Experimental Advanced Superconducting Tokamak Tokamak Research Articles

Comparison of transport coefficients before and after density pump-out induced by resonant magnetic perturbation using a BOUT++ six-field model on the EAST tokamak

Many experiments have demonstrated that resonant magnetic perturbation (RMP) can affect the turbulent transport at the edge of the tokamak. Through the Experimental Advanced Superconducting Tokamak (EAST) density modulation experiment, the particle transport coefficients were calculated using the experimental data, and the result shows that the particle transport coefficients increase with RMP. In this study, the six-field two-fluid model in BOUT++ is used to simulate the transport before and after density pump-out induced by RMP, respectively referred as the case without RMP and the case with RMP. In the linear simulations, the instabilities generally decreases for cases with RMP. In the nonlinear simulation, ELM only appears in the case without RMP. Additionally, the particle transport coefficient was analyzed, and the result shows that the particle transport coefficient becomes larger for the case with RMP, which is consistent with the experimental conclusion. Moreover, its magnitude is comparable to the results calculated from experimental data.

High-frequency fluctuation and EHO-like mode in the H-mode pedestal on the EAST tokamak

In the pedestal region of the Experimental Advanced Superconducting Tokamak (EAST) during high confinement mode plasma operations with radio-frequency heating, two distinct fluctuations are observed: high-frequency fluctuations (HFFs) and edge harmonic oscillation-like (EHO-like) modes. The HFFs are characterized by intermittent fluctuations with a broadband frequency range of 1−3 MHz and a poloidal wave number ( kθ ) greater than 0.9 cm−1 . On the other hand, the EHO-like mode exhibits characteristics similar to magnetohydrodynamics (MHD)-like modes with n = 1−5 and lower poloidal wave numbers ( kθ⩽0.12 cm−1 ). During the pedestal establishing phase following the L–H transition, a significant concurrent presence of HFF and EHO-like modes in high-density pedestal regions has been noted. In this phase, the EHO-like mode not only modulates the amplitude of the HFF but also engages in nonlinear interactions. The occurrence of EHO-like mode and HFF is associated with particle transport toward the divertor, though it is notably less than that caused by edge coherent modes. During the inter-edge localized mode (ELM) period, a significant decrease in the Dα baseline is observed whenever the low frequency fluctuation (LFF) weakens and the HFF grows, prior to each large ELM. One possible explanation is that the rapid increase of E×B shear stabilizes the LFF and destabilizes the HFF, which lowers the pedestal transport and enables the further growth of the pedestal until the onset of the ELM.

First results of CO2 dispersion interferometer on EAST tokamak

A dispersion interferometer (DI) has been installed and operates on the Experimental Advanced Superconducting Tokamak (EAST). This DI system utilizes a continuous-wave 9.3 μm CO2 laser source to measure line-averaged electron densities accurately. In contrast to conventional interferometers, the DI does not require substantial vibration isolations or compensating systems to reduce the impact of vibrations in the optical path. It also employs a ratio of modulation amplitudes, ensuring it remains immune to the variations in detected intensities. Without a variation compensation system, the DI system on EAST reaches a density resolution of less than 1.8 × 10−2π rad and a temporal resolution of 20 μs. The measurements made by the POlarimeter-INTerferometer (POINT) system and the far-infrared hydrogen cyanide (HCN) interferometer are remarkably consistent with the DI’s results. The possibility of fringe jumps and the impact of refraction in high-density discharge can be significantly decreased using a shorter wavelength laser source. A rapid density change of 3 × 1019 m−3 during 0.15 s has been measured accurately in shot No. 114755 of EAST. Additionally, the DI system demonstrates dependability and stability under 305 s long-pulse discharges in shot No. 122054.

