 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 asumptions. 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.