Nonlinear gyrokinetic analysis of linear Ohmic confinement to saturated Ohmic confinement transition

This work presents a nonlinear gyrokinetic analysis, which addresses one of the long-lived conundrums in Ohmically heated fusion plasmas. The widely observed linear Ohmic confinement (LOC) to saturated Ohmic confinement (SOC) transition of the energy confinement time $\tau_E$ with increasing density is successfully reproduced from nonlinear gyrokinetic simulations for the first time. Suppression of trapped electron turbulence by collisional detrapping due to increasing density is found to be responsible for the transition. Microturbulence transition from TEM to ITG is found to coincide, but not a necessary condition for the LOC-SOC transition. Damping of nonlinearly generated zonal flow by increasing collisionality with density can be responsible for the energy confinement degradation in SOC regime.