Observation of Doppler shift modulated by the internal kink mode using conventional reflectometry in the EAST tokamak

In this paper we present a new experimental observation using a conventional reflectometry technique, poloidal correlation reflectometry (PCR), in the Experimental Advanced Superconducting Tokamak (EAST). The turbulence spectrum detected by the PCR system exhibits an asymmetry and induced Doppler shift during the internal kink mode (IKM) rotation phase. This Doppler shift is the target measurement of Doppler reflectometry, but captured by conventional reflectometry. Results show that the Doppler shift is modulated by the periodic changes in the effective angle between the probing wave and cutoff layer normal, but not by plasma turbulence. The fishbone mode and saturated long-lived mode are typical IKMs, and this modulation phenomenon is observed in both cases. Moreover, the value of the Doppler shift is positively correlated with the amplitude of the IKM, even when the latter is small. However, the positive and negative frequency components of the Doppler shift can be asymmetric, which is related to the plasma configuration. A simulated analysis is performed by ray tracing to verify these observations. These results establish a clear link between and IKM rotation, and are helpful for studying the characteristics of IKM and related physical phenomena.

Line identification of extreme ultraviolet spectra from aluminum ions in EAST Tokamak plasmas

Extreme ultraviolet (EUV) spectra emitted from aluminum in the 5-340 Å wavelength range were observed in Experimental Advanced Superconducting Tokamak (EAST) discharges. Several spectral lines from aluminum ions with different degrees of ionization were successfully observed with sufficient spectral intensities and resolutions using three fast-time-response EUV spectrometers. The line identification uses three independent state-of-art computational codes for the atomic structure calculations, which provide the wavelengths and the collisional and radiative transition rate coefficients. These programs are hullac (Hebrew University—Lawrence Livermore Atomic Code), autostructure, and fac (Flexible Atomic Code). Using three different codes allows us to resolve some ambiguities in identifying certain spectral lines and assess the validity of the theoretical predictions.

Characterization of impurity distribution and composition on the shutter plate for an optical diagnosis in EAST tokamak using laser-induced breakdown spectroscopy

The quantification of impurities in tokamak devices is of great significance as it allows for a better understanding of the plasma performance and the lifetime of plasma-facing components (PFCs). This study focuses on the characterization of the impurity distribution and composition on the surface of a shutter plate for an optical diagnosis in the Experimental Advanced Superconducting Tokamak (EAST) by Laser-Induced Breakdown Spectroscopy (LIBS). The primary impurity elements, including copper (Cu), tungsten (W), molybdenum (Mo), lithium (Li), and Sodium(Na), were identified by analyzing the LIBS spectra obtained from the surface of the shutter plate. The spatial distribution of these impurities on both sides of the plate was investigated. The analysis results indicate an increase in impurity content on the outer surface of the shutter plate as proximity to Last Closed Flux Surface (LCFS), followed by a near-linear decrease with the distance from the LCFS. The results of the inner ring on the inner side facing the glass window show fewer metal impurities than the outer rings on the edge for the inner side of the plate. Moreover, the thickness of the deposition layer was determined by estimating the Average Ablation Rate (AAR) through the measurement by confocal microscopy. In addition, Calibration-Free LIBS method were performed to determine the relative content of impurities. The results of this study provide valuable references for studying the migration and transportation of impurities.

Optimization of power feedback control system for HCN interferometer on EAST Tokamak

The Hydrogen Cyanide (HCN) interferometer stands as an indispensable diagnostic tool, designated to measure the line-integrated electron density on the Experimental Advanced Superconducting Tokamak (EAST), thereby offering essential density feedback signals for EAST operation. The HCN laser used in the interferometer is a continuous glow discharge gas laser. However, owing to variations in external ambient temperature, the HCN laser exhibits output power instability. The current power automatic feedback control system for the HCN laser interferometer exhibits low actuator adjustment precision, thereby limiting its ability to achieve maximum output power. Moreover, the system may cause a temporary power drop to zero during adjustment, while EAST operation requires the HCN laser interferometer to maintain high power continuously. In order to solve the issues, this paper developed an innovative actuator that ingeniously amalgamates the virtues of piezoelectric ceramic and stepper motor. This novel actuator, demonstrating a broad adjustment range and high precision, is intended to replace the present one in the HCN interferometer power control system. While the stepper motor executes coarse adjustment, the piezoelectric ceramic enables precise adjustment, thus enhancing the adjustment accuracy of the automatic power control system to a sub-micrometer level and an adjustment range greater than 5 mm. Accurate determination of the regulator's adjustment direction can significantly improve the efficiency of adjustment. The slope of the laser output power peak decreases as it approaches the maximum value, and increases as it moves away from the maximum value. Therefore, this system replaces the original threshold algorithm with a slope algorithm. By correctly judging the adjustment direction, the efficiency of adjustment is improved. Furthermore, this algorithm can keep the output power stable on a power peak. There will be no instances of zero power output during the adjustment, allowing the power to remain stable above the set threshold for a long time. Currently, this system has been successfully applied to the HCN interferometer, and testing has found that the system can maintain stability for at least 16 hours. Furthermore, even when the laser output power was intentionally reduced to below the set threshold, the system was able to respond quickly and adjust the output, rapidly restoring it to above the predefined threshold. Multiple tests have shown the strong robustness of the system, indicating it meets the stringent demands of the EAST's complex environment.

