The impact of impurities and fusion-αs on STEP turbulence

Spherical Tokamak for Energy Production (STEP) is a UKAEA-led prototype powerplant based on a high-β ST concept. Under such conditions, the plasma turbulence is predicted to be highly electromagnetic with hybrid-Kinetic Ballooning Modes dominating transport [1,2]. In this presentation, building on the work of [1,2,3] we will report on the first nonlinear flux-tube gyrokinetic simulations (at mid-radius) of a candidate design point in the presence of kinetic impurities and a (hot) Maxwellian fusion-α distribution.

The most complete simulations undertaken are with GENE in the presence of flow-shear at the diamagnetic level and include 5 kinetic species: e, DT, ArXe, He (ash), α (where DT represents a single density-weighted D+T species etc.). The impurities are seen to play an important role in reducing the heat and particle transport.

The αs are seen to play an extremely significant role through two channels: (1) through their contribution to the equilibrium β', a strongly stabilizing impact on the linear modes across all perpendicular wavenumbers is seen; (2) by participating as kinetic ions, they destabilize large-scale modes. Nonlinearly, the latter translates to an enhancement in the overall heat and particle fluxes by well over an order of magnitude. This effect is in sharp contrast to the stabilizing impact αs have on low-β turbulence.

[1] D. Kennedy et al 2023 Nucl. Fusion 63 126061

[2] M Giacomin et al 2024 Plasma Phys. Control. Fusion 66 055010

[3] E. Tholerus et al 2024 Nucl. Fusion 64 106030