Constraining the divertor heat width in ITER

A model is developed which constrains heat width, λ_r based on global power balance, momentum conservation, pedestal stability and sheath heat transmission. The model relies on measurements of the ratio of separatrix to pedestal pressure; a ratio ∼5% is found to be expected for ITER. Applying this model indicates a constraint that the allowed λ_r ∼ 10–30 mm for ITER if the divertor is in the high-recycling regime as expected ( T < 20 eV) while a λ_r ∼ 1–3 mm requires a separatrix pressure approximately equal to the top pedestal pressure in violation of physical reasoning and the concept of a pedestal. A weaker constraint is applied in the model that upstream separatrix temperature simultaneously satisfies power balance. The constrained model cannot satisfy power balance with λ_r < 3 mm, and in order to obtain λ_r ∼ 5 mm requires divertor plasma temperature >100 eV, a condition which would have very negative consequences for the divertor, but has never been observed experimentally.