The design and test of the electrical insulation break for the EAST Divertor primary heat transfer system

Experimental Advanced Superconducting Tokamak (EAST) is one of the important steady-state divertor Tokamak physical experiment platforms in the world. The main functions of the high-temperature and high-pressure Divertor Primary Heat Transfer System (DIV PHTS) are to provide high-pressure cooling water for the divertor to remove the heat load deposited on the divertor under the plasma operation mode and to provide high-temperature nitrogen gas for the divertor system to achieve vacuum condition during the baking mode. The stability and reliable operation of the DIV PHTS play an important role in the EAST tokamak plasma physics experiment. The high-temperature and high-voltage electrical insulation break are one of the important components of the system. The main function of the Electrical Insulation Break (EIB) is to break the closed circuit of the heat transfer system to avoid eddy current during plasma discharge affecting the magnetic field distribution characteristics of the tokamak. On the other way, it also needs to meet the requirement, there is no leak of the cooling water and nitrogen gas during cooling and baking operation mode. This paper proposes the design of an electrical insulating break (EIB) to meet the design requirements of EAST tokamak at first. Then the electric field analysis is carried out by using a finite element model to evaluate its electrical insulation characteristics. And the sealing structural characteristics are verified by applying the finite element method under two design modes. Finally, the sealing test result under the high-temperature baking and high-pressure cooling modes are presented to validate the design.

Experimental study of core MHD events in thousand-second improved confinement plasma on the EAST tokamak

Recently, stationary plasma with a world-record pulse length of 1056 s was achieved on the Experimental Advanced Superconducting Tokamak (EAST). In this work, the core magnetohydrodynamics (MHD) events as well as mode coupling processes have been investigated in EAST long pulse operation with electron heating dominant and pure radio frequency wave heating low collisionality, by using several diagnostics and the nonlinear numerical code M3D together. A saturated m/n = 1/1 kink mode was observed in the core region, where a stable internal transport barrier was found in the electron temperature channel. The frequencies and 2D structures of these modes were studied by a combination of soft X-ray (SXR) imaging and electron cyclotron emission diagnostics. The frequency of the m/n = 1/1 mode exhibited a feature of chirping down with time, and this chirping rate corresponds to the rate of electron diamagnetic drift frequency change. A twisted pattern (‘Taichi structure in shape’) was reconstructed by SXR tomography of the m/n = 1/1 mode. The perturbations of electron temperature and density caused by m/n = 1/1 are different in size, the latter one being much smaller. The destabilization of m/n = 1/1 was due to the strong central heating combined with electron cyclotron resonance heating and lower hybrid current drive. In the presence of the m/n = 1/1 mode, a negative current was generated on the magnetic axis, which anomalously broadened the core current profile. A m/n = 3/2 tearing mode triggered by m/n = 1/1 mode was also observed. The m/n = 3/2 mode has a smaller frequency than the m/n = 1/1 mode, and carries an m/n = 3/2 island with detectable size. A novel 3D MHD model that separately evolves the plasma density and temperature is applied to the m/n = 3/2 triggered by m/n = 1/1. It is found that a toroidal current density at the q = 1.5 surface, caused by non-axisymmetric density perturbation during the m/n = 1/1 nonlinear growth phase, was generated and hence the destabilization of the m/n = 3/2 tearing mode. Both the modeled electron temperature and density perturbations agree well with experimental observations. In the last section of this paper, the interaction between the m/n = 1/1 mode and fast electrons as well as the active control of this mode are also presented.

