Accurate plasma-chemical modeling of methane conversion requires reliable ionization data in relevant gas mixtures such as Ar:CH4. In this study, the apparent reduced effective ionization coefficient, αea/N, which includes contributions from mixture-specific ionization processes was determined experimentally using a steady-state Townsend discharge in Ar:CH4 mixtures (e.g., 99%Ar:1%CH4, 97%Ar:3%CH4, 95%Ar:5%CH4, 90%Ar:10%CH4, 75%Ar:25%CH4, 50%Ar:50%CH4, and 25% Ar:75%CH4) over a broad pressure range of 10–800 Torr and reduced electric field strengths E/N ranging from 40 to 1200 Td. The measured αea/N values were compared with theoretical αe/N values calculated using the Boltzmann equation solver BOLSIG+. Deviations between experimental αea/N and calculated αe/N exceeded 20% at low E/N. Improved agreement was achieved by including Penning ionization from Ar excited states, specifically Ar (3p53d, 13.84 eV) and Ar (3p54p, 12.91–13.48 eV). These results demonstrate that Penning ionization induced by Ar excited states (3p54p, 12.91 eV and upwards) must be considered for accurate modeling of Ar:CH4 plasmas, particularly at moderate to atmospheric pressures and low E/N.
The heat flux along magnetic field lines in a multi-fluid Hall thruster model is varied to investigate the effect of electron energy closures on performance metrics and local plasma properties in the thruster discharge. A scaling factor is applied to the traditional Spitzer–Harm heat flux along field lines to adjust the energy closure. Model outputs are compared to experimental measurements from a 4.5 kW Hall thruster of discharge current, key efficiency modes, ion velocity along channel centerline, and electron temperature along both channel centerline and a magnetic field line. It is found that the adiabatic case with no heat flux along field lines provides the best match to experimental data. Most critically, a model with adiabatic heat flux is able to capture the non-isothermality of field lines exhibited in experimental measurements. These results are discussed in the context of thermal transport, power balancing, and the efficacy of magnetic shielding in Hall thrusters.
Forty-five years ago, a coordinate system was shown to exist that gave simple but exact expressions whenever and wherever a toroidal plasma equilibrium ∇→p=j→×B→ exists. These coordinates, now called Boozer coordinates, which revolutionized the stellarator program, are also applicable to tokamaks. Here expressions for Faraday's law, the safety factor, and the internal inductance are derived. Their constraints should be useful in the design of tokamak power plants and for the thoughtful allocation of resources to minimize the time and the cost to the achievement of practical fusion power. Simple explanations are obtained for (1) why disruptions in tokamaks are so common, (2) why current-profile control though difficult may be required, especially during plasma shutdowns, and (3) why only pulsed tokamaks seem possible. Lack of familiarity with Boozer coordinates can make simple but exact expressions appear naive. Complicated derivations with dubious assumptions have been interpreted as “more rigorous.”
The effect of the stratified state of a low-pressure neon discharge plasma column, which serves as the radiating element of an asymmetric dipole antenna, on its characteristics is experimentally and numerically investigated. Time-resolved measurements of the integral plasma luminosity revealed that the observed strata are moving and have a velocity of about 80–100 m/s. Their characteristic size is about 1 cm, and they correspond to P-type striations. The electron density distribution within the striations, estimated using the transmitted wave method, shows an axial density contrast of approximately 3.5. The stratified discharge regime induces modulation of the radiated microwave signal, with the modulation frequency correlated with plasma luminosity bursts associated with moving strata. The modulation depth of the antenna gain increases with microwave frequency and reaches about 10% at 10 GHz. Significant amplitude distortion of the radiated microwave signal associated with discharge stratification must be taken into account in the design of plasma antenna systems. The obtained results are important for the development of high-speed adaptive radio systems.
