Experimental study of ELM-induced filament structures using the VUV imaging system on EAST

A high-speed vacuum ultraviolet(VUV) imaging system has been developed on the Experimental Advanced Superconducting Tokamak(EAST), which selectively measures line emission with a central wavelength of 13.5 nm(CVI, n=4–2). It has been employed to study edge/pedestal plasma behavior in EAST. Edge localized mode(ELM)-induced filament structures have been captured by the VUV imaging system during the ELMy high confinement mode discharge with both high temporal and spatial resolutions. The typical features(i.e.poloidal width and pitch angle) of the observed filaments are quantitatively characterized based on the VUV imaging data, and the dependence of these features on basic plasma parameters is analyzed. It is found that the poloidal width is proportional to the heating power, and the pitch angle is inversely proportional to the edge safety factor q.95 The scatterplot shows a positive trend between the poloidal width and the ELM amplitude defined by the relative change in stored energy. These results are based on the condition that the perturbation induced by ELMs is confined to a narrow layer in the plasma.     

Observation of filament-like structures in ELMy H-mode plasma with a VUV imaging system developed on the EAST tokamak

On the EAST tokamak, filament-like structures have been observed in ELMy H-mode discharges with a high-speed vacuum ultraviolet (VUV) imaging system. The topos, chronos and their weight can be obtained simultaneously by performing the so-called singular value decomposition (SVD) analysis of raw VUV imaging data. The fluctuation amplitude is observed to be suppressed and enhanced gradually in the edge localized mode (ELM) crash and pedestal recovery phase in the chronos, respectively, while filament-like structures can only be found in the pedestal recovery phase on the topos. The mode structure, i.e. m/n=36/9 (m and n are the poloidal and toroidal mode number, respectively) with ρ₀=0.95, w₀=0.07 (ρ₀and w₀ denote the mode location and mode width, respectively) is derived by a comparison of the synthetic images and the experimental imaging data.     

Development of vacuum ultraviolet spectroscopy for measuring edge impurity emission in the EAST tokamak

The dominant wavelength range of edge impurity emissions moves from the visible range to the vacuum ultraviolet(VUV) range, as heating power increasing in the Experimental Advanced Superconducting Tokamak(EAST). The measurement provided by the existing visible spectroscopies in EAST is not sufficient for impurity transport studies for high-parameters plasmas. Therefore, in this study, a VUV spectroscopy is newly developed to measure edge impurity emissions in EAST. One Seya-Namioka VUV spectrometer(McPherson 234/302) is used in the system, equipped with a concave-corrected holographic grating with groove density of 600 grooves mm~(–1). Impurity line emissions can be observed in the wavelength range ofλ=50–700 nm, covering VUV, near ultraviolet and visible ranges. The observed vertical range is Z=-350–350 mm. The minimum sampling time can be set to 5 ms under full vertical binning(FVB) mode. VUV spectroscopy has been used to measure the edge impurity emission for the 2019 EAST experimental campaign. Impurity spectra are identified for several impurity species, i.e., lithium(Li), carbon(C), oxygen(O), and iron(Fe). Several candidates for tungsten(W) lines are also measured but their clear identification is very difficult due to a strong overlap with Fe lines. Time evolutions of impurity carbon emissions of CII at 134.5 nm and CIII at97.7 nm are analyzed to prove the system capability of time-resolved measurement. The measurements of the VUV spectroscopy are very helpful for edge impurity transport study in the high-parameters plasma in EAST.     

Analysis of electron temperature,impurity transport and MHD activity with multi-energy soft x-ray diagnostic in EAST tokamak

A new edge tangential multi-energy soft x-ray(ME-SXR) diagnostic with high temporal(≤ 0.1 ms) and spatial(~1 cm) resolution has been developed for a variety of physics topics studies in the EAST tokamak plasma. The fast edge electron temperature profile(approximately from r a~ 0.6 to the scrape-off layer) is investigated using ME-SXR diagnostic system. The data process was performed by the ideal ‘multi-foil' technique, with no priori assumptions of plasma profiles. Reconstructed ME-SXR emissivity profiles for a variety of EAST experimental scenarios are presented here for the first time. The applications of the ME-SXR for study of the effects of resonant magnetic perturbation on edge localized modes and the first time neon radiating divertor experiment in EAST are also presented in this work. It has been found that neon impurity can suppress the 2/1 tearing mode and trigger a 3/1 MHD mode.     

