On possibility of turbulence wave number spectra reconstruction using radial correlation reflectometry in Tore Supra and FT-2 tokamaks

摘要

Radial correlation reflectometry (RCR) is a microwave technique for measuring the properties of electron density fluctuations in tokamaks and more precisely its use is" "devoted to tokamak turbulence characterization. The principle >of this method is to launch simultaneously two frequencies into the plasma, which are reflected at two different cut-off layers, and then to. process the coherence decay of two scattering signals with increasing difference of probing frequencies in order to determine the correlation length [1]. It is often supposed that the distance between cut offs at which the correlation of two reflectometry signals is suppressed is equal to the turbulence correlation length. Unfortunately according to both numerical and analytical studies [2-3], this assumption is incorrect. As it was shown already in 1D numerical computations performed in the Born approximation [2], the scattering signal cross correlation function (CCF) decrease is much slower than the decay of turbulence CCF. This slow decay of RCR CCF was attributed to the contribution of small angle scattering off very long scale fluctuations. Later this observation was confirmed also in full-wave 1D numerical modeling for small level of turbulent density fluctuations and at the same time' it was shown that nonlinear effects induce a reduction of the correlation length [4]. Recently the comprehensive analysis has been performed 'in 1D model [5] showing the possibility to overcome this difficulty and determine the turbulence correlation length and even the spectrum