Speaker: Augustus Azelis, Department of Physics Graduate Student
Abstract: In toroidal ion temperature gradient (ITG) driven turbulence, it remains a challenge to understand heat flux reduction at and above the threshold of linear instability for a range of driving gradients called the Dimits regime. A known but unexplained feature of this regime is the observation of temporally intermittent turbulent fluctuations and resulting transport. Preexisting theory for the Dimits shift successfully attributed heat flux reduction to resonance in mode coupling, but this analysis was based on a cumulant-discard method which neglected intermittency and also did not produce any bifurcation demarcating variation in transport with driving gradient above and below the nonlinear critical threshold. In this work, weak turbulence closures are employed to produce dynamical equations for a fourth order cumulant as well as the heat flux itself. The former predicts conditions under which intermittent behavior may develop while the latter is a direct attempt to model said phenomenon. Preliminary analysis has found strong cumulant growth near the linear threshold which can be attributed to resonances in triplet correlation times and nonlinear coupling coefficients. This suggests possible coincidence between the mechanism responsible for heat flux suppression and the inherent non-Gaussian tendencies of the Dimits regime. Analytical work is compared against solutions of the reduced two-field fluid model for toroidal ITG driven turbulence from the numerical solver Dedalus.