Energy distribution of lost high-energy runaway electrons based on their bremsstrahlung emission in the EAST tokamak.

We report in this paper the study towards revealing the energy distribution of lost high-energy runaway electrons based on their bremsstrahlung emission. The high-energy hard x-rays are originated from the bremsstrahlung emission of lost runaway electrons in the experimental advanced superconducting tokamak (EAST) tokamak, and the energy spectra are measured using a gamma spectrometer. The energy distribution of the runaway electrons is reconstructed from this hard x-ray energy spectrum using a deconvolution algorithm. The results indicate that the energy distribution of the lost high-energy runaway electrons can be obtained with the deconvolution approach. In the specific case in this paper, the runaway electron energy was peaked around 8 MeV, covering from 6 MeV to 14 MeV.

Long pulse operation of neutral beam injector on EAST tokamak

The Experimental Advanced Superconducting Tokamak (EAST) is one of the fully superconducting tokamak. A powerful neutral beam injection system, which consists of two injectors (EAST-NBI), was employed on EAST for plasma heating and current driving. The designed parameters for each injector are as follows: deuterium beam energy is 50–80 keV, beam power is 2–4MW and the beam duration is 10–100 s with modulation.In order to support the planned physics research on EAST, the NBI system was expected to work with long pulse on DC mode. So, the NBI system was optimized, including the cooling channels optimization of high power ion source and high heating flux parts, and the feedback control for stable beam extraction. The characteristics of the NBI system after optimization was studied in the lasted EAST campaign. The NBI system can work well with arc power feedback regulation and the temperature rise of high heating flux parts was decreased after upgrade, hence it can be operated with 100 s. Long pulse operation with 100 s will be tested in the next campaign of EAST experiment.

The internal transport barrier formation on EAST tokamak during the fishbone instability

The internal transport barrier (ITB) which is related to the fishbone instability has been observed on the Experimental Advanced Superconducting Tokamak (EAST) in ELMy H-mode discharges. An interpretation of the formation of the ITB on EAST tokamak is provided, based on both analytical and numerical calculations. The fishbone instability induces the redistribution of fast ions and leads to the accumulation of fast ions in a local region where the ITB is going to appear. Correspondingly, the gradients of fast ions are enhanced, where the ion temperature gradient (ITG) mode exists. Fast ions can interact with the ITG mode through the dilution, Shafranov shift and wave-particle resonance mechanisms. It is found that the ITG mode is stabilized by fast ions and the stabilizing effects are determined mainly by the density, temperature and their gradients of fast ions. The enhanced density and temperature gradients of fast ions lead to a stronger stabilizing effect on ITG mode. Compared with the stabilizing effect before the appearance of fishbone instability, the stabilization on ITG mode is enhanced after the fishbone instability, which is beneficial to the formation of the ITB.

Observation of tearing mode triggering by sawtooth crash with high-Z impurity accumulation in EAST

The high-Z impurity accumulation in the plasma core could degrade the plasma performance and may affect sawtooth crash and trigger tearing modes (TMs) in experimental advanced superconducting tokamak (EAST). Triggering of m/n = 3/2 TM (m and n are the poloidal and toroidal mode numbers, respectively) and m/n = 2/1 TM by a large sawtooth crash is observed and studied under the condition of high-Z impurity accumulation in core plasma in EAST tokamak. This work mainly focuses on the relationship among impurity, sawtooth crashes, and TMs. When there is a certain amount of impurity in the plasma core, sawtooth crash leads to impurity transport, resulting the changes in impurity concentration, and by inducing the density perturbation near the rational surface, TMs become destabilized. The experimental analyses indicate that the outward impurity transport after a sawtooth crash plays an important role in the destabilization of TMs.

