Near-Resonant Heat Flux Reduction in Gyrokinetic Dimits-Shift Analysis and Applications to a Quasilinear Model

The onset of turbulent heat flux at a higher temperature gradient than the critical gradient of linear instability (known as the Dimits shift) is a well-known feature in fusion plasmas. It is investigated in relation to the saturation mechanism for toroidal Ion Temperature Gradient (ITG) instability using a fluid model with threshold physics. It is shown that resonance in the nonlinear coupling between the modes that dominate energy transfer in saturation can lead to suppression of turbulence and transport just above the linear critical gradient.  Resonant effects occur in both the triplet correlation time and the nonlinear coupling coefficients.  The triplet correlation time is sensitive to the eddy turnover rate, which broadens the resonance, reduces the triplet correlation time and increases the heat flux. The resonance concept is then tested in gyrokinetic simulations with ITG turbulence which has a Dimits shift, by use of artificial complex frequencies to break the resonance.

The standard quasilinear heat flux estimate, widely used in lieu of costly nonlinear computation, fails to predict important features of turbulence scalings, including the Dimits shift. The quasilinear model is improved by incorporating the more complete saturation physics described above, in particular the triplet correlation time, which can be approximated from linear inputs. Methods to further improve the quasilinear flux by calculating and including the coupling coefficients are also under investigation.