Joint TEXT Tokamak Research Articles

Comparative Analysis of the Magnetic Field in J-TEXT Tokamak

Joint TEXT (J-TEXT) is an iron-core tokamak. The iron core can reduce the requirement of heating power supplies, and it also has a significant effect on the poloidal magnetic field during J-TEXT discharge. However, it is difficult to simulate an iron-core tokamak discharge process for many tokamak simulation codes (TSCs), such as TSC and DINA. Thus, the influence of the iron core on the J-TEXT magnetic field is analyzed by the theoretical model and the ANSYS model and compared with experimental results. ANSYS simulation shows that the influence of the iron core on the toroidal magnetic field can be ignored, but it has a significant impact on the poloidal magnetic field. By the comparison of ANSYS simulation and experimental results, the correction coefficient of the poloidal field with and without iron core is obtained, which can be used in some non iron-core TSC.

Development of Q-band X-mode microwave reflectometry for the measurement of edge density profile

A Q-band X-mode homodyne microwave reflectometry has been developed for the measurement of edge electron density profile on the Joint TEXT tokamak (J-TEXT). A voltage controlled oscillator (VCO) with a frequency quadrupler is used as the microwave source. An arbitrary waveform generator (AWG) is adopted to control the sweeping period (20 μs) for the VCO. Homodyne method is chose to simplify the structure of the hardware circuit. In addition, a single antenna scheme which using a directional coupler to realize the sharing of an antenna for transmitting and receiving to save the resources of diagnostic windows has been adopted. For the data processing, the Hilbert transform has been used to demodulate the signal instead of the I/Q phase detector. This reflectometry has been installed on J-TEXT and effectively measured edge density profile.

Design of Q-band FMCW reflectometry for electron density profile measurement on the Joint TEXT tokamak

The Q-band (33–50 GHz) fast sweep frequency modulated continuous wave (FMCW) reflectometry has been recently developed for electron density profile measurement on the Joint TEXT tokamak. It operates in ordinary mode (O-mode) with a 20 μs sweeping period, covering the density range from 1 × 1019 m−3 to 3 × 1019 m−3. On the bench test, a Yttrium Iron Garnet (YIG) filter is used for the dynamic calibration of the voltage controlled oscillator (VCO) to obtain a linear frequency sweep. Besides, the use of a power combiner helps to improve the side-band suppression level of the single side-band modulator (SSBM). The reconstructed density profiles are presented, which demonstrate the capability of the reflectometry.

Electromagneto-mechanical coupling analysis of a test module in J-TEXT Tokamak during plasma disruption

In this paper, the dynamic response during plasma disruption of a test blanket module in vacuum vessel (VV) of the Joint TEXT (J-TEXT), which is an experimental Tokamak device with iron core, was simulated by applying a program developed by authors on the ANSYS platform using its parametric design language (APDL). The moving coordinate method as well as the load transfer and sequential coupling strategy were adopted to cope with the electromagneto-mechanical coupling effect. To establish the numerical model, the influence of the iron core on the eddy current and electromagnetic (EM) force during disruption was numerically investigated at first and the influence was found not significant. Together with the geometrical features of the J-TEXT Tokamak structure, 180° sector models without magnetic core were finally established for the EM field and the structural response simulations. To obtain the source plasma current, the plasma current evolution during disruption was simulated by using the Tokamak Simulation Code (TSC). With the numerical models and the source plasma current, the dynamic response of both the VV structure and the test module were calculated. The numerical results show that the maximum stress of the test module is in safe range, and the magnetic damping effect can weaken vibration of the test module. In addition, simulation without considering the coupling effect was carried out, which shows that the influence of coupling effect is not significant for the peak stress of the J-TEXT disruption problem.

Error Field Analysis of Joint TEXT Tokamak

This paper presents the study and analysis of the error field of Joint TEXT tokamak. Error fields are first calculated through both the finite-element method and the theoretical method. Then, a Fourier decomposition of the error field is performed on the plasma surface to evaluate the lower mode (m, n) = (1, 1), (2, 1), and (3, 1) error fields. The results of the two methods are notably consistent. Superposition and linearity are also verified for small misalignments. The major sources of error fields are discussed by comparing the error fields caused by different misalignments of various coils. A Monte Carlo simulation, which combines the contribution of all the coils, is performed to obtain the total error fields. A comparison between the calculated and measured error fields is also conducted. The results indicate that the analysis method presented in this paper is very effective in predicting the error field.

Density modulation experiment to determine transport coefficients on Joint-TEXT Tokamak.

Density modulation experiments have been conducted on Joint-TEXT (J-TEXT) Tokamak Ohmic discharge to investigate particle transport based on a model with constant diffusion plus inward convection. Like the HCN interferometer, the newly developed three-wave polarimeter-interferometer system (POLARIS) is used to measure the perturbed density. The comparison of results between the HCN interferometer and POLARIS is given. The consistent results indicate the validity of the analysis scheme. At lower densities, the typical particle confinement time τp is found to increase with electron density, while it saturates at higher densities.

