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.     

Plasma equilibrium calculation in J-TEXT tokamak

Plasma equilibrium has been calculated using an analytical method. The plasma profiles of the current density, safety factor, pressure and magnetic surface function are obtained. The analytical solution of the Grad–Shafranov(GS) equation is obtained by the variable separation method and compared with the computed results of the equilibrium fitting code EFIT.     

Equilibrium Reconstruction and Integration of EFIT with Diagnoses in J-TEXT Tokamak

The EFIT program is integrated with the high resolution laser polarimeter interferometer system(POLARIS), the soft X-ray imaging diagnostic system(SXR) and the electron cyclotron emission radiometer(ECE) in the J-TEXT tokamak. Then some internal information about Faraday angle and the position of safety factor q=1 can be obtained as a constraint to EFIT. The modified EFIT code is used to calculate the internal parameters such as flux function, safety factor q, pressure and current density.     

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.     

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.     

Real Time Equilibrium Reconstruction Algorithm in EAST Tokamak

The EAST (HT-7U) superconducting tokamak is a national project of China on fusion research, with a capability of long-pulse (∽1000 s) operation. In order to realize a longduration steady-state operation of EAST,some significant capability of real-time control is required. It would be very crucial to obtain the current profile parameters and the plasma shapes in real time by a flexible control system. As those discharge parameters cannot be directly measured,so a current profile consistent with the magnetohydrodynamic equilibrium should be evaluatedfrom external magnetic measurements, based on a linearized iterative least square method, which can meet the requirements of the measurements. The arithmetic that the EFIT (equilibrium fitting code) is used for reference will be given in this paper and the computational efforts are reduced by parametrizing the current profile linearly in terms of a number of physical parameters.In order to introduce this reconstruction algorithm clearly,the main hardware design will be listed also.     

Development of the HCN heterodyne collective scattering system on J-TEXT

A heterodyne collective scattering system has been designed and developed to investigate the turbulent transport of core plasma on J-TEXT. A dual-HCN laser which consists of two separately pumped HCN gas lasers at 337 μm has been developed as the laser source of the scattering system. The intermediate frequency (IF) is ∼1 MHz when there is a 4 μm cavity length difference and capable to maintain stability more than 5 h without manual operation. Detection channels at three different angles (2 ≤ k⊥ ≤ 12 cm−1) have been installed with Schottky barrier diode mixers of 893 GHz. The sampling frequency of the acquisition system is 6 MHz to observe low-frequency density fluctuations. Initial experimental results have been detected and more results can be expected in future experiments.     

The Construction of Plasma Density Feedback Control System on J-TEXT Tokamak

The plasma density feedback control system(PDFCS) has been established on the Joint Texas Experimental Tokamak(J-TEXT) for meeting the need for an accurate plasma density in physical experiments.It consists of a density measurement subsystem,a feedback control subsystem and a gas puffing subsystem.According to the characteristic of the gas puffing system,a voltage amplitude control mode has been applied in the feedback control strategy,which is accomplished by the proportion,integral and differential(PID) controller.In this system,the quantity calibration of gas injection,adjusted responding to the change of the density signal,has been carried out.Some experimental results are shown and discussed.     

Design and first result of combined Langmuir-magnetic probe on J-TEXT tokamak

In order to measure boundary electrostatic and magnetic fluctuations simultaneously, a combined Langmuir-magnetic probe (CLMP) has been designed and built on joint-Texas experimental tokamak. The probe consists of 8 graphite probe pins and a 3D magnetic probe, driven by a mechanical pneumatic device. By means of simulation, the shielding effect of the graphite sleeve on the magnetic fluctuation signal is explored, and the influence of the eddy current was reduced by cutting the graphite sleeve. In the experiment, it has been verified that the mutual inductance of electromagnetic signals can be ignored, and a 70–90 kHz electromagnetic mode is observed around the last closed magnetic surface. The establishment of CLMP provides data for the exploration of the coupling of electrostatic and magnetic fluctuations.     

Study of the fast electron behavior in electron cyclotron current driven plasma on J-TEXT

In J-TEXT tokamak, fast electron bremsstrahlung diagnostic with 9 chords equipped with multi-channel analyzer enables detailed studies of the generation and transport of fast electrons. The spatial profiles and energy spectrum of the fast electrons have been measured in two ECCD cases with either on-axis or off-axis injection, and the profiles processed by Abel-inversion are consistent with the calculated power deposition locations. Moreover, it is observed that the energy of fast electrons increases rapidly after turning off the ECCD, which may be attributed to the acceleration by the recovered loop voltage at low electron density.     

Suppression of runaway current by electrode biasing and limiter biasing on J-TEXT

The avoidance of runaway electrons (REs) generated during plasma disruption is of great concern for the safe operation of tokamak devices. Experimental study on the suppression of runaway current by electrode biasing (EB) and limiter biasing (LB) has been performed on the J-TEXT tokamak, which could be an alternative way to suppress the runaway current. The experimental results show that the higher the voltage value, the smaller the runaway current in both EB and LB experiments. The runaway current can be completely suppressed at an electrode biased voltage of +450 V and a limiter biased voltage of +300 V. The comparison of the energy spectra during the runaway plateau phase shows that the maximum energy max (ERE) and radiation temperature THXR of hard x-rays (HXRs) are significantly reduced after the application of +200 V limiter biased voltage. The electric field generated by the biased voltage may be the key factor to suppress the runaway current, and the measured radial electric field increases obviously after the voltage is applied. This may result in an increase in the loss of REs to realize the suppression of runaway current.     

