A brief review of the development and optimization of the three-dimensional magnetic configuration system in the J-TEXT tokamak

The three-dimensional (3D) magnetic configuration system in the J-TEXT tokamak has featured in many experimental studies. The system mainly consists of three subsystems: the static resonant magnetic perturbation (SRMP) system, the dynamic resonant magnetic perturbation (DRMP) system and the helical coil system. The SRMP coil system consist of two kinds of coils, i.e. three six-loop coils and two five-loop coils. It can suppress tearing modes with a moderate strength, and may also cause mode locking with larger amplitude. The DRMP coil system consists of 12 single-turn saddle coils (DRMP1) and 12 double-turn saddle coils (DRMP2). Its magnetic field can be rotated at a few kHz, leading to either acceleration or deceleration of the tearing mode velocity and the plasma rotation. The helical coil system consists of two closed coils, and is currently under construction to provide external rotational transform in J-TEXT. The 3D magnetic configuration system can suppress tearing modes, preventing and avoiding the occurrence of major disruption.     

Introduction to resonant magnetic perturbation coils of the J-TEXT Tokamak

To investigate the interactions between both the static and rotating resonant magnetic perturbations (RMP) and the tokamak plasma, two sets of coils, namely static RMP (SRMP) and dynamic RMP (DRMP), are constructed on the J-TEXT tokamak. SRMP is reconstructed from TEXT-U and mainly produces static m/n = 1/1, 2/1 and 3/1 resonant perturbation field, where m and n are the poloidal and toroidal mode numbers, respectively. DRMP, newly designed and installed inside the vacuum vessel, can generate pure 2/1 RMP. DRMP is also designed to operate in the AC mode and can produce rotating 2/1 RMP which will be used to study the tearing mode control. Due to the effect of the eddy current in the vacuum vessel wall, the amplitudes of the 2/1 component will be attenuated to about 1/3.6 of the DC value when the operation frequency is larger than 500 Hz. However, DRMP can still provide sufficient large rotating 2/1 perturbation for tearing mode related studies.     

Overview of the J-TEXT progress on RMP and disruption physics

The J-TEXT tokamak has been operated for ten years since its first plasma obtained at the end of 2007. The diagnostics development and main modulation systems, i.e. resonant magnetic perturbation (RMP) systems and massive gas injection (MGI) systems, will be introduced in this paper. Supported by these efforts, J-TEXT has contributed to research on several topics, especially on RMP physics and disruption mitigation. Both experimental and theoretical research show that RMP could lock, suppress or excite the tearing modes, depending on the RMP amplitude, frequency difference between RMP and rational surface rotation, and initial stabilities. The plasma rotation, particle transport and operation region are influenced by the RMP. Utilizing the MGI valves, disruptions have been mitigated with pure He, pure Ne, and a mixture of He and Ar (9:1). A significant runaway current plateau could be generated with moderate amounts of Ar injection. The RMP has been shown to suppress the generation of runaway current during disruptions.     

Design of the power system for dynamic resonant magnetic perturbation coils on the J-TEXT tokamak

A set of in-vessel saddle coils has been installed on J-TEXT tokamak. They are proposed for further researches on controlling tearing modes and driving plasma rotation by static and dynamic resonant magnetic perturbations (RMPs). The saddle coils will be energized by DC with the amplitude up to 10 kA, or AC with maximum amplitude up to 5 kA within the frequency range of 1–5 kHz. At DC mode two antiparallel 6-pulse phase thyristor rectifiers are chosen to obtain bidirectional current, while at AC mode an AC–DC–AC converter including a series resonant inverter can generate current of various amplitudes and frequencies. The paper presents the design of the power supply system, based on the definition of the power supply requirements and the feasibility of implementation of the topology and control strategy. Some simulation and experimental results are given in the end.     

The anode power supply for the ECRH system on the J-TEXT tokamak

The electron cyclotron resonance heating(ECRH) system with a 60 GHz/200 k W/0.5 s gyrotron donated by the Culham Science Center is being developed on the J-TEXT tokamak for plasma heating, current drive and MHD studies. Simultaneously, an anode power supply(APS) has been rebuilt and tested for the output power control of the gyrotron, of which the input voltage is derived from an 80 k V negative cathode power supply. The control strategy by controlling the grid voltage of the tetrode TH5186 is applied to obtain an accurate anode climbing voltage, of which the output voltage can be obtained from 0-30 k V with respect to the cathode power supply. The characteristics of the APS, including control, protection, modulation, and output waveform, were tested with a100 k V/60 A negative cathode power supply, a dummy load and the ECRH control system. The results indicate that the APS can meet the requirements of the ECRH system on J-TEXT.     

