Machine learning opens up new possibilities for research of plasma confinement. Specifically, models constructed using machine learning algorithms may effectively simplify the simulation process. Previous first-principles simulations could provide physics-based transport information, but not fast enough for real-time applications or plasma control. To address this issue, this study proposes SExFC, a surrogate model of the Gyro-Landau Extended Fluid Code (ExFC). As an extended version of our previous model ExFC-NN, SExFC can capture more features of transport driven by the ion temperature gradient mode and trapped electron mode, using an extended database initially generated with ExFC simulations. In addition to predicting the dominant instability, radially averaged fluxes and radial profiles of fluxes, the well-trained SExFC may also be suitable for physics-based rapid predictions that can be considered in real-time plasma control systems in the future.

Microturbulence excited by ion temperature gradient (ITG)-dominant and trapped electron mode (TEM)-dominant instabilities is investigated by employing an extended fluid code (ExFC) based on the so-called Landau fluid model, which includes the trapped electron dynamics. Firstly, the global effect is emphasized through direct comparison of ITG and TEM instability domains based on local and global simulations. The global effect makes differences in both linear instability and nonlinear transport, including the fluxes and the structure of zonal flow. The transitions among ITG, TEM, and ITG & TEM (ITG & TEM represents that ITG and TEM coexist with different wavelengths) instabilities/turbulence depend not only on the three key drive forces $({R/L_{\rm n}, R/L_{\rm Te}, R/L_{\rm Ti}})$ but also on their global (profile) effects. Secondly, a lot of electrostatic linear gyro-fluid simulations are concluded to obtain a distribution of the instability.

Numerical research is conducted to investigate the effects of plasma boundary shape on the tearing mode triggering explosive bursts in toroidal tokamak plasmas. In this work, $m/n=2/1$ mode is responsible for the triggering of the explosive burst. Plasma boundary shape can be adjusted via the adjustment of the parameters triangularity ${\delta}$ and elongation ${\kappa}$. The investigations are conducted both under low $\beta$ (close to zero) and under finite $\beta$ regimes. In the low $\beta$ regime, triangularity and elongation both have stabilizing effect on the explosive burst, and the stabilizing effect of elongation is stronger. Under a large elongation (${\kappa =2.0}$), the elongation effect can evidently enhance the stabilizing effect in a positive triangularity regime, but barely affects the stabilizing effect in a negative triangularity regime. In the finite $\beta$ regime, the explosive burst is delayed in comparison with that in the low $\beta$ regime. Similar to the low $\beta$ cases, the effects of triangularity and elongation both are stabilizing. Under a large elongation (${\kappa =2.0}$), the elongation effect can evidently enhance the stabilizing effect on the explosive burst in a positive triangularity regime, but impair the stabilizing effect in a negative triangularity regime. The explosive burst disappears in the large triangularity case (${\delta =0.5}$), indicating that the explosive burst can be effectively prevented in experiments via carefully adjusting plasma boundary shape. Moreover, strong magnetic stochasticity appears in the negative triangularity case during the nonlinear phase.

We report the laser-chirp controlled terahertz (THz) wave generation from two-color-laser-induced air plasma. Our experimental results reveal that the THz wave is affected by both the laser energy and chirp, leading to radiation minima that can be quantitatively reconstructed using the linear-dipole-array model. The phase difference between the two colors, determined by the chirp and intensity of the laser, can account for the radiation minima. Furthermore, we observe an asynchronous variation in the generated THz spectrum, which suggests a THz frequency-dependent phase matching between the laser pulse and THz wave. These results highlight the importance of laser chirp during the THz wave generation and demonstrate the possibility of modulating the THz yields and spectrum through chirping the incident laser pulse. This work can provide valuable insights into the mechanism of plasma-based THz wave generation and offer a unique means to control THz emissions.

