Experimental Study Of The High-Speed Vacuum Ultraviolet(VUV)imaging System On EAST

In discharge with an H mode plasma,the burst of Edge Localized Modes(ELMs)can cause periodic particles and energy transport from the core to the scrape-off layer(SOL).In long-pulse and high-performance operation on EAST and the future fusion reactors,this part of the particles and energy will reach the divertor target along the magnetic field lines,causing severe target sputtering and high heat load and resulting in damage of the target materials.The impurity radiation will increase simultaneously,which can cause the serious degradation of plasma confinement and divertor life.Therefore,the development of targeted diagnostic techniques is of great practical significance for the development of ELM-related physics research.A high-speed vacuum ultraviolet(VUV)imaging system has been developed on EAST for the typical H-mode plasma parameters of EAST.The preliminary experimental research has been carried out based on this system.The optics of the system consists of three key parts,i.e.,an inverse Schwarzschild telescope composed of two narrowband-reflection multi-layer mirrors made of molybdenum(Mo)and silicon(Si),a micro-channel plate(MCP)equipped with a P47 phosphor screen and a high-speed camera with CMOS sensors.This system can selectively measure the VUV photons with a wavelength of 13.5 nm which mainly come from the line emission from intrinsic carbon(C VI:n=4?2 transition).Generally,the multi-layer mirrors have a significant reflectance for VUV photons with a wavelength longer than 23.7 nm as well.So a zirconium(Zr)filter is installed in front of the MCP detector to exclude the contribution from this part.The two-dimensional density fluctuation information can be obtained by measuring the line emission from C ?.There are complex plasma information in the VUV video data measured in experiments.In order to extract the valid physics information,the image data is preprocessed with the method of singular value decomposition(SVD)in this paper.In order to obtain the two-dimensional distribution of disturbance from the integral image data,further analysis of the preprocessed image data is required.Whether tomographic inversion or comparison of simulated and experimental image,it is the key to calculate the system geometry matrix.Therefore,in this paper,the method for calculating the geometric matrix of the EAST VUV imaging system is developed with the magnetic field line tracking method based on EFIT.In ELMy H-mode plasma experiments,the ELM events has been observed by this system.It was found that the VUV image showed obvious intensity variation in different periods of the ELM indicating that the system has a high time resolution for ELM.One spatial structure like the filament is obtained by analyzing the VUV imaging data captured during the pedestal recovery phase with the SVD method.For further analysis of the ELM,the individual ELM period is divided into two phases(the ELM crash and pedestal recovery phase)according to the evolution of the line-integrated density measured by the POlarimeter-INTerferometer(POINT)diagnostics.SVD analysis are performed for the two phases with the same time window.In pedestal recovery phase,obvious fluctuations are presented on chronos components and the filament-like structures are observed on topos components,while such structures do not appear on topos in ELM crash phase,which is qualitatively consistent with the prediction of Peeling-Ballooning theory.The mode structure,i.e.m/n=36/9(m and n are poloidal and toroidal mode number,respectively)with po?0.95,w0=0.07(p0 and wo denote the mode location and mode width,respectively)is derived by a comparison of the synthetic images and the experimental imaging data.In addition,through the conditional average analysis of VUV imaging data,the filament-like structure is extracted and the drift velocity and direction information along the poloidal direction is obtained.At present,the whole system is installed on the horizontal D-port of the EAST and views the plasma with its major optical axis parallel to the major radius.The major axis of the optics is located 155mm above the plasma mid-plane.It is difficult to estimate the radial evolution of this filament-like structure during ELM event.Imaging with a tangential view may help to catch the radial evolution.And upgrade on the optics to realize a tangentially viewing is proposed.