Global gyrokinetic simulations of electromagnetic turbulence in STEP

Abstract

This paper presents gradient-driven global electromagnetic gyrokinetic simulations for a conceptual burning flat-top operating point of STEP (Tholerus et al 2024 Nucl. Fusion 64 106030), STEP-EC-HD, and investigates how non-local effects influence the nonlinear saturation and transport of the electromagnetic turbulence at finite β. Local gyrokinetic simulations have shown that including δB∥ is essential for the dominant hybrid kinetic ballooning modes, or hKBMs, to be unstable in STEP (Kennedy et al 2024 Nucl. Fusion 64 086049). Using the long-wavelength δB∥ solver (Sheffield et al 2024 Plasma Phys. Control. Fusion 67 015028) implemented in GENE (Jenko et al 2000 Phys. Plasmas 7 1904–10), this work demonstrates that the linear mode spectrum can be accurately captured in global geometry, which results in good agreement with an ensemble of local flux-tube simulations. The global framework reproduces the hKBMs identified in Kennedy et al (2023 Nucl. Fusion 63 126061), while microtearing modes remain challenging to resolve due to their shorter radial scales. Nonlinear simulations reveal clear evidence of an electromagnetic transition to states with extremely large heat fluxes, consistent with local simulations and with the predicted loss of zonal-flow regulation for this proposed operating point (Kennedy et al 2026 Nucl. Fusion). These findings establish the capability of global gyrokinetics to capture finite- β dynamics in STEP-like plasmas and motivate future work to identify the conditions governing this transition.