Low-temperature synthesis of Nb$_3$Sn thin-film cavity is of great significance in the field of superconducting radio frequency (SRF). The bronze process can grow only stable Nb$_3$Sn phase at 700 ℃, so it is considered to be the most promising process for low-temperature synthesis of Nb$_3$Sn thin-film cavity. We successfully fabricated the worldwide first Nb$_3$Sn thin-film cavity by bronze process. We technically solved the key problems of precursor preparation, characterized and analyzed the uniformity of the Nb$_3$Sn film, and tested the performance of the cut-out samples and the whole cavity of the Nb$_3$Sn film. It is obtained that the $Q_0$ value of the cavity at 4.2 K is about $1.2\times10^{9}$, which is greater than the performance of the bulk-niobium cavity under the same conditions. This result means that the preparation of Nb$_3$Sn by bronze process has the great potential to more practical copper-based Nb$_3$Sn thin-film cavity, which is expected to achieve a substantial improvement in the performance of SRF cavity and comprehensive engineering applications.

The scaling law of divertor heat flux width is one of the key topics of magnetic confinement fusion, which is almost inversely proportional to the poloidal magnetic field on some opened divertor tokamaks. This work focuses on the scaling laws of the closed divertor heat flux width in the HL-2A tokamak under different discharge conditions, such as the Ohmic, L- and H-modes. The results indicate that there are basic similarities of the scaling laws of the heat flux width between the opened and closed divertors. However, a larger spreading width in the private flux region is found, which is relevant to a small expansion factor of the magnetic flux in the closed divertor.

A branch of high-frequency Alfvénic modes is observed on the HL-2A tokamak. The electromagnetic mode can be driven unstably in the plasma with an off-axis neutral beam heating. Its mode frequency keeps almost unchanged or presents a slow-sweeping behavior, depending on the detail current evolution. The poloidal and toroidal mode numbers are $m/n=1/1$. The mode has a quite short duration ($\leq$20 ms) and usually appears 5–10 ms after the neutral beam being injected into the plasma. Hybrid simulations based on M3D-K have also been carried out. The result suggests that co-passing energetic particles are responsible for the mode excitation. The simulated mode structures are localized nearby location of minimum safety factor ($q_{\rm min}$) and agree with the structures obtained through tomography of soft x-ray arrays. Further, the modes are localized in the continuum gap and their frequencies increase with variation of $q_{\rm min}$ in a wide range. Last but not least, the characteristic of unchanged frequency on experiment is also reproduced by the nonlinear simulation with a fixed safety factor. All those evidences indicate that the $n=1$ high-frequency mode may belong to a reversed shear Alfvén eigenmode.

Plasma technology has widespread applications in many fields, whereas the methods for manipulating plasma transport are limited to magnetic control. In this study, we used a simplified diffusion-migration approach to describe plasma transport. The feasibility of the transformation theory for plasma transport was demonstrated. As potential applications, we designed three model devices capable of cloaking, concentrating, and rotating plasmas without disturbing the density profile of plasmas in the background. This research may help advance plasma technology in practical fields, such as medicine and chemistry.

Uniformly dispersed nickel single atoms (SAs) are experimentally prepared on ultralight N-doped graphene aerogels (Ni-SA@NRGA). The experimental results show that Ni-SAs in graphene aerogels can improve the conduction, polarization losses, and impedance matching properties of the Ni-SA@NRGA. As a result, the minimum reflection loss ($R_{\rm L,min}$) of Ni-SA@NRGA is $-$49.46 dB with a matching thickness of 2.0 mm and the broadest efficient absorption bandwidth is 3.12 GHz at a low thickness of 1.5 mm. Meanwhile, even with a matching thickness of 1.2–2.0 mm, the $R_{\rm L,min}$ value of Ni-SA@NRGA can reach $-$20 dB. The current study demonstrates the significance of incorporating metal single atoms into graphene aerogel for electromagnetic wave absorption.

