Experimental Advanced Superconducting Tokamak Device Research Articles

Design of the CFC composite brazed HFS first wall on the NBI shine-through area of the EAST device

The high field side (HFS) first wall is the inner wall nearest to the vertical center axis in tokamak. To avoid high Z impurity sputtering due to plasma interaction and neutral beam injector (NBI) radiation, graphite is adopted as the plasma facing material (PFM) of the HFS first wall on the NBI shine-through area (NBISTA). In the experimental advanced superconducting tokamak (EAST), graphite tiles are bolted to the CuCrZr heat sinks by screw on the NBISTA, but cracks, fractures and ablations were observed on the graphite tiles after experimental campaigns because of insufficient heat removal capability. Thus, a new design of the HFS first wall on the NBISTA was proposed by adopting the carbon fiber composite (CFC) and brazing technology. In the new design, CFC tiles were brazed to the CuCrZr heat sink, and the oxygen-free copper (OFC) sheet was adopted between CFC tiles and CuCrZr heat sinks as stress-buffer layer. To avoid potential brazing failures or CFC tiles detachments, a modular pattern active cooling panels was proposed for the HFS first wall on the NBISTA. Steady heat transfer analysis of those panels was carried out and results show that the heat removal capacity satisfied the design requirements. Meanwhile, feasibility of the CFC brazing process was verified by nondestructive testing (NDT), mechanical property testing and high heat flux (HHF) testing, and testing results indicate that the CFC/OFC/CuCrZr bonding processed by one-step brazing had good mechanical property and heat removal capability.

Effect of continuously flowing liquid Li limiter on particle and heat fluxes during H-mode discharges in EAST

• Particle and heat fluxes were successfully controlled by using continuously flowing liquid Li (FLiLi) limiter in the H mode discharges with high plasma heating power in EAST. • By using flowing liquid Li limiter, the fuel recycling could effectively reduce by ∼ 50% and Zeff could decrease from 2.3 to 1.6. • The decreased recycling and impurity radiation led to the achievement of high energy confinement plasma with the average stored energy increased up to ∼ 290 kJ. • Due to Li vapor shielding effect, ∼40% plasma heat flux was dissipated before arrived at the Li limiter. Particle and heat fluxes were successfully controlled by using a continuously flowing liquid Li (FLiLi) limiter in the H-mode discharges with high plasma heating power in the Experimental Advanced Superconducting Tokamak device. There were strong interactions between the FLiLi limiter and high-power plasma with a ∼ 8.3 MW source heating power, and successively, a bright Li radiation ring was produced, which effectively decreased fuel particle recycling by approximately 50%. Due to Li efflux from FLiLi during a series of high-power discharges, an obvious real-time wall conditioning effect was produced, and fuel particle recycling further decreased. Moreover, the value of Z eff decreased from 2.3 to 1.6 due to a decrease in impurity sources; this was attributed to the accumulation of Li deposited on the first wall, which effectively protected the wall materials. The decreased recycling and impurity radiation achieved high-energy confinement plasma, and the average stored energy increased up to ∼ 290 kJ. Moreover, due to the effect of Li vapor shielding, nearly 30% plasma heat flux was dissipated before it arrived at the Li limiter. These results promote further exploration of liquid Li solutions for the critical challenge of heat flux handling and particle control in fusion power plants.

Design and preliminary realization of component installation scheme generation system in virtual EAST

In the experimental advanced superconducting tokamak (EAST) experiment, the properties, states of plasma and the engineering parameters of the EAST device are measured and analyzed by various diagnostic methods. The approaches of diagnosis and measurement are constantly updated, and the diagnostic systems should be updated accordingly. However, position conflicts would inevitably occur due to the traditional diagnostic marker from diagnosis personnel according to the installation location of the part drawing, leading to modification after the rediscussion with other diagnosis personnel. In addition, the confined environments of EAST result in inconvenient installation. Therefore, the technology of scheme generation of an immersive component installation should be urgently pursued. Aimed at the actual requirements of physical experiments, the digital technology of component installation and design process based on multiuser and multidetail level, as well as the technology of virtual installation and cooperative discussion based on hierarchical collision detection are proposed. Taking the electromagnetic measurement diagnosis system as an example, the scheme generation system of immersive virtual EAST component installation is established to provide a reference for researchers to access the actual installation process, reduce the conflict of component positions, and improve the installation efficiency of the components. Preliminary realization and design of component installation scheme generation system are described.

