Spherical tokamaks (STs) have been shown to possess properties desirable for a fusion power plant such as achieving high plasma β and having increased vertical stability. To understand their confinement properties in a reactor relevant regime a 1GW fusion power spherical tokamak plasma equilibrium was analysed using linear gyrokinetics to determine the type of micro-instabilities that arise. Kinetic ballooning modes (KBMs) and micro-tearing modes (MTMs) are found to be the dominant instabilities. The parametric dependence of these linear modes was determined and from the insights gained, the equilibrium was optimised to find a regime marginally stable to all micro-instabilities. This work indicates the physics needed to perform transport modelling in high β ST regimes such as for STEP.