Rutherford scattering formula plays an important role in plasma classical transport. It is urgent to investigate influence of magnetic field on the Rutherford scattering since the high magnetic field has been widely used in nowadays magnetic confinement fusion, inertial confinement fusion, and magneto-inertial fusion. In order to elucidate the magnetic field effect in a concise manner, we study the electron-ion collisions transverse to the magnetic field. The scattering angle is defined using the directions of electron velocity before and after collision, which is obtained analytically. It is found that the scattering angle can be influenced by finite magnetic field significantly. The theoretical results agree well with numerical calculation by checking the dependence of scattering angle on the magnetic field.

The excitation condition of reversed shear Alfvén eigenmodes (RSAEs) has been investigated during sawtooth-like oscillation in the EAST tokamak. The sawtooth-like phenomena can be reproduced in the configuration of reversed magnetic shear, and the threshold gradient of electron temperature is formed accordingly, together with the increasing of the confinement of thermal particles. The distribution function of energetic ions density is altered dramatically when the neutral beam is switched from NBI1L (tangent) to NBI1R (perpendicular), which can be captured by the measurement of radial neutron camera. The RSAEs are excited easily in the vicinity of $q_{\min}$ (1.99 m $\leq R \leq 2.06$ m) for the injection of neutral beam with perpendicular direction, which should be excited by the steep gradient of energetic ions density. Furthermore, the excitation of RSAEs and the formation of threshold gradient of electron temperature can take place concurrently, which means that the neutral beam with perpendicular injection is beneficial for the establishment of internal transport barrier.

The kinetic effects of thermal particles and fast ions on internal kink (IK) mode are numerically investigated by the MHD-kinetic hybrid code MARS-K. It is shown that either thermal particles or fast ions have stabilizing influence on IK. However, the former can not fully stabilize IK, and the later can suppress the IK. In addition, the synergistic effect from thermal particles and fast ions induces more stronger damping on IK. The kinetic effects from particles significantly raise the critical value of poloidal beta ($\beta_{\rm p}^{\rm crit}$) for driving IK in the toroidal plasma. This implies a method of controlling IK or sawtooth in the high-$\beta_{\rm p}$ discharge scenario of tokamak. It is noted that, at the $q=1$ rational surface, mode structure becomes more sharp due to the self-consistent modification by particles' kinetic effect.

Fusion born $\alpha$ particle confinement is one of the most important issues in burning plasmas, such as ITER and CFETR. However, it is extremely complex due to the nonequilibrium characteristics, and multiple temporal and spatial scales coupling with background plasma. A numerical code using particle orbit tracing method (PTC) has been developed to study energetic particle confinement in tokamak plasmas. Both full orbit and drift orbit solvers are implemented to analyze the Larmor radius effects on $\alpha$ particle confinement. The elastic collisions between alpha particles and thermal plasma are calculated by a Monte Carlo method. A triangle mesh in poloidal section is generated for electromagnetic fields expression. Benchmark between PTC and ORBIT has been accomplished for verification. For CFETR burning plasmas, PTC code is used for $\alpha$ particle source and slowing down process calculation in 2D equilibrium. In future work, 3D field like toroidal field ripples, Alfvén and magnetohydrodynamics instabilities perturbation inducing $\alpha$ particle transport will be analyzed.

Multiple broadband Alfvénic chirping modes (CMs), with frequencies in the wide range of $f\sim35$–150 kHz and chirping down rapidly, are found in HL-2A neutral beam injection plasmas, and the CMs can even coexist. The frequency chirping down process can be completed within $\sim$1 ms, and the frequency shift can reach 30–50 kHz. The CMs propagate in ion diamagnetic drift directions poloidally. The toroidal mode number is confirmed to be $n=1$, $2$, $3$ and $4$ for the $f\sim35$–65, 55–90, 70–120 and 100–150 kHz CMs, respectively. The CMs are more like to be energetic-particle continuum modes (EPMs), since the modes almost locate on the Alfvén continuum.