Optical plasma boundary detection and its reconstruction on EAST tokamak

Plasma boundary detection and reconstruction are important not only for plasma operation but also for plasma facing materials. Traditional methods, for example, EFIT code, which is constrained by electromagnetic measurement, and is very challenging for detecting the plasma boundary in long-pulse burning plasma devices such as ITER. A novel algorithm for the reconstruction of the plasma boundary using one visible camera has been developed on experimental advanced superconducting tokamak (EAST) for fusion reactors. A U-Net convolutional neural network was used to identify the plasma boundary and the pixel coordinates of the boundary points were fitted with EFIT via the XGBoost model. This algorithm can transform the boundary from the image plane to the poloidal plane of the Tokamak based on machine learning without traditional spatial calibration, and then the reconstruction of the plasma configuration shall be realized based on a monocular visible light camera. The reconstruction accuracy of this algorithm is relatively high. The average error on the test set was only 7.36 mm (<1 cm) and satisfied the accuracy requirements of control for EAST tokamak. This result can contribute to the development of the plasma boundary reconstruction and operation based on one visible camera.

The design and construction of divertor primary heat transfer system for EAST Tokamak

The Experimental Advanced Superconducting Tokamak (EAST) had been under major upgrading from 2020 to 2021 in China. In this campaign of major upgrading, the new lower tungsten divertor is successfully installed to achieve the targets of long pulse and high β H-mode plasma operation. High temperature and high pressure Divertor primary heat transfer system (DIV PHTS) is also developed to remove the plasma deposition heat load on the Divertor. This paper presents a description of the design and construction of DIV PHTS for EAST Tokamak. Starting from the requirements specified in the Technical Specifications of DIV PHTS, the process to meet the Divertor cooling and baking requirement is designed and validated by the supported thermal-hydraulic analysis. Then, the piping stress analysis under different types of load combinations is performed to verify the structural design requirements. General layout design is also performed based on the component requirement and the allocated space in the EAST Tokamak hall. Finally, the procurement and test of the system main equipment is also given.

Structural design and analysis of EAST TCWS pipeline

EAST (Experimental Advanced Superconducting Tokamak) Tokamak Cooling Water System (TCWS) provides cooling and baking functions to the in-vessel component and vacuum vessel of the tokamak. A section of high stiffness pipes in the system is difficult to compensate for the reaction from thermal expansion caused by high temperature nitrogen baking operation. Under seismic loads, additional acceleration may have serious impact on piping network and produce more complex reactions to equipment.This paper describes EAST Tokamak Cooling Water System (TCWS) system layout design, piping support design, methodology and results of pipe stress analysis. The sustained and occasional load responses of piping network are analyzed, natural frequencies of the Baking system pipes are calculated. The calculation results are verified by the experimental data pipeline displacement at 200°C. The process piping code ASME B31.3 is used for pipe stress analysis. The least available stress margin is 7% and the calculated stresses are in acceptable limit. The maximum displacement of the pipeline is within the requirements of the spatial layout. The results would be used in further loop optimization and analysis of fire case.

Software platform for imaging diagnostic exploitation applied to edge plasma physics and real-time PFC monitoring

In current tokamaks, imaging diagnostics (visible/infrared) have become increasingly used for real-time control of the temperature of the Plasma Facing Components (PFC) and for off-line physical analysis. Developing a full acquisition and control system based on a network of multiple cameras is a complex task requiring advanced software tools. Furthermore, such systems produce a large quantity of data encouraging end users to share tools and codes for data access and analysis.In this paper, we introduce a software platform, named Thermavip, dedicated to these tasks. The main contribution of this software is to gather under the same framework different functionalities for (1) firm real-time video acquisition and PFC monitoring, (2) offline data access and display, (3) signal and video processing applied to plasma physics, (4) video annotation and thermal events database management. This software platform is widely used on the WEST (W Environment in Steady-state Tokamak) tokamak (France) for offline study of infrared (IR) and almost all WEST diagnostics data, and online display of IR videos and temperature time traces for critical components. Thermavip platform is also used on the HADES (High heAt LoaD tESt) facility (France) and the EAST (Experimental Advanced Superconducting Tokamak) tokamak (China) for offline signal analysis, and on the Wendelstein 7-X (W7-X) stellarator (Germany) for the IR diagnostic acquisition, PFC real-time (RT) monitoring, online display and offline video analysis. In addition, Thermavip provides a full featured video annotation tool used on both WEST and W7-X to build and manage thermal events datasets. These annotations are used as input dataset to train deep-learning models, like Region Based Convolutional Neural Networks, for automatic detection and classification of thermal events based on IR movies.The Thermavip framework is composed of a unique C++ Software Development Kit (SDK) providing high-level classes for offline multi-sensor data analysis, firm real-time processing and online visualization of these sensor data. Its strength comes from its unique component block architecture, allowing to build multicore and distributed processing pipelines for both offline and firm real-time applications. Thermavip architecture relies on a versatile plugin mechanism to extend its functionalities using C++ code or Python scripts, facilitating the integration on new machines.

