Ming Xu

The continuous energy and pitch-angle distribution of hydrogen fast ions generated by ion cyclotron range of frequencies (ICRF) minority heating is measured in EAST using an imaging neutral particle analyzer (INPA). The measured two-dimensional fast-ion distribution exhibits a triangular structure: as energy increases, the pitch-angle distribution becomes progressively narrower, with its peak located around 0.4 < | v ∥ / v | < 0.5 . This behavior is consistent with resonance localization theory, which associates the pitch-angle peak with trapped orbits whose turning points are close to the ion cyclotron resonance layer. Through diagnostic response correction, the reconstructed spectrum shows a progressive enhancement of the high-energy minority-ion tail with rising ICRF power. A clear anisotropic evolution of the fast-ion distribution is also observed between discharges with different electron-temperature profiles. In the discharge with higher core electron temperature, the signal enhancement appears mainly for fast ions at energies above 100 keV and | v ∥ / v | ⩾ 0.5 , together with a broader measured pitch-angle distribution. This behavior is interpreted as the combined result of collisional pitch-angle scattering, fast-ion accumulation, and finite-orbit-width effects. The enhanced signal region corresponds to wide-orbit fast ions whose inner orbit legs extend to the hotter inner plasma region, making them more sensitive to changes in the electron-temperature profile. These results demonstrate the capability of INPA to resolve the anisotropic evolution of ICRF-heated fast ions and provide experimental insight into the optimization of ICRF heating schemes in EAST and future fusion devices.

The reversed shear Alfvén eigenmode (RSAE) and the relationship with the internal transport barrier (ITB) have been investigated in the Experimental Advanced Superconducting Tokamak for the high qmin scenario. The RSAE with low toroidal mode numbers (1 ≤n< 5) is distributed within the normalized minor radius range of 0.2 ≤ρ≤ 0.4. Furthermore, two distinct RSAE branches with similar toroidal mode numbers but different radial coverage and phase profiles are observed. The excitation of RSAE leads to a redistribution of fast ions, with the density change predominantly in the central region enclosed by the minimum safety factor (qmin), as measured by the radial neutron camera. A micro-instability characterized by thermal particle transport has been detected by the POLarimeter-INTerferometer diagnostic, which is excited concurrently after the upward sweeping frequency of RSAE. The inward shift of the inner q = 2 surface is indicated by a sawteeth-like collapse event (Δr≈ 2 cm, Δr/a≈ 5%, a is the plasma minor radius), accompanied by a comparable outward shift in the R/LTi profile [R/LTi is the normalized ion temperature gradient (ITG) scale length]. These profile modifications, which subsequently suppress thermal particle transport, are correlated with a decrease in magnetic shear. The formation of Ti-ITB, characterized by R/LTi≥ 10, is observed following the RSAE instability under conditions of high neutral beam injection and ion cyclotron resonance heating power. A negative radial electric field (Er) is observed at the position of qmin≈2, and the E×B shear rate significantly increases after the formation of ITB. Furthermore, the RSAE experiences partial suppression due to the improved ITG; however, it becomes fully suppressed after ITB formation and subsequently recovers following the ITB collapse.

The basic features and relationship with zonal flows of multiple branches of beta-induced Alfvén eigenmodes (BAEs) are investigated in the EAST tokamak. Those branches of BAEs, excited by tangentially injected neutral beam injection, are located in the central region ( 0.2 ⩽ ρ ⩽ 0.4 ), with frequencies in the range of 50 ⩽ f ⩽ 70 kHz and low toroidal mode numbers 1 ⩽ n ⩽ 5 . The excitation of BAEs is clearly characterized by three typical features: an increase in the ω E × B shear locally, accompanied by simultaneous rises in both central rotation velocity and electron temperature. Radial profiles of the mode phase angle from electron cyclotron emission (ECE) diagnostics vary significantly with n . Global gyrokinetic simulations of core BAEs are performed in both linear and nonlinear regimes. In the linear regime, the unstable BAE branches are consistent with experiment on toroidal mode number, real frequency and the mode structure phase angle. The radial variation of BAE phase angle is primarily caused by bulk plasma kinetic effects rather than the energetic particle (EP) non-perturbative drive. In the nonlinear regime, single- n BAE without zonal fields saturates at a large amplitude that triggers the non-adiabatic frequency chirping on a fast timescale of sub-millisecond, while the self-organized zonal flow beat-driven by BAE greatly reduces BAE saturation amplitude below the chirping threshold, which is in consistency with experimental observation. Moreover, it is confirmed that the criterion of zonal flow regulation on dissipative-type instability (such as AEs) becomes ω E × B > ω r rather than traditional ω E × B > γ L for reactive-type instability. In general,the self-organized zonal flow can improve plasma confinement through regulating both BAEs and drift-wave turbulence.