This research applies experimental measurements and NUBEAM, ONETWO and TRANSP modules to investigate the shine-through (ST) loss ratio and beam heating percentage of neutral beam injection on EAST. Measurements and simulations confirm that the ST loss ratio increases linearly with beam energy, and decreases exponentially with plasma density. Moreover, using the multi-step fitting method, we present analytical quantitative expressions of ST loss ratio and beam heating percentage, which are valuable for the high parameter long-pulse experiments of EAST.

Using the particle-in-cell simulations, we report an efficient scheme to generate a slow wave structure in the electron density of a plasma waveguide, based on the array laser–plasma interaction. The spatial distribution of the electron density of the plasma waveguide is modulated via effective control of the super-Gaussian index and array pattern code of the lasers. A complete overview of the holding time, and the bearable laser's intensity of the electron density structure of the plasma waveguide, is obtained. In addition, the holding time of the slow wave structure of the plasma waveguide is also controlled by adjusting the frequency of the array laser beam. Finally, effects due to ion motion are discussed in detail.

We reproduce nonlinear behaviors, including frequency chirping and mode splitting, referred to as bump-on-tail instabilities. As has been reported in previous works, the generation and motion of phase-space hole-clump pairs in a kinetically driven, dissipative system can result in frequency chirping. We provide examples of frequency chirping, both with and without pure diffusion, in order to illustrate the role of the diffusion effect, which can suppress holes and clumps; Asymmetric frequency chirpings are produced with drag effect, which is essential to enhance holes, and suppress clumps. Although both diffusion and drag effect suppress the clumps, downward sweepings are observed, caused by a complicated interaction of diffusion and drag. In addition, we examine the discrepancies in frequency chirping between marginally unstable, and far from marginally unstable cases, which we elucidate by means of a dissipative system. In addition, mode splitting is also produced via BOT code for a marginal case with large diffusion.

The critical gradient mode (CGM) is employed to predict the energetic particle (EP) transport induced by the Alfvén eigenmode (AE). To improve the model, the normalized critical density gradient is set as an inverse proportional function of energetic particle density; consequently, the threshold evolves during EP transport. Moreover, in order to consider the EP orbit loss mechanism in CGM, ORBIT code is employed to calculate the EP loss cone in phase space. With these improvements, the AE enhances EPs radial transport, pushing the particles into the loss cone. The combination of the two mechanisms raises the lost fraction to 6.6%, which is higher than the linear superposition of the two mechanisms. However, the loss is still far lower than that observed in current experiments. Avoiding significant overlap between the AE unstable region and the loss cone is a key factor in minimizing EP loss.

We numerically investigate the Coriolis force effect on the suppression of an explosive burst, triggered by the neo-classical tearing mode, in reversed magnetic shear configuration tokamak plasmas, using a reduced magnetohydrodynamic model, including bootstrap current. Previous works have shown that applying differential poloidal rotation, with rotation shear located near the outer rational surface, is an effective way to suppress an explosive burst. In comparison with cases where there is no Coriolis force, the amplitude of differential poloidal rotation required to effectively suppress the explosive burst is clearly reduced once the effect of Coriolis force is taken into consideration. Moreover, the effective radial region of the rotation shear location is broadened in cases where the Coriolis force effect is present. Applying rotation with shear located between the radial positions of $q_{\rm min}$ and the outer rational surface always serves to effectively suppress explosive bursts, which we anticipate will reduce operational difficulties in controlling explosive bursts, and will consequently prevent plasma disruption in tokamak experiments.