Current status of ECE system on EAST tokamak

The electron cyclotron emission (ECE) diagnostic on the experimental advanced superconducting tokamak (EAST) has had a major upgrade since 2020, when EAST heating system also went through a significant upgrade, including one NBI system changed from counter-current to co-current (moving from port F to port D), and the antenna and the installation port of LHW and ICRF system have also been changed. The quasi-optical (QO) antenna of P port ECE system has been redesigned, the main purpose of which is to add one oblique ECE view. The angle with respect to perpendicular to the magnetic field is about 10°, which will facillitate measurement of the electron velocity distribution altered by LHW system. The ellipsoidal mirror has also been moved close to the plasma, about 70 cm away from the plasma center, and the poloidal beam waist radius in the plasma has been optimised to be less than 3 cm. The CECE system has also been moved from port G to port C. The frequency coverage of the CECE system has been upgraded to 104-132 GHz by adding one radio frequency (RF) module. Also in the intermediate frequency (IF) module, 8 narrow-band filters have been added to improve the spacial coverage of the system. On port F, a new superheterodyne radiometer with narrow-band filters in IF module has been installed. It consists of eight channels, the radial coverage is about 8 cm, the main purpose of this new system is to study the fine structure of magnetic island.

A Robust and Fast Data Management System for Machine-Learning Research of Tokamaks

In recent years, machine-learning (ML) research methods have received increasing attention in the tokamak community. The conventional database (i.e., MDSplus for tokamak) of experimental data has been designed for small group consumption and is mainly aimed at simultaneous visualization of a small amount of data. The ML data access patterns fundamentally differ from traditional data access patterns. The typical MDSplus database is increasingly showing its limitations. We developed a new data management system suitable for tokamak ML research based on experimental advanced superconducting tokamak (EAST) data. The data management system is based on MongoDB and hierarchical data format version 5 (HDF5). Currently, the entire data management has more than 3000 channels of data. The system can provide highly reliable concurrent access. The system includes error correction, MDSplus original data conversion, and high-performance sequence data output. Furthermore, some valuable functions are implemented to accelerate ML model training of fusion, such as a bucketing generator, the concatenating buffer, and distributed sequence generation. This data management system is more suitable for fusion ML model research and development than MDSplus, but it cannot replace the MDSplus database. The MDSplus database is still the backend for EAST tokamak data acquisition and storage.

Calorimetry system for heat load in long-pulse discharges on EAST tokamak

A calorimetry system has been developed in Experimental Advanced Superconducting Tokamak (EAST). It is employed to evaluate the heat load exhausted by cooling water. Full poloidal coverage is achieved after the upgrade in 2019. The experimental results show that the heat load exhausted by cooling water is 64%–86% of the injected energy. Moreover, the heat load increases faster in the case with a higher heating power, and this is qualitatively confirmed by the simulation. In addition, a longer time is required for the temperature of the cooling water to achieve its maximum for a higher heating power. And in the decay phase of the water temperature, it shows an exponential characteristic. The averaged time constant of the thermal decay obtained by exponential fitting for the upper and the lower divertor are 12.1 s and 107.9 s, respectively. It indicates that the heat removal capability of the upper tungsten divertor outperforms the lower graphite divertor.