DIII-D research towards long-pulse high-performance tokamak operation

The ultimate potential of a tokamak, embodied in the ARIES-AT fusion power plant design [1], calls for plasmas that are quite unlike most regimes in current tokamaks. The high b_P regime is the most similar to ARIES-AT, in terms of the key features of high bootstrap current fraction and Shafranov shift stabilization of turbulence. ARIES-AT employed L-mode edge profiles to address power and particle exhaust issues. DIII-D shows that the high b_P regime approaches this solution from the H-mode side, leveraging a synergy between the large-radius internal transport barrier (ITB) strength, and the edge pedestal weakness: pedestal degradation caused by detached divertor operation leads to stronger ITB, maintaining or even increasing the global energy confinement quality. Furthermore, a degraded pedestal (low pressure, high density) moves into a small/no ELM operating space [2]. The values of b_N (>3.5) and q₉₅ (<7) achieved on DIII-D yield fully noninductive operation in ITER or a reactor.



[1] S.C. Jardin et al., Fusion Eng. Des. 80 (2006) 25

[2] L. Wang et al., Nature Comm. 12 (2021) 1365