Nanoparticle formation, confinement, and loss in a low-current DC glow discharge in acetylene with vertically oriented electrodes are investigated experimentally and numerically. Laser light scattering reveals the formation of a nanoparticle cloud in the negative glow region at acetylene pressures above 0.15 Torr, whereas no stable cloud is observed at lower pressures. The cloud is localized near the region of maximum plasma emission and evolves in time owing to particle growth and gravity-driven redistribution. A robust nonlocal loss mechanism is observed, whereby nanoparticles leave the confinement region and deposit on the discharge tube walls at positions significantly shifted toward the anode. Transmission electron microscopy demonstrates pronounced spatial size selection, with small nanoparticles confined near the cloud, whereas only large particles and aggregates are transported into the Faraday dark space. Two-dimensional particle-in-cell simulations combined with a force-balance analysis reveal a shallow axial potential pit in the negative glow and a spatially varying near-wall sheath that govern nanoparticle confinement and escape. The calculations show that the smallest nanoparticles carry, on average, less than one elementary charge because of the exceptionally low electron temperature in the negative glow, allowing stochastic charge fluctuations to produce neutral particles that can escape radially to the chamber walls. Larger nanoparticles remain electrostatically confined by the near-wall potential barrier. Because it is localized along the discharge axis, nanoparticles can drift along the confinement boundary toward the anode and bypass the barrier, escaping to the chamber wall. This mechanism provides a physical explanation for the observed nonlocal nanoparticle loss.
A relativistic electron beam passing above a slow-wave structure emits high-power Cherenkov radiation (CR). When the beam is magnetized by an axial magnetic field, the cyclotron motion of the beam may affect the polarization of the radiation. In this study, we measure a degree of circular polarization (CP) of 0.1-THz CR generated from a magnetized electron beam where the cyclotron frequency is sufficiently lower than the radiation frequency. Right-hand (RH) and left-hand (LH) CP components of the radiation power are observed separately by using a CP divider. The RH-CP component, which rotates in the same direction of the cyclotron rotation, is dominant in the elliptical polarization of CR when the observed power is relatively low. On the other hand, the LH-CP component tends to increase as the observed power increases and becomes dominant in the polarization. The change in the handedness of the elliptically polarized CR may be associated with Faraday rotation and the effect of superposed modes with different azimuthal indices. Our results contribute to the development of Cherenkov-type high-power sources in terms of polarization control as well as the evaluation of the electrons in Cherenkov interaction.
We develop a theory of terahertz surface-plasmon generation in hollow metallic nanotubes driven by a beat-induced nonlinear current. Two co-propagating femtosecond laser beams produce a second-order ponderomotive force at the difference frequency, generating an axial surface current confined within the metal skin depth. This nonlinear current acts as a boundary source that excites cylindrical surface-plasmon modes when an axial phase-synchronism condition is satisfied. We derive the dispersion relation and obtain an analytical expression for the terahertz amplitude, demonstrating that the emission frequency and efficiency are governed predominantly by geometric confinement rather than the intrinsic material response. The radiated power scales with the nanotube radius and interaction length, enabling tunable and scalable terahertz output. The results identify a geometry-controlled nonlinear coupling mechanism and establish metallic nanotubes as compact plasmonic platforms for coherent terahertz generation.
The cyclic pitch motion of helicopter blades readily induces the dynamic stall phenomenon, which constrains the aerodynamic performance of the rotor system. To address this problem, research on improving airfoil dynamic stall using plasma actuation was carried out. By adjusting the actuation frequency and duty cycle, the induced jet characteristics of a dielectric barrier discharge (DBD) plasma actuator and its control effect on airfoil dynamic stall were investigated. Subsequently, based on the particle swarm optimization (PSO) algorithm, intelligent optimization for the combination of actuation parameters was achieved through the interactive iteration between the experimental environment and the algorithm. The results show that the effectiveness of plasma actuation in controlling dynamic stall is closely related to the characteristics of the DBD-induced jet. Specifically, the dimensionless frequency (F+) has a relative control advantage at different stages of the airfoil motion: high-frequency, small-scale vortices induced by a high F+ help to increase the maximum lift coefficient and delay stall, whereas when the F+ is 0.6 or 1.5, the formed low-frequency, large-scale vortices perform better in promoting flow reattachment during the downstroke. Regarding the duty cycle (Dc), the control effect of unsteady actuation is superior to that of steady actuation. Optimized by the PSO algorithm, the global optimum solution was obtained in the 8th generation of particles, corresponding to a F+ and Dc of 1.635 and 23%, respectively, achieving a 10.6% increase in the average lift.
