Effective Modeling of Turbulent Transport via Reduced Dynamics in Helical Plasmas

Quantitative prediction of turbulent plasma transport is crucial in magnetic confinement research. While gyrokinetic simulations provide valuable insights, their high computational cost limits practical applications. To mitigate this, a reduced transport model was developed by incorporating time-averaged representations of turbulence and zonal flows from simulations [1, 2]. In contrast, by explicitly considering the time evolution of each fluctuation component, we found that the system dynamics are confined to a restricted subspace spanned by turbulent component, zonal flow amplitude, and transport coefficient. Including the degrees of freedom associated with this subspace improves model accuracy [3]. Further extensions of the model have previously been considered, but uncertainties remained in determining its functional form. To resolve this, we explored the structure of the model's objective function in parameter space—not merely seeking its minimum value, but optimizing the structure itself in the context of helical plasmas [4]. This approach reduces the arbitrariness inherent in conventional models and allows for a more definitive model formulation.