Speaker
Description
Recently stellarators have made significant progress with respect to improvements in the classical guiding center energetic particle confinement via shape optimization [1]. However, as has been observed with tokamaks once the neoclassical confinement is reduced to a low level, the confinement properties of energetic particle (EP) populations may be more dominated by instability-induced transport phenomena [2]. In particular, the various Alfvén eigenmodes (AE) that can be resonantly destabilized by EP components provide channels for enhanced transport of EPs. To address this issue, the FAR3d gyro-Landau closure model [3] has been employed. This is a reduced model that accurately models the mode coupling physics which is important to the analysis of these instabilities in 3D configurations, while using a simplified parallel EP resonance closure model to excite the instabilities. This provides a means to rapidly evaluate the linear stability of many configurations and scenarios and a feasible method to study the nonlinear saturation and resulting transport, considering interactions between multiple modes. Stellarators offer the possibility to operate at high density regimes where the slowing-down time is decreased, resulting in lower drives for EP instabilities. This effect is analyzed for a reactor-sized quasi-helical stellarator, Wistell-D, through evaluating linear stability along constant fusion power contours (0.5, 1, 2, and 3 GW) in electron density and temperature space for the n = 1, 2, and 3 mode families. For this example, suppression of AE’s is seen up to 1 GW, but residual modes persist at higher power levels, even at high densities. This work is then extended into the nonlinear regime via simulations of the Wistell-D and a reactor scaled version of W7-X; toroidal modes up to n = 30, are followed to saturation. Zonal flow/current generation and mild flattening of the EP density profile are observed from which global EP particle and energy fluxes are derived. The flux-gradient relations exhibit non-local characteristics, as has been seen in applications of FAR3d to tokamaks [4], and motivates going beyond the usual diffusive and critical gradient models. This type of EP transport can be characterized using machine-learning surrogate models which can interface with integrated simulation models. The efficient performance of FAR3d and its demonstrated application to a wide range of stellarator configurations also motivate its use as a target function for stellarator shape optimization to further explore possibilities for suppression of AE instabilities in 3D configurations.
[1] M. Landreman, S. Buller, and M. Drevlak. “Optimization of quasi-symmetric stellarators
with self-consistent bootstrap current and energetic particle confinement,” Physics of Plasmas 29.8
(2022).
[2] W. W. Heidbrink, N. N. Gorelenkov, Y. Luo, M. A. Van Zeeland, “Anomalous Flattening of the Fast-Ion Profile during Alfvén-Eigenmode Activity,” Phys. Rev. Lett., Vol. 99, 245002 (2007).
[3] J. Varela, D. Spong, L. Garcia, Y. Ghai, J. Ortiz and FAR3d project collaborators, “Stability optimization of energetic particle driven modes in nuclear fusion devices: the FAR3d gyro-fluid code,” Frontiers in Physics, 12:1422411 (2024)
[4] D.A. Spong, Y. Ghai, J. Varela, L. Garcia, "Nonlinear Alfven instability simulation and EP transport for ITER reversed shear (steady-state) and monotonic q-profile regimes," Nuclear Fusion, 2025; 61(11):116061.
Acknowledgements - This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, under Award DE-AC05-00OR22725.