Kinetic-magnetohydrodynamic hybrid simulations have been carried out to investigate the transition between different types of Alfvén eigenmodes (AEs) based on the Experimental Advanced Superconducting Tokamak. Two evolution processes of q -profile, starting with a reverse shear q -profile with minimum q min < 2 have been analyzed: an ascending one based on the magnetic diffusion effect and a descending one inferred from the coexistence of the double tearing mode. The experimental phenomenon, where the type of AE transit from the beta induced AE (BAE) to the reverse shear AE (RSAE) and up-sweeping of RSAE frequency with multiple toroidal number n is reproduced in both processes. In the ascending process, the transition from BAE to RSAE happens at q min = 2 when the q = 2 rational surface vanishes. In the descending process, the transition happens at q min = 1.92 without the vanishing of q = 2 rational surface and is accompanied by a sudden shift in mode location. By comparing frequency and growth rate patterns with experimental observations during the mode transition, the descending process satisfies experiment better. In addition, different physical factors affecting the mode transition have been investigated, including the q -profile shape and energetic particle (EP) pressure. It is found that smaller magnetic shear at rational surface favors the excitation of BAE, and the RSAE frequency is influenced by second order derivative of the q -profile at its minimum. Moreover, the excitation of BAE is not sensitive to the peak location of EP pressure gradient, but the radial phase variation of the mode structure is positively linked to the distance between the EP drive location and the mode location, which is also influenced by the local magnetic shear. These systematic studies on the effects of different physical factors on AEs can provide guidance for controlling Alfvén instabilities in the future.

The experimental measurements reveal that a notable characteristic of the imaging neutral particle analyzer (INPA) passive signal in Experimental Advanced Superconducting Tokamak is its bimodal distribution in the pitch angle ( v ∥ / v ). The synthetic images of INPA passive signal, obtained through synthetic diagnostic, are in good agreements with the experimental images. The measurements reveal two distinct pitch peaks in the passive signals. The synthetic diagnostic results indicate that the two observed pitch peaks predominantly originate from fast ions with distinct initial deposition locations: the low pitch peak arises from particles initially deposited on the low-field side, while the high pitch peak corresponds to those initially deposited on the high-field side. Furthermore, experimental observations demonstrate that the change of the electron density, which directly affects the initial deposition distribution of fast ions, leads to variations in the ratio between these two pitch peaks. Within similar electron density ranges, the proportion of high pitch peak increases with elevated β N , indicating the increasing fraction of fast ions. The variation in the ratio between two pitch peaks is observed during long-lived mode, further reflecting the relationship between fast ions and bimodal distribution. During supersonic molecular beam injection, the passive signal intensity exhibits an order of magnitude amplification while preserving the two pitch peaks. INPA passive signals, which carry information about the fast ions’ distribution, can serve as a novel diagnostic for inferring fast ions transport.