Design and realization of the J-TEXT tokamak central control system

The Joint Texas Experimental Tokamak (J-TEXT), a medium-sized conventional tokamak, serves as a user experimental facility in the China-USA fusion research community. Development of a flexible and easy-to-use J-TEXT central control system (CCS) is of supreme importance for users to coordinate the experimental scenarios with full integration into the discharge operation. This paper describes in detail the structure and functions of the J-TEXT CCS system as well as the performance in practical implementation. Results obtained from both commissioning and routine operations show that the J-TEXT CCS system can offer a satisfactory and effective control that is reliable and stable. The J-TEXT tokamak achieved high-quality performance in its first-ever experimental campaign with this CCS system.     

Equilibrium Reconstruction and Its Integration in J-TEXT

The equilibrium fitting code(EFIT) and its application in the J-TEXT tokamak are integrated by the Matlab language. The function of analysis and visualization to the results is added. In addition, the experiment data measured by soft X-ray(SXR) are used to calculate plasma equilibrium as a constraint condition. The improved EFIT code is used for J-TEXT discharge and the profiles of plasma parameters such as flux function, safety factor q, pressure and current density are obtained from the reconstructed configurations.     

On the Transition Regime of Nonlinear Error Field Penetration in Toroidal Plasmas

The error field penetration is numerically studied in the frame of the visco-resistive magnetohydrodynamics(MHD) model.A transition scaling is obtained to link the Rutherford and Waelbroeck regimes in the nonlinear phase of error field penetration process.Furthermore,a transition density scaling of[b_r/B_T]_(crit)~n_e~(1/2) obtained in accord with recent experimental observations in the J-TEXT tokamak.     

The timing system on the J-TEXT tokamak

This paper describes the timing system designed to control the operation time-sequence and to generate clocks for various sub-systems on J-TEXT tokamak. The J-TEXT timing system is organized as a distributed system which is connected by a tree-structured optical fiber network. It can generate delayed triggers and gate signals (0 μs–4000 s), while providing reference clocks for other sub-systems. Besides, it provides event handling and timestamping functions. It is integrated into the J-TEXT Control, Data Access and Communication (J-TEXT CODAC) system, and it can be monitored and configured by Experimental Physics and Industrial Control System (EPICS). The configuration of this system including tree-structured network is managed in XML files by dedicated management software. This system has already been deployed on J-TEXT tokamak and it is serving J-TEXT in daily experiments.     

Design of Digital Multi-Radian Phase Detector on J-TEXT

A real time system used to detect phase difference between two sinusoidal signals is proposed in this paper. The system is designed to process the phase signal of the far-infrared (FIR) hydrogen cyanide (HCN) interferometer on J-TEXT. It is based on zero-crossing detection and makes use of the digital circuit. Compared with a traditional zero-crossing phase detector, it doesn’t need to sacrifice the time resolution to expand the phase range. The phase difference is divided into two parts, the integer part and the fraction part. In each detecting cycle, they are detected separately. It outputs digital signals that are more stable for transmission. A prototype was built on J-TEXT using discrete components. A practical method is proposed to deal with the counting error caused by the deviation of electronic components in manufacture. Reasonable results were obtained on the prototype. The phase resolution reaches 2π/64 in test, and can still be improved by raising the clock frequency.     

Recent progress of the ECRH system and initial experimental results on the J-TEXT tokamak

