Understanding the origin of the tokamak thermal quench (TQ) remains a critical unresolved issue, particularly the causal link between initial localized temperature collapse and the final core energy collapse. This work presents a detailed experimental investigation of the TQ process on the J-TEXT tokamak, utilizing high-resolution diagnostics at multiple toroidal locations to observe disruptions with rotating precursor modes. We demonstrate that the initial localized temperature collapse near the q = 2 surface and the core temperature collapse are not independent events but are sequential phases of the continuous evolution of a cold structure (CS), thereby establishing a direct causal link between the two events. This CS originates near the inner separatrix of the pre-existing m / n = 2/1 magnetic island, and it is observed to be in-phase locked with the island. This CS is identified as a helical structure with a dominant m / n = 1/1 character, when it develops towards the core. The continuous radially inward and poloidal expansion of this structure drives an asymmetric temperature collapse. This provides a physical explanation to the increase of measured core electron temperature during TQ, which results from the rotation of remaining helical hot core. The observations are also applicable to auxiliary-heated plasmas on J-TEXT. The initial collapse is discussed, which is likely triggered by the excitation of 3/2 island. Additionally, a reduced model is developed to describe the magnetic topology and to simulate the temperature evolution. These findings indicate that the stochastic field during TQ originates from a localized region during precursors and evolves to the global stochasticity at the final energy loss.

This paper summarizes recent experimental and theoretical progress achieved on the J-TEXT tokamak during the past two years, with an emphasis on the physics and applications of three-dimensional (3D) magnetic fields. Key hardware upgrades include a new ICRF system and the advanced 3D magnetic coil systems, consisting of the resonant magnetic perturbation (RMP) coils, the island divertor (ID) coils and the external rotational transform (ERT) coils. And these upgrades enable novel investigations into MHD instabilities, disruptions, transport, and divertor solutions. A major finding is the neoclassical tearing mode (NTM) triggered formation of electron internal transport barrier, where magnetic island nonlinearly interacts with turbulence to suppress transport and steepen core temperature gradients, and reveals a new mechanism for confinement improvement. Furthermore, internal kink mode driven neoclassical toroidal viscosity torque is identified as a key mechanism governing intrinsic rotation. The application of ERT coils successfully creates a Tokamak-Stellarator hybrid configuration, demonstrating complete suppression of NTMs and a 20% increase in stable plasma current. For boundary control, an ID configuration is established, reducing peak heat loads by ∼50% and enabling detachment via supersonic molecular beam injection fuelling. In disruption physics, synergetic control using RMP and O-point aligned electron cyclotron resonance heating efficiently suppresses locked modes and disruption, while low- n magnetic perturbations are capable of runaway electron suppression. Thermal quench timescale, estimated by a unified model with stochastic magnetic field and turbulence, matches experimental observations. Additionally, AI-driven disruption prediction frameworks, incorporating adaptive anomaly detection and cross-machine domain adaptation, are developed, with interpretability analyses linking predictions to physical mechanisms. Studies on turbulence and transport elucidate the role of electrode biasing, turbulence spreading on the edge cooling, density limit, while helium ash removal dynamics are also investigated. The unique up-down symmetric poloidal divertor configuration is measured and modelling to identify the electric field induced drift effect as a key driver for asymmetry, and hence target biasing is designed and experimentally studied for control these asymmetries. These collective advances in heating, 3D field control, stability, and AI forecasting provide critical solutions for managing plasma confinement and mitigating risks in future MCF devices.

