Application of the singular value decomposition method for inversion of interferometer measurements in fusion plasmas

We present applications of a recently developed method, the singular value decomposition inversion (SVD-I), for inverting line integrated multichannel interferometer measurements in fusion devices. The method expands the local electron density profiles into a limited series of orthogonal basis functions formed from the singular value decomposition of Thomson scattering measurements, which are obtained at much lower frequencies than the interferometer data. Because these basis functions are formed from a local measurement of the electron densities in the same plasmas, they are well adapted to the class of density profiles to be reconstructed. Using this expansion, the inversion of the interferometer data are reduced to solving an overdetermined set of equations. SVD-I is significantly faster than regularization methods and more accurate for reconstructing hollow profiles. This method, which combines the high bandwidth of interferometer systems with the spatial accuracy of Thomson scattering, is applied to invert data from a 14-chord interferometer on the TCV tokamak. (C) 2004 American Institute of Physics.