In order to broaden the range of the plasma parameters and provide experimental conditions for physical research into high-performance plasma, the development of the electron cyclotron resonance heating (ECRH) system for the J-TEXT tokamak was initiated in 2017. For the first stage, the ECRH system operated successfully with one 105 GHz/500 kW/1 s gyrotron in 2019. More than 400 kW electron cyclotron (EC) wave power has been injected into the plasma successfully, raising the core electron temperature to 1.5 keV. In 2022, another 105 GHz/500 kW/1 s gyrotron completed commissioning tests which signifies that the ECRH system could generate an EC wave power of 1 MW in total. Under the support of the ECRH system, various physical experiments have been carried out on J-TEXT. The electron thermal transport in ECRH plasmas has been investigated. When ECRH is turned on, the electron thermal diffusivity significantly increases. The runaway current is elevated when a disruption occurs during ECRH heating. When the injected EC wave power is 400 kW, the conversion efficiency of runaway current increases from 35% to 75%. Fast electron behavior is observed in electron cyclotron current drive (ECCD) plasma by the fast electron bremsstrahlung diagnostic (FEB). The increase in the FEB intensity implies that ECCD could generate fast electrons. A successful startup with a 200 kW ECW is achieved. With the upgrade of the ECRH system, the J-TEXT operational range could be expanded and further relevant research could be conducted.     

Application of 3D MHD equilibrium calculation to RMP experiments in the J-TEXT tokamak

The application of resonant magnetic perturbation(RMP) coils could break the initial axisymmetry and change the magnetic topology in tokamak systems. To understand the plasma equilibrium response to the RMP fields, three-dimensional(3 D) non-linear magnetohydrodynamics equilibrium calculations have been carried out using the HINT code for an RMP field-penetration experiment on J-TEXT. The HINT code does not assume perfectly nested flux surfaces, and is able to consider directly the change of magnetic topology due to the RMP field penetrations. Correlations between 3 D equilibrium calculations and experimental observations are presented. The magnetic topologies calculated by HINT were compared with the field topologies obtained from a vacuum approximation method. It turns out that the effects of redistribution of plasma pressure and current due to the formation of magnetic islands at various resonant rational surfaces should be considered self-consistently for understanding the change of magnetic structure. Such changes include changes in the shape and size of magnetic islands, and the distribution of stochastic fields around the magnetic islands and at the plasma boundary, which plays an important role for plasma-wall interactions.     

Magnetic diagnostics for magnetohydrodynamic instability research and the detection of locked modes in J-TEXT

Magnetohydrodynamic(MHD) instabilities are widely observed during tokamak plasma operation. Magnetic diagnostics provide important information which supports the understanding and control of MHD instabilities. This paper presents the current status of the magnetic diagnostics dedicated to measuring MHD instabilities at the J-TEXT tokamak; the diagnostics consist of five Mirnov probe arrays for measuring high-frequency magnetic perturbations and two saddle-loop arrays for low-frequency magnetic perturbations, such as the locked mode. In recent years, several changes have been made to these arrays. The structure of the probes in the poloidal Mirnov arrays has been optimized to improve their mechanical strength, and the number of in-vessel saddle loops has also been improved to support better spatial resolution. Due to the installation of high-field-side(HFS) divertor targets in early 2019,some of the probes were removed, but an HFS Mirnov array was designed and installed behind the targets. Owing to its excellent toroidal symmetry, the HFS Mirnov array has, for the first time at J-TEXT, provided valuable new information about the locked mode and the quasi-static mode(QSM) in the HFS. Besides, various groups of magnetic diagnostics at different poloidal locations have been systematically used to measure the QSM, which confirmed the poloidal mode number m and the helical structure of the QSM. By including the HFS information, the 2/1 resonant magnetic perturbation(RMP)-induced locked mode was measured to have a poloidal mode number m of ～2.     

The J-TEXT CODAC system design and implementation

Inspired by the ITER COntrol, Data Access and Communication (CODAC) and ITER instrumentation and control system, J-TEXT tokamak has upgraded its control system with J-TEXT CODAC system. The J-TEXT CODAC system is based on Experimental Physics and Industrial Control System (EPICS). The J-TEXT CODAC system covers everything in the J-TEXT control system including both central and plant control systems, similar to the ITER I&C system. J-TEXT CODAC system is built around a single central control system called Central CODAC system. All the control functions including conventional control, interlock, safety and other common services are supervised by CCS. The J-TEXT CODAC system has been implemented and tested on J-TEXT. It not only tests some of the ideas in ITER CODAC in real life, but also explores the feasibility of new approaches that is unique in J-TEXT CODAC system.     

