Yifei Jin

In magnetically confined plasmas, energetic-particle (EP)-driven instabilities can induce redistribution and losses of energetic ions, thereby affecting plasma heating and confinement. In this work, fast-ion-driven beta-induced Alfvén eigenmode (BAE) excited by ion cyclotron resonance heating (ICRH) was investigated in low-density EAST plasmas using experimentally reconstructed equilibria and profiles. Experimentally, strong ICRH led to a significant increase in ion temperature and plasma stored energy, accompanied by the excitation of an Alfvén eigenmode at 64kHz under 1.6 MW heating power, while no such mode was observed at reduced power. Continuum analysis identified this mode as a core-localized BAE. To interpret these observations, integrated simulations combining TORIC, ASCOT, and GTC were performed. A modified fast-ion distribution function was implemented to capture the strong velocity-space anisotropy of ICRH-generated fast ions. Linear gyrokinetic simulations reproduced the observed BAE frequency and identified a threshold in fast-ion pressure for mode excitation. Orbit-following simulations with realistic wall geometry demonstrated that the BAE induces significant phase-space redistribution and enhances the loss of high-energy fast ions (energy >600keV). Separately, the ASCOT power-deposition analysis shows that the strong BAE reduces the collisional power transferred from fast ions to thermal ions, thereby suppressing the fast-ion contribution to ion heating and limiting the increase in ion temperature. As the measured BAE activity decreases in Phase B, recovery of this ion-heating channel explains the subsequent rise in ion temperature. These results clarify the interplay between ICRH-generated fast ions, BAE activity, and ion heating, and provide insights into optimizing fast-ion confinement and heating efficiency in future burning plasma devices.

In magnetically confined fusion plasmas heated only by electron cyclotron resonance heating (ECRH), ion temperature typically saturates in the 1–3 keV range. The EAST tokamak, equipped with an ITER-like tungsten divertor, has operated deuterium H-mode discharges with high-power pure ECRH. In a specific discharge, a continuous ion acceleration process is observed, with ion energy ranging from an off-axis supra-thermal component above 6 keV to an isolated Gaussian peak reaching 1.8 MeV. The acceleration occurs at the location of a large 2/1 magnetic island, and an EGAM-like electrostatic mode at ∼ 23 kHz is detected by microwave diagnostics. The MeV-range cutoff energy, the mode power, and the island width exhibit pairwise strong linear correlations ( R 2 > 0.9 ), and the three are spatially coincident. Transfer entropy analysis and a 0D collisional-radiative model suggest a mechanism from island to mode to ion acceleration, excluding a purely electric-field-driven process. These observations provide experimental evidence of MeV-range ion acceleration accompanied by an EGAM-like mode in a pure ECRH plasma without auxiliary ion heating, offering a new data point for understanding ion acceleration under ECRH operation in tungsten-wall devices, particularly with degraded boron wall coating.

Evolution of trapped electron mode (TEM) turbulence was observed in H-mode plasmas in the Experimental Advanced Superconducting Tokamak. Unstable high- k ( k ρ > 0.1 ) TEM and the suppressed low- k ( k ρ < 0.1 ) TEM are observed when R / L T e increases and R / L n e decreases at T e / T i ∼ 1 . Low- k TEM, sensitive to density gradient, drives finite electron heat transport and outward particle transport. While high- k TEM, sensitive to electron temperature gradient, drives stronger electron heat transport and the inward particle transport when density gradients are below 3 . The experimental characteristics of low- k and high- k TEMs, as well as their dependence on density and temperature gradients, were systematically investigated. Experimental identification of TEM turbulence is crucial for understanding transport in burning plasma dominated by electron heating with T e / T i ∼ 1 .

Modes with toroidal symmetry (n=0) play a pivotal role in enhancing confinement performance in magnetically confined fusion devices. Here, we report the experimental discovery of a global n=0 mode prior to the formation of an ion internal transport barrier (ITB) in neutral beam-heated plasmas on the EAST tokamak. This mode manifests simultaneously in the ITB region and the plasma edge. We identify that the zonal structure within the ITB region facilitates barrier formation, while the edge-localized, poloidal mode number m=1 dominant structure significantly improves fast ion confinement. Further analysis reveals that this edge n=0 mode is driven by fast ion radial transport induced by core sawtooth crashes, representing a novel non-local excitation mechanism. This mechanism is substantiated by hybrid simulations using the M3D-K code. These findings provide a new perspective on the non-local coupling between core and edge dynamics, offering crucial insights for alpha particle heating and transport in future fusion reactors.

