The study of the dynamic response of the plasma is a valuable tool for investigating transport processes in a tokamak. A deductive approach is adopted, in order to make the best use of this experimental information. A system identification method is developed to estimate the transfer function of the system from the time evolution of its parameters to any excitation. The form of the identified transfer function is linked with a representation of the transport in terms of poles (eigenvalues) and eigenmodes. These are directly deduced from the raw data, with no restriction on the underlying processes, and there is consequently no need to adjust any simplified transport model to the experimental data. This method is illustrated by analysing the injection of pellets on Tore Supra. The density and the temperature transfer functions were observed to share the same poles, with the corresponding eigenmodes grouped in pairs with identical profiles. This implies the presence of a coupling between the particle flow and the heat flow. A criterion based on compatibility with the observed transfer function is developed to select among several possible coupling mechanisms. The selection suggests a particle diffusion coefficient depending on the density and the temperature gradient
Model for particle transport in high-density plasmas