Zhoujun Yang - Academia.edu (original) (raw)
Papers by Zhoujun Yang
Plasma Physics and Controlled Fusion
The coupling of multiple MHD modes can lead to mode locking and major disruption in tokamak plasm... more The coupling of multiple MHD modes can lead to mode locking and major disruption in tokamak plasmas. In the J-TEXT tokamak, the coupling between two small modes, i.e., m/n = 2/1 and 3/1 modes (m and n are poloidal and toroidal mode numbers, respectively), appears when the edge safety factor is reduced to the vicinity of 3. After the mode coupling, the toroidal phase difference between the 2/1 and 3/1 modes equals 0 in the low field side midplane. This phase relation of coupled modes leads to mutual destabilization and even major disruption. A control scheme to avoid disruption caused by coupled modes by resonant magnetic perturbations (RMP) is presented. It is found that the application of RMP significantly changes the evolution of the coupled modes. The coupling of the 2/1 and 3/1 modes occurs earlier as the RMP amplitude increases. The RMP with moderate amplitude can suppress the growth of 2/1 and 3/1 coupled modes and hence avoid disruption. These results provide a possible strat...
Plasma Science and Technology
To extend the operation region of the Joint-Texas Experimental tokamak (J-TEXT) to the divertor c... more To extend the operation region of the Joint-Texas Experimental tokamak (J-TEXT) to the divertor configuration and even H-mode, the divertor configuration discharge has been realized for the first time in J-TEXT tokamak. Along with establishing a power supply for divertor configuration, the construction of relevant diagnostics, and the installation of the divertor target in the high-field side, the divertor discharge has been tested. And through the equilibrium calculation and position stability analysis, the control strategy has been evolved to be more stable. High-density experiments and auxiliary heating experiments have been carried out in the divertor configuration. The special midplane single-null (MSN) divertor configuration exhibits to be more stable than the limiter configuration in the density limit condition and can reach a higher density in the experiment. In the ECRH experiment, the power injection enhances the electron temperature and density, while more heat outflux is...
Plasma Science and Technology
Machine learning research and applications in fusion plasma experiments are one of the main subje... more Machine learning research and applications in fusion plasma experiments are one of the main subjects on J-TEXT. Since 2013, various kinds of traditional machine learning, as well as deep learning methods have been applied to fusion plasma experiments. Further applications in the real-time experimental environment have proved the feasibility and effectiveness of the methods. For disruption prediction, we started by predicting disruptions of limited classes with a short warning time that could not meet the requirements of the mitigation system. After years of study, nowadays disruption prediction methods on J-TEXT are able to predict all kinds of disruptions with a high success rate and long enough warning time. Furthermore, cross-device disruption prediction methods have obtained promising results. Interpretable analysis of the models are studied. For diagnostics data processing, efforts have been made to reduce manual work in processing and to increase the robustness of the diagnost...
Cornell University - arXiv, Aug 19, 2022
Predicting disruptions across different tokamaks is a great obstacle to overcome. Future tokamaks... more Predicting disruptions across different tokamaks is a great obstacle to overcome. Future tokamaks can hardly tolerate disruptions at high performance discharge. Few disruption discharges at high performance can hardly compose an abundant training set, which makes it difficult for current data-driven methods to obtain an acceptable result. A machine learning method capable of transferring a disruption prediction model trained on one tokamak to another is required to solve the problem. The key is a disruption prediction model containing a feature extractor that is able to extract common disruption precursor traces in tokamak diagnostic data, and a transferable disruption classifier. Based on the concerns above, the paper first presents a deep fusion feature extractor designed specifically for extracting disruption precursor features from common diagnostics on tokamaks according to currently known precursors of disruption, providing a promising foundation for transferable models. The fusion feature extractor is proved by comparing with manual feature extraction on J-TEXT. Based on the feature extractor trained on J-TEXT, the disruption prediction model was transferred to EAST data with mere 20 discharges from EAST experiment. The performance is comparable with a model trained with 1896 discharges from EAST. From the comparison among other model training scenarios, transfer learning showed its potential in predicting disruptions across different tokamaks.
