Wei Shen

In magnetic confinement fusion plasmas, the main product of deuterium-tritium fusion is energetic alpha particles, which transfer energy to the background plasma by collisions and become thermalized helium ash. Helium ash accumulation would cause fuel dilution and reduce plasma confinement performance and fusion gain. In this work, the transport of helium ash in China fusion engineering test reactor (CFETR) is simulated by the advanced transport solver TGYRO, which invokes the NEO and TGLF codes to compute neoclassical and turbulent transport, respectively. By analyzing the neoclassical and turbulent transport of helium ash, it is found that ITG turbulence is dominant for helium ash transport in CFETR. In addition, with low helium ash fractions, the redistributed helium ash density profile is higher than the initial setting profile because the source rate exceeds the outward transport, while at high helium ash fractions it becomes lower than the initial profile. Inside the internal transport barrier, ITG turbulence is suppressed, leading to a significant reduction in the outward transport of helium ash. The transport process of helium ash can also affect micro-instabilities. In particular, an increase in helium ash concentration may stabilize ITG turbulence. Redistributed energetic alpha particles due to Alfvén eigenmodes affect the helium ash transport by modifying the helium ash source profile, but they do not significantly alter the helium ash distribution. Our work advances the understanding of the dominant physical mechanism governing helium ash transport in future fusion devices.

The linear stability and nonlinear dynamics of fishbones with high order harmonics have been investigated by the global kinetic-magnetohydrodynamic (MHD) code M3D-K in the Experimental Advanced Superconducting Tokamak (EAST). Based on EAST discharge parameters with neutral beam injection (NBI) heating, linear simulations show that the modes with different toroidal mode numbers are primarily driven by trapped energetic particles (EPs) through toroidal precession resonance. The n = 1 mode is the typical fishbone, while there exists a transition for n = 2 mode from fishbone-like mode to energetic particle mode (EPM) when the EP pressure exceeds a critical value. For the n = 3 mode, the mode frequency remains almost unchanged with different EP pressure. Nonlinear simulations show that with weak EP drive, the m / n = 2 / 2 mode is driven by the MHD nonlinear coupling of the fundamental m / n = 1 / 1 mode. In this regime, the frequency of the m / n = 2 / 2 harmonic is twice that of the m / n = 1 / 1 mode, with both modes exhibiting downward frequency chirping. With strong EP drive, beta-induced Alfvén eigenmodes (BAEs) with higher mode frequencies emerge for both m / n = 1 / 1 and m / n = 2 / 2 components, and the m / n = 3 / 3 component is driven unstable with frequency chirping up. These simulation results highlight that nonlinear MHD coupling and fast ion redistribution are critical for understanding the dynamics of fishbones with high order harmonics.

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.

Nonlinear simulations of Alfvénic instabilities in EAST Ion Cyclotron Resonance Heating (ICRH) experiments are carried out using the magnetohydrodynamic-kinetic hybrid code MEGA. Multi-n simulations identify unstable energetic particle modes (EPMs) with toroidal mode numbers n=1 and n=2, along with an n=3 toroidal Alfvén eigenmode (TAE), all of which collectively induce measurable fast-ion redistribution. The resonant excitation mechanisms for both trapped and co-current passing fast ions are elucidated. Nonlinear analysis reveals frequency bifurcation, with the mode evolving into high-frequency TAE and low-frequency BAE-like branches via upward and downward chirping. Phase-space diagnostics directly visualize the wave-particle phase-locking between the downward-chirping branch and resonant trapped fast ions, a key process responsible for their outward transport. Finally, systematic parameter scans further establish the dependence of the mode stability on fast-ion parameters.

In fusion plasmas, α particles are crucial for achieving self-sustained burning conditions. However, toroidal Alfvén eigenmodes (TAE) can induce radial transport of α particles, causing energy loss and potentially quenching the plasma discharge. Recent studies have shown that in the Chinese Fusion Engineering Test Reactor (CFETR) tokamak, the most unstable TAE modes exhibit significantly higher toroidal mode numbers ( n = 7 ∼ 10 ) compared to existing devices ( n = 1 ∼ 4 ). This implies complex α particle transport with larger radial scales under high- n TAE perturbations. This study investigates the impact of single- n TAE perturbations on α particle transport using the PTC code, focusing on CFETR 2019 steady-state scenario. Three phases of TAE evolution are analyzed: linear growth, weak saturation, and strong saturation. During growth phase, resonant α particles exhibit enhanced energy variations and radial transport compared to non-resonant particles. These particles are transported radially from resonance positions towards the core and boundary of the plasma. In the nonlinear saturated phase, resonant particles form island structures in the Θ − P ϕ phase space, where Θ represents the wave-particle phase, and P _φ is the toroidal angular momentum. Notably, island widths Δ P ϕ are wider for high- n TAE, indicating increased radial transport. When TAE saturation amplitudes are large, high- n resonance islands overlap radially, leading to substantially enhanced transport scales for associated α particles, particularly pronounced for n = 9 ∼ 12 modes.

