Simulations of collisional Trapped-Electron-Mode turbulence with the global gyrokinetic $\delta f$ Particle-in-Cell code ORB5

Global collisional gyrokinetic simulations of Trapped-Electron-Mode (TEM) instabilities, for which the drive is the electron temperature gradient, are presented. The numerical tool is the Particle-In-Cell code ORB5, upgraded with linearized electron collision operators. Electrons are treated according to the so-called hybrid model, considering kinetic trapped electrons and adiabatic passing electrons. The linear TEM growth rates are found to be damped by electron collisions. The effect of the ratio $T_{e}/T_{i}$ on the collisional damping is studied. The accuracy of the Lorentz model is tested against the full linearized operator predictions. The issue of $\rho^{*}$ effects in TEM simulations is addressed. A critical electron temperature gradient for linear TEM instabilities is established and compared to the temperature gradient dependence of the turbulence level in non-linear global simulations. Finally, the effects of the zonal flow shearing rate on TEM turbulence are investigated in both collisionless and collisional global ORB5 simulations.