The spatial structure and temporal evolution of coherent MHD modes in fusion plasma devices have been so far inferred from the experimental signals by using spectral techniques. Considering the data as a collection of n-dimensional discrete time series xi(t) permits one to introduce in this context a signal processing method based on the singular value decomposition (SVD) of a rectangular matrix. This paper shows that, whereas the SVD is equivalent to the discrete Fourier transform in the case of travelling sinusoidal waves, in more realistic cases it is a clear improvement on the spectral methods, since it disentangles the poloidal structure without a priori knowledge. Various applications of SVD, first on artificial signals and then on magnetic and SXR signals detected in the JET Tokamak in several plasma regimes, are shown with the purpose of illustrating the power and limits of the method.
A new method for the inversion of interferometry data using basis functions derived from singular value decomposition of local measurements in tokamak plasmas