Joint Texas Experimental Tokamak Tokamak Research Articles

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.

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.

Design of a shattered pellet injection system on J-TEXT tokamak.

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.

Response of plasma rotation to resonant magnetic perturbations in J-TEXT tokamak

The response of plasma toroidal rotation to the external resonant magnetic perturbations (RMP) has been investigated in Joint Texas Experimental Tokamak (J-TEXT) ohmic heating plasmas. For the J-TEXT’s plasmas without the application of RMP, the core toroidal rotation is in the counter-current direction while the edge rotation is near zero or slightly in the co-current direction. Both static RMP experiments and rotating RMP experiments have been applied to investigate the plasma toroidal rotation. The core toroidal rotation decreases to lower level with static RMP. At the same time, the edge rotation can spin to more than 20 km s−1 in co-current direction. On the other hand, the core plasma rotation can be slowed down or be accelerated with the rotating RMP. When the rotating RMP frequency is higher than mode frequency, the plasma rotation can be accelerated to the rotating RMP frequency. The plasma confinement is improved with high frequency rotating RMP. The plasma rotation is decelerated to the rotating RMP frequency when the rotating RMP frequency is lower than the mode frequency. The plasma confinement also degrades with low frequency rotating RMP.

The fast reciprocating magnetic probe system on the J-TEXT tokamak.

The fast reciprocating magnetic probe (FRMP) system is newly developed on the Joint Texas Experimental Tokamak (J-TEXT) to measure the local magnetic fluctuations at the plasma edge. The magnetic probe array in the FRMP consists of four 2-dimensional magnetic probes arranged at different radial locations to detect local poloidal and radial magnetic fields. These probes are protected by a graphite and boron nitride casing to improve the frequency response of each probe; they are mounted on the head of a movable rod, which is oriented along radial direction at the top of the torus. In the experiments, multiple core diagnostics show that the insertion of the FRMP has little impact on the equilibrium of the plasma. Local magnetic fluctuations inside the last closed flux surface are successfully measured by the FRMP.

The Construction of Plasma Density Feedback Control System on J-TEXT Tokamak**supported by the National Magnetic Confinement Fusion Science Program (Nos. 2014GB103001 and 2013GB106001) and National Natural Science Foundation of China (Nos. 11305070 and 11105028)

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 of charge exchange recombination spectroscopy for the joint Texas experimental tokamak.

The old diagnostic neutral beam injector first operated at the University of Texas at Austin is ready for rejoining the joint Texas experimental tokamak (J-TEXT). A new set of high voltage power supplies has been equipped and there is no limitation for beam modulation or beam pulse duration henceforth. Based on the spectra of fully striped impurity ions induced by the diagnostic beam the design work for toroidal charge exchange recombination spectroscopy (CXRS) system is presented. The 529 nm carbon VI (n = 8 - 7 transition) line seems to be the best choice for ion temperature and plasma rotation measurements and the considered hardware is listed. The design work of the toroidal CXRS system is guided by essential simulation of expected spectral results under the J-TEXT tokamak operation conditions.

Service-Oriented Remote Operation System for J-TEXT Tokamak

To fully use the tokamak facility, it is necessary to allow researchers to remotely participate in the tokamak experiments via the Internet. Being able to control the operation of the tokamak is a key point. In this paper, the design and implementation of the remote operation system for the Joint Texas Experimental Tokamak (J-TEXT) are presented. To make this system flexible, the service-oriented design is applied. Instead of just providing software or website that controls devices and instruments on the tokamak, a series of Application Programming Interfaces are exposed as web services. All the operations of the J-TEXT tokamak are wrapped in these services. The representational state transfer design practice has been adopted to ensure that these services are accessible by various clients. A remote operation website based on these services has been developed. However, users can build their own websites or remote control rooms, which consume these services to operate the J-TEXT tokamak. An authorization and authentication system is included to prevent the users from consuming unauthorized services. The proposed remote operation system is compatible with Experimental Physics and Industrial Control System and has been integrated within the J-TEXT control, data access, and communication system.

