Qiming Hu

Effect of externally applied resonant magnetic perturbations on resistive tearing modes

Static resonant magnetic perturbations (RMPs) generated by saddle coil current have been applied in J-TEXT tokamak experiments in order to study their effects on tearing mode instabilities. With increasing RMP amplitude in time during the discharge, the mode stabilization is first observed, but a large locked mode follows if the RMP amplitude is increased to a very large value, indicating that the RMP amplitude is important in determining the plasma response and the tearing mode behaviour. By careful adjustment of the RMP amplitude, the (partial) stabilization of the m/n?=?2/1 tearing mode by RMPs of moderate amplitude has been achieved without causing mode locking (m and n are the poloidal and toroidal mode numbers). To compare with experimental results, nonlinear numerical modelling based on reduced MHD equations has been carried out. With experimental parameters as input, both the mode locking and mode stabilization by RMPs are also obtained from numerical modelling. Further calculations have been carried out to study the plasma parameters affecting the mode stabilization by RMPs, including the plasma rotation frequency, viscosity, Alfv?n velocity and the RMP amplitude. It is found that the suppression of the tearing mode by RMPs of moderate amplitude is possible for a sufficiently high ratio of plasma rotation velocity to the Alfv?n speed. A larger plasma viscosity enhances the mode stabilization.

Recent research work on the J-TEXT tokamak

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.

Dependence of plasma responses to an externally applied perturbation field on MHD oscillation frequency on the J-TEXT tokamak

The plasma response to externally applied perturbation fields is investigated on the J-TEXT tokamak using a set of static resonant magnetic perturbation (SRMP) coils. Several different experimental results are obtained including partial or complete suppression of the existing m/n = 2/1 tearing mode, mode locking or non-uniform magnetohydrodynamic (MHD) oscillations. These results depend sensitively on the tearing mode frequency and the amplitude of the perturbation field. It is found that mode locking is most likely to happen at a lower rotation frequency (< ~5 kHz) and the threshold for mode locking has a linear relation with MHD frequency. However, complete suppression of the tearing mode happens in a region where the MHD frequencies are higher (~6 kHz). The experimental observations are explained by numerical simulations based on reduced MHD equations. The error field contributes to an offset between the mode-locking thresholds for the two opposite spatial phases of the SRMP, through which the intrinsic error field of J-TEXT can be estimated.

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.

First observation of rotation acceleration of magnetic island by using rotating resonant magnetic perturbation on the J-TEXT tokamak

A set of in-vessel saddle coils called dynamic resonant magnetic perturbation (DRMP) for generating rotating resonant magnetic perturbations has recently been constructed on the J-TEXT tokamak. The phenomenon of tearing mode locking to DRMP and rotating together with the DRMP field has been observed. There is an apparent decrease of the island width during the locking and unlocking procedure. Similar results are obtained in the numerical simulation.

Analytical compensation of axisymmetric equilibrium fluxes picked up by locked mode detectors in tokamaks

In the detection of locked modes using saddle loops, the problem of how to remove the axisymmetric equilibrium flux picked up by the loops has still to be solved. The problem becomes more difficult when there are conductive structures located near the saddle loops. In this paper, we present an analytical model based on lumped eddy current circuits and use it to interpret the measured equilibrium flux and the corresponding eddy current fluxes. Using this model, precise compensation for fluxes induced by the horizontal field coils and the toroidal field coils, with relative errors of less than 1%, has been realized for the saddle loops in the Joint Texas Experimental Tokamak. This paper also presents a new method to compensate for the detection of equilibrium flux by the locked mode detector.

Enhanced particle transport caused by resonant magnetic perturbations in the J-TEXT tokamak

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

Observation of beta-induced Alfvén Eigenmode in J-TEXT tokamak

High-frequency oscillations have been frequently observed under the conditions of tearing modes and runaway electrons in J-TEXT Ohmic plasmas. It is found the frequencies of these oscillations range from 20 to 45 kHz, being consistent with the beta-induced Alfven Eigenmodes (BAEs) with the same order of the low-frequency gap induced by finite beta effects and the coupling of the shear Alfven wave with the compressional response of the plasma. The exciting conditions for BAEs are investigated, which indicate that runaway electrons, as well as magnetic perturbations contributed by magnetic islands, are indispensable in the excitation of BAEs. In addition, externally applied static resonant magnetic perturbations (RMPs) are used to excite BAEs successfully for the first time in J-TEXT, as indicated by high frequency oscillations (~30 kHz). Further studies show that BAEs can be excited only when the coil current of RMP is stronger than 4 kA, and the strength of BAEs becomes stronger with stronger RMP. To assess the verification of the BAEs, the frequencies of observed modes are compared to the calculated frequencies of the BAE frequency gap in the Alfven continuum, namely the continuum accumulation point (CAP), and they are found to be close.

The operation region and MHD modes on the J-TEXT tokamak

The operation region and the parameter region of magnetohydrodynamic (MHD) modes are analyzed for J-TEXT Ohmic discharges. The operation region is described by the Hugill diagram, which combines low-q and high density limits. It is found that the operation region has expanded over the years on J-TEXT. In detail, the high density limit has increased from less than 0.5n G to 0.7n G and the low-q limit has lowered from 2.8 to 2.2; this is due to the reduced impurity content that results from coating graphite on the wall. Furthermore, the operation region has further expanded to 0.85n G and q a ~ 2.0, respectively—a result of suppressing the disruptive precursor MHD by using externally-applied resonant magnetic perturbations (RMPs). Here, n G and q a are the Greenwald density limit and edge safety factor, respectively. Corresponding to the results of the operation region, the parameter regions of MHD modes are presented. It is found that a m/n = 2/1 tearing mode (TM) appears for a wide parameters region with 2.4 < q a < 4 and n e < 3 × 1019 m−3—here m and n are the poloidal and toroidal mode numbers. Furthermore, other MHD modes such as m/n = 5/2, 3/1, 4/1 and 7/2, appear only when their rational surfaces are close to the plasma edge or m/n ~ q a, and these MHD modes may transit to a 2/1 TM when changing the plasma parameters. In addition, correlation analysis between the amplitude and frequency of the dominant 2/1 TM for different plasma conditions reveals that there is a threshold between normal discharges and density-limit discharges, which would be a reference to predict density-limit disruptions.

Upgrade of the Mirnov probe arrays on the J-TEXT tokamak

The magnetic diagnostic of Mirnov probe arrays has been upgraded on the J-TEXT tokamak to measure the magnetohydrodynamic instabilities with higher spatial resolution and better amplitude-frequency characteristics. The upgraded Mirnov probe array contains one poloidal array with 48 probe modules and two toroidal arrays with 25 probe modules. Each probe module contains two probes which measure both the poloidal and the radial magnetic fields (Bp and Br). To ensure that the Mirnov probe possess better amplitude-frequency characteristics, a novel kind of Mirnov probe made of low temperature co-fired ceramics is utilized. The parameters and frequency response of the probe are measured and can meet the experiment requirement. The new Mirnov arrays have been normally applied for a round of experiments, including the observation of tearing modes and their coupling as well as high frequency magnetic perturbation due to the Alfvén eigenmode. In order to extract useful information from raw signals, visualization processing methods based on singular value decomposition and cross-power spectrum are applied to decompose the coupled modes and to determine the mode number.