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.
Various types of plasma events emerge in specific parameter ranges and exhibit similar characteristics in diagnostic signals, which can be applied to identify these events. A semi-supervised machine learning algorithm, k-means clustering algorithm, is utilized to investigate and identify plasma events in the J-TEXT plasma. This method can cluster diverse plasma events having homogeneous features, and then these events can be identified if given few manually labeled examples based on physical understanding. A survey of clustered events reveals that the k-means algorithm can make plasma events (rotating tearing mode, sawtooth oscillations, and locked mode) gathering in Euclidean space composed of multi-dimensional diagnostic data, like soft x-ray emission intensity, edge toroidal rotation velocity, the Mirnov signal amplitude and so on. Based on the cluster analysis results, an approximate analytical model is proposed to rapidly identify plasma events in the J-TEXT plasma. The cluster analysis method is conducive to data markers of massive diagnostic data.
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.
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