The Calibration of High-Speed Camera Imaging System for ELMs Observation on EAST Tokamak

A tangential fast visible camera has been set up in EAST tokamak for the study of edge MHD instabilities such as ELM.To determine the 3-D information from CCD images,Tsai's two-stage technique was utilized to calibrate the high-speed camera imaging system for ELM study.By applying tiles of the passive stabilizers in the tokamak device as the calibration pattern,transformation parameters for transforming from a 3-D world coordinate system to a 2-D image coordinate system were obtained,including the rotation matrix,the translation vector,the focal length and the lens distortion.The calibration errors were estimated and the results indicate the reliability of the method used for the camera imaging system.Through the calibration,some information about ELM filaments,such as positions and velocities were obtained from images of H-mode CCD videos.     

Space-resolved vacuum-ultraviolet spectroscopy for measuring impurity emission from divertor region of EAST tokamak

The measurement of impurity distribution in the divertor region of tokamaks is key to studying edge impurity transport. Therefore, a space-resolved vacuum-ultraviolet (VUV) spectrometer is designed to measure impurity emission in the divertor region on EAST. For good spectral resolution, an eagle-type VUV spectrometer with 1 m long focal length with spherical holograph grating is used in the system. For light collection, a collimating mirror is installed between the EAST plasma and the VUV spectrometer to extend the observing range to cover the upper divertor region. Two types of detectors, i.e. a back-illuminated charge-coupled device detector and a photomultiplier-tube detector, are adopted for the spectral measurement and high-frequency intensity measurement for feedback control, respectively. The angle between the entrance and exit optical axis is fixed at 15°. The detector can be moved along the exit axis to maintain a good focusing position when the wavelength is scanned by rotating the grating. The profile of impurity emissions is projected through the space-resolved slit, which is set horizontally. The spectrometer is equipped with two gratings with 2400 grooves/mm and 2160 grooves/mm, respectively. The overall aberration of the system is reduced by accurate detector positioning. As a result, the total spectral broadening can be reduced to about 0.013 nm. The simulated performance of the system is found to satisfy the requirement of measurement of impurity emissions from the divertor area of the EAST tokamak.     

A Tangentially Visible Fast Imaging System on EAST

A tangentially visible imaging system has been developed on the Experimental Advanced Superconducting Tokamak(EAST) to record the full cross-section plasma visible radiation with speed up to 7500 frames per second at full resolution or much faster for a limited number of pixels.The edge localized mode(ELM) filaments during ELM eruptions in H-modes are captured.Using these pictures,the current in the filament is estimated.     

EAST等离子体图像采集与位置识别技术

本工作采用快速CCD相机采集并保存托卡马克等离子体放电时的图像。根据放电时等离子体图像的位置特点,采用改进的主动轮廓模型算法实时获得等离子体边界位置,并通过最小二乘法对其边界进行拟合,以此获得等离子体的中心位置,为进一步进行等离子体的实时控制提供了基础。该方法已成功应用于EAST装置的放电实验中,并取得了良好的效果。     

Observation of blobs using gas puff imaging diagnostic on HL-2A tokamak

A gas puff imaging (GPI) diagnostic has been developed and applied to measure edge plasma turbulence on HL-2A tokamak. The principle and experimental setup of GPI are described. GPI is applied to investigate blobs in the edge and scrape-off layer (SOL). Statistical characterizations of GPI line emission intensity are calculated, including the probability density functions (PDF), skewness and kurtosis of the intensity, which are found to be consistent with measurements by Langmuir probes. Besides, the track of blob motions is recorded by time sequence of individual frames. The characteristics of original images and the relatively high-frequency (>10 kHz)/low-frequency (1-10 kHz) components images are illustrated. The observation of blob’s structures and high-speed motions proves the success and high performance of the GPI diagnostic.     

Thermo-mechanical analysis of RMP coil system for EAST tokamak

Resonant magnetic perturbation (RMP) has been proved to be an efficient approach on edge localized modes (ELMs) control, resistive wall mode (RWM) control, and error field correction (EFC), RMP coil system under design in EAST tokamak will realize the above-mentioned multi-functions. This paper focuses on the thermo-mechanical analysis of EAST RMP coil system on the basis of sensitivity analysis, both normal and off-normal working conditions are considered. The most characteristic set of coil system is chosen with a complete modelling by means of three-dimensional (3D) finite element method, thermo-hydraulic and thermal-structural performances are investigated adequately, both locally and globally. The compromise is made between thermal performance and structural design requirements, and the results indicate that the optimized design is feasible and reasonable.     

