Xingbao Lyu

Zero-dimensional collisional-radiative models (0D-CRMs) commonly represent particle transport and radiative transfer through diffusion frequencies and escape factors. In non-uniform discharges, the actual particle distributions deviate from the geometric eigenmodes assumed in 0D-CRM, leading to significant bias in the inferred plasma parameters. Using a two-dimensional axisymmetric fluid simulation of a helium discharge as the reference solution, this work examines how bias in the diffusion frequency modifies the 2 1 S and 2 3 S populations and propagates to higher excited states. A straightforward approach is to treat the diffusion frequencies as additional Bayesian variables with uniform priors, but the resulting inverse mapping is strongly non-unique and prone to noise amplification. To address this issue, an additional physical constraint is introduced into the likelihood function. Under fixed electron temperature and electron density, the populations of highly excited states exhibit a quasi-linear relationship with the 2 1 S and 2 3 S populations; this structure is quantified by a singular value decomposition based subspace residual and incorporated into the likelihood function. The constrained approach contracts the posterior distribution, improving the robustness and accuracy of 0D-CRM-based optical emission spectroscopy diagnostics under diffusion frequency bias.

Superparamagnetic materials have attracted increasing attention for high-frequency microwave absorption because superparamagnetic relaxation can partially overcome the high-frequency limitations of conventional magnetic absorbers. Herein, Fe₃O₄/rGO composite powders were prepared by electrostatic self-assembly and subsequently incorporated into an epoxy matrix, and magnetic-field-induced alignment was introduced during curing. Owing to the synergistic effects of interfacial polarization, magnetic dissipation, and improved impedance matching, the optimized composites exhibited markedly enhanced microwave absorption performance. In particular, when the rGO content was 10 wt% and an external magnetic field was applied, the composite achieved effective absorption across the entire X-band (8–12 GHz) within a thickness range of 1–3 mm, together with a minimum reflection loss of −40.3 dB. The enhanced performance is attributed to the combined contributions of abundant heterogeneous interfaces, superparamagnetic relaxation, and field-induced orientation of Fe₃O₄-decorated rGO sheets. This work provides a simple physical strategy for the microstructural regulation of magnetic–dielectric composites toward high-performance microwave absorption.

This article studies the performance of a microwave plasma interference switch (MPIS) in a 10-MW-class S-band microwave pulse compressor (MPC). It analyzes the impacts of pin and plate electrodes and distance on the output gain, cavity gain, and self-breakdown characteristics of the MPC. Frequency-domain simulations and experimental validation are conducted using a vector network analyzer (VNA) and high-power microwave tests, in order to determine the optimal structural parameters for the resonant cavity. The obtained results show that large-distance pin electrodes provide the highest output gain (up to 17.7 dB) and are less prone to self-breakdown at repetition frequencies below 25 Hz. On the contrary, small-distance plate electrodes have higher performance at higher repetition frequencies compared with small-distance pin electrodes due to their reduced self-breakdown. The 2-D fluid simulation results of the plasma switch demonstrate its evolution process and allow us to understand the experimental results.

Superparamagnetic materials break the Snoek’s limit of ferromagnetic materials, offering significant potential for the design and development of broadband microwave-absorbing metamaterials. This study presents the design and fabrication of a novel metamaterial with a periodic truncated pyramid structure, based on superparamagnetic Fe3O4 nanoparticles/hollow glass microspheres/epoxy resin composite (Fe3O4NP/HGS/EP). By optimizing the fabrication process and geometric structure, the proposed material achieves effective ultra-wideband electromagnetic wave absorption across the entire C-band and X-band, with an absorption rate exceeding 90%. Experimental validation confirms the exceptional electromagnetic absorption performance of the composite material, which is primarily attributed to the synergistic effects of superparamagnetic relaxation, multiple electromagnetic loss mechanisms, and the significantly improved impedance matching enabled by the gradient structure.

