Experimental Study Of ELM Filamentary Structures Using The VUVI System On EAST

Edge local modes(ELMs)are a quasi-periodic MHD phenomenon,which usually occurs on the plasma edge pedestal region.ELMy H-mode based on type-I ELM is the basic operation scenario of ITER device in the future.The instability caused by the burst of ELMs will cause the periodic crash of edge pressure gradient,and a large amount of particle flow and energy flow will pass through the separatrix and scrape-off layer(SOL)to reach the first wall and divertor target of the device.This transport process is closely related to the filamentary structure induced by ELM.Therefore,the research on the filamentary structure of ELM is of great significance for the in-depth understanding of the evolution process of ELM and the improvement of the relevant physical model.In recent years,a high-speed vacuum ultraviolet imaging(VUVI)diagnostic system has been developed on EAST,and has been put into experiments and obtained a large amount of data related to the ELMy H-mode experiment.This system is mainly for selective measurement of vacuum ultraviolet photons with a central wavelength of 13.5nm(C Ⅵ,n=4-2).The calculation results show that the line radiation measured by the VUVI system mainly comes from the pedestal region under the typical H-mode experimental discharge conditions of EAST.Therefore,the two-dimensional density fluctuation information of the plasma can be obtained in the pedestal region by the VUVI system by measuring the line emission from C Ⅵ.The included angle between the primary optical axis of the diagnostic system,which is 250mm higher than the midplane,and the centerline of the horizontal D port is 22.50 from top of view.In the process of H-mode experiment,the VUVI diagnostic system usually shoots at a frame rate of 20kfps,and the pixel resolution at the target plane is about 2mm.This paper is mainly based on the experimental data obtained from the ELMy H mode experiment of the diagnositics to carry out the filament-related experimental research.In the ELMy H-mode plasma discharge experiment,the VUVI system have successfully observed ELM events.The two dimensional structure information images of the filamentary structure in the poloidal direction and the toroidal direction are successfully extracted by using a method of background subtraction when ELM burst.In order to quantitatively describe the characteristics of the filamentary structures after extraction,two parameters,the poloidal width and the pitch angle of the filamentary structures,were introduced in this paper.By Gaussian fitting the image data of different toroidal positions,the poloidal width and peak position of the filamentary structure can be obtained.The included angle between the direction of filamentary structure and the toroidal direction,i.e.the pitch angle,can be obtained by connecting the poloidal positions of these peaks.Generally,the ELM-induced filaments are a density perturbation structures along the direction of the magnetic field line.For comparison purposes,this paper also uses the magnetic field line tracing method based on EFIT balance to invert the three-dimensional magnetic field line structure in the pedastal region,and projects the magnetic field line structure onto the two-dimensional target plane of the VUVI diagnostic system in the pedastal region.The projected magnetic field lines are superimposed on the VUVI image,and the direction is the same as the filamentary structure captured by the VUVI camera.Based on the above characterization method of ELM filamentary structures extracted from VUVI images,this paper mainly focuses on the analysis of the poloidal width and pitch angle of the filament during the Type-I ELMs burst.The analysis results show that the poloidal width of the filament structure increases with the increase of the heating power;the pitch angle show a decreasing trend with the increase of the edge safety factor q95 when the toroidal field BT is a constant.Comparing the pitch angle of the filament with the direction of the magnetic field line in the edge pedestal region(R=2.28m),the direction is basically the same.In addition,the relationship between the relative change of the stored energy and the poloidal width of the filament during an ELM burst is analyzed,and the results show that the relative change of the stored energy increases with the increase of the poloidal width.The filament width of Type-Ⅲ ELMs is also analyzed,and its poloidal width also increases with the increase of heating power.The poloidal width of Type-III ELMs is much smaller by comparing with the filament width of Type-I ELMs.Further analyze the influence of resonance magnetic perturbation(RMP)on the filament width comparing the filament width with and without RMP in the same shot data.The result shows that the filament width with RMP comparing without RMP is reduced to a certain extent.