Magnetohydrodynamic (MHD) instabilities are widely observed during tokamak plasma operation. Magnetic diagnostics provide important information which supports the understanding and control of MHD instabilities. This paper presents the current status of the magnetic diagnostics dedicated to measuring MHD instabilities at the J-TEXT tokamak; the diagnostics consist of five Mirnov probe arrays for measuring high-frequency magnetic perturbations and two saddle-loop arrays for low-frequency magnetic perturbations, such as the locked mode. In recent years, several changes have been made to these arrays. The structure of the probes in the poloidal Mirnov arrays has been optimized to improve their mechanical strength, and the number of in-vessel saddle loops has also been improved to support better spatial resolution. Due to the installation of high-field-side (HFS) divertor targets in early 2019, some of the probes were removed, but an HFS Mirnov array was designed and installed behind the targets. Owing to its excellent toroidal symmetry, the HFS Mirnov array has, for the first time at J-TEXT, provided valuable new information about the locked mode and the quasi-static mode (QSM) in the HFS. Besides, various groups of magnetic diagnostics at different poloidal locations have been systematically used to measure the QSM, which confirmed the poloidal mode number m and the helical structure of the QSM. By including the HFS information, the 2/1 resonant magnetic perturbation (RMP)-induced locked mode was measured to have a poloidal mode number m of ∼2.
The spectrum effect on the penetration of resonant magnetic perturbation (RMP) is studied with upgraded in-vessel RMP coils on J-TEXT. The poloidal spectrum of the RMP field, especially the amplitudes of 2/1 and 3/1 components, can be varied by the phase difference between the upper and lower coil rows, Δϕ = ϕtop – ϕbottom, where ϕtop and ϕbottom are the toroidal phases of n = 1 field of each coil rows. The type of RMP penetration is found to be related to Δϕ, including the RMP penetration of either 2/1 or 3/1 RMP and the successive penetrations of 3/1 RMP followed by the 2/1 RMP. For cases with the penetration of only one RMP component, the penetration thresholds measured by the corresponding resonant component are close for various Δϕ. However, the 2/1 RMP penetration threshold is significantly reduced if the 3/1 locked island is formed in advance. The changes in the rotation profile due to 3/1 locked island formation could partially contribute to the reduction of the 2/1 thresholds.
The coupling of multiple MHD modes can lead to mode locking and major disruption in tokamak plasmas. In the J-TEXT tokamak, the coupling between two small modes, i.e., m/n = 2/1 and 3/1 modes (m and n are poloidal and toroidal mode numbers, respectively), appears when the edge safety factor is reduced to the vicinity of 3. After the mode coupling, the toroidal phase difference between the 2/1 and 3/1 modes equals 0 in the low field side midplane. This phase relation of coupled modes leads to mutual destabilization and even major disruption. A control scheme to avoid disruption caused by coupled modes by resonant magnetic perturbations (RMP) is presented. It is found that the application of RMP significantly changes the evolution of the coupled modes. The coupling of the 2/1 and 3/1 modes occurs earlier as the RMP amplitude increases. The RMP with moderate amplitude can suppress the growth of 2/1 and 3/1 coupled modes and hence avoid disruption. These results provide a possible strategy for the suppression of neoclassical tearing mode (NTM) seed islands on ITER or future fusion reactors.
It has been demonstrated in J-TEXT experiments that a biased electrode or electrode biasing (EB) in the scrape-off layer (SOL) can drive SOL helical current filaments (HCFs). The bright helical radiation belts of carbon impurities in the SOL indicate that SOL current flows along the magnetic field lines. Based on the experimental phenomenon, three SOL current models (model A, B, C) have been set-up in order to understand the spatial structure of SOL current and the perturbed magnetic field it generates. Model A is a simplified calculation of HCFs in the cylindrical geometry, and takes into account the presence of cross-field current by a linear decay of current along magnetic field lines. Model B take into account the actual toroidal geometry and the complex path of HCFs connected from the electrode to the limiters. By including the radial dependence of the resistivity into model B, model C is developed and describes the SOL current more perfectly than the other two models. Furthermore, the model C shows that SOL helical current can produces stronger boundary resonant magnetic perturbations (RMPs) at the last closed flux surface (LCFS) due to the consistent helixity of SOL current filaments and the boundary rational surface, which may be a new way to generate RMPs to control the edge-localized modes (ELMs). The equivalent inductance and resistance of HCFs at different edge safety factor q a are measured by applying a square wave voltage. The results show that the inductance and resistance of the HCFs are related to q a and the radial position of the biased electrode r EB , in a qualitatively consistent manner as that predicted by model C.
The phase difference Δξ between locked islands (2/1 and 3/1) has been found to influence the heat transport on the thermal quench during disruptions by numerical modeling [Hu Q. et al., Nucl. Fusion 59, 016005 (2019)]. To verify this experimentally, a set of resonant magnetic perturbation (RMP) coils are required to excite coupled magnetic islands with different Δξ. The spectrum analysis shows that the current RMP coils on J-TEXT can only produce sufficient 2/1 and 3/1 RMP fields with limited phase difference of Δξ∈[-75°, 75°]. In order to broaden the adjustable range of Δξ, a set of coils on the high field side (HFS) is proposed to generate 2/1 and 3/1 RMP fields with Δξ = 180°. As a result, RMPs with adjustable Δξ∈[-180°,180°] and sufficient amplitudes could be achieved by applying the HFS coils and the low field side (LFS) coils. RMPs with different Δξ are conducive to understand the mechanism of triggering the thermal quench by the modes coupling. This work provides a feasible solution for flexible adjustment of the phase difference between m and m+1 RMP, which might facilitate the study of major disruptions and their control.
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