  In this work, we present first-of-their-kind nonlinear local gyrokinetic
simulations of electromagnetic turbulence at mid-radius in the burning plasma
phase of the conceptual high-$\beta$, reactor-scale, tight-aspect-ratio tokamak
STEP (Spherical Tokamak for Energy Production). A prior linear analysis in D.
Kennedy et al. 2023 Nucl. Fusion 63 126061 reveals the presence of unstable
hybrid kinetic ballooning modes, where inclusion of the compressional magnetic
field fluctuation, $\delta B_{\parallel}$, is crucial, and subdominant
microtearing modes are found at binormal scales approaching the ion-Larmor
radius. Local nonlinear gyrokinetic simulations on the selected surface in the
central core region suggest that hybrid kinetic ballooning modes can drive
large turbulent transport, and that there is negligible turbulent transport
from subdominant microtearing modes when hybrid kinetic ballooning modes are
artificially suppressed (through the omission of $\delta B_{\parallel}$).
Nonlinear simulations that include perpendicular equilibrium flow shear can
saturate at lower fluxes that are more consistent with the available sources in
STEP. This analysis suggests that hybrid kinetic ballooning modes could play an
important role in setting the turbulent transport in STEP, and possible
mechanisms to mitigate turbulent transport are discussed. Increasing the safety
factor or the pressure gradient strongly reduces turbulent transport from
hybrid kinetic ballooning modes in the cases considered here. Challenges of
simulating electromagnetic turbulence in this high-$\beta$ regime are
highlighted. In particular the observation of radially extended turbulent
structures in the absence of equilibrium flow shear motivates future advanced
global gyrokinetic simulations that include $\delta B_\parallel$.