Preliminary assessment of genotoxic effects induced by radiation from EAST using Vicia faba micronucleus assay

During long-pulse deuterium plasma operations in the Experimental Advanced Superconducting Tokamak (EAST), a mixed radiation field is generated, which is mainly composed of fusion neutrons, gamma rays, and x-rays. More accurate and effective dose monitoring methods have been developed and established to determine the ionizing radiation intensity both for the stable operation of the device and for the radiation safety of personnel. As far as we know, there are few reports about the biological effects of radiation induced by fusion neutrons and γ radiation, which are of vital importance for the assessment of radiation hazards presented by fusion devices, such as EAST, to human beings and the environment. In this study, three positions in the EAST hall were selected to detect genotoxic effects induced by nuclear fusion radiation using a Vicia faba micronucleus (MN) test for the first time. The doses of neutrons and gamma rays at these places were measured by thermoluminescence dosimeters four times between June 2019 and May 2020. The radiation doses decreased as the distances from the EAST device shell gradually increased from S1 to S3. The radiation in the EAST hall resulted in a significant induction of MN in the Vicia faba root tip cells compared to a negative control, which was different from the MN frequency induced by fission neutrons, γ-rays and other kinds of radiation in previous studies. These results indicate the existence of potential genotoxic effects induced by radiation from EAST which is different from other radiation and suggest that personnel should not be permitted to enter the experimental hall during the discharge process, and that radiation protection measures should be taken during necessary maintenance to avoid radiation damage. These newly acquired results will certainly increase our knowledge about the biological effects induced by radiation from nuclear fusion and provide good data support for developing more effective environmental and personnel fusion radiation protection.

Improvement of power quality in EAST power supply system based on hybrid active filter

Abstract During the operation of the nuclear fusion experiment device EAST (Experimental Advanced Superconducting Tokamak), low-frequency harmonic current with high amplitude was further resonantly amplified by the capacitive impedance in the poloidal field power supply system, which is endangering the reliable operation and power system of EAST device. A low-frequency harmonic resonance scheme is proposed in this paper to repress low-order harmonics generated by EAST poloidal field power supply. The low-frequency resonance suppressor (LFRS) based on hybrid active filter can effectively restrain the low-order harmonic current and its resonance amplification. The low-frequency harmonic suppression principle of LFRS is analyzed and the control equation of LFRS is established. Meanwhile, an active damping strategy is designed to control the resonance peak of the LCL filter and improve the overall stability of the LFRS. The results show the correctness of the theoretical analysis and provide a technical reference for suppression of low-frequency harmonics in other Tokamak devices.

Design of Central Controller for EAST ECRH System Based on FPGA

Electron cyclotron resonance heating is one of the important heating methods for controlled nuclear fusion reactions, which can make up for the shortcomings of the traditional heating methods. To this end, a 4MW long pulse electron cyclotron heating system was designed on the Experimental Advanced Superconducting Tokamak device. This paper mainly introduces the design of the gyrotron controller of ECRH system based on FPGA chip. The control logic of the central controller is based on state machine. Throughout the entire control process, each signal in the system is monitored in real time. Once an abnormality occurs, the main control logic will cut off the high voltage power of the gyrotron as required to protect the gyrotron from damage. In addition to the normal control logic, the FPGA also embeds abnormality diagnosis logic, which monitors the control state transitions of the main control logic and various input signals in real time to determine whether an abnormality has occurred in the system and can diagnose the cause of the abnormality. Timing simulation was done to the FPGA program. The result shows that the controller can respond to the total control trigger signal in real time, accurately control the switching of the cathode and anode high voltage power supply, diagnose and handle various abnormal conditions, and meet the control requirements of the ECRH system.