Influence of neoclassical toroidal viscosity (NTV) torque on intrinsic toroidal rotation caused by internal kink mode (IKM) in the J-TEXT tokamak is studied. It is observed that the toroidal rotation in the counter-current direction decreases after the on-axis electron cyclotron resonance heating (ECRH) is turned on, during which the IKM is strongly enhanced. The change of plasma rotation due to ECRH decreases with increasing plasma density, which is due to a decreasing of the IKM’s amplitude. This gives direct evidence that the change in intrinsic rotation strongly depends on the amplitude of the IKM. It is shown in the modeling that the decrease in the amplitude of the IKM reduces the NTV torque, and hence the influence on the toroidal rotation becomes weaker. The modeled dependence of rotation variation on plasma density agrees well with the observations. These results indicate that the NTV torque caused by the IKM plays a key role in changing the intrinsic core toroidal rotation in ECRH plasmas in the J-TEXT tokamak.

For the first time, an island divertor configuration was successfully implemented in the J-TEXTtokamak to improve heat exhaust and impurity control. The magnetic island is generated byapplying external resonant magnetic perturbation fields, and the intersection between the edgeisland and the divertor target is then controlled by adjusting the edge safety factorqa, therebyachieving the island divertor configuration. The overall confinement is maintained in spite of theloss of the edge volume. The island divertor configuration significantly reduces peak heat-loadon the divertor target by approximately 50% and improves impurity screening. Additionally, iteffectively modulates radiation around the magnetic island's X-point, potentially enhancing thestability and control of radiative divertor operations. These findings highlight the island divertorconfiguration as a promising strategy for advancing heat exhaust and impurity control intokamak operations

A deep learning-based disruption prediction algorithm has been implemented on a new tokamak, HL-3. An Area Under receiver-operator characteristic Curve of 0.940 has been realized offline over a test campaign involving 72 disruptive and 240 non-disruptive shots, despite the limited training data available from the initial two campaigns. In addition to the well-documented challenge of insufficient training data, a previously unanticipated issue is addressed that the data distribution of a new device is continuously drifting. The plasma scans across a broad parameter space, bringing a drifting distribution of disruption causes and diagnostic data. This problem is often overlooked in previous implementations on steadily operating tokamaks, necessitating greater attention in future tokamaks like ITER. To address these challenges, innovative modules including predict-first neural network, data augmentation, and pseudo data placeholders are developed and implemented, which promotes the accuracy by up to 20%. A series of advantages are also brought by the modules, including the robustness in handling missing input channels, and the interpretability to identify which parameter of plasma is under abnormal condition. The results demonstrate that, with dedicated data collection and algorithm implementation, the issues of limited data and drifting distribution can be overcome, and further, the deep learning-based algorithm can reliably provide disruption alarms on a new tokamak.

Experiments conducted on the J-TEXT tokamak have provided the first evidence that the Beta-induced Alfvén Eigenmode (BAE) is localized inside the isolated helical flux tube of its edge m / n = 3/1 magnetic island. The observations show that the BAE forms a standing wave inside the magnetic island, with its nodes located at the X- and O-points of the magnetic island. When the island is cut open by contact with the limiter plates, the BAE is found to remain inside the remnant closed island in the Scrape-Off Layer, but its amplitude decreases as the width of the remnant island becomes smaller.

On the J-TEXT tokamak, the dynamics of edge magnetic topology during the opening of the edge magnetic islands induced by the external Resonant Magnetic Perturbation (RMP) are investigated. The edge island chain is pushed outward by increasing plasma toroidal current to intersect the poloidally and toroidally localized divertor plate, forming an open magnetic island in the Scrape-Off Layer (SOL). The location of the strike points on the divertor plate predicted by the HINT code is in agreement with the experimental observations. The influence of the magnetic topology on edge plasma profiles has also been investigated using Langmuir probes. The properties of edge T e , n e , P e and E r profiles are studied as function of three magnetic structures in q a -dependence experiments and two magnetic structures in RMP configuration-dependence experiments. Some common features are observed. Inside the edge closed and SOL remnant islands, flat P e but non-flat T e and n e profiles are detected. When transitioning from the edge closed island to the partially open island with remnant island, the local flat P e profile is shifted outward accompanied by a narrower flattened P e region. The steep slopes of T e , n e and P e are measured in the SOL regions characterized by a long connection length L c , and the longer L c , the steeper slopes. E r exhibits a negative well inside the remnant island with infinite L c and in the SOL regions where L c significantly exceeds than the electron mean free path lambda e , but develops a positive value in the SOL regions where L c is relatively short.