Velocity fluctuations are of significant importance in plasma transport in magnetic confinement devices. Measurements of the E → × B → velocity fluctuation ( v ~ E × B ) in the core region associated with the fishbone instability have been carried out on EAST using a W-band X-mode Doppler reflectometry (DR). For internal kink-like mode, Doppler shift fluctuations of probing beams due to deformation or advection of the cut-off layer are small. The v ~ E × B amplitude inside the resonant surface is measured to be 1 − 5   km s − 1 and is linearly proportional to the poloidal magnetic fluctuation, as expected from the ideal MHD instabilities. Furthermore, the radial velocity fluctuation ( v ~ r ) inferred from the density fluctuation is comparable to v ~ E × B , which is consistent with the plasma incompressibility. This is the first measurement on the poloidal flow fluctuation generated by fishbone bursts. The potential impact of such significant flow fluctuations on turbulence in core plasma through flow shear or dynamo effect is a subject that merits further investigation.

In the EAST experiments with neutral beam injection and at a low electron density, fishbone instabilities characterized by mode numbers m/n = 1/1, 2/2 have been observed. The simultaneous growth of these modes, particularly m/n = 1/1 and m/n = 2/2, results in a decrease in neutron emission at the plasma core. As the electron density continues to decrease, the beta-induced Alfvén eigenmode (BAE) emerges, replacing the m/n = 2/2 fishbone just before a more severe sawtooth crash occurs. Numerical simulations with the global kinetic-magnetohydrodynamic (MHD) code M3D-K show that the m/n = 2/2 high frequency fishbone branch is linearly stable, but nonlinearly grows due to the coupling with the m/n = 1/1 low frequency fishbone branch. The m/n = 2/2 fishbone frequency is almost twice of the m/n = 1/1 fishbone, and both fishbone frequencies chirp down together. The mode frequencies and structures of the simulated fishbones are consistent with the experimental measurements. In addition, a BAE with m/n = 2/2 is observed in the late nonlinear stage of the simulation. Energetic particle nonlinearity is dominant for the m/n = 2/2 fishbone saturation and transition to BAE. In particular, the transition from m/n = 2/2 fishbone to BAE is found to be the consequence of fast ion redistribution due to the fishbone instability.

An x-ray imaging diagnostic system using the Timepix3 detector has recently been developed and installed on the Experimental Advanced Superconducting Tokamak (EAST). The diagnostic system measures the temporal evolution, spatial distribution, and energy spectrum of x-ray signals during plasma discharge with a 16° diagonal field of view, spatial resolution of 4 mm, and broad spectral-energy response range (5–200 keV). Equipped with event-driven capability, the Timepix3 detector captures the coordinates, energy, and arrival time of each photon event with a temporal resolution of 1.56 ns. Herein, the energy resolution and spatial resolution performances of the detector were tested in counting and time-over-threshold modes. The energy resolution of the detector in both modes was within 5 keV. The temporal resolution of the detector and the temporal evolution of x-ray signals were analyzed during plasma discharge experiments on the EAST, harnessing the event-driven capability of the detector for latter analysis. Furthermore, the detector-measured signals were compared with conventional diagnostic signals, confirming the reliability of the Timepix3 detector. Overall, this paper presents the parameter testing results and preliminary experimental diagnostic outcomes.

An imaging neutral particle analyzer (INPA) has been successfully designed and implemented in the Experimental Advanced Superconducting Tokamak to detect fast ions (FIs). The system capitalizes on carbon foils to ionize charge exchange neutral particles and utilizes scintillator screens to differentiate the energy and radial distribution of the particles. The detection characteristics of the pitch angle profile and energy resolution are illustrated, and the modeling-based energy calibration has been validated with the experimental data. Furthermore, the FI redistribution during sawtooth crashes has been effectively recorded by the INPA, manifesting its functionality.

Recent observations of the successive formations of the 4 / 1 , 3 / 1 , and 2 / 1 magnetic islands as well as the subsequent braking of the 2 / 1 mode during a low-Z impurity penetration process in EAST experiments are well reproduced in our 3D resistive MHD simulations. The enhanced parallel current perturbation induced by impurity radiation predominately contributes to the tearing mode growth, and the 2 / 1 island rotation is mainly damped by the impurity accumulation as results of the influence from high n modes.