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.

Full-Wave Modeling of Doppler Backscattering from Filaments

“…After normalization BQFs for the both linear and nonlinear look similar as it is shown in figure 2(b) and the relation for the Doppler shift k⊥ = k х = 2k 0 sinα is preserved correspondingly. It is important to note that similarly shaped BQOs were observed in experiments on the Globus-M tokamak (see Figure 2c) [3,4]. The similarity of BQFs in the linear and nonlinear case, on the one hand, explains the fact that similar BQFs were observed The calculations for the case of multiple frequency probing were carried out for the same radial position of the moving filament r = 9cm.…”

“…Various diagnostics are employed for this purpose. A few years ago the authors proposed to use the Doppler backscattering (DBS) method for filaments investigation in the Globus-M spherical tokamak [3,4]. Later this diagnosis was successfully applied to the study of the filaments in the ASDEX Upgrade tokamak [5,6].…”

Review of Advanced Implementation of Doppler Backscattering Method in Globus-M

“…One of them is ELM filaments. They were observed during ELMs triggered by sawtooth oscillations [60] in discharges with H-mode initiated by NBI when plasma density was relatively small n e < 1.2 × 10 19 m −3 [61].…”

“…The second filament type is inter-ELM filaments. Such filaments were discovered between ELMs in discharges with H-mode initiated by NBI when plasma density was higher than the critical value n e = 1.5 × 10 19 m −3 [61]. Unlike radially elongated ELM filaments, inter-ELM filaments were located in a very narrow layer of <2 cm.…”

“…One of them is ELM filaments. They were observed during ELMs triggered by sawtooth oscillations [60] in discharges with H-mode initiated by NBI when plasma density was relatively small ne < 1.2 × 10 19 m −3 [61]. If the plasma density was higher, the waveform during ELMs ceased to resemble quasi-coherent oscillations, although the signal level increased greatly during ELMs.…”