To explore innovative approaches for optimizing tokamak configurations and combining the advantages of both tokamaks and stellarators, the J-TEXT tokamak recently underwent an upgrade by installing the External Rotational Transform (ERT) coil system. This system consists of two rings for producing a helical magnetic field. Due to space limitations, the ERT coil system is installed inside the vacuum vessel. The ERT coils feature a modular rail structure designed to navigate the intricate vacuum vessel environment. The successful installation of the ERT coil system on J-TEXT has yielded preliminary experimental results that align with the design objectives.

The J-TEXT capability is enhanced compared to two years ago with several upgrades of its diagnostics and the increase of electron cyclotron resonance heating (ECRH) power to 1 MW. With the application of electron cyclotron wave (ECW), the ECW assisted plasma startup is achieved; the tearing mode is suppressed; the toroidal injection of 300 kW ECW drives around 24 kA current; fast electrons are generated with toroidal injected ECW and the runaway current conversion efficiency increases with ECRH power. The mode coupling between 2/1 and 3/1 modes are extensively studied. The coupled 2/1 and 3/1 modes usually lead to major disruption. Their coupling can be either suppressed or avoided by external resonant magnetic perturbation fields and hence avoids the major disruption. It is also found that the 2/1 threshold of external field is significantly reduced by a pre-excited 3/1 mode, which can be either a locked island or an external kink mode. The disruption control is studied by developing prediction methods capable of cross tokamak application and by new mitigation methods, such as the biased electrode or electromagnetic pellet injector. The high-density operation and related disruptions are studied from various aspects. Approaching the density limit, the collapse of the edge shear layer is observed and such collapse can be prevented by applying edge biasing, leading to an increased density limit. The density limit is also observed to increase, if the plasma is operated in the poloidal divertor configuration or the plasma purity is increased by increasing the pre-filled gas pressure or ECRH power during the start-up phase.

The low elastic modulus of glass fiber-reinforced polymer (GFRP) materials used in civil engineering may lead to insufficient structural stiffness in GFRP-aluminum space truss structures, limiting their ability to meet the service limit state requirements. To enhance flexural stiffness, a prestressed carbon fiber-reinforced polymer (CFRP) tendon system was developed and demonstrated. Full-scale three-point bending tests were performed to evaluate the flexural response of GFRP space truss girders, both with and without CFRP tendons. Four prestressing schemes were investigated, revealing the effect of the tendon system in enhancing stiffness. A simplified, design-oriented theoretical model using the equivalent continuum method and the force method was developed to aid structural design calculations. The model's formulas account for variable joint stiffness and equivalent shear deformation, enabling accurate stiffness evaluations. Parametric analyses were conducted on the prestress level, the girder-to-tendon stiffness ratio, and the geometric parameters of the CFRP tendon system. The results indicated that the four prestressing schemes enhanced the flexural stiffness and reduced the internal forces, validating the effectiveness of the novel prestressed FRP space truss structure. The proposed model accurately describes the prestressing enhancement mechanism and offers theoretical support for structural design.

Toroidal coupling between m / n = 2/1 and m / n = 3/1 modes frequently occurs in the J-TEXT, where m ( n ) is the poloidal (toroidal) mode number. These coupled modes destabilize each other, leading to confinement degradation and even triggering a major disruption. This paper presents two control strategies for preventing the mode coupling through the application of a proper static resonant magnetic perturbation (RMP) field. Experimental results demonstrate that moderate 2/1 RMP can suppress the small, rotating 2/1 mode thus prevent coupling between the 2/1 and 3/1 modes. The 3/1 static RMP can excite a large 3/1 locked island while leave the small 2/1 mode rotating at 8 kHz. Enlarging the frequency difference between 2/1 and 3/1 modes makes mode coupling more difficult. Both strategies can break the frequency coupling condition between the 2/1 and 3/1 modes, and hence avoid coupling and mutual destabilizing.