Tokamak Plasma Flows Induced by Local RF Forces

The tokamak plasma flows induced by the local radio frequency(RF) forces in the core region are analyzed. The effective components of local RF forces are composed of the momentum absorption term and the resonant parallel momentum transport term(i.e. the parallel component of the resonant ponderomotive forces). Different momentum balance relations are employed to calculate the plasma flows depending on different assumptions of momentum transport.With the RF fields solved from RF simulation codes, the toroidal and poloidal flows by these forces under the lower hybrid current drive and the mode conversion ion cyclotron resonance heating on EAST-like plasmas are evaluated.     

Numerical simulation of plasma response to externally applied resonant magnetic perturbation on the J-TEXT tokamak

Nonlinear magnetohydrodynamic (MHD) simulations of an equilibrium on the J-TEXT tokamak with applied resonant magnetic perturbations (RMPs) are performed with NIMROD (non-ideal MHD with rotation, open discussion). Numerical simulation of plasma response to RMPs has been developed to investigate magnetic topology, plasma density and rotation profile. The results indicate that the pure applied RMPs can stimulate 2/1 mode as well as 3/1 mode by the toroidal mode coupling, and finally change density profile by particle transport. At the same time, plasma rotation plays an important role during the entire evolution process.     

Realization of divertor configuration discharge in J-TEXT tokamak

To extend the operation region of the Joint-Texas Experimental tokamak (J-TEXT) to the divertor configuration and even the H-mode, the divertor configuration discharge has been realized for the first time in the J-TEXT tokamak. Along with the establishment of a power supply for the divertor configuration, the construction of relevant diagnostics, and the installation of the divertor target on the high-field side, divertor discharge has been tested. Through the equilibrium calculation and position stability analysis, the control strategy has evolved to be more stable. High-density experiments and auxiliary heating experiments have been carried out on the divertor configuration. The special midplane single-null (MSN) divertor configuration is shown 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 loaded on the divertor target tiles and causes more intensive recycling and impurity release. The future plan for the divertor configuration operation in the J-TEXT tokamak is also included.     

Investigation of electron cyclotron current drive efficiency on the J-TEXT tokamak

Electron cyclotron current drive (ECCD) efficiency research is of great importance for the neoclassical tearing mode (NTM) stabilization. Improving ECCD efficiency is beneficial for the NTM stabilization and the ECCD power threshold reduction. ECCD efficiency has been investigated on the J-TEXT tokamak. The electron cyclotron wave (ECW) power scan was performed to obtain the current drive efficiency. The current drive efficiency is derived to be approximately η₀ = (0.06–0.16) × 10¹⁹ A m⁻² W⁻¹ on the J-TEXT tokamak. The effect of the residual toroidal electric field has been included in the determination of the current drive efficiency, which will enhance the ECCD efficiency. At the plasma current of I_p = 100 kA and electron density of n_e = 1.5 × 10¹⁹ m⁻³, the ratio of Spitzer conductivity between omhic (OH) and ECCD phases is considered and the experimental data have been corrected. The correction results show that the current drive efficiency η1 caused by the fast electron hot conductivity decreases by approximately 79%. It can be estimated that the driven current is approximately 24 kA at 300 kW ECW power.     

Neural Network Prediction of Disruptions Caused by Locked Modes on J-TEXT Tokamak

Prediction of disruptions caused by locked modes using the Back-Propagation （BP） neural network is completed on J-TEXT tokamak. The network, which is based on the BP neural network, uses Mirnov coils and locked mode coils signals as input data, and outputs a signal including information of prediction of locked mode. The rate of successful prediction of locked modes is more than 90%. For intrinsic locked mode disruptions, the network can give a prewarning signal about 1 ms ahead of the locking-time. For the disruption caused by resonant magnetic perturbation （RMPs） locked modes, the network can give a prewarning signal about 10 ms ahead of the locking-time.     

Design of the high voltage isolation transmission module with low delay for ECRH system on J-TEXT

As a flexible auxiliary heating method,the electron cyclotron resonance heating(ECRH) has been widely used in many tokamaks and also will be applied for the J-TEXT tokamak.To meet requirements of protection and fault analysis for the ECRH system on J-TEXT,signals of gyrotrons such as the cathode voltage and current,the anode voltage and current,etc should be transmitted to the control and data acquisition system.Considering the high voltage environment of gyrotrons,isolation transmission module based on FPGA and optical fiber communication has been designed and tested.The test results indicate that the designed module has strong anti-noise ability,low error rate and high transmission speed.The delay of the module is no more than 5 μs which can fulfill the requirements.