Reduced models for ETG transport in the pedestal

Electron temperature gradient (ETG) driven turbulence is likely a major contributor to electron heat transport in the pedestal. Nonlinear gyrokinetic simulations have demonstrated realistic transport levels for several discharges. There exist, however, few (if any) reduced models for pedestal ETG transport, which is the topic of this presentation. In the pedestal, ETG turbulence does not rely on streamers to enhance transport beyond the standard mixing length arguments—low transport is, after all, the defining characteristic of a transport barrier. Rather, significant heat fluxes follow simply from the enormous pedestal gradients compensating for low diffusivities. Pedestal ETG modes are rather exotic in nature, having, for example, multiple branches (toroidal and slab) in different wavenumber ranges. Moreover, many ETG modes can be unstable simultaneously at a single wavenumber and growth rates can peak at finite ballooning angle. These characteristics make reduced modeling of slab ETG challenging. We will present a reduced model of ETG pedestal transport that effectively reproduces the nonlinear transport levels across a database of several dozen nonlinear simulations. This reduced model is a generalization of the standard quasilinear approach.