In this work, several key scaling laws of the quasi-static magnetic compression of field reversed configuration（FRC） plasma（Spencer et al 1983 Phys. Fluids 26 1564） are amended from a series of two-dimensional FRC MHD equilibriums numerically obtained using the Grad–Shafranov equation solver NIMEQ. Based on the new scaling for the elongation and the magnetic fields at the separatrix and the wall, the empirically stable limits for the compression ratio, the fusion gain, and the neutron yield are evaluated, which may serve as a more accurate estimate for the upper ceiling of performance from the magnetic compression of FRC plasma as a potential fusion energy as well as neutron source devices.

The coupling of multiple magnetohydrodynamic (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 (RMPs) 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 strategy for the suppression of neoclassical tearing mode (NTM) seed islands on International Thermonuclear Experimental Reactor (ITER) or future fusion reactors.

The identification of magnetohydrodynamic (MHD) modes is a crucial issue in the control of magnetically confined plasmas. This paper proposes a novel method for identifying the evolution of MHD modes from a signal with a low signal-to-noise ratio. The proposed method generates a weighted directed graph from the time-frequency spectrum and calculates the evolution of the mode frequency by solving the shortest path. This method addresses the limitations posed by the lack of data channels and the disturbance of noise in the estimation of mode frequency and yields much better results compared to traditional methods. It is demonstrated that, using this method, the evolution of an unlocked tearing mode was more accurately calculated on the J-TEXT tokamak. This method remains feasible even with a low signal-to-noise ratio of 0.5, as shown by its uncertainty. Furthermore, with appropriate parameters, this method can be applied to not only signals with MHD modes, but also to general signals with continuous modes.

Measurement of locked mode (LM) is important for the physical research of Magnetohydrodynamic (MHD) instabilities and plasma disruption. The n = 0 pick-up need to be extracted and subtracted to calculate the amplitude and phase of the LM. A new method to extract this pick-up has been developed by predicting the n = 0 pick-up brn=0 by the LM detectors based on Neural Networks (NNs) in J-TEXT. An approach called Power Multiple Time Scale (PMTS) has been developed with outstanding regressing effect in multiple frequency ranges. Three models have been progressed based on PMTS NNs. PMTS could fit the brn=0 on the LM detectors with little errors both in time domain and frequency domain. The n>0 pick-up brn>0 generated by resonant magnetic perturbations (RMPs) can be obtained after subtracting the extracted brn=0. This new method uses only one LM instead of 4 LM detectors to extract brn=0. Therefore, the distribution of the LM detectors can also be optimized based on this new method.

A significant reduction of the external resonant magnetic perturbation (RMP) required for exciting the locked mode (LM) has been observed experimentally due to the presence of an existing LM on J-TEXT. In a plasma with edge safety factor q a slightly smaller than 3, 3/1 and 2/1 modes are observed to be destabilized successively by a rotating RMP field. The 3/1 mode is excited first and identified as an external kink mode, whose excitation threshold decreases with the operating q a increasing from 2.67 to 2.8 with the absence of a 3/1 rational surface. Shortly after excitation of the 3/1 external kink mode, 2/1 field penetration occurs and the 2/1 mode is excited, as identified by the phase between the 2/1 magnetic response and the RMP field jumping by π . The 2/1 mode is excited with a dominant kink structure and then converts into tearing mode for a duration of several milliseconds. During the excitation of the 2/1 mode, the phase difference between the 2/1 and 3/1 modes evolves from π to around 0 and the 2/1 mode amplitude grows to a saturation value comparable with that of the 3/1 mode. The presence of the 3/1 mode leads to a remarkable reduction of the external RMP current required for the 2/1 mode excitation, which can be explained by the contribution of a 2/1 resonant field from the 3/1 mode via the toroidal coupling effect. The larger 3/1 mode amplitude can lead to a greater reduction. This work reveals that the mode can be excited more easily with a reduced amplitude of RMP or error field due to the impact of the pre-existing mode and this might be a new consideration for error field correlation predictions and active magnetohydrodynamics control actuator designs.

