The study of filaments by the Doppler backscattering method in the ‘Globus-M’ tokamak

Abstract

Investigation of filamentary structures was performed on the spherical tokamak Globus-M by the Doppler backscattering (DBS) method. Filaments were detected in the H-mode regime both during the edge localized mode event and in the periods between them. They manifested themselves as bursts of quasi-coherent fluctuations (BQFs) of the output signals of the DBS quadrature detector. To confirm BQF appearing due to backscattering from a moving filament, special experiments were carried out with two DBS systems located at different poloidal angles. Using the new filament research method one could evaluate their velocity, mode number, and radial and poloidal positions. Using this technique, it was possible to estimate the size of the filaments. Experiments have shown that the filaments move predominantly in the direction of the electron diamagnetic drift with a velocity exceeding the velocity of background small-scale fluctuations. Filaments were recorded more frequently when the density gradient on the plasma periphery increased.