Design of FPGA based high-speed data acquisition and real-time data processing system on J-TEXT tokamak

Tokamak experiment requires high-speed data acquisition and processing systems. In traditional data acquisition system, the sampling rate, channel numbers and processing speed are limited by bus throughput and CPU speed. This paper presents a data acquisition and processing system based on FPGA. The data can be processed in real-time before it is passed to the CPU. It provides processing ability for more channels with higher sampling rates than the traditional data acquisition system while ensuring deterministic real-time performance. A working prototype is developed for the newly built polarimeter–interferometer diagnostic system on the Joint Texas Experimental Tokamak (J-TEXT). It provides 16 channels with 120MHz maximum sampling rate and 16bit resolution. The onboard FPGA is able to calculate the plasma electron density and Faraday rotation angel. A RAID 5 storage device is adopted providing 700MB/s read–write speed to buffer the data to the hard disk continuously for better performance.

Experimental study of the runaway current in the J-TEXT Tokamak

Major plasma disruptions in tokamaks often generate runaway currents, which contain electrons with energies of several tens of megaelectron-volts (MeV). These currents can cause substantial damage when control is lost and the current hits the limiters or the vessel wall. The interaction between the runaway electrons and the impurities inside the plasma results in soft X-ray emission, which can provide detailed information about the runaway generation process and the confinement of runaway electrons. A vertical soft X-ray array at the top of Joint Texas Experimental Tokamak (J-TEXT) was used to study the runaway beams resulting from major disruptions. Runaway electron production and confinement of runaway current were observed by using soft X-ray images.

Observation of inverse sawtooth activity on central soft X-ray emission during impurity injection in the J-TEXT tokamak

Abstract Short- and long-time lasting inverse sawtooth activities during neon (Ne) gas injection, not during helium (He) or argon (Ar) though, have been clearly observed on soft X-ray emissions from chords viewing the central plasma region of the Joint Texas Experimental Tokamak (J-TEXT). A self-consistent interpretation of the inverse sawtooth activity depending on the impurity radiation characteristics is proposed. It is evidenced that the radiation loss caused by the injection of noble gas not only depends on the amount of the noble gas itself, but also depends on the temperature region where the noble gas can reach.

Robust control design for the plasma horizontal position control on J-TEXT Tokamak

It is extremely important for tokamak to control the plasma position during routine discharge. However, the model of plasma in tokamak usually contains much of the uncertainty, such as structured uncertainties and unmodeled dynamics. Compared with the traditional PID control approach, robust control theory is more suitable to handle this problem. In the paper, we propose a H∞ robust control scheme to control the horizontal position of plasma during the flat-top phase of discharge on Joint Texas Experimental Tokamak (J-TEXT) tokamak. First, the model of our plant for plasma horizontal position control is obtained from the position equilibrium equations. Then the H∞ robust control framework is used to synthesize the controller. Based on this, an H∞ controller is designed to minimize the regulation/tracking error. Finally, a comparison study is conducted between the optimized H∞ robust controller and the traditional PID controller in simulations. The simulation results of the H∞ robust controller show a significant improvement of the performance with respect to those obtained with traditional PID controller, which is currently used on our machine.

Experimental Studies of Electrostatic Fluctuations and Turbulent Transport in the Boundary of J-TEXT Tokamak Using Reciprocating Probe

As the basic of a deeper investigation on the turbulent transport, the fluctuation property in the boundary of the newly-reconstructed Joint Texas Experimental Tokamak (J-TEXT) is studied experimentally using the reciprocating Langmuir four-tip probe, which has been built and operated as the primary diagnostic tool in the boundary of J-TEXT tokamak. In this paper, spatial profiles of the plasma-edge parameters are obtained, such as electron temperature, plasma density, plasma potential, poloidal electric field and their fluctuations. The results indicate the existence of a Er×BT shear layer at the vicinity of last closed flux surface (LCFS), with the fluctuations suppressed in varying degrees. The turbulence-induced particle and energy fluxes can be calculated by the local plasma parameters above. Convection dominates the cross-field turbulent transport in boundary plasma. Electrostatic fluctuations properties are also studied in detail with the help of numerical analysis. Statistical analysis on density fluctuation shows that, the intermittency can affect the turbulence in the scrape-off layer (SOL).

