The effect of divertor magnetic balance on H-mode performance in DIII-D

We report on recent experiments for which the magnetic balance of highly triangular (δ ≈ 0.8), unpumped H-mode plasmas was varied. Changes in divertor heat loading and particle flux were observed when the magnetic configuration was varied from a balanced double-null (DN) divertor to a slightly unbalanced DN divertor. For attached plasmas, the variation in heat flux sharing between divertors is very sensitive near balanced DN. This sensitivity can be shown to be consistent with the measured scrape-off width of the parallel divertor heat flux density, 1?||. At magnetic balance we find that the peak heat flux density at the divertor in the ∇5 ion drift direction is twice that of the other divertor. Most of the heat flux go to the outboard divertor targets in a balanced double-null, where the peak heat flux density at the outer divertor targets may exceed that of the inner divertor targets by tenfold. However, the variation of the peak particle flux density between divertors is less sensitive to changes in magnetic balance. These particle and heat flux 'asymmetries' in DN plasmas are consistent with the presence of E × B poloidal particle drifts in the scrape-off layer and private flux region [1]. Regardless of how the divertors were magnetically balanced, D₂ gas puffing always reduced energy confinement to the range τ_E/τ_E89P≈1.3-1.6. When this energy confinement range was reached, τ_E/τ_E89P remained nearly constant up to near the H-mode density limit.