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.     

Towards advanced divertor configurations on the J-TEXT tokamak

Developing advanced magnetic divertor configurations to address the coupling of heat and particle exhaust with impurity control is one of the major challenges currently constraining the further development of fusion research. It has therefore become the focus of extensive attention in recent years. In J-TEXT, several new divertor configurations, including the high-field-side single-null poloidal divertor and the island divertor, as well as their associated fundamental edge divertor plasma physics, have recently been investigated. The purpose of this paper is to briefly summarize the latest progress and achievements in this relevant research field on J-TEXT from the past few years.     

基于EPICS的J-TEXT CODAC系统

J-TEXT装置是华中科技大学恢复建造的中型托卡马克装置,已于2007年放电运行,其控制系统采用分布式结构,由多个子系统组成。为提高子系统集成、维护和更新的效率,并有效地管理各子系统、控制装置的运行状态及保障设备和人员安全,J-TEXT装置参考ITER CODAC的设计思路,结合J-TEXT装置的需求设计了J-TEXT CODAC系统。J-TEXT CODAC系统为装置各子系统提供统一的设计模型和相关设计标准,使用EPICS软件作为通讯中间层,设计了全局控制系统、时序和同步控制系统、联锁保护系统,并将原有控制系统改造、集成到J-TEXT CODAC系统中。目前该系统已部署在J-TEXT装置上,在2012年春季以来的多轮实验中运行良好。     

The application of limiter target electrostatic measurement system in J-TEXT tokamak

The limiter target electrostatic measurement system including limiter grounding current sensors and Langmuir probes have been newly developed for the measurement of the limiter target area on the Joint-Texas Experimental tokamak(J-TEXT).Current sensors fixed between graphite limiters and the vacuum vessel walls are used to measure the currents between limiters and vessel wall.Simultaneously,a rectangular poloidal array containing 54 Langmuir probes is embedded in the graphite tiles of limiters for a more localized measurement.Based on this system,the effect of both the plasma's inherent behavior,including plasma motion and the 2/1 tearing mode,and the electrode biasing on probe and sensor signals have been observed and analyzed in the experiments.     

Recent progress on the Tokamak Fusion Test Reactor

The deuterium-tritium (D-T) experiments on the Tokamak Fusion Test Reactor (TFTR) have yielded unique information on the confinement, heating and alpha particle physics of reactor scale D-T plasmas as well as the first experience with tritium handling and D-T neutron activation in an experimental environment. The D-T plasmas produced and studied in TFTR have peak fusion power of 10.7 MW with central fusion power densities of 2.8 MWm^–3 which is similar to the 1.7 MWm^–3 fusion power densities projected for 1,500 MW operation of the International Thermonuclear Experimental Reactor (ITER). Detailed alpha particle measurements have confirmed alpha confinement and heating of the D-T plasma by alpha particles as expected. Reversed shear, highl _i and internal barrier advanced tokamak operating modes have been produced in TFTR which have the potential to double the fusion power to 20 MW which would also allow the study of alpha particle effects under conditions very similar to those projected for ITER. TFTR is also investigating two new innovations, alpha channeling and controlled transport barriers, which have the potential to significantly improve the standard advanced tokamak.     

Recent plasma control progress on EAST

In recent 2 years, various algorithms to control plasma shape, current and density have been implemented or improved for EAST tokamak. These plasma control performances have been verified by either simulated or actual experimental operation, and thus plasma control basis has been established for the long pulse operation and high performance H-mode plasma operation with low hybrid wave (LHW) and ion cyclotron resonance frequency (ICRF) heating. Startup simulation has been done by using TOKSYS code for the plasma breakdown in either 3.1 Wb or 4.5 Wb initial poloidal flux state and the scenarios proved to be robust and used for routine operation. Various shape configurations have been well feedback controlled by using ISOFLUX limited, double-null or single null algorithms based on RTEFIT equilibrium reconstruction. For the long pulse operation, strike point control and magnetics drift compensation have been implemented in the plasma control system (PCS). To improve the operation safety and efficiency, the verification of magnetic diagnostics before plasma breakdown has been demonstrated adequate to prevent a discharge in case of key sensor failure.     

Preliminary Study of the Magnetic Perturbation Effects on the Edge Density Profiles and Fluctuations Using Reflectometers on EAST

The resonant magnetic perturbation(RMP) coils have been successfully designed and installed on the Experimental Advanced Superconducting Tokamak(EAST).Using the reflectometer systems,the density profile and the density fluctuations during magnetic perturbations(MPs) phase have been investigated.During the experiments,two different cases are studied separately:steady MPs and rotating MPs.In both cases,a strongly density pump-out has been observed.In the steady MPs cases,an enhancement of the low frequency(60 kHz)density fluctuations in H-mode phase has been observed.The plasma density boundary out-shifts~ 5%caused by the MPs.The pedestal density gradient is reduced by 50%,while the radial location nearly stays unchanged.In the rotating MPs,the line-averaged density,the D_α emission at the divertor region and the spectrum of the density fluctuations are modulated.The results suggest that the low frequency(60 kHz) density fluctuations may contribute to the strong density pump-out.     

Development of the pellet injection system on the J-TEXT tokamak

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 experiments show that pellet fueling efficiency can reach 50%. The energy confinement time increased by about 7.5‒10 ms after pellet injection.