Tearing mode suppression by using resonant magnetic perturbation coils on J-TEXT tokamak

Abstract A series of experiments on the interactions between external resonant magnetic perturbations (RMP) and plasmas has recently been conducted, using static resonant magnetic perturbation (SRMP) coils on the Joint Texas Experimental Tokamak (J-TEXT). The tearing mode can be completely suppressed by applying SRMP. However, the locked mode is likely to be stimulated under a larger SRMP field even though the tearing mode has been first suppressed. A numerical simulation shows three typical regimes of RMPʼs effects on the tearing mode, which are consistent with experimental results.

New printed circuit boards magnetic coils in the vacuum vessel of J-TEXT tokamak for position measurement

Four sets of magnetic diagnostic coils, which are printed on machinable ceramic printed circuit boards (PCB), are designed, fabricated, installed, and tested in the Joint Texas Experimental Tokamak (J-TEXT) vacuum vessel for detecting the plasma radial and vertical displacements relative to the geometric center of the vacuum vessel in Ohmic discharges. Each coordinate is determined by a pair of variable cross-section Rogowski and saddle coils, which measure the tangential and normal magnetic fields (relative to the coil surface). These coils are suitable for mass production and offer advantages in vacuum compatibility and temperature tolerance that are important for J-TEXT. Position measurements using PCB coils are compared with those from soft x-ray image system and match the position well.

The vacuum system of the J-TEXT tokamak

A new vacuum system is designed and developed on the Joint Texas Experimental tokamak (J-TEXT), which consists of two pumping stations, a Taylor discharge cleaning (TDC) system and gas puffing. With this system, the vacuum vessel of the J-TEXT tokamak can easily reach the vacuum conditions which allow the operation of the machine. The sub-control system of the vacuum pumping systems with the self-protection function is reliable and easy to use. Inside the chamber, the base pressure can be lowered to 6.4×10−6 Pa, and the total leakage and outgassing rates in 12 h is below 1.5×10−8 Pa m3/s. Moreover, the TDC sub-system is used to clean the first facing walls of vacuum chamber to guarantee good wall conditions of the device before each run of the experimental campaign on the J-TEXT tokamak. The basic functions of the vacuum system meet the essential requirements for the present experiments on the J-TEXT tokamak.

Soft X-ray imaging diagnostic system on the J-TEXT tokamak

A soft X-ray (SXR) imaging system composed of three miniature SXR pinhole array camera designed and installed on the joint texas experimental tokamak (J-TEXT) for the study of plasma equilibrium and fluctuation properties during ohmic discharges. The imaging system functions well, giving good resolution and maneuverability. Combined with the use of the biorthogonal decomposition technique, it helps us to analyze the sawtooth crashes and their basic parameters such as reversal surface and modes. Furthermore, the plasma displacement is measured using SXR emission and the results are compared with those from magnetic diagnostic tools. The comparisons verify that the SXR imaging system is a legitimate alternative for non-magnetic plasma offset observation. In addition, it can form the basis of a real-time feedback control system for magnetic-confined plasma.

Startup of Plasma Current in J-TEXT Tokamak Prompted by the Hα Line Emission Criterion

An Hα line-emission detection system was developed on the joint texas experimental tokamak (J-TEXT), which is used to determine the Hα emission level during the gas breakdown and hereafter to control the startup of the plasma current. The detector consists of an Hα interference filter, a focusing lens, a photodiode and a preamplifier. In the J-TEXT operation, the Hα emission is taken as a monitor signal which is highly sensitive to the generation of a plasma. Furthermore, the power supply control system using the above signal as an input is capable of determining whether and when to fire the Ohmic heating capacitor banks, which are applied to drive the plasma current ramp-up. The experimental results confirm that the Hα emission criterion is acceptable for controlling the plasma current promotion in the J-TEXT tokamak.

The printed circuit board Rogowski coils for plasma current measurements on the J-TEXT tokamak

Compared with the conventional Rogowski coils made by winding wire loops, a printed circuit board (PCB) based on Rogowski coils has some favourable advantages for current measurements, such as large effective area, good linearity, high frequency response, precise geometry and thin profile. This type of Rogowski coil has been widely applied in electrical systems as the current sensor. Nevertheless, the PCB Rogowski coils are introduced for the first time to measure the plasma current on the Joint Texas Experimental Tokamak (J-TEXT). The measurements have shown that this novel PCB Rogowski coil works perfectly and is very suitable for studies of disruption, equilibrium reconstruction, etc.

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.