On the importance of parallel and perpendicular magnetic field fluctuations for electromagnetic instabilities in STEP
This paper discusses the importance of parallel and perpendicular magnetic field perturbations in gyrokinetic simulations of electromagnetic instabilities and turbulence at mid-radius in the burning plasma phase of the conceptual high-β, reactor-scale, tight-aspect- ratio tokamak STEP. Previous studies have revealed the presence of unstable hybrid kinetic ballooning modes (KBMs) and subdominant microtearing modes (MTMs) at binormal scales approaching the ion-Larmor radius. It was found that the hybrid kinetic ballooning mode requires the inclusion of parallel magnetic field perturbations to be linearly unstable. Here, the extent to which the inclusion of parallel (and perpendicular) magnetic field perturbations can be relaxed is explored through gyrokinetic simulations. In particular, the frequently used MHD-approximation (dropping δB∥ and setting the ∇B drift frequency equal to the curvature drift frequency) is discussed and simulations explore whether this approximation is useful for modelling STEP plasmas. It is shown that the MHD-approximation can reproduce some of the linear properties of the full STEP gyrokinetic system, but nonlinear simulations using the MHD-approximation result in very different transport states. It is demonstrated that agreement can be improved in conditions that are more compatible with the assumptions that underlie the MHD-approximation. Furthermore, it is shown that the reason STEP is so sensitive to δB∥ fluctuations is simply because the plasma sits so close to marginality and it is demonstrated that in slightly more strongly driven conditions the hybrid-KBM is unstable without δB∥. Crucially, it is demonstrated that the state of large transport typically predicted by local electromagnetic gyrokinetic simulations of STEP plasmas is not due to δB∥ physics.