[en] Magnetic diagnostics is one of basic measurement systems for Tokamak plasma current and shape control. The accuracy of magnetic data will influence the plasma shape reconstruction. In this paper, the calibration of magnetic diagnostics is carried out on the EAST Tokamak. The overall uncertainties of magnetic sensors are analyzed from the calibration and vacuum shots. The uncertainty results are used as fitting weight in plasma shape reconstruction code (EFIT). Based on EFIT simulation and experiment data fitting results, the sensitivity of the magnetic data uncertainty in the plasma shape reconstruction is analyzed. (author)

[en] Based on the basic principle of measuring magnetic field by magnetic probe, a solenoid calibration system is constructed by a long solenoid, alternating current power, standard probe and data acquisition system in order to get the accurate magnetic field data. The NS value of magnetic probe on EAST is calibrated accurately by the solenoid calibration system and the data of the calibration is analysed. The obtained results are what we expected and provide the prerequisite for accurate magnetic field measurement in tokamak. (authors)

[en] In the discharge of EAST tokamak, it is observed that (2,1) neoclassical tearing mode (NTM) is triggered by mode coupling with a (1,1) internal mode. Using singular value decomposition (SVD) method for soft X-ray emission and for electron cyclotron emission (ECE), the coupling spatial structures and coupling process between these two modes are analyzed in detail. The results of SVD for ECE reveal that the phase difference between these two modes equals to zero. This is consistent with the perfect coupling condition. Finally, performing statistical analysis of r_1_/_1, ξ_1_/_1 and w_2_/_1, we find that r_1_/_1 more accurately represents the coupling strength than ξ_1_/_1, and r_1_/_1 is also strongly related to the (2,1) NTM triggering, where r_1_/_1 is the width of (1,1) internal mode, ξ_1_/_1 is the perturbed amplitude of (1,1) internal mode, and w_2_/_1 denot es the magnetic island width of (2,1) NTM. (paper)

[en] Accurate detection of a magnetic island in real time is one of the important issues for the tearing mode (TM) and neoclassical tearing mode (NTM) control. This paper presents a real-time detection system for the magnetic island of NTM control in the EAST Plasma Control System (PCS). Diagnosis is based on magnetic periodic perturbation and electron temperature fluctuation caused by the magnetic island. Therefore, a Mirnov measurement has been selected to calculate the island's parameters, such as island width, frequency of island rotation, and toroidal number. The electron cyclotron emission (ECE) system can detect the island position, which is calculated by two fast detection algorithms called correlation analysis and Hilbert transform. For future NTM control, real-time equilibrium reconstruction (rt-EFIT) is needed to locate the rational q-surface where the island is detected. This fast detection system is able to detect an island within 3 ms. It can be integrated into PCS to provide effective parameters of the island for NTM control by using EC resonance heating (ECRH) in the next experiment of EAST. (paper)

[en] Pressure gradient driven m = 1 internal kink mode destabilization that follows an L-H transition is observed in the operational region of the EAST tokamak, which manifests in periodic oscillations in soft x-ray (SXR) and Mirnov coil signals. Using tomography with the high resolution soft x-ray detection array, we find that the rotation direction of the 1/1 kink mode is in the ion diamagnetic drift direction in poloidal cross-section. A large displacement of the hot core is attributable to the shift of the 1/1 internal kink mode. In contrast to stationary oscillations with fixed frequency, various frequency chirping behavior is observed with this 1/1 kink mode. Furthermore, we also occasionally observe that a 2/1 neoclassical tearing mode (NTM) is triggered by a 1/1 internal kink mode via mode coupling in a high-performance plasma. The spatial structure of a 2/2 mode, which is the harmonic mode of the 1/1 kink mode, is also presented in this paper. Large amounts of medium-Z impurities accumulate in the central plasma region where the 1/1 kink mode instability bursts. Finally, we also find that the frequency beating associated with a 1/1 kink mode is a consequence of plasma rotation. Based on all of these observations, we propose that the plasma pressure gradient, the driving force in kink modes, is plausibly the product of an intense concentration of impurities, which are related to plasma rotation.