Fibre Bragg grating sensors for fusion diagnostics: Temperature monitoring of a tungsten mono-block mock-up under high heat flux

The optimum functioning of the divertor is very important for the realisation of the long-pulse H-mode discharge of a tokamak device. This necessitates the development of a component monitoring system for its operation status including temperature and strain. This study introduces a promising technique for monitoring temperature and strain sensor systems based on the fibre Bragg grating (FBG). It was used to monitor the temperature of a W/Cu mono-block mock-up during a high heat flux (HHF) test for the first time, demonstrating its suitability as a temperature monitoring system for the tungsten divertor. Temperatures up to 1000 °C were measured under HHF using five femtosecond FBG sensors inscribed in an optic fibre encapsulated in a stainless-steel capillary tube. The FBG sensors exhibit higher sensitivity to temperature variation while maintaining the same measurement accuracy relative to the five thermocouples (TCs) in symmetrical locations. An ANSYS fluent simulation based on the finite element method yielded an output consistent with the experimental results. The temperature distribution obtained through one channel of optical fibre sensors demonstrates the advantage of FBG sensors as a multipoint measurement method. These sensors are preferred to TCs because the complex installation process and utilisation of a mass of wires are eliminated. In view of the success of this work, the installation of an FBG-based temperature monitoring system as a novel diagnostic system is planned on the upgraded lower divertor of the Experimental Advanced Superconducting Tokamak device.

A large field of view visible cameras image acquisition and processing technology in EAST device

A large field of view visible image acquisition and processing system was designed to address the discharge visual needs for estimating the state of plasma operation and the first wall conditions during the plasma discharge of the Experimental Advanced Superconducting Tokamak (EAST) device. The entire system, which consisted of software and hardware, was introduced in detail. Moreover, three sets of the visible camera diagnostic systems with the different field-of-view images acquired by three industrial personal computers (IPCs) were used for forming the large field of view through image stitching. Three IPCs, and data storage and image processing servers are connected by a reflective memory network, which can realize the collection and fast transmission of image data during discharge. In addition, an image stitching algorithm based on block matching scale-invariant feature transformation was adopted. To accelerate the processing of the algorithm, parallelization was achieved by using the Graphics Processing Unit (GPU). Through the mesa test and experimental results, the proposed system provides a feasible way of transmitting and stitching images quickly and fulfills the EAST experimental demands.

Multiple striated heat fluxes patterns on the EAST first wall generated by lower hybrid wave absorption in the scrape-off layer

Multiple striated heat fluxes on the Experimental Advanced Superconducting Tokamak (EAST) first wall are observed with lower hybrid wave (LHW) heating on the EAST device. Our analysis indicates that the heat source for the striated heat flux is from LHW absorption in the scrape-off layer (SOL). Magnetic field line tracing from the front of the LHW antenna grills is in good agreement with the striated heat flux profile. In addition, the relative intensity of the LHW absorption in the SOL is consistent with the measured striated heat flux. Furthermore, the heat flux at inner strike points and outer strike points significantly decreased when the ion cyclotron resonance frequency (ICRF) wave was switched off, while there was no change for the striated heat flux, which suggests that the striated heat flux is independent of the injected power to the core plasma.

