Recent Progress in our Understanding of Electromagnetic Turbulence in a Conceptual Spherical Tokamak FPP (STEP)

Electromagnetic microinstabilities are likely to limit performance in future advanced steady state tokamak plasmas and are expected to dominate transport in high β next generation spherical tokamaks (STs) such as STEP. While gyrokinetic (GK) simulations have thus far proven to be a very accurate tool in modelling turbulent transport in predominantly electrostatic regimes, obtaining saturated nonlinear simulations in higher β plasmas with unstable kinetic ballooning modes (KBMs) and microtearing modes (MTMs) has proven computationally challenging. Recent simulations of STEP-relevant equilibria that retain only MTMs and exclude KBMs (by neglecting compressional perturbations) saturate cleanly at very modest electron heat flux. However, local GK simulations find that including δB∥ in such plasmas can unleash a hybrid KBM-like (hKBM) instability which drives very large heat fluxes (orders of magnitude greater than the available heating power) in the absence of strong equilibrium shear flows. These simulations underscore that understanding and mitigating hKBM-induced turbulence will be essential for the development of consistent flat-top operating regimes for future high performance ST devices. This presentation will discuss recent advances in our understanding of electromagnetic turbulence in high β STs and will cover: (i) linear and nonlinear local simulations of hKBM turbulence, exploring sensitivities to local parameters and actuators to avoid the high-transport state; (ii) a quasi-linear inspired reduced transport model for the hKBM turbulence; (iii) first flux-driven simulations for a high β ST, which support the existence of a high performance transport steady state in STEP; and (iv) first-of-their-kind global nonlinear electromagnetic simulations of STEP to include δB∥ . The latter global simulations support conclusions drawn from local GK simulations, and suggest that high-transport states in some equilibrium conditions are not an artefact of the local approximation. We will conclude by summarising the most pressing priorities to be addressed in future work.