Joint Texas Experimental Tokamak Research Articles

Investigation on the edge cooling threshold of the density limit in the J-TEXT tokamak with limiter and divertor configurations

Cooling of the plasma edge is widely considered to be a key element in the density limit (DL) of tokamaks. This paper investigates the critical edge cooling threshold of the DL by exploring various plasma configurations in the joint-Texas experimental tokamak. Notably, significant differences in the edge electron temperature in the vicinity of the last closed flux surface were observed between the limiter and divertor configurations. However, the electron temperature drops to a similar level in the vicinity of q= 3 surface close to DL, independent of the magnetic field configuration. In addition, to evaluate the reliability of the critical edge cooling threshold, experiments were conducted by increasing the carbon impurity content to enhance the edge cooling rate. These experiments involved two approaches to increase the carbon impurity content: methane injection and penetration of a graphite solid source. Results from these experiments indicate that the temperature threshold of the q = 3 surfaces remains consistent even with a stronger edge cooling rate. The consistency observed in the electron temperature threshold near the q = 3 surface, regardless of magnetic configuration or edge cooling rate, could help refine existing theoretical and simulation works and improve the prediction accuracy of the DL disruption.

The behaviour of the argon transport with electron cyclotron resonance heating in J-TEXT

The transport of impurities is very important for burning plasma; in particular, the accumulation of highly charged impurities will lead to the deterioration of plasma performance and trigger disruption by radiation losses. In order to study impurity transport in Joint Texas EXperimental Tokamak electron cyclotron resonance heating (ECRH) plasmas, argon has been injected at the discharge flattop in an amount approximately 1% that of fuelling hydrogen gas to ensure no significant effect on the discharge. Experimental results show that precursor oscillations with a sawtooth appearance are beneficial for argon ion transport to the wall. The argon behaviour has been modulated by ECRH, with the power deposited inside/outside the sawtooth inverse radius. The ECRH power deposited outside the sawtooth inverse radius can induce larger precursor oscillations in the end of sawtooth and promote the argon transport. The ECRH power deposited inside the sawtooth inverse radius can induce oscillations in the mid-phase of the sawtooth and decrease the core toroidal rotation velocity, which can enhance argon transport. The results of the analysis that these oscillations can lead to an outward convection velocity, which means that the argon ions are transported to the wall. In addition, the oscillations in the mid-phase of the sawtooth can strongly enhance the argon transport.

Observation of the poloidally asymmetrical density perturbation of sawtooth collapse on J-TEXT

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 found that the increase in density in the region between the inversion radius and mixing radius is poloidally asymmetrical, while the increase in temperature is 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, and this can shed new light on the understanding of sawtooth collapse.

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.

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.

Effect of equilibrium radial position on horizontal instability of middle single null divertor configuration in the iron core tokamak

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 amplitude and guarantees the controllable horizontal displacement.

Upgrade of vacuum ultraviolet spectroscopy system on J-TEXT tokamak.

The vacuum ultraviolet (VUV) spectroscopy system on the Joint Texas Experimental Tokamak has been upgraded to achieve fast acquisition for the study of impurity transport in transient modulated experiments. In this upgrade, the previous high-energy charge-coupled device detector was replaced by a microchannel plate with a CsI-coated photocathode and P43 phosphor to transform the VUV light to visible light, which is then acquired by a high-speed electron-multiplying charge-coupled device. Two-stage focusing was achieved using a reference slit plate illuminated successively by a green light source and the Lyman series hydrogen spectral lines from the vacuum-conditioning plasma. The spatial resolution was evaluated as ∼4mm based on the level of image blurring from the alignment plate. A response time of ∼2ms was obtained with the ten-vertical-track setup.

Fast Protection Unit Designed for the Electron Cyclotron Resonance Heating System on J-TEXT

Two types of fast protection units are designed and established to ensure the safe operation of the electron cyclotron resonance heating (ECRH) system on Joint Texas Experimental Tokamak (J-TEXT). One is the overvoltage and overcurrent fast protection unit; and the other is the arc fast protection unit. All of them are used to send protection signals to the control unit for switching off the high-voltage power supplies of the gyrotron when there is a fault. The circuits based on two types of fast protection units are designed and tested. The response time of the overvoltage and overcurrent fast protection circuit is less than 900 ns, and the response time of the arc fast protection circuit is less than $1.2~\mu \text{s}$ . The test results show that these two fast protection units can fulfill the safe operation requirements of the ECRH system on J-TEXT.

Study of intrinsic rotation by the Gyrokinetic Electromagnetic Numerical Experiment code in the Joint Texas Experimental Tokamak

The core toroidal plasma intrinsic rotation has been studied by experiments and simulations in the Joint Texas Experimental Tokamak (J-TEXT). The direction of core intrinsic rotation in the J-TEXT plasma is counter-current. As the plasma density ramps up, the rotation velocity increases in the counter-current direction. By comparing four different electron densities, linear local gyrokinetic simulations have been performed by the Gyrokinetic Electromagnetic Numerical Experiment code for the first time on J-TEXT. It is found that the most dominant turbulence is the ion temperature gradient at $0.2a$ , where $a$ is the minor radius of the plasma and this is unchanged during the plasma density ramp up. By scanning the radial wave vectors, it is found that the residual stress term reverses from negative to positive when the plasma density exceeds a certain threshold. The pinch term is larger than the residual stress term at all four electron densities, which means that the pinch term is always dominant in the core of a J-TEXT plasma.

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.