As a key parameter in magnetic reconnection, the reconnection rate determines the speed at which magnetic energy is converted into plasma kinetic energy. It is well established that the reconnection rate of collisionless magnetic reconnection is on the order of 0.1—a value sufficiently high to account for various explosive phenomena in space environments, such as solar flares and geomagnetic substorms. Recently, Lu et al. [Sci. Bull. 70, 2766 (2025)] proposed a first-principles theoretical model of the reconnection rate during non-steady collisionless reconnection in a Harris-type current sheet. As magnetic reconnection proceeds, the ion and electron outflows increase continuously over time. Plasma becomes depleted in the electron and ion diffusion regions, while the magnetic field piles up downstream, causing the opening angles of the diffusion regions to grow continuously. As a result, the reconnection rate rises progressively and eventually reaches a peak value, which is approximated as 0.2n0/nb (where n0 is the peak density in the current sheet and nb is the density of the background plasma) and exhibits no dependence on the ion-to-electron mass ratio. However, in that model, the ion and electron outflows are solely governed by the Lorentz force term. In the present paper, we additionally incorporate contributions from the electric field and pressure gradient terms and analyze their combined influence on the electron and ion outflows and then the peak reconnection rate. Our results indicate that the electric field and pressure gradient terms weaken electron and ion outflows, which, in turn, reduces the peak reconnection rate. In the new model, the peak reconnection rate is roughly 1/5 times that in Lu et al. [Sci. Bull. 70, 2766 (2025)], which scales approximately as 0.09n0/nb.
This work presents the three-dimensional (3D) extension of the FENNECS code, a particle-in-cell framework developed to simulate the dynamics of non-neutral plasmas in complex geometries. The development is motivated by the study of spontaneous electron cloud formation in gyrotron electron guns, which can induce parasitic currents and lead to operational disruptions. The inclusion of 3D effects allows the modeling of the diocotron instability, which plays a major role in limiting the cloud density and driving electron losses. These features are inherently absent from the previous 2D version, where the axisymmetric assumption suppresses all azimuthal dynamics. The 3D capabilities are verified in simplified configurations. Simulations of the diocotron instability in an axially uniform annular electron cloud are found to be in excellent agreement with analytical linear theory. Finite-length effects are also investigated by simulating a cloud confined in a Penning–Malmberg trap, with the code reproducing the expected trends predicted by a linear model. FENNECS 3D is then used to simulate the TRapped Electrons eXperiment, a dedicated setup designed to reproduce the trapping conditions of gyrotron electron guns. Simulations performed under experimentally realistic conditions demonstrate the periodic, self-consistent growth and disruption of the electron cloud due to the diocotron instability. The simulated currents agree quantitatively with experimental measurements in both frequency and amplitude. This work establishes a robust and versatile framework for investigating electron trapping phenomena in gyrotrons and other devices where similar mechanisms are present and represents a significant step forward in the numerical modeling of non-neutral plasmas.
The Kinetic-scale Energy and momentum Transport eXperiment (KiNET-X) sounding rocket explored the coupling of injected barium ions (Ba+) to the ambient ionospheric plasma at two altitudes by releasing two canisters of barium neutrals, each forming an ionized cloud. KiNET-X had instruments on the main payload and on two small deployed subpayloads (Bobs) to measure the Ba+ density (nBa+). The main payload observed nBa+ at a nearly fixed pitch angle to the local geomagnetic field (B→), whereas the Bobs scanned a range of pitch angles. A simple model based on tracing each cloud's particles agreed with the main payload's observed nBa+ profile from the first release; however, for the second release, non-ideal “skidding” motion was required to capture features of the observed nBa+ profile. This paper is a companion to Moses et al. [Phys. Plasmas 32, 042109 (2025)] that analyzes the Bobs' nBa+ profiles. Comparison between the Bobs' measured and modeled nBa+ profiles indicate that pitch angle scattering occurred in each release. Non-idealized pitch angle distributions may arise in the Ba+ clouds from the presence of electric fields parallel to B→ or wave–particle pitch angle diffusion. Pitch angle scattering processes can be proxied in our model by increasing the field-of-view for the density calculation. These different model configurations capture different portions of the measured subpayloads' density and pitch angle profiles. There is perhaps a combination of skidding and scattering that is not easily represented in our model.