Plasma Science and Technology
The detailed density perturbations provided by the advanced polarimeter–interferometer system (Po... more The detailed density perturbations provided by the advanced polarimeter–interferometer system (Polaris) during sawtooth collapse on the Joint Texas Experimental Tokamak (J-TEXT) are reported in this article. During a sawtooth collapse and the crash of plasma pressure at the center, it is interestingly found that the increase in density in the region between the inversion radius and mixing radius is poloidally asymmetrical, while the increase in temperature are poloidally symmetrical. The poloidal location where the density increases is dependent on the phase of the precursory m/n=1/1 kink mode. It is always out of phase with the hot-core of the m/n=1/1 mode. The behaviors of density perturbations during sawtooth collapse observed in J-TEXT are beyond the expectations of the standard model, which can shed new light on the understanding of sawtooth collapse.
Plasma Science and Technology
Pellet injection is an attractive technology for core-fueling and magnetohydrodynamic study in ma... more Pellet injection is an attractive technology for core-fueling and magnetohydrodynamic study in magnetic-confinement fusion devices like tokamaks and stellarators. It can inject solid hydrogen/deuterium pellets into the plasma with deeper density deposition compared with other fueling methods, such as gas puffing. A three-barrel H2 pellet injection system was installed on the J-TEXT tokamak and experiments were carried out. The pellets are formed in three barrels cooled by a cryocooler and compressor system at around 9 K, and are 0.8 mm/1 mm diameter and 0.8 mm length. The pellet is launched by helium propellant gas and injected from the low-field side of the plasma. The normal range of pellet speed is 210–310 m s−1 for different propellant gas pressures. Due to the three-barrel structure, the number of injected pellets can be adjusted between one and three. Pellets can be launched sequentially with arbitrary time intervals, which enables flexible applications. The results of the exp...
Fusion Engineering and Design, 2021
A tangential X-ray imaging crystal spectrometer (XICS) has been upgraded in the J-TEXT tokamak. T... more A tangential X-ray imaging crystal spectrometer (XICS) has been upgraded in the J-TEXT tokamak. The XICS system has been used to measure ion and electron temperature, plasma rotation velocities, selected impurity densities, and can be used for impurity transport. The XICS on J-TEXT was designed to receive Ar XVII emissions from the-0.5a to +0.5a region with a spatial resolution of 1.8 cm in the vertical direction, a is the minor radius. The temporal evolution of argon impurity density profiles after an argon gas puffed were observed with a time resolution of up to 2 ms. The behaviour of argon was investigated under different discharge conditions. The experimental results show that, as the density increased, the impurity confinement time obviously increased. Simulation results obtained by STRAHL showed that the convection velocity changed from negative to positive as the electron density increased. This indicated that impurities moved more favourably toward the plasma boundary.
Bulletin of the American Physical Society, 2014
The Review of scientific instruments, 2021
Dispersion interferometry (DI) is a promising method for density measurement. Compared with the t... more Dispersion interferometry (DI) is a promising method for density measurement. Compared with the traditional interferometer, the DI is immune to mechanical vibration and can avoid the fringe jump error. In addition, a simple optical configuration is also one of the advantages of the DI. The electron density of the Huazhong University of Science and Technology field-reversed configuration (HFRC) device can reach 1020 m-3 with a pulse length of 50 µs. In this case, the DI based on the CO2 laser on the HFRC device adopts the heterodyne technique based on the acousto-optic modulator, which can increase the temporal resolution to 40 MHz. It can realize density fluctuation measurements in the MHz range. The test of each optical element, especially the nonlinear crystal, has been completed. The AgGaSe2 crystal can produce a second harmonic wave of about 52.5 µW when the incident CO2 power is 10 W. Based on these designs and tests, a DI system can be expected on the HFRC device.
Journal of Instrumentation, 2019
A space-resolved Vacuum Ultraviolet (VUV) spectroscopy system based on a 1-meter normal incidence... more A space-resolved Vacuum Ultraviolet (VUV) spectroscopy system based on a 1-meter normal incidence spectrometer has been developed on J-TEXT tokamak. To obtain the impurity emission profile, the Optical path and absolute intensity calibration are implemented. An alignment plate customized with several different shaped apertures is mounted between the entrance of the spectrometer and the tokamak. The exact viewing angle is identified via the on-site test. The collisional-radiative line ratio method based on the impurity transport code STRAHL is applied for the absolute intensity calibration. Pairs of lines with different upper energy levels for the same ions, e.g. C III 465 nm/C III 97.7 nm, C IV 580 nm/C IV 154.8 nm are chosen to achieve the calibration. A calibrated visible spectrometer is used to provide the absolute intensity of the visible branch. The line emission profile in the VUV range could be evaluated from the visible line via STRAHL and the atomic data from ADAS to take into account the effect of temperature on the ratio of the selected line pair. Based on the calibrated coefficient, the radial emission profiles of C III 97.7 nm and C IV 154.8 nm are obtained.