The resonant magnetic perturbation (RMP) system is a powerful auxiliary system on tokamaks. On the J-TEXT tokamak, a set of new in-vessel coils is designed to enhance the amplitude of the RMP. The new coils are designed to be two-turn saddle coils. These two-turn saddle coils have been optimized in terms of their structure, support, and protection components to overcome the limitations of the narrow in-vessel space, resulting in a compact coil module that can be accommodated in the vessel. To verify the feasibility of this design, an electromagnetic simulation is performed to investigate the electrical parameters and the generated field of the coils. A multi-field coupled simulation is performed to investigate the capacity of heat dissipation. As a result of these efforts, the new RMP coils have been successfully installed on the J-TEXT tokamak. It has significantly enhanced the RMP amplitude and been widely applied in experiments.

This paper aims to investigate the axial compression behaviour of perforated glass fibre reinforced plastic (GFRP) circular tubes. A total of 12 GFRP medium-length tubes with and without circular holes were tested under axial compression. The influence of hole size and hole quantity on the failure mode and mechanical behaviour of perforated GFRP tubes were carefully evaluated. The overall buckling failure and material strength failure, mainly determined by the hole size, were observed in the axial compression tests. The critical load and axial stiffness of the perforated tubes decreased further with the increase of hole sizes and quantities, compared to those of the intact GFRP tubes. To reveal the damage mechanism of perforated GFRP tubes, three-dimensional digital image correlation (3D-DIC) technique was used to analyse the evolution of full-field strain around the hole. It was demonstrated that the damage mechanisms of the overall buckling failure and material strength failure were respectively compression-shear destruction and tension-shear destruction in the high strain concentration regions around the hole. Finally, design-oriented equations were proposed to predict the critical load and axial stiffness of perforated GFRP tubes under axial compression, and 95% confidence interval was used for interval estimation. The proposed equations were in good agreement with the test results. In addition, the prediction results of axial stiffness are generally more accurate than that of critical load.

It has been found that a higher-frequency rotating resonant magnetic perturbation (RRMP) can suppress a large tearing mode (TM) and avoid subsequent disruption (Li et al 2020 Nucl. Fusion 60 056022). To clarify the mechanism of the stabilizing effect of the higher-frequency RRMP, experiments on the effects of RRMPs with different frequencies on the TM amplitude are presented in this paper. After eliminating the destabilizing effect of RRMP, the statistical analysis reveals a notable negative correlation between changes in the amplitude of the TM and changes in the mode frequency. Further investigation indicates an absence of a suppressive effect of the toroidal flow on the TM. Instead, the data shows a clear linear relationship between changes in the TM amplitude and the flow shear in proximity to the resonant surface, with a high coefficient of determination ( r 2 ). Those experimental results suggest that the flow shear plays a significant role in the suppression of TMs and offer a plausible method to prevent disruptions in future devices.

Plasma Position Reflectometry (PPR) is planned to provide plasma position and shape information for plasma operation in future fusion reactors. Its primary function is to calibrate the drift of the magnetic signals due to the integral nature of magnetic measurement. Here, we attempt to measure plasma position using ordinary mode (O-mode) and extraordinary mode (X-mode) reflectometry systems on two tokamaks. A new physical model based on the phase shift is proposed to deduce the relative movement of the cut-off layer without density inversion. We demonstrate the plasma position measurements by absolute measurement from density profile inversion and relative measurement from phase shift. The combination of X-mode and O-mode reflectometers can minimize the limitations of single polarization reflectometry and further increase the accuracy of plasma position measurement. These results could provide an important technical basis for the further development of a real-time control system based on PPR.