Synergy between ion temperature gradient turbulence and neoclassical processes in global gyrokinetic particle-in-cell simulations

Based on the CYCLONE case, simulations of collisional electrostatic ion temperature gradient (ITG) microturbulence carried out with the global gyrokinetic particle-in-cell (PIC) code ORB5 are presented. Considering adiabatic electrons, an increase in ion heat transport over the collisionless turbulent case due to ion-ion collisions is found to exceed the neoclassical contribution. This synergetic effect is due to interaction of collisions, turbulence, and zonal flows. When going from a collisionless to a collisional ITG turbulence simulation, a moderate reduction of the average zonal flow level is observed. The collisional zonal flow level turns out to be roughly independent of the finite collisionality considered. The Dimits shift softening by collisions [Z. Lin et al., Phys. Rev. Lett. 83, 3645 (1999)] is further characterized, and the shearing rate saturation mechanism is emphasized. Turbulence simulations start from a neoclassical equilibrium [T. Vernay et al., Phys. Plasmas 17, 122301 (2010)] and are carried out over significant turbulence times and several collision times thanks to a coarse-graining procedure, ensuring a sufficient signal/noise ratio even at late times in the simulation. The relevance of the Lorentz approximation for ion-ion collisions, compared to a linearized Landau self-collision operator, is finally addressed in the frame of both neoclassical and turbulence studies.