Role of zonal flows in trapped electron mode turbulence through nonlinear gyrokinetic particle and continuum simulation

Trapped electron mode (TEM) turbulence exhibits a rich variety of collisional and zonal flow physics. This work explores the parametric variation of zonal flows and underlying mechanisms through a series of linear and nonlinear gyrokinetic simulations, using both particle-in-cell and continuum methods. A new stability diagram for electron modes is presented, identifying a critical boundary at eta(e)=1, separating long and short wavelength TEMs. A novel parity test is used to separate TEMs from electron temperature gradient driven modes. A nonlinear scan of eta(e) reveals fine scale structure for eta(e)greater than or similar to 1, consistent with linear expectation. For eta(e)< 1, zonal flows are the dominant saturation mechanism, and TEM transport is insensitive to eta(e). For eta(e)>1, zonal flows are weak, and TEM transport falls inversely with a power law in eta(e). The role of zonal flows appears to be connected to linear stability properties. Particle and continuum methods are compared in detail over a range of eta(e)=d ln T-e/d ln n(e) values from zero to five. Linear growth rate spectra, transport fluxes, fluctuation wavelength spectra, zonal flow shearing spectra, and correlation lengths and times are in close agreement. In addition to identifying the critical parameter eta(e) for TEM zonal flows, this paper takes a challenging step in code verification, directly comparing very different methods of simulating simultaneous kinetic electron and ion dynamics in TEM turbulence.