Achieving temporary divertor plasma detachment with MARFE events by pellet injection in the EAST superconducting tokamak

A new pellet injection system has been equipped on the experimental advanced superconducting tokamak(EAST) in the 2012 campaign,with a pellet size of Ф 2 mm?×?2 mm,a frequency of1 Hz–10 Hz and velocity of 150 m s~(-1)–300 m s~(-1).The deuterium pellet is well-known for plasma fuelling as well as for? triggering the edge localized mode(ELM).In the 2012 campaign,pellet injection experiments were successfully carried out on EAST.Temporary plasma detachment achieved by deuterium pellets has been observed in a double null(DN) divertor configuration,with multi-pellet injections at a repetition frequency of 2 Hz.The partial detachment of the outer divertors and complete detachment of the inner divertors was achieved after 35 ms of each pellet injection,which have a duration of 30–60 ms with the maximum degree of detachment(DOD) reaching 3.5 and 37,respectively.Meanwhile,the multifaceted asymmetric radiation from the edge(MARFE) phenomena was also observed at the high field side(HFS) near both the lower and upper X-points with radiation loss suddenly increased to about 15%–70%,which may be the main cause of divertor plasma detachment.The temporary detachment induced by pellet injection may act as a new way to study divertor detachment behaviors.     

The Upgrade of Fast Ferrite Tuning Matching System for ICRF in EAST

The fast ferrite tuning (FFT) real-time matching system has been designed and tested for the ion cyclotron range of frequency (ICRF) in EAST tokamak, which is necessary to transfer ICRF power to the plasma against variations in the antenna impedance. Through the test results, we proved this FFT system is feasible in EAST. Therefore this system have been upgraded recently to achieve real-time matching by the upgrading of the coil power supply and optimizing of the tuning structure. Finally the new FFT system achieved a response time of 10 ms and operated with a peak power of 1.5 MW, which satisfied the requirements of matching system in EAST.     

Recent results of fusion triple product on EAST tokamak

High fusion triple product has been obtained in the advanced scenarios with high normalized beta (β_N) on the Experimental Advanced Superconducting Tokamak (EAST).A record value of n_(i0)T_(i0)τ_E~1.0?×?10~(19)m~(-3)ke V s for EAST deuterium plasma has been achieved,which is due to the formation of strong and broad internal transport barriers (ITBs) in n_e,T_eand T_iprofiles.Analysis shows that the strong ITB formation could be attributed to the reduction of transport from ITG modes.Based on the analysis,the physical mechanisms and methods to furtherimprove the plasma performance are discussed.     

Spatial Resolution Measurements of C,Si and Mo Using LIBS for Diagnostics of Plasma Facing Materials in a Fusion Device

Recently,a laser-induced breakdown spectroscopic(LIBS) system has been developed for in situ measurements of the chemical compositions of plasma facing materials(PFMs)in the Experimental Advanced Superconducting Tokamak(EAST).In this study,a LIBS system,which was used in a similar optical configuration to the in situ LIBS system in EAST,has been developed to investigate the spatial distribution of PFM elements at 1CP4 Pa.The aim of this study was to understand the nature of the spatial distribution of atoms or ions of different elements in the plasma plume and optimize the signal to background ratio for the in situ LIBS diagnosis in EAST.The spatial profiles of the LIBS signals of C,Si,Mo and the continuous background were measured.Moreover,the influence of laser spot size and laser energy density on the LIBS signals of C,Si,Mo and H was also investigated.The results show that the distribution of the C,Si and Mo peaks' intensities first increased and then decreased from the center to the edge of the plasma plume.There was a maximum value at R≈1.5 mm from the center of the plasma plume.This work aims to improve the understanding of ablating plasma dynamics in very low pressure environments and give guidance to optimize the LIBS system in the EAST device.     

Improvement of divertor triple probe system and its measurements under full graphite wall on EAST

Full graphite wall of experimental advanced superconducting tokamak (EAST) has been developed in the spring of 2008. A new divertor triple probe diagnostics system (DTPDs) is built for EAST during this upgrade. The tip shape and connected structure of the probe are optimized for variational magnetic field directions and DTPDs maintenance. The experiment has been carried out with a full graphite wall for EAST, and near double-null diverted plasma is achieved successfully. The evolutions of electron temperature, density, particle flux and power densities along the divertor targets have been obtained with DTPDs.     

