  The transport of heat out of tokamak plasmas by turbulence is the dominant
mechanism limiting the performance of fusion reactors. Turbulence can be driven
by the ion temperature gradient (ITG) and suppressed by toroidal sheared flows.
Numerical simulations attempting to understand turbulence are crucial for
guiding the design of future reactors. We investigate ion-scale turbulence via
gyrokinetic simulations in the outer core of the Mega Ampere Spherical Tokamak
(MAST). We perform a parameter scan in the values of the ITG and the flow
shear. We show that nonlinear simulations reproduce the experimental ion heat
flux and that the experimental values of the ITG and the flow shear lie close
to the turbulence threshold. We demonstrate that the system is subcritical in
the presence of flow shear, i.e., the system is formally stable, but
transitions to a turbulent state given a large enough initial perturbation. We
propose a scenario for the transition to turbulence previously unreported in
tokamak plasmas: close to the threshold, the plasma is dominated by a low
number of coherent long-lived structures; as the system is taken away from the
threshold into the more unstable regime, the number of these structures
increases until they fill the domain and a more conventional turbulence
emerges. We make comparisons of correlation properties between our simulations
and experimental measurements of density fluctuations from the MAST BES
diagnostic. We apply a synthetic diagnostic to our simulation data and find
reasonable agreement of the correlation properties of the simulated and
experimental turbulence, most notably of the correlation time. We show that the
properties of turbulence are essentially functions of the ion heat flux. We
find that turbulence close to the threshold is strongly affected by flow shear,
whereas far from threshold, the turbulence resembles a conventional ITG-driven
regime.
