Dynamics of Wave-Number Spectrum of Plasma Turbulence

Full text: Direct numerical simulations of gyro-kinetic Vlasov equation are the main tools for studying anomalous transport and thus estimating the confinement time. One of the ways the validity of these codes can be verified is by studying the wave-number spectrum of density fluctuations, which can be measured directly in a tokamak. With this motivation, we have derived a simple model for the evolution of turbulence fluctuation spectra, which includes neighboring interactions leading to the usual two dimensional dual cascade phenomenon as well as disparate scale interactions corresponding to refraction by large scale structures. When disparate scale interactions are dominant, which is a relevant limit for fusion plasmas, a simple spectrum for the density fluctuations of the form |nk|2 ∼ k-3/(1 + k2)2 is obtained. This simple prediction is then compared to, and found to be in fair agreement with Tore Supra CO2 laser scattering data. In the same spirit, the wavenumber spectrum obtained from nonlinear gyrokinetic simulations of Tore Supra tokamak discharges are compared with turbulence measurements. The minor discrepancies in this comparison suggest that intermittency and non-local interactions both in space and in k-space play a role in the form of the wave number spectrum in the observed region in experiment, while in most gyrokinetic simulations such effects are either excluded or poorly treated. This shows that the physics understanding of the nonlinear interaction processes are essential for a careful quantitative comparison. (author)