A novel approach for simultaneous power exhaust and edge-localized mode (ELM) control is presented in the Experimental Advanced Superconducting Tokamak discharges, which utilize an ITER-like tungsten divertor. Real-time injection of boron (B) powder and neon (Ne) gas overcomes their limitations encountered when used separately. Pure Ne seeding leads to a narrow operational window constrained by core impurity accumulation and H-mode to L-mode back transitions, while pure solid B injection (SBI) is insufficient for effective divertor cooling. In comparison, their combined use achieves a stable, stationary, ELM-suppressed H-mode with adequate power exhaust. This synergistic scenario features partial energy detachment at the outer divertor while maintaining good plasma confinement ( H ₉₈ ∼ 1) with minimal degradation. Two key features of this scenario are: (1) the SBI triggers a persistent Edge Harmonic Mode (EHM), which provides a crucial continuous particle transport channel, preventing Ne and tungsten/molybdenum accumulation without flushing out by ELM, and (2) the B + Ne mixture allows for active optimization of the radiated power profile. Core radiation can be reduced by substituting a portion of the Ne with B, leveraging their complementary non-coronal equilibrium radiation efficiencies. This combined B + Ne injection scheme presents a promising pathway toward integrated core-edge scenarios, offering the potential to minimize total impurity throughput while leveraging an actuator (powder injection) already being considered for ITER.

We report the first in-depth comparison of the impact of toroidal magnetic field direction on solid boron injection used for Edge-Localized Mode (ELM) control, power exhaust, and core high- Z impurity control in the Experimental Advanced Superconducting Tokamak. With favorable ion ∇ B drift towards the upper X-point in an upper-single-null configuration, boron injection effectively suppresses ELMs, produces a detachment of the inner divertor target, and leads to improved energy confinement. ELM suppression in this configuration is accompanied by the excitation of an Edge Harmonic Mode. In contrast, with unfavorable ion ∇ B drift away from the upper X-point, boron injection also suppresses ELMs but leads to a more symmetric detachment state of both the inner and outer divertor targets, while plasma energy confinement is slightly degraded despite similar boron injection levels; a different low-frequency coherent mode without multiple harmonics is observed. Measurements from toroidally separated views show that the divertor response to boron injection is essentially toroidally symmetric, supporting the use of two-dimensional SOLPS-ITER modeling with a toroidally uniform impurity source. These experimental observations are qualitatively consistent with SOLPS-ITER simulations, which highlight the critical role of E × B drift effects in setting the Bt -dependent in–out asymmetry of detachment and in asymmetrically transporting particles and injected impurities within the scrape-off layer and private-flux region. These findings underscore the importance of drift physics and real-time wall conditioning in controlling low- Z impurity transport and optimizing edge solutions for integrated, ELM-stable, high-performance tokamak operation.

Improved energy confinement mode (I-mode) regime offers promising confinement characteristics with temperature pedestal and low-confinement mode (L-mode) like particle transport. Lower Hybrid Current Drive (LHCD) capability in I-mode is experimentally and numerically investigated. Experimental results show that compared to L-mode plasma, enhanced LHCD efficiency is observed during I-mode plasma, with a broader current profile. Ray-tracing simulations suggest that the observed improvement may be attributed to altered LH wave power deposition caused by the temperature pedestal, which leads to a broader driven current profile. These results underscore the significant impact of edge conditions on LHCD. The edge localized mode-free nature of I-mode provides a stable platform for LHCD, making it attractive for steady-state operation. Note that the potential role of reduced parametric decay instability (PDI) in the edge region cannot be fully excluded due to the lack of direct 2.45 GHz RF spectral measurements and kinetic edge profile in scrape-off layer. Consequently, both interpretations, modified wave propagation and suppressed PDI, remain plausible and further verification by full-wave modeling and dedicated experiments are necessary. Considering that both the PDI and the confinement regimes are closely related to the boundary parameters, the LHCD performance across different confinement regimes under the high-density platform will be further investigated.

Accurate inference of electron temperature ( T _e ) profiles is essential for understanding energy transport and impurity dynamics in magnetic confinement fusion plasmas. Traditional spectral diagnostics based on satellite line ratios become unreliable in high- T _e regimes due to reduced line intensity. Here we present a framework that combines Gaussian Process Regression (GPR) and convolutional neural networks (CNN) to reconstruct T _e profiles from impurity emission spectra measured by a high-resolution x-ray crystal spectrometer on EAST. The GPR module leverages a non-stationary Gibbs kernel to interpolate spatially discrete ECE data with uncertainty quantification, while the CNN model learns the nonlinear mapping between impurity line intensity profiles and electron temperature. The model is trained and validated on a argon spectra dataset comprising over 2300 discharges and extended to tungsten spectra for high- Z impurity scenarios. It demonstrates robust performance across diverse plasma conditions, with the coefficient of determination ( R ² ) > 0.9 and mean absolute errors below 0.5 keV. The model’s generalization capability is further confirmed using synthetic data generated by STRAHL simulations. Our results highlight the feasibility of real-time surrogate diagnostics for future fusion experiments and control systems, providing a scalable approach for high-temperature profile inference using x-ray spectrometer.

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.