Interferometric study of density fluctuations in a tokamak plasma

Abstract

Density fluctuations in the LT-4 tokamak plasma are investigated using a Phase Scintillation Interferometer operating at 10.6/Ltm which is sensitive to density fluctuations of δnₑ/nₑ> 10⁻¹⁴. The plasma is imaged across a linear detector array which can be rotated to record projections in any direction, from toroidal to poloidal. The theory of forward scattering from plasmas is developed from the Rytov approximation and aspects of the Fourier diffraction projection theorem relevant to plasma scattering. The result is a clear conceptual picture of diffraction from arbitrary extended refractive media, from which important analytical tools are developed. The Phase Scintillation Interferometer is used to image density perturbations produced by large scale magnetohydro dynamic (MHD) modes in the plasma associated with Mimov oscillations. Structural characteristics are determined, and a comparison between experimental and computed projections of the Dubois model is made which shows that the density fluctuations are consistent with a model of rotating magnetic islands. Island widths and local magnetic field fluctuations are determined and are found to compare well with measured poloidal magnetic field fluctuations. The interferometer is used in conjunction with other diagnostics to investigate minor and major disruptions in LT-4. The time frequency distribution is introduced as an important analytical tool in the characterization of the various regimes of MHD activity. Frequency and amplitude variations of an m = 3 mode during current rise appear correlated with variations in toroidal loop voltage. The mode is also found to persist throughout the whole discharge and to play a part in mode locking which precedes major disruptions. Mode frequencies are found to vary in a regular way with the safety factor q(a). Precursor oscillations before minor and major disruptions are identified. A strong m — 1 type of internal relaxation is found to follow rapid growth and locking of an m = 2 mode during minor disruptions. The interferometer is also applied to the measurement of fine scale density fluctuations in the LT-4 tokamak during periods of low level MHD activity. Line integral measurements indicate an edge fluctuation level of about 10% and broad band spectra typical of strong turbulence. Anisotropy in the spectrum of fluctuations perpendicular to the magnetic field is observed. This observation runs counter to reported measurements of isotropic fluctuations made on other tokamaks using small angle scattering techniques. Very long correlation lengths along the field lines are observed, which are consistent with nearly all models of turbulence in tokamak plasmas. The images are numerically filtered so as to isolate and display counter-propagating structures in the turbulent flow. Show more