Asymptotic behavior, non-local dynamics, and data assimilation tailoring of the reduced κ−ε model to address turbulent transport of fusion plasmas

Abstract

The high-dimensional and multiscale nature of fusion plasma flows require the development of reduced models to be implemented in numerical codes capable of capturing the main features of turbulent transport in a sufficiently short time to be useful during tokamak operation. This paper goes further in the analysis of the dynamics of the [Formula: see text] model based on the turbulent kinetic energy κ and its dissipation rate ε [Baschetti et al., Nucl. Fusion 61, 106020 (2021)] to improve the predictability of the transverse turbulent transport in simulation codes. The present 1D results show further capabilities with respect to current models (based on constant effective perpendicular diffusion) and on the standard quasi-linear approach. The nonlinear dependence of D in κ and ε estimated from two additional transport equations allows us to introduce some non-locality in the transport model. This is illustrated by the existence of parameter ranges with turbulence spreading. The paper also addresses another issue related to the uncertainties on the inherent free parameters of such reduced model. The study proposes a new approach in the fusion community based on a variational data assimilation involving the minimization of a cost function defined as the distance between the reference data and the calculated values. The results are good and show the ability of the data assimilation to reduce uncertainties on the free parameters, which remains a critical point to ensure the total reliability of such an approach.