Vertical fast electron bremsstrahlung diagnostic on J-TEXT tokamak

Fast electron bremsstrahlung (FEB) emission during Ohmic discharge experiments on the Joint Texas Experimental Tokamak (J-TEXT) has been measured by a recently developed vertical multi-channel FEB diagnostic based on CdZnTe detectors. There are 5 sight lines to observe the vertical emission of fast electrons at the high-field side with a spatial resolution of 5 cm. The FEB emission in the energy range of 30-300 keV can be measured. The generation of fast electrons accelerated by loop voltage has been confirmed during the early phase of discharge by analyzing the signals of FEB emission. The runaway electron beam instabilities have been observed with the FEB diagnostic on J-TEXT.

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.

Measurement of electron temperature fluctuations on J-TEXT via correlation ECE.

The capabilities of the joint-Texas experimental tokamak correlation electron cyclotron emission (CECE) diagnostic have recently been extended with an upgrade. Four new yttrium iron garnet (YIG) filters from 4 GHz to 18 GHz with a bandwidth of 90 ∼ 230 MHz are added to the previous 4 channels. Optical optimization of the transmission line has improved the poloidal resolution, which allows k θ < 3.08 cm-1. The improvement of video amplifiers allows the frequency and amplitude gain to be adjusted discretely from 200 kHz to 1 MHz and from 200 to 1000, respectively, for different situations. A controller is designed to remotely adjust the center frequency of the YIG filters. Based on the CECE, the distribution and the effect of magnetohydrodynamic instabilities on electron temperature fluctuations have been observed. The experiment results show good performance of the upgraded CECE diagnostic.

Design and development of a synchronized operation control system for Thomson scattering diagnostic on J-TEXT

A Thomson scattering diagnostic system is under construction at the Joint Texas Experimental Tokamak (J-TEXT). A 1064 nm Nd:YAG laser with 50 Hz repetition rate is used as the laser source. We have used a software for careful and precise control of the laser through serial communication. A time sequence operating system has been developed to synchronize the laser control and data acquisition system with the central control system (CSS). The system operates commands from the CSS of J-TEXT and generates triggers for the laser and data acquisition system in the proper sequence. It also measures an asynchronous time value that is needed for accurate time stamping. All functions are served by a field-programmable gate array development platform that is suitable for high-speed data and signal processing applications. Several embedded peripherals, including Ethernet and USB 2.0, provide communication with the CSS and the server.

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.

Research of an Energy Processing System for Disposing Plasma Electromagnetic Energy During Disruption in Tokamak

During a disruption, plasma electromagnetic energy usually damages tokamak devices severely. Thus, this paper presents a five-part energy processing system (EPS) to dispose this energy. The five parts of the EPS are as follows: the metal oxide varistors employed to limit overvoltage and to absorb energy, the resistor module applied to consume energy, the pulse capacitor module used to store energy, the inverter module used for energy feedback, and the controller. These parts are described, and their key design features are introduced. The influence of different capacitances and resistances on the amount of energy transferred is investigated. The simulation results obtained by the Power Simulation program are given. The experimental and simulation results show that about 20%–30% of the total plasma electromagnetic energy can be transferred out of the tokamak device and effectively disposed by the EPS during the disruptions in the Joint Texas Experimental Tokamak (J-TEXT) tokamak. Moreover, the loop voltage during a disruption can be inhibited and decreases about 40%.

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.

Measurement of diagnostic neutral beam parameters on J-TEXT.

A Doppler frequency shift spectrum (DFSS) system composed of two spectrometers has been developed for the joint Texas experimental tokamak to measure diagnostic neutral beam parameters including the beam energy fractions, intensity distributions, and divergences. The beam energy fractions are derived from measurements of H-alpha (Hα) emission using collisional excitation cross sections. The beam intensity distributions are obtained using an 11-channel measurement with a reconstruction technique. The beam divergences are obtained from spectrum broadening and geometric calculations. The results of preliminary investigations indicate that the DFSS system works well and can be used to obtain all of these parameters simultaneously. According to the preliminary experiment, the one-third energy fraction has the largest proportion (about 45%) of the beam energy and the full energy fraction is about 10%. The beam diameter is about 8.1 cm at a distance of 2.04 m from the accelerator. The beam divergence angle is about 3.3°. The current beam parameters are insufficient for charge-exchange measurements.

Design of Thomson scattering diagnostic system on J-TEXT.

An infrared multi-channel Thomson scattering diagnostic system is designed from the viewpoint of development of the proposed system on the Joint Texas Experimental Tokamak (J-TEXT). A 3 J/50 Hz Nd:YAG laser, which is injected vertically into plasma in the direction from top to bottom, serves as the power source of the system. The scattering light is then collected horizontally and is transmitted to an interference-filter avalanche photodiode based polychromater for spectrum analysis. The system covers the half plasma cross section, providing 14 spatial points with 2 cm resolution. The proposed system can thus satisfy the requirements of the J-TEXT at present and in the near future. A detailed description of the system design is presented in this paper.

Measurement of the electron and ion temperatures by the x-ray imaging crystal spectrometer on joint Texas experimental tokamak.

An x-ray imaging crystal spectrometer has been developed on joint Texas experimental tokamak for the measurement of electron and ion temperatures from the Kα spectra of helium-like argon and its satellite lines. A two-dimensional multi-wire proportional counter has been applied to detect the spectra. The electron and ion temperatures have been obtained from the Voigt fitting with the spectra of helium-like argon ions. The profiles of electron and ion temperatures show the dependence on electron density in ohmic plasmas.