The Implementation of Data Acquisition System for EAST Technical Diagnostic System

EAST (Experimental Advanced Superconducting Tokamak) Technical Diagnostic System (TDS) is used to monitor the outlet temperature of all superconducting coils, in case of temperature anomaly, it will trigger safety interlock system to meet EAST device safety. The data acquisition system of TDS is in charge of continuous data acquisition of the nitrogen and helium temperature signals, TDS security alarm and long-term data storage. It supports continuous data acquisition and pulse data acquisition. The data acquisition of the nitrogen temperature signals is based on the PXI technology while obtaining the helium temperature signals from Lake Shore model 224 temperature monitors with VISA standard. After data conversion, all the data will be stored in MySQL and MDSPlus for long-term storage. It should output TDS fault signal and status signal to trigger the safety interlock system to take actions after threshold evaluation of key temperature signals. It publishes part of real-time TDS data to the cryogenic system and provides an information inquiry service to the TDS administrator. The system has been used in 2018 EAST campaign.

R&D of a strain monitoring system for the EAST tungsten divertor

The tungsten divertor of the Experimental Advanced Superconducting Tokamak (EAST) device operates at varying temperatures and strong electromagnetic fields during plasma operation. In order to monitor the thermomechanical and electromagnetic responses of the structural components, a strain sensor system has been proposed to be applied in the EAST device. To develop this technology, two initial experiments on the thermal strain measurement of a tungsten divertor module of the EAST have been carried out using electrical strain gages and optical strain sensors. However, some engineering issues were exposed during the experiments. Under this condition, all these issues are summarized and discussed this time. Moreover, a comparison between the two techniques has been made when applied in the tungsten divertor. To make them more suitable for the application in the tungsten divertor of the EAST, several activities including a thermomechanical simulation, the welding technologies for the sensors’ installation and a thermal stability test on the optical strain sensors were conducted. Based on these progresses, it is expected that the strain monitoring system will be established in the EAST device in the near future.

Advances in experimental research towards high confinement and steady state operation on the Experimental Advanced Superconducting Tokamak

The final goal of nuclear fusion science and technology research is to develop safe and clean fusion energy which can solve the human energy crisis ultimately. As construction started in the early of this century, International Thermonuclear Experimental Reactor (ITER) project has pushed the fusion energy development a big step forward toward commercial application. Those tokamaks which are ITER-like magnetic confinement fusion devices have focused their fusion energy research on high temperature plasma experiments at this stage. The Experimental Advanced Superconducting Tokamak (EAST) was designed and constructed independently by Institute of Plasma Physics, Chinese Academy of Sciences. EAST has implemented lots of exploration and research on the key engineering and physical problems for fusion reactor. Especially in the aspect of high confinement and steady-state operation, EAST has made major experimental research progress, which is important to the commercial power generation of fusion reactors economically and practically. EAST device has achieved the 400 s long pulse plasma in low confinement mode in 2012, then in 2017 EAST refreshed the world record of high confinement mode plasma discharge operation time, and became the first tokamak device that has the capability of high confinement and steady-state operation with 100 s. Moreover, for the first time, EAST touched the longest time scale in the magnetically confined fusion plasma—particle equilibration time. In order to provide a good experimental platform for investigation of the first wall with high heat load, edge localized modes suppression and engineering and physical problems research towards the steady-state operation and control for future fusion reactor, a series of experiments were implemented on EAST, including tungsten-copper water cooled divertor, lithium first wall, current driving with radio frequency wave, projectile injection, precise magnetic surface configuration control, edge localized modes and high heat load mitigation, and a lot of valuable results have been obtained.

Improved plasma position detection method in EAST Tokamak using fast CCD camera

To control the steady-state operation of Tokamak plasma, it is crucial to accurately obtain its shape and position. This paper presents a method for use in rapidly detecting plasma configuration during discharge of the Experimental Advanced Superconducting Tokamak device. First, a visible/infrared integrated endoscopy diagnostic system with a large field of view is introduced, and the PCO.edge5.5 camera in this system is used to acquire a plasma discharge image. Based on the analysis of various traditional edge detection algorithms, an improved wavelet edge detection algorithm is then introduced to identify the edge of the plasma. In this method, the local maximum of the modulus of wavelet transform is searched along four gradient directions, and the adaptive threshold is adopted. Finally, the detected boundary is fitted using the least square iterative method to accurately obtain the position of the plasma. Experimental results obtained using the EAST device show that the method presented in this paper can realize expected goals and produce ideal effects; this method thus has significant potential for application in further feedback control of plasma.