To meet the demands of high-voltage power electronics, this study elucidates the role of ion flux in plasma-enhanced chemical vapor deposition in governing the sp3/sp2 ratio of diamond-like carbon (DLC) films on alumina particles, which is believed to have the potential to improve the interface between epoxy and filler particles. Independent control of ion flux revealed a non-monotonic trend in the sp3/sp2 ratio: it initially increases due to suppressed carbon relaxation at moderate flux, then decreases beyond a critical threshold due to thermal graphitization from excessive energy deposition. By integrating the subplantation model with energy analysis, this study clarifies the competition between kinetic stabilization and thermal degradation. This work provides a mechanistic understanding and a practical guideline for optimizing DLC-coated fillers in high-performance composites.
The reliability of a model that incorporates the calculation of the electron energy distribution function into a fluid model of a gas discharge is considered. This modeling approach is available in the Plasma Module of the popular COMSOL Multiphysics computational package, where it is named “Space-Dependent Electron Energy Distribution Function Modeling.” The analysis is carried out for a positive column of a DC glow discharge in argon. This work completes our previous study, which dealt with a short (without a positive column) glow discharge. As in the previous study, the analysis, supported by relevant data from the kinetic simulations, revealed fundamental inconsistencies and internal contradictions in the model under study.
A novel per-time step, per-particle efficient and accurate numerical method for modeling dynamic dust charge in dusty plasma simulations is presented. A variety of charging currents are implemented, and any charging current with an analytical expression or empirical value can be included. While work on charge-varying dust has long existed in the literature, our numerical model provides significant improvements to the spatial and temporal evolution of dust charge for molecular dynamics simulations of dusty plasma. When charge time exceeds simulation time, we allow for non-equilibrium dust charge by coupling the charge convergence to the simulation time step. The model is verified with the well-known Spitzer potential and tested with a simple laboratory RF-plasma discharge background plasma and Particle-In-Cell lunar surface output background plasma. When both gradients in the plasma profiles and gravity are included, the steady state is a dynamic equilibrium, where particles oscillate when using the dynamic dust charge model as opposed to stationary equilibrium for static dust charge. Modeling varying or dynamic dust charge allows exploring new complex dynamics with simulations, such as ion-acoustic waves, dust heating, lunar cavity dust transport, and in general, dynamic equilibrium conditions, which can arise from dynamic dust charge.
A fundamental question in inertial confinement fusion is how implosion performance, and therefore ignition thresholds and fusion gain, evolve with target size. In laser-driven direct drive fusion, the scaling of laser-drive performance with size is critical to this evolution and to extrapolating results from the 30-kJ OMEGA laser-fusion experiments to ignition-class facilities such as the National Ignition Facility. Beyond the well-known adverse effects of cross-beam energy transfer (CBET) on drive performance, here we demonstrate that effects related to the non-scaling physics of thermal conduction and electron–ion energy equilibration exert an influence on drive behavior with scale that equals or surpasses that of CBET. We find that a significant portion of the lost implosion performance with increasing scale is due to the loss of shell implosion velocity. Furthermore, we show that while modest modifications to hydro-scaled designs can recover most of the lost implosion velocity, a full hydro-equivalent performance extrapolation is difficult to achieve without CBET mitigation or subcooling below the triple point of DT.
