  In electrostatic simulations of MAST plasma at electron-gyroradius scales,
using the local flux-tube gyrokinetic code GS2 with adiabatic ions, we find
that the long-time saturated electron heat flux (the level most relevant to
energy transport) decreases as the electron collisionality decreases. At early
simulation times, the heat flux "quasi-saturates" without any strong dependence
on collisionality, and with the turbulence dominated by streamer-like radially
elongated structures. However, the zonal fluctuation component continues to
grow slowly until much later times, eventually leading to a new saturated state
dominated by zonal modes and with the heat flux proportional to the collision
rate, in approximate agreement with the experimentally observed collisionality
scaling of the energy confinement in MAST. We outline an explanation of this
effect based on a model of ETG turbulence dominated by zonal-nonzonal
interactions and on an analytically derived scaling of the zonal-mode damping
rate with the electron-ion collisionality. Improved energy confinement with
decreasing collisionality is favourable towards the performance of future,
hotter devices.
