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Gyrokinetic Simulations for Spherical Tokamak Energetic Particle-Turbulence Experiments
C. Gallaro¹†, T. Barberis¹, T. Adkins¹, P. J. Bonofiglo¹, and V. N. Duarte¹
¹Princeton Plasma Physics Laboratory, Princeton, New Jersey, USA
†cgallaro@pppl.gov
Recent experiments on the MAST-U spherical tokamak investigated the multi-scale coupling between energetic particles, Alfvénic eigenmodes (AEs), and microturbulence [1]. In particular, the experiments consisted of using neutral beam injection to create sequences within the same discharge where there is a clear presence and absence of beam ions to probe the two regimes. This multi-scale relationship has been examined in multiple advanced tokamaks such as JET [2] and DIII-D [3], demonstrating the pumping of a zonal mode via beating of toroidal Alfvén eigenmodes leading to an increase in thermal confinement. However, a precise coupling has not been quantified in a spherical tokamak where low aspect ratio and high-𝛽 alter both the AE spectrum and the nature of the dominant microinstabilities. Here we present gyrokinetic simulations using the CGYRO [4] code to determine the spectrum of turbulent fluctuations of both electrostatic and electromagnetic character for the MAST-U plasmas of interest. Based on previous studies, we expect that the dominant contributions from the linear spectrum will be microtearing modes [5], depending on the equilibrium flow shear and collisionality, as well as high-k electron-scale turbulence [6]. These simulations will be compared with experimental data collected via the Doppler backscattering system to verify the nature of the fluctuations present in the discharges. We will then compare these spectra in the presence and absence of beam ions to determine the primary direct and indirect effects of the beam ions on the turbulence. Lastly, we consider the inclusion of energetic particles in linear CGYRO simulations to comment on the spectrum of Alfvénic fluctuations excited by beam ions in the aforementioned MAST-U plasmas.
References
[1] P. Bonofiglo et al., 2026, in 19th Technical Meeting on Energetic Particles in Magnetic Confinement Systems
[2] J. Ruiz Ruiz et al., 2025 Phys. Rev. Lett. 134 095103
[3] X. D. Du et al., 2025 Phys. Rev. Lett. 135 265101
[4] J. Candy, E. A. Belli, and R. V. Bravenec, 2016 J. Comput. Phys. 324 73
[5] M. Giacomin et al., 2023 Plasma Phys. Control. Fusion 65 095019
[6] D. C. Speirs et al., 2025 Nucl. Fusion 65 046019
Acknowledgements
This work is based upon work supported by the US Department of Energy, Office of Science, Office of Fusion Energy Sciences, and has been authored by Princeton University under Contract DE-AC02-09CH11466 with the US Department of Energy. The work was supported by the DOE Early Career Research Program, project "Phase-Space Engineering of Supra-Thermal Particle Distribution for Optimizing Burning Plasma Scenarios".