A real-time laser feedback control method for the three-wave laser source used in the polarimeter-interferometer diagnostic on Joint-TEXT tokamak

A three-wave laser polarimeter-interferometer, equipped with three independent far-infrared laser sources, has been developed on Joint-TEXT (J-TEXT) tokamak. The diagnostic system is capable of high-resolution temporal and phase measurement of the Faraday angle and line-integrated density. However, for long-term operation (>10 min), the free-running lasers can lead to large drifts of the intermediate frequencies (∼100-∼500 kHz/10 min) and decay of laser power (∼10%-∼20%/10 min), which act to degrade diagnostic performance. In addition, these effects lead to increased maintenance cost and limit measurement applicability to long pulse/steady state experiments. To solve this problem, a real-time feedback control method of the laser source is proposed. By accurately controlling the length of each laser cavity, both the intermediate frequencies and laser power can be simultaneously controlled: the intermediate frequencies are controlled according to the pre-set values, while the laser powers are maintained at an optimal level. Based on this approach, a real-time feedback control system has been developed and applied on J-TEXT polarimeter-interferometer. Long-term (theoretically no time limit) feedback of intermediate frequencies (maximum change less than ±12 kHz) and laser powers (maximum relative power change less than ±7%) has been successfully achieved.

Equilibrium reconstruction based on core magnetic measurement and its applications on equilibrium transition in Joint-TEXT tokamak.

Evaluation and reconstruction of plasma equilibrium, especially to resolve the safety factor profile, is imperative for advanced tokamak operation and physics study. Based on core magnetic measurement by the high resolution laser polarimeter-interferometer system (POLARIS), the equilibrium of Joint-TEXT (J-TEXT) plasma is reconstructed and profiles of safety factor, current density, and electron density are, therefore, obtained with high accuracy and temporal resolution. The equilibrium reconstruction procedure determines the equilibrium flux surfaces essentially from the data of POLARIS. Refraction of laser probe beam, a major error source of the reconstruction, has been considered and corrected, which leads to improvement of accuracy more than 10%. The error of reconstruction has been systematically assessed with consideration of realistic diagnostic performance and scrape-off layer region of plasma, and its accuracy has been verified. Fast equilibrium transitions both within a single sawtooth cycle and during the penetration of resonant magnetic perturbation have been investigated.

High resolution polarimeter-interferometer system for fast equilibrium dynamics and MHD instability studies on Joint-TEXT tokamak (invited).

A high-performance Faraday-effect polarimeter-interferometer system has been developed for the J-TEXT tokamak. This system has time response up to 1 μs, phase resolution < 0.1° and minimum spatial resolution ∼15 mm. High resolution permits investigation of fast equilibrium dynamics as well as magnetic and density perturbations associated with intrinsic Magneto-Hydro-Dynamic (MHD) instabilities and external coil-induced Resonant Magnetic Perturbations (RMP). The 3-wave technique, in which the line-integrated Faraday angle and electron density are measured simultaneously by three laser beams with specific polarizations and frequency offsets, is used. In order to achieve optimum resolution, three frequency-stabilized HCOOH lasers (694 GHz, >35 mW per cavity) and sensitive Planar Schottky Diode mixers are used, providing stable intermediate-frequency signals (0.5-3 MHz) with S/N > 50. The collinear R- and L-wave probe beams, which propagate through the plasma poloidal cross section (a = 0.25-0.27 m) vertically, are expanded using parabolic mirrors to cover the entire plasma column. Sources of systematic errors, e.g., stemming from mechanical vibration, beam non-collinearity, and beam polarization distortion are individually examined and minimized to ensure measurement accuracy. Simultaneous density and Faraday measurements have been successfully achieved for 14 chords. Based on measurements, temporal evolution of safety factor profile, current density profile, and electron density profile are resolved. Core magnetic and density perturbations associated with MHD tearing instabilities are clearly detected. Effects of non-axisymmetric 3D RMP in ohmically heated plasmas are directly observed by polarimetry for the first time.

Toroidal Field Analysis of the J-TEXT Tokamak

The toroidal field (TF) magnet of the Joint TEXT (J-TEXT) tokamak consists of 16 D-shaped copper coils interlinked in series. In this study, the finite element simulation software ANSYS is used to calculate the toroidal magnetic field and its ripple over the plasma region. Moreover, the possible error field caused by the busbar and the mounting error or noncoaxiality among the TF coils are also calculated. Results show that the busbar lead-in and the possible outward radial shift of a single TF coil will cause a relatively large error field.

Measurement of 2/1 intrinsic error field of Joint TEXT tokamak

The amplitude and spatial phase of the intrinsic error field of Joint TEXT (J-TEXT) tokamak were measured by scanning the spatial phase of an externally exerted resonant magnetic perturbation and fitting the mode locking thresholds. For a typical plasma with current of 180 kA, the amplitude of the 2∕1 component of the error field at the plasma edge is measured to be 0.31 G, which is about 1.8 × 10(-5) relative to the base toroidal field. The measured spatial phase is about 317° in the specified coordinate system (r, θ, ϕ) of J-TEXT tokamak. An analytical model based on the dynamics of rotating island is developed to verify the measured phase.

The reconstruction and research progress of the TEXT-U tokamak in China

The TEXT/(TEXT-U) tokamak, formerly built and operated by the University of Texas at Austin in USA, was dismantled and shipped to China in 2004, and renamed as the Joint TEXT (J-TEXT) tokamak. The reconstruction work, which included reassembly of the machine and development of peripheral devices, was completed in the spring of 2007. Consequently, the first plasma was obtained at the end of 2007. At present, a typical J-TEXT ohmic discharge can produce a plasma with flattop current up to 220 kA and lasting for 300 ms, line-averaged density above 2 × 1019 m−3, and an electron temperature of about 800 eV, with a toroidal magnetic field of 2.2 T. A number of diagnostic devices used to facilitate the routine operation and experimental scenarios were developed on the J-TEXT tokamak. Hence, the measurements of the electrostatic fluctuations in the edge region and conditional analysis of the intermittent burst events near the last closed flux surface were undertaken. The observation and simple analysis of MHD activity and disruption events were also performed. The preliminary experimental results and the future research plan for the J-TEXT are described in detail.