Two novel nonlinear mode coupling processes for reversed shear Alfvén eigenmode (RSAE) nonlinear saturation are proposed and investigated. In the first process, the RSAE nonlinearly couples to a co-propagating toroidal Alfvén eigenmode (TAE) with the same toroidal and poloidal mode numbers, and generates a geodesic acoustic mode. In the second process, the RSAE couples to a counter-propagating TAE and generates an ion acoustic wave quasi-mode. The condition for the two processes to occur is favored during current ramp. Both the processes contribute to effectively saturate the Alfvénic instabilities, as well as nonlinearly transfer of energy from energetic fusion alpha particles to fuel ions in burning plasmas.

Interaction between shear Alfvén wave (SAW) and energetic particles (EPs) is one of major concerns in magnetically confined plasmas since it may lead to excitation of toroidal symmetry breaking collective instabilities, thus enhances loss of EPs and degrades plasma confinement. In the last few years, Alfvénic zoology has been constructed on HL-2A tokamak and series of EPs driven instabilities, such as toroidal Alfvén eigenmodes (TAEs), revered shear Alfvén eigenmodes (RSAEs), beta induced Alfvén eigenmodes (BAEs), Alfvénic ion temperature gradient (AITG) modes and fishbone modes, have been observed and investigated. Those Alfvénic fluctuations show frequency chirping behaviors through nonlinear wave-particle route, and contribute to generation of axisymmetric modes by nonlinear wave-wave resonance in the presence of strong tearing modes. It is proved that the plasma confinement is affected by Alfvénic activities from multiple aspects. The RSAEs resonate with thermal ions, and this results in an energy diffusive transport process while the nonlinear mode coupling between core-localized TAEs and tearing modes trigger avalanche electron heat transport events. Effective measures have been taken to control SAW fluctuations and the fishbone activities are suppressed by electron cyclotron resonance heating. Those experimental results will not only contribute to better understandings of energetic particles physics, but also provide technology bases for active control of Alfvénic modes on International Thermonuclear Experimental Reactor (ITER) and Chinese Fusion Engineering Testing Reactor (CFETR).

Due to the limitations of impedance matching and attenuation matching, carbon nanotubes (CNTs) employed alone have a weak capacity to attenuate electromagnetic wave (EMW) energy. In this work, B and N co-doped CNTs with embedded Ni nanoparticles (Ni@BNCNTs) are fabricated via an in situ doping method. Compared with a sample without B doping, Ni@BNCNTs demonstrate a superior EMW absorption performance, with all minimum reflection loss values below $-20$ dB, even at a matching thickness of 1.5 mm. The experimental and theoretical calculation results demonstrate that B doping increases conduction and polarization relaxation losses, as well as the impedance matching characteristic, which is responsible for the enhanced EMW absorption performance of Ni@BNCNTs.

The energetic-particle-induced geodesic acoustic mode (EGAM) is studied using gyrokinetic particle simulations in tokamak plasmas. In our simulations, exponentially growing EGAMs are excited by energetic particles with a slowing-down distribution. The frequencies of EGAMs are always below the frequencies of GAMs, which is due to the non-perturbative contribution of energetic particles (EPs). The mode structures of EGAMs are similar to the corresponding mode structures of GAMs. Our gyrokinetic simulations show that a high EP density can enhance the EGAM growth rate, due to high EP free energy, and that EPs' temperature and the pitch angle of the distribution modify the EGAM frequency/growth rate by means of the resonance condition. Kinetic effects of the thermal electrons barely change the EGAM frequency, and have a weak damping effect on the EGAM. Benchmarks between the gyrokinetic particle simulations and a local EGAM dispersion relation exhibit good agreement in terms of EGAM frequency and growth rate.

The influences of the temperature gradient and toroidal effects on drift-tearing modes have been studied using the Gyrokinetic Toroidal code. After the thermal force term is introduced into the parallel electron force balance equation, the equilibrium temperature gradient can cause a significant increase in the growth rate of the drift-tearing mode and a broadening of the mode structure. The simulation results show that the toroidal effects increase the growth rate of the drift-tearing mode, and the contours of the perturbation field “squeeze” toward the stronger field side in the poloidal section. Finally, the hybrid model for fluid electrons and kinetic ions has been studied briefly, and the dispersion relation of the drift-tearing mode under the influence of ion finite Larmor radius effects is obtained. Compared with the dispersion relation under the fluid model, a stabilizing effect of the ion finite Larmor radius is observed.