Review of Scientific Instruments, 2018
Disruptions have the possibility of causing severe wall damage to large tokamaks like ITER. The m... more Disruptions have the possibility of causing severe wall damage to large tokamaks like ITER. The mitigation of disruption damage is essential to the safe operation of a large-scale tokamak. The shattered pellet injection (SPI) technique, which is regarded as the primary injection method for ITER, presents several advantages relative to massive gas injection, including more rapid particle delivery, higher total particle assimilation, and more centrally peaked particle deposition. A dedicated argon SPI system that focuses on disruption mitigation and runaway current dissipation has been designed for the Joint Texas Experimental Tokamak (J-TEXT). A refrigerator is used to form a single argon pellet at around 64 K. The pellet will be shaped with a 5 mm diameter and a 1.5-10 mm length. Helium gas at room temperature will be used as a propellant gas for pellet acceleration. The pellet can be injected with a speed of 150-300 m/s. The time interval between injection cycles is about 8 min. The pellet will be shattered at the edge of the plasma and then injected into the core of plasma. The first experiments of SPI fast shutdown and runaway current dissipation have been performed.
Nuclear Fusion, 2018
The generation of runaway electrons during disruptions poses a serious threat for the operation o... more The generation of runaway electrons during disruptions poses a serious threat for the operation of ITER. The efficiency of the injection of large amounts of impurities by massive gas injection or shattered pellet injection to achieve runaway suppression might be compromised due to low gas mixture efficiency and the high Rosenbluth density for runaway suppression. The transport of runaway electrons is dominated by magnetic perturbations. The magnetic perturbations have the advantage of expelling the runaway seeds before they reach high energy. Robust runaway suppression has been reached on J-TEXT with mode locking by the application of m/n = 2/1 resonant magnetic perturbations before the thermal quench. The mode locking implemented large magnetic islands inside the plasma which acted as an explosive bomb during disruptions and led to stronger stochasticity in the whole plasma cross section. The NIMROD simulation indicates that this strong stochasticity expels the runaway seeds and results in runaway free disruptions on J-TEXT. This might provide an alternative runaway suppression technique during disruptions for large-scale tokamaks.
Review of Scientific Instruments, 2019
A new correlation electron cyclotron emission (CECE) diagnostic has recently been installed on th... more A new correlation electron cyclotron emission (CECE) diagnostic has recently been installed on the HL-2A tokamak in order to study electron temperature fluctuations. Eight radial locations are measured simultaneously through eight pairs of correlated channels. Multiplexers are employed in the intermediate frequency section instead of the conventional separated filter banks to meet strict cross-isolation specifications and lower insertion loss. Relative electron temperature fluctuations are observed by CECE for the first time on the HL-2A by using the spectral decorrelation method. The achieved minimum detectable fluctuation level is up to (Te Te) min ∼ 0.5%. When studying electron temperature fluctuations in the core region with gas puffing, the cross-power spectra show that the amplitude of the electron temperature fluctuation increases in a high temperature and low density plasma. Further analysis demonstrates that the electron temperature gradient ∇Te drives the electron temperature fluctuations together with electron heat transport.
Journal of Fusion Energy, 2015
Abstract A supersonic molecular beam injection (SMBI) system has been developed as a new fueling ... more Abstract A supersonic molecular beam injection (SMBI) system has been developed as a new fueling method for the J-TEXT tokamak. According to experimental needs, the system can provide various supersonic gas jets with the pressure range 0.2–2.0 MPa and pulse duration 0.2–10 ms. In this paper, the structure of the system is described in detail, including injectors, controller and gas line. Wave forms of the controller output show a fast response of the system. The evolution of pressure curve in the vacuum chamber indicates the system works well. Test results also show that the particle number of SMBI is linear proportional to the product of gas pressure and pulse duration. For different pressure of 0.5, 1.0 and 1.66 MPa, the beam velocities are estimated about 0.9, 1.2 and 1.4 km/s, respectively. Besides, fuelling experiments with the SMBI system are performed. Significant density increase demonstrates the fuelling abilities of both LSMBI and HSMBI.
