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A new self-saturation regime for energetic particle (EP) driven Alfvén eigenmodes (AEs) at reactor-like ρ is identified using the CGYRO [1] gyrokinetic flux-tube code. This new saturation regime could markedly reduce the predicted impact of AEs in future reactors. Previous nonlinear gyrokinetic flux-tube studies [2,3] of fast ion-driven AEs at higher values of ρ associated with modern experiments have shown highly stiff transport behavior, generally manifesting as a failure to saturate above a critical energetic particle EP driving gradient. This stiff behavior has served as the basis for the TGLF-EP+Alpha model of EP transport and profile prediction [4]. In modern machines, the AE spectrum consists of only a few modes, around five distinct values of toroidal n numbers. In a reactor, the spectrum is much denser, with AEs dominant at the lowest ~30 modes. The present CGYRO simulations, performed at various radii within the Holland compact reactor use case [5] steady-state scenario, show that self saturation can occur with modest diffusivities at EP gradient drive strength well above the critical gradient predicted by the TGLF-EP model. The predicted transport converges only with mode spacing of ∆𝑛 ≤ 2, a so-called half-torus simulation. Saturated diffusivities are found to mostly drop with ∆𝑛 as the AE spectrum becomes denser. We hypothesize that the increased nonlinear n-to-n AE coupling provides the needed saturation mechanism. We propose a finite-stiffness correction to the TGLF-EP+Alpha model for low-ρ* cases consistent with these results that predicts significantly less fusion performance degradation than the current model.
[1] J. Candy and E.A. Belli. Spectral treatment of gyrokinetic shear flow. J. Comput. Phys., 356:448, 2018. doi:10.1016/j.jcp.2017.12.020.
[2] E.M. Bass and R.E. Waltz, Phys. Plasmas 17, 112319 (2010). https://doi.org/10.1063/1.3509106
[3] E. M. Bass and R. E. Waltz Phys. Plasmas 24, 122302 (2017) https://doi.org/10.1063/1.4998420
[4] He Sheng, R. E. Waltz, and G.M. Staebler, Phys. Plasmas 24, 072305 (2017). https://doi.org/10.1063/1.4989716
[5] C. Holland et al., Journal of Plasma Physics. 2023;89(4):905890418. https://doi.org/10.1017/S0022377823000843