Integration of X-point radiator divertor operation with high beta hybrid core plasmas in DIII-D

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Abstract Recent DIII-D experiments have demonstrated the compatibility of complete divertor detachment with a high-beta core using the high beta hybrid scenario plasmas with an ITER-similar shape and nitrogen impurity seeding. With complete divertor detachment (near-zero divertor particle flux and temperature), the radiation peaks inside the X-point indicating the achievement of the X-Point Radiator (XPR) regime, which is a highly dissipative divertor operation scenario that may be attractive for future reactors. SOLPS-ITER modelling with full drifts is able to qualitatively reproduce the experimental measured boundary plasma conditions and radiation patterns from attached to XPR detached divertor state for these high-beta hybrid plasmas. Experiments found that when the radiation peak is inside but close to the X-point, as noted ‘shallow XPR’, complete divertor detachment and high-beta high-confinement core (βN ~3.0, H98~1.25) could be simultaneously achieved. However, this plasma remains ELMing with giant ELMs (W/W ~ 3-4%). With stronger N2 impurity injection, the plasma enters a ‘deep XPR’ regime where the radiation peak is close to inboard side of the pedestal and core radiation is about a factor of 2 higher. With deep XPR, the ELMs are strongly mitigated. However, the confinement is significantly reduced to H98<1.0, which is attributed to the 50% lower pedestal pressure and 30% colder pedestal temperature. SOLPS-ITER simulations highlight the key role of impurity radiation, neutrals and divertor closure in the formation of an XPR and the effects of drifts on the distribution of plasma and radiation near the X-point, all of which are important for the divertor design and operation in future reactors. The modelling also exhibits qualitatively good agreement with experimental observations on the pedestal performance responding to the radiation dynamics, which provides physics insight on the core-edge integration between divertor dissipation and high-performance core that is critically important for future tokamak fusion reactors.

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