A Rapid Nondestructive Inspection Method Applied to EAST Lower Divertor by Infrared Thermography

Lower divertor, as the most critical part of experimental advanced superconducting tokamak (EAST), has functions of protecting the vacuum vessel, particles exhausting, and heat load handling. Doped graphite with SiC coating is adopted as plasma-facing material, which is bolted to active water-cooling heat sink by an M8 screw with a soft graphite sheet inserted between graphite tile and heat sink to improve the capacity of heat transmission currently. In order to guarantee excellent performance of lower divertor, every striking target, which contains 27th graphite tiles, must be inspected for its assembly quality before being installed in EAST device. This paper introduces a rapid nondestructive inspection method for EAST lower divertor by infrared (IR) thermography. The poor assembly quality, such as unevenness of contact surface of graphite tile or heat sink, overlap of the soft graphite sheet, and unqualified values of torque, can be detected qualitatively by comparison of the surface temperature evolution of graphite tiles in symmetric positions of the same divertor target when water flowing in the tube welded into heat sink is changed from hot to cold. The 3-D thermal finite-element analyses are performed to simulate the case of target with excellent assembly quality and defect-free tiles and as a reference for the experimental observations obtained by IR thermography eventually.

Preliminary realization of immersive EAST system using virtual reality

Aiming at the complex and confined environments of experimental advanced superconducting tokamak (EAST) and the need to improve the ability of resident and visiting scientists to access and understand EAST, the immersive EAST system based on virtual reality is developed. Compared to previous virtual EAST systems, model reality, rendering speed, immersion and interaction functions are greatly increased. In the previous system, models were transformed from CATIA and imported to 3DS Max for subsequent processing. While those models have the advantages of being very precise and easily generated, they come at the price of decreased rendering speed and poor rendering quality because of a large loss of triangle faces. In this regard, EAST models are rebuilt and materials created by photos of EAST device are assigned to models. Moreover, because virtual reality is quite effective at creating experiment scenarios thanks to their ability to provide the feeling of immersive operation and collaborative environments, model libraries are built during this research for future scheme generation and hardware installation training. The immersive EAST system is developed in the Unity3D environment using a Client/Server architecture, the HTC Vive and handheld devices are used to provide the immersive scene, intuitive controls and realistic haptic feedback. Users can roam in the virtual scene and interact with the immersive EAST using the handheld devices and communicate with other users in the system. The establishment of the system provides the framework for a comprehensive and cooperative experiment and training environment for EAST.

Improvement of virtual EAST system based on WebVR

As the EAST (Experimental Advanced Superconducting Tokamak) experimental data is analyzed by more and more collaborators, the former virtual EAST system based on WebGL needs to be improved to be more convenient for sharing data and communicating between collaborators. The improved virtual EAST system is established as an open web-portal without plug-ins or third party components by using Three.js. This portal offers a more highly-interactive interface for scientists and collaborators to roam inside the EAST device, view the complex structure, and acquire diagnostic metadata via a user-friendly control panel. In order to enhance the system's interactivity and user's immersion within the 3D virtual scene, WebVR technology which provides support for exposing virtual reality devices is used in this new system. Moreover, the EAST wiki system and web model editor are integrated into the new system to collect information and store the EAST models. The wiki system displays the information of the EAST device and diagnostic systems, which provides collaborative modifications and content update. The web model editor offers an open interface for users to edit and upload models through a web browser. Compared with the former system, the improved system adds more models and experimental data visualization. This paper presents the implementation details for optimizing models and improving performance.