The Review of scientific instruments, 2014
Significant progress has been made in the imaging and visualization of magnetohydrodynamic and mi... more Significant progress has been made in the imaging and visualization of magnetohydrodynamic and microturbulence phenomena in magnetic fusion plasmas. Of particular importance has been microwave electron cyclotron emission imaging (ECEI) for imaging Te fluctuations. Key to the success of ECEI is a large Gaussian optics system constituting a major portion of the focusing of the microwave radiation from the plasma to the detector array. Both the spatial resolution and observation range are dependent upon the imaging optics system performance. In particular, it is critical that the field curvature on the image plane is reduced to decrease crosstalk between vertical channels. The receiver optics systems for two ECEI on the J-TEXT device have been designed to ameliorate these problems and provide good performance with additional field curvature adjustment lenses with a meniscus shape to correct the aberrations from several spherical surfaces.
Plasma Science and Technology
The behavior of multiple modes before and during minor disruption with the external resonant magn... more The behavior of multiple modes before and during minor disruption with the external resonant magnetic perturbations (RMPs) has been studied on a J-TEXT tokamak. The main component of RMPs is m/n = 2/1, where m and n are the numbers of the poloidal and toroidal modes, respectively. During the mode-locking caused by RMPs, it is found that before a minor or a major disruption (if there is no minor disruption), strong oscillations in both electron temperature and density occur if the edge safety factor q a > 3. The analysis shows that the oscillations are caused by the m/n = 3/1 mode. In addition, using the ECE, Mirnov coils and 2D electron cyclotron emission imaging diagnostic systems, it is found that a thermal collapse occurs on the inner side of the 2/1 magnetic island during the minor disruption, and before the collapse, a 3/2 island increases, after the collapse, the 3/2 island may disappear. Moreover, the study also shows that these 3/1, 2/1 and 3/2 modes play roles in the the...
Nuclear Fusion, 2022
Collapse of the edge flow shear as the line-averaged density approaches the Greenwald density lim... more Collapse of the edge flow shear as the line-averaged density approaches the Greenwald density limit has been observed as a precursor to the enhanced edge particle flux characteristic of proximity to the density limit regime. Here, we report the use of a biased electrode to sustain the edge shear layer in high density discharges, in which the shear layer would otherwise collapse. A stable increase in line-averaged density is observed along with a strong increase in edge density. These experiments were carried out on the J-TEXT tokamak. The Reynolds stress at the edge is enhanced, and the zonal flow sustained, while density perturbation levels, the flux of turbulence internal energy (i.e., turbulence spreading), and particle and heat flux all decrease significantly. Electron adiabaticity increases, and bias voltage modulation experiments show that an increase in the edge shear leads the increase in adiabaticity. These results suggest that external edge E × B flow shear drive may be of...
Physics of Plasmas, 2022
The unique high-field side middle single null divertor configuration in the Joint-Texas Experimen... more The unique high-field side middle single null divertor configuration in the Joint-Texas Experimental Tokamak is attractive in studying detachment, impurity control, L-H transition, and negative triangularity. Its equilibrium configuration with a horizontally elongated (or flattened) poloidal cross section causes position instability and difficulty in equilibrium control. This work disassembles the vertical magnetic field of each group of poloidal field windings and calculates the field decay index ( n-index) at different radial positions. It is found that the instability growth time τg of horizontal displacement increases as the equilibrium radial position of plasma moves outward, which is beneficial to the position control. The simulation and experiments verified the effect of equilibrium position on horizontal instability. The equilibrium position slightly outward from the geometric center of the vacuum chamber effectively decreases the horizontal displacement oscillation amplitud...
Fusion Engineering and Design, 2022
Journal of Instrumentation, 2018
A Q-band X-mode homodyne microwave reflectometry has been developed for the measurement of edge e... more A Q-band X-mode homodyne microwave reflectometry has been developed for the measurement of edge electron density profile on the Joint TEXT tokamak (J-TEXT). A voltage controlled oscillator (VCO) with a frequency quadrupler is used as the microwave source. An arbitrary waveform generator (AWG) is adopted to control the sweeping period (20 μs) for the VCO. Homodyne method is chose to simplify the structure of the hardware circuit. In addition, a single antenna scheme which using a directional coupler to realize the sharing of an antenna for transmitting and receiving to save the resources of diagnostic windows has been adopted. For the data processing, the Hilbert transform has been used to demodulate the signal instead of the I/Q phase detector. This reflectometry has been installed on J-TEXT and effectively measured edge density profile.
