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Description
Recent DIII-D experiments have demonstrated the compatibility of divertor detachment with a high normalized beta core, a significant step towards solving the core-edge integration issue for steady-state fusion. The high-confinement, high-beta operation leveraging both high beta hybrid and high poloidal-beta approaches greatly improves core performance under dissipative divertor operation. Experimental analysis and simulations reveal that a closed divertor with impurity seeding and opaque edge, a widened pedestal with optimized radiation front, and improved core stability play key roles on the improvement of core-edge integration. These results pave a promising path to improve the integration of high-performance core and dissipative edge.
Using a high-density hybrid scenario, simultaneously high-confinement high-beta core (H98 ~1.1, βN ~3.1), low collisionality pedestal with v*ped~0.5 and Te,ped ~1keV, naturally small ELMs with frequency >500Hz, and partial detachment with significant pressure loss near the strike point have been obtained. With moderate gas puffing, the pedestal density gradient was reduced via edge neutral opacity, which promotes divertor dissipation and the achievement of small ELMs. Increasing heating power does not significantly increase the detachment threshold, which is attributed to the broadening of upstream profiles. Additional divertor nitrogen gas puffing further enhances the divertor dissipation to achieve partial detachment.
Separately, using high-poloidal-beta plasma operation, DIII-D experiments have demonstrated the compatibility of high-confinement core with nearly-full-discharge divertor detachment. Partial detachment with divertor temperature Te<10eV was achieved shortly after the L-H transition and sustained for the entire flattop phase, while the energy confinement is significantly improved with H98 increased to 1.7 and maintained, which is attributed to the formation of a large-radius internal-transport-barrier (ITB).
Starting from the low-density hybrid scenario, shallow XPR (peak radiation right inside the X-point) with complete divertor detachment and high beta (βN ~3.0, H98~1.25) have been simultaneously achieved using an ITER-similar shape and nitrogen puffing. With stronger N2 impurity injection, the plasmas exhibit a deep XPR regime with 2x core radiation peaking in the pedestal, while ELMs are strongly mitigated. However, the confinement is significantly reduced to H98<1.0, which is attributed to the 50% lower pressure and 30% colder pedestal temperature. By leveraging the large-radius ITB in high poloidal-beta plasmas, deep XPR plasma with ELM suppression could be integrated with the high beta core, while the confinement maintained. The ITB compensates the potential performance degradation from the weaker pedestal due to the formation of XPR, even with radiation front deeply inside the separatrix