Interface width effect on the spherical Rayleigh–Taylor instability in the weakly nonlinear regime is studied by numerical simulations. For Legendre perturbation mode $P_n$ with wave number $k_n$ and interface half-width $L$, the commonly adopted empirical linear growth rate formula $\gamma_n^{\rm em}(L)=\gamma_n/\sqrt{1+k_nL}$ is found to be sufficient in spherical geometry. At the weakly nonlinear stage, the interface width affects the mode coupling processes. The development of the mode $P_{2n}$ is substantially influenced by the interface width. Moreover, the nonlinear saturation amplitude increases with the interface width.

The terahertz (THz) temporal waveform and spectrum from a longitudinal electrically biased femtosecond filament is studied experimentally. The initial direction of the electron motion inside the unbiased filament plasma is deduced from the transformation of the THz temporal waveform with applied fields of opposite polarities. Furthermore, a spectrum shift to lower frequency of the THz spectrum is observed in the presence of a biased field. It agrees well with theoretical predictions.

The Rayleigh–Taylor instability at the weakly nonlinear (WN) stage in spherical geometry is studied by numerical simulation. The mode coupling processes are revealed. The results are consistent with the WN model based on parameter expansion, while higher order effects are found to be non-negligible. For Legendre mode perturbation $P_n(\cos\theta)$, the nonlinear saturation amplitude (NSA) of the fundamental mode decreases with the mode number $n$. When $n$ is large, the spherical NSA is lower than the corresponding planar one. However, for large $n$, the planar NSA can be recovered by applying Fourier transformation to the bubble/spike near the equator and calculating the NSA of the converted trigonometric harmonic.

Carbon nanotube@polypyrrole (CNT@PPy) hybrids have been successfully fabricated via a simple in situ chemical oxidation polymerization. The thickness of the PPy shell can be finely controlled in the range of 3.0–6.4 nm. The dielectric loss of core-shell hybrids can be tuned by the shell thickness, resulting in a well-matched characteristic impedance that can enhance electromagnetic wave (EMW) absorption performance. Minimum reflection loss of the hybrid with moderate PPy shell thickness can reach $-51.4$ dB at 11.8 GHz with a matching thickness of merely 2 mm. Furthermore, the minimum reflection loss values of the hybrid are below $-30$ dB even at thickness in the range of 1.4–1.9 mm, endowing the possibility of practical application of the hybrids in electromagnetic wave absorption field.

High $\beta_{\rm p}$ scenario is foreseen to be a promising candidate operational mode for steady-state tokamak fusion reactors. Dedicated experiments on EAST and data analysis find that density gradient $\nabla n$ is a control knob to improve energy confinement in high $\beta_{\rm p}$ plasmas at low toroidal rotation as projected for a fusion reactor. Different from previously known turbulent stabilization mechanisms such as ${\boldsymbol E} \times {\boldsymbol B}$ shear and Shafranov shift, high density gradient can enhance the Shafranov shift stabilizing effect significantly in high $\beta_{\rm p}$ regime, giving that a higher density gradient is readily accessible in future fusion reactors with lower collisionality. This new finding is of great importance for the next-step fusion development because it may open a new path towards even higher energy confinement in the high $\beta_{\rm p}$ scenario. It has been demonstrated in the recent EAST experiments, i.e., a fully non-inductive high $\beta_{\rm p}$ ($\sim $2) H-mode plasma ($H_{98y2}\ge 1.3$) has been obtained for a duration over 100 current diffusion times, which sets another new world record of long-pulse high-performance tokamak plasma operation with the normalized performance approaching the ITER and CFETR regimes.