Plasma Physics and Controlled Fusion
The coupling of multiple MHD modes can lead to mode locking and major disruption in tokamak plasm... more The coupling of multiple MHD modes can lead to mode locking and major disruption in tokamak plasmas. In the J-TEXT tokamak, the coupling between two small modes, i.e., m/n = 2/1 and 3/1 modes (m and n are poloidal and toroidal mode numbers, respectively), appears when the edge safety factor is reduced to the vicinity of 3. After the mode coupling, the toroidal phase difference between the 2/1 and 3/1 modes equals 0 in the low field side midplane. This phase relation of coupled modes leads to mutual destabilization and even major disruption. A control scheme to avoid disruption caused by coupled modes by resonant magnetic perturbations (RMP) is presented. It is found that the application of RMP significantly changes the evolution of the coupled modes. The coupling of the 2/1 and 3/1 modes occurs earlier as the RMP amplitude increases. The RMP with moderate amplitude can suppress the growth of 2/1 and 3/1 coupled modes and hence avoid disruption. These results provide a possible strat...
Plasma Science and Technology
To extend the operation region of the Joint-Texas Experimental tokamak (J-TEXT) to the divertor c... more To extend the operation region of the Joint-Texas Experimental tokamak (J-TEXT) to the divertor configuration and even H-mode, the divertor configuration discharge has been realized for the first time in J-TEXT tokamak. Along with establishing a power supply for divertor configuration, the construction of relevant diagnostics, and the installation of the divertor target in the high-field side, the divertor discharge has been tested. And through the equilibrium calculation and position stability analysis, the control strategy has been evolved to be more stable. High-density experiments and auxiliary heating experiments have been carried out in the divertor configuration. The special midplane single-null (MSN) divertor configuration exhibits to be more stable than the limiter configuration in the density limit condition and can reach a higher density in the experiment. In the ECRH experiment, the power injection enhances the electron temperature and density, while more heat outflux is...
Plasma Science and Technology
Machine learning research and applications in fusion plasma experiments are one of the main subje... more Machine learning research and applications in fusion plasma experiments are one of the main subjects on J-TEXT. Since 2013, various kinds of traditional machine learning, as well as deep learning methods have been applied to fusion plasma experiments. Further applications in the real-time experimental environment have proved the feasibility and effectiveness of the methods. For disruption prediction, we started by predicting disruptions of limited classes with a short warning time that could not meet the requirements of the mitigation system. After years of study, nowadays disruption prediction methods on J-TEXT are able to predict all kinds of disruptions with a high success rate and long enough warning time. Furthermore, cross-device disruption prediction methods have obtained promising results. Interpretable analysis of the models are studied. For diagnostics data processing, efforts have been made to reduce manual work in processing and to increase the robustness of the diagnost...
Cornell University - arXiv, Aug 19, 2022
Predicting disruptions across different tokamaks is a great obstacle to overcome. Future tokamaks... more Predicting disruptions across different tokamaks is a great obstacle to overcome. Future tokamaks can hardly tolerate disruptions at high performance discharge. Few disruption discharges at high performance can hardly compose an abundant training set, which makes it difficult for current data-driven methods to obtain an acceptable result. A machine learning method capable of transferring a disruption prediction model trained on one tokamak to another is required to solve the problem. The key is a disruption prediction model containing a feature extractor that is able to extract common disruption precursor traces in tokamak diagnostic data, and a transferable disruption classifier. Based on the concerns above, the paper first presents a deep fusion feature extractor designed specifically for extracting disruption precursor features from common diagnostics on tokamaks according to currently known precursors of disruption, providing a promising foundation for transferable models. The fusion feature extractor is proved by comparing with manual feature extraction on J-TEXT. Based on the feature extractor trained on J-TEXT, the disruption prediction model was transferred to EAST data with mere 20 discharges from EAST experiment. The performance is comparable with a model trained with 1896 discharges from EAST. From the comparison among other model training scenarios, transfer learning showed its potential in predicting disruptions across different tokamaks.
