In the J-TEXT tokamak, the penetration of resonant magnetic perturbations (RMPs) has been studied by using a set of in-vessel RMP coils. It is found that, once the RMP amplitude exceeds a critical value, the applied RMP can lead to field penetration and excitation of a large locked mode in the tearing-stable plasma. The sawtooth oscillations disappear and the confinement deteriorates significantly accompanied by tearing mode excitation. For the plasma with an initial high frequency tearing mode, the RMP can suppress the tearing mode, and field penetration followed with a further increased RMP. The relationship between the RMP penetration threshold and the electron density has been investigated for tearing-stable plasmas. It is found that the penetration threshold increases with the density and scales proportionally to in the ranges of (0.7–2.7) × 1019 m−3. Using the experimental parameters as input, the numerical modelling based on two-fluid equations gives the scaling of , which approximately agrees with the experimental density scaling.

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The effect of resonant magnetic perturbations (RMPs) on particle transport has been studied in J-TEXT tokamak. It is found that for the discharges with an existing saturated 2/1 resistive tearing mode, applied RMPs of moderate amplitude lead to a decrease in electron density with a relative amplitude 3% < |ne/ne0| < 10% in plasma core, and the mode stabilization and electron temperature increase are observed simultaneously in this case. Sufficiently large amplitude of RMPs, however, lead to locked modes and much larger decrease in the electron density as well as in the electron temperature, with |ne/ne0| ~ 20%. For the discharges without 2/1 tearing modes, applied RMPs cause a relative density decrease |ne/ne0| ~ 10% (30%) before (after) field penetration. Using the two-fluid equations and experimental parameters as input, the numerical results approximately agree with experimental observations.

An overview of the recent research work on the J-TEXT tokamak over the last two years is presented. A series of experiments and simulations of the interaction between resonant magnetic perturbations (RMPs) and plasma were carried out on the J-TEXT tokamak. The results show that the m/n = 2/1 (m and n are the poloidal and toroidal mode numbers, respectively) mode locking is obtained with sufficiently large RMPs. And suppression of the m/n = 2/1 tearing mode by moderate magnetic perturbation amplitude is also observed. With experimental parameters as input, both mode locking and mode suppression by RMPs are simulated by nonlinear numerical modelling based on reduced magnetohydrodynamic equations. The simulations are in good agreement with the experimental observations. Density modulation using gas puffing is carried out on J-TEXT to evaluate the particle transport parameters in a typical J-TEXT discharge, including diffusion coefficient and convective velocity. Inverse sawtooth-like activity caused by neon gas injection is observed. The inverse sawtooth-like activity occurs only when the amount of neon impurity exceeds a threshold. Nevertheless, other impurities such as helium and argon cannot trigger such events. With the aid of a soft x-ray detector array, the runaway electron beam following disruptions is visible directly. A high-resolution far infrared polarimeter/interferometer, based on a three-wave technique, was developed and it observes the perturbations associated with sawtooth and tearing mode activities; the first result of the current density profile reconstruction is provided. An x-ray imaging crystal spectrometer is designed to receive the Kα line of Ar XVII and its satellites. The electron temperature obtained from line ratios of the W line to its satellites is 750 eV, and the ion temperature deduced from the Doppler broadening of the W line is 330 eV.

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.

To investigate the interactions between external resonant magnetic perturbations (RMPs) and a tokamak plasma, a set of saddle coils are designed for the J-TEXT. A method for designing RMP field structure based on Fourier transform is used in this paper. Since the coils will be mounted inside the vacuum vessel where the pressure is about , an airproof multilayer structure is adopted to avoid contaminating the vacuum environment. Besides, stress and thermal analysis and the AC response of magnetic field are also given.

Characteristics of tokamak current quenches are an important issue for the determination of electro-magnetic forces that act on the in-vessel components and vacuum vessel during major disruptions. The characteristics of current quenches in spontaneous disruptions in the J-TEXT tokamak have been investigated. It is shown that the waveforms for the fastest current quenches are more accurately fitted by linear current decays than exponential, although neither is a good fit in many slower cases. The minimum current quench time is about 2.4 ms for the J-TEXT tokamak. The maximum instantaneous current quench rate is more than seven times the average current quench rate in J-TEXT.

This study aimed to unlock the m/n = 2/1 locked mode (LM) performed in J-TEXT tokamak by using rotating resonant magnetic perturbations (RMPs), where m and n are the poloidal and toroidal mode numbers, respectively. In the experiments, to maintain the LM, mode locking occurs by using static RMPs generated by a set of saddle coils. After mode locking, another rotating RMP with frequency of several kilo-Hz is applied to drive the static LM to rotate. The unlocking of LM is realized by using rotating RMP with different frequency and amplitude. It is found that the unlocking process contains two stages, i.e. the oscillating stage and the unlocking stage. In the oscillating stage, the rotating RMP with amplitude that is not strong enough causes the LM to oscillate around its locked phase and produces magnetic fluctuation to behave as a standing wave-like structure in poloidal direction. When the amplitude of the rotating RMP is strong enough, it first causes the LM to oscillate and then transforms to mode unlocking quickly in less than 1 ms, namely the unlocking stage. Further analysis shows that the unlocking of LM is determined by the torque balance between the viscous torque and the electromagnetic torques exerted by both the static and the rotating RMP. In addition, the unlocking process is sensitive to both the amplitude and the frequency of the rotating RMP as well as the amplitude of static RMP. Nonlinear numerical modeling based on reduced MHD equations is also performed to understand the unlocking process, and numerical results qualitatively agree with the experimental ones.