Reconstruction of the Density Profile for the EAST Tokamak Based on Polarimeter/Interferometer and Microwave Reflectometer Systems

A plasma density profile reconstruction procedure based on the Park matrix method has been developed for both circular and elongated plasma configuration on the Experimental Advanced Superconducting Tokamak(EAST).This method incorporates the line integrated electron density measured by the HCN interferometer and polarimeter/interferometer(POINT) system,the equilibrium fit(EFIT) based on magnetic measurements and the edge electron density profile provided by the microwave reflectometer.It is shown that when the magnetic flux surfaces are slightly corrected,the fitting error is less than 5% in comparison with the measurement data.     

First Achievement of Plasma Heating for EAST Neutral Beam Injector

As one of the most effective methods for plasma heating,a neutral beam injector(NBI) achieved plasma heating and current driving for the first time in EAST 2014 experimental campaign.According to the research plan of the EAST physics experiment,the first NBI(EASTNBI-1) has been built and become operational in 2014.In this article,the latest experiment results of EAST-NBI-1 are reported as follows:(1) EAST achieves H-mode plasma in the case of NBI heating alone,(2) EAST achieves 22 s long pulse stable H-mode plasma in the case of sinndtaneous NBI and lower hybrid wave(LHW) heating.The measurement data show that the loop voltage decreased and the plasma stored energy increased obviously.The results indicate that EAST-NBI-1 has achieved plasma heating and current driving,and thus lays a foundation for the construction of EAST-NBI-2,which will be built in a few months this year.     

The Alignment and Assembly for EAST Tokamak Device

EAST （HT-7U） is a large fusion experimental device. It is a full superconducting tokamak with 1 MA of plasma current, 1000 s of plasma duration, high elongation and triangularity. It mainly consists of superconducting magnets of poloidal and toroidal field （PF ＆ TF）, vacuum vessel （VV）, thermal radiation shield （TRS） and cryostat vessel （CV）. The significant difficulty for assembly of EAST is tight installation tolerances, which are in the order of several tenth of a millimeter. In particular, the alignment of plasma facing components to the magnetic axis of the device is less than ±0.5 mm. At present, a reasonable assembly process of EAST has been defined, and based on it, the alignment method for EAST, including the survey control network, the location of the main components in different directions, the magnetic axis determination and the accurate positioning of the plasma facing components inside of the vacuum vessel and so on, has been defined by using the sophisticated optical metrology system （SOMS）. This paper describes the assembly procedure of EAST and the installation tolerances associated with the main components. Meanwhile, how to establish the assembly survey control network, magnetic axis determination methods, are introduced in detail.     

The design and the manufacturing process of the superconducting toroidal field magnet system for EAST device

The toroidal field (TF) magnet system of EAST (HT-7U), which consists of 16 superconducting coils enclosed in steel cases, has been manufactured to generate the magnetic field of 3.5 T at the plasma center to maintain plasma in a tokamak configuration with a current up to 1 MA. The TF coils have an approximately D-shape geometry of 2.6 m wide and 4.0 m high, and operate at a maximum field of 5.8 T. The conductor used in the TF coil is NbTi/Cu cable-in conduit (CIC) conductor, and its operating current is 14.3 kA.In March 2006, the first cooling down of the EAST device has been carried out successfully. The total of TF magnet system has been cooled down from room temperature to 4.5 K, and the TF system has been energized up to 8.2 kA with 5 A/s ramp rate. In September 2006, full performances of the TF magnet system have been reached, and the device of EAST has delivered its first plasma. In addition, the TF magnet system has been routinely operated with a current maintained constant on a whole day basis, for a preliminary program of more than 500 shots.In this paper, the main parts of the design, developmental tests, and the fabrication and assembly of TF coils are described in detail.     

Study on Cavity Modes for D(H) Ion Cycloton Range Frequency Heating Scenarios in EAST

For low single-pass absorption of ion cyclotron range frequency (ICRF) wave in the EAST plasma cavity modes are expected to be excited between the low field side (LFS) antenna and the hybrid cut-off layer. The toroidal spectrum for D(H) minority heating scenarios in EAST is modeled by using FELICE（Finite Elements Ion Cyclotron Emulator）, a full wave code based on plane-stratified geometry. The excitation of cavity modes is studied. The methods for suppressing cavity modes are also discussed, to increase the efficiency of minority ion heating.