Plasma Science and Technology
The detailed density perturbations provided by the advanced polarimeter–interferometer system (Po... more The detailed density perturbations provided by the advanced polarimeter–interferometer system (Polaris) during sawtooth collapse on the Joint Texas Experimental Tokamak (J-TEXT) are reported in this article. During a sawtooth collapse and the crash of plasma pressure at the center, it is interestingly found that the increase in density in the region between the inversion radius and mixing radius is poloidally asymmetrical, while the increase in temperature are poloidally symmetrical. The poloidal location where the density increases is dependent on the phase of the precursory m/n=1/1 kink mode. It is always out of phase with the hot-core of the m/n=1/1 mode. The behaviors of density perturbations during sawtooth collapse observed in J-TEXT are beyond the expectations of the standard model, which can shed new light on the understanding of sawtooth collapse.
Plasma Science and Technology
Pellet injection is an attractive technology for core-fueling and magnetohydrodynamic study in ma... more Pellet injection is an attractive technology for core-fueling and magnetohydrodynamic study in magnetic-confinement fusion devices like tokamaks and stellarators. It can inject solid hydrogen/deuterium pellets into the plasma with deeper density deposition compared with other fueling methods, such as gas puffing. A three-barrel H2 pellet injection system was installed on the J-TEXT tokamak and experiments were carried out. The pellets are formed in three barrels cooled by a cryocooler and compressor system at around 9 K, and are 0.8 mm/1 mm diameter and 0.8 mm length. The pellet is launched by helium propellant gas and injected from the low-field side of the plasma. The normal range of pellet speed is 210–310 m s−1 for different propellant gas pressures. Due to the three-barrel structure, the number of injected pellets can be adjusted between one and three. Pellets can be launched sequentially with arbitrary time intervals, which enables flexible applications. The results of the exp...
Fusion Engineering and Design, 2021
A tangential X-ray imaging crystal spectrometer (XICS) has been upgraded in the J-TEXT tokamak. T... more A tangential X-ray imaging crystal spectrometer (XICS) has been upgraded in the J-TEXT tokamak. The XICS system has been used to measure ion and electron temperature, plasma rotation velocities, selected impurity densities, and can be used for impurity transport. The XICS on J-TEXT was designed to receive Ar XVII emissions from the-0.5a to +0.5a region with a spatial resolution of 1.8 cm in the vertical direction, a is the minor radius. The temporal evolution of argon impurity density profiles after an argon gas puffed were observed with a time resolution of up to 2 ms. The behaviour of argon was investigated under different discharge conditions. The experimental results show that, as the density increased, the impurity confinement time obviously increased. Simulation results obtained by STRAHL showed that the convection velocity changed from negative to positive as the electron density increased. This indicated that impurities moved more favourably toward the plasma boundary.
Bulletin of the American Physical Society, 2014
The Review of scientific instruments, 2021
Dispersion interferometry (DI) is a promising method for density measurement. Compared with the t... more Dispersion interferometry (DI) is a promising method for density measurement. Compared with the traditional interferometer, the DI is immune to mechanical vibration and can avoid the fringe jump error. In addition, a simple optical configuration is also one of the advantages of the DI. The electron density of the Huazhong University of Science and Technology field-reversed configuration (HFRC) device can reach 1020 m-3 with a pulse length of 50 µs. In this case, the DI based on the CO2 laser on the HFRC device adopts the heterodyne technique based on the acousto-optic modulator, which can increase the temporal resolution to 40 MHz. It can realize density fluctuation measurements in the MHz range. The test of each optical element, especially the nonlinear crystal, has been completed. The AgGaSe2 crystal can produce a second harmonic wave of about 52.5 µW when the incident CO2 power is 10 W. Based on these designs and tests, a DI system can be expected on the HFRC device.
Journal of Instrumentation, 2019
A space-resolved Vacuum Ultraviolet (VUV) spectroscopy system based on a 1-meter normal incidence... more A space-resolved Vacuum Ultraviolet (VUV) spectroscopy system based on a 1-meter normal incidence spectrometer has been developed on J-TEXT tokamak. To obtain the impurity emission profile, the Optical path and absolute intensity calibration are implemented. An alignment plate customized with several different shaped apertures is mounted between the entrance of the spectrometer and the tokamak. The exact viewing angle is identified via the on-site test. The collisional-radiative line ratio method based on the impurity transport code STRAHL is applied for the absolute intensity calibration. Pairs of lines with different upper energy levels for the same ions, e.g. C III 465 nm/C III 97.7 nm, C IV 580 nm/C IV 154.8 nm are chosen to achieve the calibration. A calibrated visible spectrometer is used to provide the absolute intensity of the visible branch. The line emission profile in the VUV range could be evaluated from the visible line via STRAHL and the atomic data from ADAS to take into account the effect of temperature on the ratio of the selected line pair. Based on the calibrated coefficient, the radial emission profiles of C III 97.7 nm and C IV 154.8 nm are obtained.
