Collisional damping of ETG-mode-driven zonal flows.

We study collisional damping of electron zonal flows in toroidal electron temperature gradient (ETG) turbulence due to the friction between trapped and untrapped electrons. With the assumption of adiabatic ions, the collisional damping is shown to occur on fast time scales approximately 0.24epsilon(1/2)tau(e). The comparison with the growth rate of electron zonal flows indicates that the shearing by electron zonal flows is unlikely to be a robust mechanism for regulating ETG turbulence. This finding vitiates the claims of several simulation studies that have ignored the effects of collisional damping of electron zonal flows and offers a possible partial explanation of the high levels of electron thermal transport observed in the National Spherical Torus Experiment.