Studies of impurity-induced turbulence stabilization and implications for breakeven in SPARC

Recent work with nonlinear gyrokinetic profile predictions with PORTALS-CGYRO [1] has shown that SPARC’s core may benefit from increased fuel dilution. It is found that not only does reducing the concentration of deuterium and tritium increase the energy confinement time, but also fusion power can be higher despite the lower density of ions to fuse. This is caused by the stabilization of ion temperature gradient driven turbulence, which results in a shift in the nonlinear critical gradient. The higher steady-state ion temperature gradients that are achievable at the same input power —at the expense of lower density— are beneficial in the early-campaigns of SPARC, where very high temperatures (as those in full-current H-mode conditions) are not expected. In this work, we perform a detailed gyrokinetic (CGYRO [2]) and gyrofluid (TGLF [3]) analysis of the turbulence suppression via fuel dilution and explore in what regions of the parameter space we would expect it to be beneficial to fusion power production, in both SPARC and ARC-class devices.

[1] P. Rodriguez-Fernandez et al Phys. Plasmas 31, 062501 (2024)

[2] J. Candy et al Journal of Computational Physics 324, 73–93 (2026)

[3] G. M. Staebler et al Phys. Plasmas 12(10), 102508 (2005)