The impact of Tungsten radiation and MHD instabilities on steady-state reactor plasmas

DIII-D experiments have demonstrated robust access to a naturally sawtooth-free, high-confinement regime compatible with steady-state conditions and tungsten (W) and W-equivalent radiators simulating the integration of a metal-wall to a high-performance scenario. The “hybrid” scenario, characterized by low q_min~1, has been adapted to the ITER shape and access parameters for the first time, and sustained in stationary conditions at β_N≈3.3, H₉₈≈1.4 and reactor-relevant q₉₅≈5.7 without harmful MHD instabilities. A full-profile ρ* scaling model shows that these plasmas project to fusion gain Q~5 and fusion power >300 MW in ITER at a reduced current of ≈8 MA, satisfying the requirements of early high fusion power achievement at low plasma current and subsequently of the ITER steady-state mission or a high-field long pulse FPP design. Xe was injected as a W-equivalent radiator at low core T_e≈3-6 keV, leading to minimal core radiated fraction of ≈10%, as Xe radiates mostly in the divertor and no impurity penetration to the core is observed on the time scale of >3.5 s (≈40 τ_E). The core exhibits a bifurcation between two benign tearing instabilities: the larger n=2 island prevents any activity at the q=1 surface, while the n=3 mode is associated with fishbones or in limited cases with small sawteeth. This is consistent with the hypothesis that the q profile is maintained above 1 by the anomalous current diffusion caused by “flux pumping” by the magnetic island, and that the anomalous diffusion scales with mode amplitude.