Upgraded flowing liquid lithium limiter for improving Li coverage uniformity and erosion resistance in EAST device.

We report on design and technology improvements for a flowing liquid lithium (FLiLi) limiter inserted into auxiliary heated discharges in the experimental advanced superconducting tokamak device. In order to enhance Li coverage uniformity and erosion resistance, a new liquid Li distributor with homogenous channels was implemented. In addition, two independent electromagnetic pumps and a new horizontal capillary structure contributed to an improvement in the observed Li flow uniformity (from 30% in the previous FLiLi design to >80% in this FLiLi design). To improve limiter surface erosion resistance, hot isostatic press technology was applied, which improved the thermal contact between thin stainless steel protective layers covering the Cu heat sink. The thickness of the stainless steel layer was increased from 0.1 mm to 0.5 mm, which also helped macroscopic erosion resilience. Despite the high auxiliary heating power up to 4.5 MW, no Li bursts were recorded from FLiLi, underscoring the improved performance of this new design.

RAMI analysis for EAST upper full tungsten divertor water cooling system

EAST (Experimental Advanced Superconducting Tokamak) device is a D-shaped full superconducting tokamak with actively water cooled plasma facing components. As the EAST project technical risk control and possible failure mitigation approach, RAMI (Reliability, Availability, Maintainability and Inspectability) analysis was required for the upper full tungsten divertor water cooling system, which insures the system to cool the PFCs with high efficient and safety without leakage. At the same time the cooling water could be at suitable pressure, temperature and velocity, water parameters of cooling circuit also could be monitored. A functional breakdown was prepared and all functions are described using the IDEFØ (Integration DEfinition Function – language Ø) method. FMECA (Failure Mode, Effects and Criticality Analysis) was performed to the system initial risk. There are 31 risks for the initial state, including 7 major risks. No major risk remains after taking into account all the actions. RBDs (Reliability Block Diagrams) were prepared to calculate the reliability and availability of each function. The initial availability of upper divertor cooling water system was 93.63%, after optimization the expected availability was 98.56% over 11,520h. It was assessed that the upper full tungsten divertor water cooling system RAMI analysis results meet the EAST project requirement during this design phase and the results will be qualified further when the system design is updated.

Plasma image edge detection based on the visible camera in the EAST device.

The controlling of plasma shape and position are essential to the success of Tokamak discharge. A real-time image acquisition system was designed to obtain plasma radiation image during the discharge processes in the Experimental Advanced Superconducting Tokamak (EAST) device. The hardware structure and software design of this visible camera system are introduced in detail. According to the general structure of EAST and the layout of the observation window, spatial location of the discharging plasma in the image was measured. An improved Sobel edge detection algorithm using iterative threshold was proposed to detect plasma boundary. EAST discharge results show that the proposed method acquired plasma position and boundary with high accuracy, which is of great significance for better plasma control.

Quasi-optical design for systems to diagnose the electron temperature and density fluctuations on EAST.

A system to simultaneously diagnose the electron temperature and density fluctuations is proposed for Experimental Advanced Superconducting Tokamak device. This system includes a common quasi-optical antenna, a correlation electron cyclotron emission (CECE) system that is used to measure the electron temperature fluctuations and a Doppler backscattering (DBS) system that is used to measure the electron density fluctuations. The frequency range of the proposed CECE system is 108-120 GHz, and this corresponds to a radial coverage of normalized radius ((R - R0)/a, R0 = 1850 mm, a = 450 mm) from 0.2 to 0.67 for the plasma operation with a toroidal magnetic field of 2.26 T. This paper focuses on the design of the quasi-optical antenna and aims at optimizing the poloidal resolution for different frequency bands. An optimum result gives the beam radius for the CECE system of 13-15 mm and this corresponds to a wave number range of kθ < 2.4 cm-1. The beam radius is 20-30 mm for V band (50-75 GHz) and 15-20 mm for W band (75-110 GHz).