Review of Scientific Instruments, 2018
Disruptions have the possibility of causing severe wall damage to large tokamaks like ITER. The m... more Disruptions have the possibility of causing severe wall damage to large tokamaks like ITER. The mitigation of disruption damage is essential to the safe operation of a large-scale tokamak. The shattered pellet injection (SPI) technique, which is regarded as the primary injection method for ITER, presents several advantages relative to massive gas injection, including more rapid particle delivery, higher total particle assimilation, and more centrally peaked particle deposition. A dedicated argon SPI system that focuses on disruption mitigation and runaway current dissipation has been designed for the Joint Texas Experimental Tokamak (J-TEXT). A refrigerator is used to form a single argon pellet at around 64 K. The pellet will be shaped with a 5 mm diameter and a 1.5-10 mm length. Helium gas at room temperature will be used as a propellant gas for pellet acceleration. The pellet can be injected with a speed of 150-300 m/s. The time interval between injection cycles is about 8 min. The pellet will be shattered at the edge of the plasma and then injected into the core of plasma. The first experiments of SPI fast shutdown and runaway current dissipation have been performed.
Nuclear Fusion, 2018
The generation of runaway electrons during disruptions poses a serious threat for the operation o... more The generation of runaway electrons during disruptions poses a serious threat for the operation of ITER. The efficiency of the injection of large amounts of impurities by massive gas injection or shattered pellet injection to achieve runaway suppression might be compromised due to low gas mixture efficiency and the high Rosenbluth density for runaway suppression. The transport of runaway electrons is dominated by magnetic perturbations. The magnetic perturbations have the advantage of expelling the runaway seeds before they reach high energy. Robust runaway suppression has been reached on J-TEXT with mode locking by the application of m/n = 2/1 resonant magnetic perturbations before the thermal quench. The mode locking implemented large magnetic islands inside the plasma which acted as an explosive bomb during disruptions and led to stronger stochasticity in the whole plasma cross section. The NIMROD simulation indicates that this strong stochasticity expels the runaway seeds and results in runaway free disruptions on J-TEXT. This might provide an alternative runaway suppression technique during disruptions for large-scale tokamaks.
Review of Scientific Instruments, 2019
A new correlation electron cyclotron emission (CECE) diagnostic has recently been installed on th... more A new correlation electron cyclotron emission (CECE) diagnostic has recently been installed on the HL-2A tokamak in order to study electron temperature fluctuations. Eight radial locations are measured simultaneously through eight pairs of correlated channels. Multiplexers are employed in the intermediate frequency section instead of the conventional separated filter banks to meet strict cross-isolation specifications and lower insertion loss. Relative electron temperature fluctuations are observed by CECE for the first time on the HL-2A by using the spectral decorrelation method. The achieved minimum detectable fluctuation level is up to (Te Te) min ∼ 0.5%. When studying electron temperature fluctuations in the core region with gas puffing, the cross-power spectra show that the amplitude of the electron temperature fluctuation increases in a high temperature and low density plasma. Further analysis demonstrates that the electron temperature gradient ∇Te drives the electron temperature fluctuations together with electron heat transport.
Journal of Fusion Energy, 2015
Abstract A supersonic molecular beam injection (SMBI) system has been developed as a new fueling ... more Abstract A supersonic molecular beam injection (SMBI) system has been developed as a new fueling method for the J-TEXT tokamak. According to experimental needs, the system can provide various supersonic gas jets with the pressure range 0.2–2.0 MPa and pulse duration 0.2–10 ms. In this paper, the structure of the system is described in detail, including injectors, controller and gas line. Wave forms of the controller output show a fast response of the system. The evolution of pressure curve in the vacuum chamber indicates the system works well. Test results also show that the particle number of SMBI is linear proportional to the product of gas pressure and pulse duration. For different pressure of 0.5, 1.0 and 1.66 MPa, the beam velocities are estimated about 0.9, 1.2 and 1.4 km/s, respectively. Besides, fuelling experiments with the SMBI system are performed. Significant density increase demonstrates the fuelling abilities of both LSMBI and HSMBI.