[en] The pellet-induced snake oscillation was observed by soft x-ray (SXR) diagnostic in EAST for the first time after a fueling-sized pellet penetrated the q   =  1 surface. The snake phenomenon has a long lifetime with a helicity of m   =  1 and n   =  1. Basic behaviors of the snake, including the triggering condition, interaction with the sawtooth and snake rotation frequency, were discussed in detail by multiple core diagnostics. The snake location was also analyzed through observation of the vertical SXR arrays and raw SXR brightness profiles. It is clear that the snake resided in a broad region between the magnetic axis and the q   =  1 surface derived from equilibrium reconstruction. This investigation is beneficial for the understanding of the snake formation for EAST and future devices, like ITER and DEMO. (paper)

[en] A radiation-driven m  =  2 island was observed in the experimental advanced superconducting tokamak (EAST) ohmic plasma, near the density limit. The mode onset occurs when the the ohmic heating input is less than the radiative cooling loss, which agrees with the mode onset behavior of the thermo-resistive model. The evolution of the equilibrium during the mode process was obtained using the ONETWO transport code, with inputs comprising the experimental electron temperature and density profiles. A large m  =  2 island can drive an m  =  1 sideband mode, which leads to an internal crash that appears as a large change in temperature that occurs not only in the q  =  2 region but also in the core. (paper)

[en] Ideal magnetohydrodynamic theory for localized interchange modes is developed for toroidal plasmas with anisotropic pressure. The work extends the existing theories of Johnson and Hastie [Phys. Fluids 31, 1609 (1988)], etc., to the low n mode case, where n is the toroidal mode number. Also, the plasma compressibility is included, so that the coupling of the parallel motion to perpendicular one, i.e., the so-called apparent mass effect, is investigated in the anisotropic pressure case. The singular layer equation is obtained, and the generalized Mercier's criterion is derived.

[en] Reverse-sheared Alfvén eigenmodes (RSAEs) have been observed by using an interferometer and ECE diagnostics in NBI heated ELMy H-mode plasma on EAST tokamak. A typical feature of these modes is a fast frequency sweeping upward from ∼80 kHz to ∼110 kHz in hundred milliseconds during which the plasma temperature, density and rotation keeps no change. Only core channels of the interferometer can observe these modes, implying a core localized mode. The ECE measurement further showed that these modes located at about ρ = 0.37–0.46, just around the position of q _min with ρ ∼ 0.4. These core localized modes are very weak in the magnetic fluctuations measured by mirnov probes mounted at the machine vacuum vessel. A multiple frequency fluctuation component, seemingly the so-called ‘grand cascades’, was also clearly observed on the ECE signal at ρ = 0.46. During the phase, a transient internal transport barrier (ITB) in ion temperature and toroidal rotation was observed and the ITB foot was just close to the position of q _min. A modulation of RSAE frequency by ELM event was observed and this modulation could be attributed to rotation decrease or q _min increase due to ELM. Further study of these modes in EAST can provide valuable constraints for the q profile measurement and will be important for the long pulse operation. (paper)

[en] Neoclassical tearing modes (NTM) are observed in discharges with auxiliary heating LH+ICRF and LH only during H-mode in EAST. The m/n = 2/1 NTM is triggered by strongly coupling with an m/n = 1/1 internal mode. Here, LH and ICRF are the abbreviations of lower hybrid resonance heating and ion cyclotron resonance frequency heating, respectively. The mode number of the NTM is m/n = 2/1, where m is the poloidal mode number and n is the toroidal mode number. Just before the triggering of NTMs, an m/n = 1/1 internal mode appears in the soft x-ray emission at plasma centre when the intensity of hard x-ray (I_HX) reaches a critical value. The mode, characterized by frequency chirping in the spectrum, may be related to suprathermal electrons produced by LH. The saturated magnetic island width w_sat of the NTM is strongly correlated with poloidal β_p. Normalized β_N,onset and the magnetic island critical width w_crit increase with electron temperature T_e. (paper)