Daniel Kennedy, UK Atomic Energy Authority, Culham Science Centre, UK

"Electromagnetic Instabilities in the Spherical Tokamak for Energy Production (STEP)"

Abstract: Electromagnetic microinstabilities are likely 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 at low β, obtaining saturated nonlinear simulations of plasmas with unstable kinetic ballooning modes (KBMs) and microtearing modes (MTMs) has proven computationally and conceptually challenging. However, recent investigations that retain only MTMs and exclude KBMs (by neglecting compressional perturbations) reveal strong sensitivity of the heat flux to parallel dissipation and velocity space resolution. With sufficient resolution or dissipation to avoid numerical instabilities, recent MTM-only simulations saturate cleanly at much reduced electron heat flux. We find that including compressibility can unleash a KBM-like instability which drives very large heat fluxes (orders of magnitude greater than the available heating power). The saturated fluxes are sensitive to equilibrium flow shear, and at mid-radius diamagnetic levels of flow shear can reduce the fluxes to much lower values.