Alfvén wave instabilities driven by energetic particles are common in fusion devices. Understanding their behavior is essential for the good confinement of fusion plasma. For this purpose, a series of experiments are performed on EAST to investigate the excitation of toroidal Alfvén eigenmodes (TAEs). Experimentally, it was found that AEs with frequencies around 80, 134, 157 kHz are excited by ion cyclotron resonance heating (ICRH) hydrogen minority heating scenario. Moreover, the excitation of AEs is independent of the wall-coating materials, but determined with the generation and confinement of the fast ions. Statistical analysis suggests that the TAEs can only be excited when the ICRH power is larger than 2 MW and the H ₉₈ factor is larger than 1.15. In line with the experiments, a set of simulations using TORIC, ASCOT, and NOVA-C is performed. The simulation results show that with the experimental density profile and safety factor, the measured TAE with frequency of 134 kHz is well reproduced by taking into account the plasma toroidal rotation frequency. However, the appearance of the 80 kHz mode cannot be captured in the simulations. The ICRH-generated fast hydrogen ions mainly have banana orbits and that their perpendicular velocity is much larger than that of the parallel ions. The stability analysis with NOVA-C suggests that the radial gradient of the fast H ion distribution is the only driving source of TAEs. For TAEs locate in the center of Alfvén continuum, the D ion Landau damping is the dominant damping term; For most other TAEs which have intersections with the continuum boundary, the continuum damping becomes the dominant damping mechanism.

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.

Alfvén eigenmodes (AE) activity is observed in the EAST high β _N and low B_T discharge 93910, operation scenario dedicated to explore the ITER baseline scenario. AEs are triggered after the plasma is contaminated by Tungsten that causes an abrupt variation of the thermal plasma and energetic particles (EPs) profiles. The aim of the present study is to analyze the AE stability in the 93910 discharge using the gyro-fluid code FAR3d, identifying the AE stability trends by comparing the plasma before and after the Tungsten contamination. Tungsten contamination causes the destabilization of Toroidal AEs (TAE) and Energetic particle modes (EPMs) in the same frequency range and radial location with respect to the experimental observation and M3D-K/GTAW code results. Next, a set of parametric studies are performed to analyze the effect of the thermal plasma and EP parameters on the AE stability. The analysis indicates a lower EP β threshold for the AEs destabilization if the EP energy increases, an improved AE stability of on-axis NBI configurations due to the stronger continuum damping in the inner plasma region as well as a large enhancement of the EP drive as the thermal ion density increases due to a higher ratio of the EP and Alfven velocities. Consequently, the simulations indicate the increment of the thermal ion density after the Tungsten contamination could be the main cause of the AE/EPM destabilization.

The aim of this work is to analyze the alpha particle transport induced by Alfvén eigenmodes (AEs) based on the latest design of China Fusion Engineering Test Reactor (CFETR). Firstly, the stability of AEs is analyzed in order to understand the physical characteristics of AEs in CFETR with latest design parameters. AEs driven by alpha particles are analyzed using the gyrokinetic ion/fluid electron hybrid code GEM. The transport of alpha particles is investigated using a reduced energetic particle transport model based on the resonance broadened quasi-linear model. GEM results show that AEs with many toroidal mode numbers can be destabilized simultaneously by alpha particles in CFETR, and the toroidal mode number of the most unstable mode is n = 8 . It is found that the excitation threshold of the alpha particle central beta for the n = 8 mode is substantially below the expected alpha particle beta value of CFETR. All these results indicate that AEs can be driven strongly in CFETR plasmas. Furthermore, simulation results using the reduced transport model show that, due to multiple unstable AEs, the alpha particle density decreases in the core and alpha particles are transported from the core region ( r / a < 0.35 ) to the outer region ( r / a > 0.35 ). In addition, we find that the minimum value of the safety factor q min and the central beta value of alpha particle have significant effects on the alpha particle transport, and the AE-induced alpha particle transport level becomes lower with smaller linear growth rates. Finally, it is found that the radial range of the redistributed alpha particle density profile depend on the q min radial location.

负三角形变位形下的托卡马克具有更低的湍流输运和更好的能量约束,被认为是未来聚变堆一个更好的选择。为了探索负三角形变位形下高能量粒子激发不稳定性的特征,使用动理学-磁流体混合模型程序M3D-K开展了此位形下高能量离子激发鱼骨模的线性不稳定性和非线性演化的模拟研究。基于类EAST参数条件,模拟发现负三角形变解稳理想内扭曲模不稳定性,但会致稳鱼骨模不稳定性。非线性模拟发现在没有磁流体非线性效应时,负三角形变位形下的鱼骨模更不容易饱和,可能的解释是相比于正三角形变位形,在负三角形变位形下的高能量离子轨道更接近与芯部,因而更容易驱动鱼骨模不稳定性。这些结果表明考虑高能量粒子激发的鱼骨模不稳定性后,负三角形变位形相比于正三角形变位形并没有明显优势。