The Review of scientific instruments, 2014
Significant progress has been made in the imaging and visualization of magnetohydrodynamic and mi... more Significant progress has been made in the imaging and visualization of magnetohydrodynamic and microturbulence phenomena in magnetic fusion plasmas. Of particular importance has been microwave electron cyclotron emission imaging (ECEI) for imaging Te fluctuations. Key to the success of ECEI is a large Gaussian optics system constituting a major portion of the focusing of the microwave radiation from the plasma to the detector array. Both the spatial resolution and observation range are dependent upon the imaging optics system performance. In particular, it is critical that the field curvature on the image plane is reduced to decrease crosstalk between vertical channels. The receiver optics systems for two ECEI on the J-TEXT device have been designed to ameliorate these problems and provide good performance with additional field curvature adjustment lenses with a meniscus shape to correct the aberrations from several spherical surfaces.
Plasma Science and Technology
The behavior of multiple modes before and during minor disruption with the external resonant magn... more The behavior of multiple modes before and during minor disruption with the external resonant magnetic perturbations (RMPs) has been studied on a J-TEXT tokamak. The main component of RMPs is m/n = 2/1, where m and n are the numbers of the poloidal and toroidal modes, respectively. During the mode-locking caused by RMPs, it is found that before a minor or a major disruption (if there is no minor disruption), strong oscillations in both electron temperature and density occur if the edge safety factor q a > 3. The analysis shows that the oscillations are caused by the m/n = 3/1 mode. In addition, using the ECE, Mirnov coils and 2D electron cyclotron emission imaging diagnostic systems, it is found that a thermal collapse occurs on the inner side of the 2/1 magnetic island during the minor disruption, and before the collapse, a 3/2 island increases, after the collapse, the 3/2 island may disappear. Moreover, the study also shows that these 3/1, 2/1 and 3/2 modes play roles in the the...
Nuclear Fusion, 2022
Collapse of the edge flow shear as the line-averaged density approaches the Greenwald density lim... more Collapse of the edge flow shear as the line-averaged density approaches the Greenwald density limit has been observed as a precursor to the enhanced edge particle flux characteristic of proximity to the density limit regime. Here, we report the use of a biased electrode to sustain the edge shear layer in high density discharges, in which the shear layer would otherwise collapse. A stable increase in line-averaged density is observed along with a strong increase in edge density. These experiments were carried out on the J-TEXT tokamak. The Reynolds stress at the edge is enhanced, and the zonal flow sustained, while density perturbation levels, the flux of turbulence internal energy (i.e., turbulence spreading), and particle and heat flux all decrease significantly. Electron adiabaticity increases, and bias voltage modulation experiments show that an increase in the edge shear leads the increase in adiabaticity. These results suggest that external edge E × B flow shear drive may be of...
Physics of Plasmas, 2022
The unique high-field side middle single null divertor configuration in the Joint-Texas Experimen... more The unique high-field side middle single null divertor configuration in the Joint-Texas Experimental Tokamak is attractive in studying detachment, impurity control, L-H transition, and negative triangularity. Its equilibrium configuration with a horizontally elongated (or flattened) poloidal cross section causes position instability and difficulty in equilibrium control. This work disassembles the vertical magnetic field of each group of poloidal field windings and calculates the field decay index ( n-index) at different radial positions. It is found that the instability growth time τg of horizontal displacement increases as the equilibrium radial position of plasma moves outward, which is beneficial to the position control. The simulation and experiments verified the effect of equilibrium position on horizontal instability. The equilibrium position slightly outward from the geometric center of the vacuum chamber effectively decreases the horizontal displacement oscillation amplitud...
Fusion Engineering and Design, 2022
Journal of Instrumentation, 2018
A Q-band X-mode homodyne microwave reflectometry has been developed for the measurement of edge e... more A Q-band X-mode homodyne microwave reflectometry has been developed for the measurement of edge electron density profile on the Joint TEXT tokamak (J-TEXT). A voltage controlled oscillator (VCO) with a frequency quadrupler is used as the microwave source. An arbitrary waveform generator (AWG) is adopted to control the sweeping period (20 μs) for the VCO. Homodyne method is chose to simplify the structure of the hardware circuit. In addition, a single antenna scheme which using a directional coupler to realize the sharing of an antenna for transmitting and receiving to save the resources of diagnostic windows has been adopted. For the data processing, the Hilbert transform has been used to demodulate the signal instead of the I/Q phase detector. This reflectometry has been installed on J-TEXT and effectively measured edge density profile.