Turbulent transport is the next big challenge in optimized stellarators, since it limits energy confinement. The characteristics of turbulent transport can be described by the heat fluxes and related transport coefficients. Having a model for the heat flux would be helpful to measure the performance of a magnetic geometry without the need for expensive simulations.
A reduced model originally designed for the Large Helical Device (LHD) stellarator relates the Ion Temperature Gradient (ITG) generated turbulence and zonal flows as a saturation mechanism; with the final goal of predicting the heat diffusivity. The coexistence of ITG-generated turbulence and zonal flows is well established in tokamaks. After the implementation of the model on the geometry of stellarator Wendelstein 7-X, the model has to be reduced. There has been found a linear relation between linear and nonlinear simulations of turbulence, from which the coefficient can be used to reduce the model. For zonal flows such a relation has not been established. Therefore we aim to investigate different characteristics of the linear zonal flow response that represent the nonlinear zonal flow strength.
Such characteristics are identified and compared against functions of the nonlinear turbulent spectrum and the nonlinear zonal flow strength. The data that is used is retrieved using the GENE code. The characteristics identified are the residual value, the zonal flow decay time, the integrals over the real and imaginary part, the angular frequency and the decay constant. No relations have been found between the identified characteristics and nonlinear simulations of turbulence and the zonal flow response in Wendelstein 7-X. The identified characteristics and the used functions of the nonlinear turbulent spectrum and the nonlinear zonal flow strength are found to be not adequate for determining such a relation.