Hypervelocity impacts generate plasma plumes containing significant quantities of micrometer/sub-micrometer dust grains. The charging dynamics of these grains significantly modulate the kinetic evolution and electromagnetic response characteristics of the plasma. To elucidate the control mechanisms of charged dust on the electromagnetic properties of impact-generated plasma, this study establishes a theoretical model for the complex electrical conductivity of dusty plasma in hypervelocity impact environment. Rooted in electron transport kinetic theory and the linearized Boltzmann equation, the model rigorously incorporates both inelastic electron–dust charging collisions and elastic Coulomb scattering mechanisms to derive an analytical expression for macroscopic conductivity. Numerical analyses reveal that the presence of dust induces a distinct frequency dependence in plasma conductivity: in the low-frequency regime, dust grains suppress conductivity by enhancing electron momentum dissipation; conversely, in the high-frequency regime, dust effects lead to a slight enhancement in conductivity. Furthermore, the study identifies that thermionic emission, while reducing the net negative charge on dust, significantly increases the electron–dust Coulomb collision frequency, thereby further attenuating low-frequency conductivity. Parametric studies demonstrate that plasma temperature, dust number density, and particle radius exert nonlinear modulation effects on conductivity. Finally, a power-law particle size distribution is introduced to correct the mono-disperse assumption. The results confirm that non-uniform size distributions yield a higher effective collision frequency compared to average-size models. This work not only refines the electromagnetic transport theory for Hypervelocity impact plasmas but also provides a critical theoretical basis for assessing electromagnetic damage and designing protection systems for spacecraft.
Coupling reactor-scale plasma simulations with device-scale three-dimensional (3D) feature evolution models enables predictive profile simulation under realistic plasma boundary conditions. This coupling is essential for accurate analysis and mechanistic interpretation of etched feature evolution during plasma etching processes. However, quantitatively validated multiscale simulations that reliably couple bulk plasma behavior with surface reaction databases for realistic 3D profile prediction remain limited. This study proposes a multiscale simulation framework that integrates reactor-scale plasma predictions with a device-scale 3D profile evolution platform to investigate crystalline Si etching under HBr/Cl2 mixture plasma conditions at varying gas mixing ratios. Reactor-scale simulations predict plasma properties in a virtual sheath regime, providing the characteristics of ion and neutral particles impinging on 3D surfaces. These characteristics are incorporated into a 3D feature profile simulator that computes real-time shape evolution using an effective 3D mesh, Compute Unified Device Architecture-accelerated ballistic transport, and an OpenMP-parallelized surface reaction model. The plasma–surface reaction set estimates etch yields and surface reaction coverages as functions of the HBr/Cl2 mixing ratio and ion energy over a wide range of plasma process conditions. Realistic transport mechanisms include incident ion and neutral distributions from the bulk plasma, reflected ions, and re-emitted neutrals from feature sidewalls. The predicted profile evolution agrees well with the experimental results. Additionally, the model provides quantitative information on the etch rate, ion flux, ion energy, radical fluxes, and reaction coverages, enabling mechanistic interpretation of etching characteristics in HBr/Cl2 mixture plasma etching of crystalline Si.
Time-resolved, absolute populations of metastable molecules, N2(A3Σu+,v=0,1), number densities of metastable atoms, N(2D) and N(2P), and the number density of molecular ions, N2+, are measured in nitrogen and N2–NO mixtures in a heated plasma flow reactor, at temperatures up to T = 1000 K. The flow in the reactor is excited by the ns pulse discharge bursts, generating a diffuse plasma with well-defined boundaries. The measurements are made by the tunable diode laser absorption spectroscopy, atomic resonance absorption spectroscopy, and pulsed UV cavity ring down spectroscopy. The results are compared to the kinetic modeling predictions. The results illustrate the generation and decay of N2(A), a precursor for metastable excited atoms, during the discharge burst and in the afterglow. The N2(A) populations decay significantly during the burst, mainly due to quenching by the ground state N atoms. The excited atoms, N(2D) and N(2P), decay in the afterglow is due to their quenching by N2 molecules and N atoms, respectively. The apparent yield of metastable N atoms during the quenching of N2(A), inferred from comparison with the modeling predictions, is significantly lower than expected, below 1%. Time-resolved N2+ measurements in the afterglow exhibit its gradual decay, dominated by the conversion to N4+ and electron–ion recombination. At the present conditions, the effect of associative ionization of excited metastable species on the electron and ion densities in the afterglow appears negligible. Enhancing the metastable N atom yield may require the use of different precursors, such as Ar metastables generated in N2–Ar plasmas.