Spatial evolution of microscopic collision mechanisms in non-equilibrium plasmas is a key challenge in energy transfer research. This work introduces a diagnostic method: By mapping probe bias voltage, spatial position, electron energy, and collision type, we reconstruct the spatial gradient of the collision network in the negative glow region using a single Langmuir probe. Key aspects of this approach include converting voltage scans into axial movement coordinates and correlating EEDF peak energies to specific collision “fingerprints” (e.g., 9.4 eV for Penning ionization of H2S by metastable He, 19.8 eV for superelastic collisions between metastable He and fast electrons). Experiments in helium glow discharges reveal enhanced metastable helium excitation near the cathode sheath and low-energy electron de-excitation at the center, intensifying atom–atom collisions. This method allows in situ, calibration-free identification of impurities like H2S, CH4, and air components. EEDF evolution under discharge perturbations provides a comprehensive view of collision kinetics, revealing complex interactions between energy transfer, collision cross sections, and particle transport. This approach overcomes the spatial limitations of traditional point measurements, offering a high-resolution tool for plasma micro-kinetics research.

This paper has established a relatively comprehensive model for ultra-low frequency (ULF) current induced by thermal pressure gradients and its propagation. In the ULF current excitation model, we decomposed the current into a constant term unaffected by altitude and a product with a function significantly influenced by altitude. Combining this with the EISCAT background, we determined that for modulation frequencies below 5 Hz, the optimal height for ULF current excitation corresponds to the critical frequency height. We calculated the ionospheric currents at heating altitudes of 332 km for modulation frequencies of 5 Hz; the corresponding maximum currents were 1.03 × 10−10 A·m−2. By incorporating the current into the ULF waves propagation model based on magnetoionic theory, we found that the electromagnetic field energy is mainly concentrated in the horizontal direction, indicating that the energy primarily propagates outward through magnetosonic waves. The dominant components are the electric field component Ey and the magnetic field component Bz, whose maximum values reached 1.1 μV·m−1 and 1.5 pT. Unfortunately, magnetosonic waves cannot propagate downward due to the sharp variation in the real part of the refractive index between 200 and 300 km. However, the shear Alfvén waves component By can propagate downward, and there is still an intensity of approximately 0.1 pT at the bottom of the ionosphere, which is because the refractive index of shear Alfvén waves is most uniform in the parallel magnetic field direction, allowing By to propagate parallel to the magnetic field effectively.

A systematic investigation of the emerging toroidal vortex flow formation in a long-lived plasmoid composed of a high-current pulse atmospheric discharge with an electrolytic anode was carried out by using a particle image velocimetry (PIV) setup, as well as a specially assembled laser light sheet visualization system. More specifically, a laser light sheet imaging method of the investigated vortex flow was carried out by using a powerful green laser, which formed a flat divergent beam. The reflection of the laser beam reflected from dust particles in the atmosphere or from the smallest droplets of water vapor, especially prelaunched into the discharge zone to improve the contrast, rendered the visualization of the gas flow feasible, which was then recorded by a video camera. From our analysis, it was demonstrated that the emerging long-lived plasmoid gradually acquired a characteristic ring shape from the initial spherical shape and was able to maintain its volume for several hundred milliseconds after switching off the discharge current due to the formation of a vortex structure. The proposed method of measuring the particle image velocity made it thus possible to determine the velocity and direction of the flow in the formed toroidal vortex structure of the plasmoid.

Powerful high-frequency (HF) radio waves can modulate the ionosphere and lead to the current generation. Along these lines, in this work, a numerical model for the investigation of ultralow-frequency (ULF) currents excited by radio waves heating the ionosphere in mid-latitudes was established. The impact of both neutral winds and pressure gradient was taken into account by the proposed model. Combining the ionospheric background data of Beijing, the characterization of ULF ionospheric currents (1 Hz) was numerically calculated and discussed. From our analysis, it was demonstrated that the electron temperature relaxation time gradually increased with increasing the altitude during the day and night. The temperature relaxation time of 1 s was attributed to an altitude of 140 km during the day and 180 km at night. The maxima of neutral wind term current was 22 nA $\cdot$ m $^{-2}$ at daytime, corresponding to the position 102 km. Interestingly, at nighttime, the maximum value of the current was reduced to 0.032 nA $\cdot$ m $^{-2}$ , corresponding to 112 km. The electronic pressure gradient term current exhibited a maximum value of 8 nA $\cdot$ m $^{-2}$ at the position 104 km during the day, while, at nighttime, the maxima of the current was 0.19 nA $\cdot$ m $^{-2}$ , corresponding to the position near 224 km. It was also proved that the properties of the neutral wind term current are strongly dependent on the ionospheric conductivity but are constrained by the electron temperature and the relaxation time. In addition, the current induced by the electronic pressure gradient term is mainly correlated with the electronic Hall conductivity and the electron temperature relaxation time.