Abstract The origin of cosmic rays from outside the Solar system are unknown, as they are deflected by the interstellar magnetic field. Supernova remnants are the main candidate for cosmic rays up to PeV energies but due to lack of evidence, they cannot be concluded as the sources of the most energetic Galactic CRs. We investigate discrete ejecta produced in state transitions of black hole X-ray binary systems as a potential source of cosmic rays, motivated by recent >100 TeV γ-ray detections by LHAASO. Starting from MAXI J1820+070, we examine the multi-wavelength observations and find that efficient particle acceleration may take place (i.e. into a robust power-law), up to ∼2 × 1016μ−1/2 eV, where μ is the ratio of particle energy to magnetic energy. From these calculations, we estimate the global contribution of ejecta to the entire Galactic spectrum to be $\sim 1~{{\ \rm per\ cent}}$, with the cosmic ray contribution rising to $\sim 5~{{\ \rm per\ cent}}$ at PeV energies, assuming roughly equal energy in non-thermal protons, non-thermal electrons and magnetic fields. In addition, we calculate associated γ-ray and neutrino spectra of the MAXI J1820+070 ejecta to investigate new detection methods with CTAO, which provide strong constraints on initial ejecta size of order 107 Schwarzschild radii (10−5 pc) assuming a period of adiabatic expansion.

Radiation reaction, the force experienced by an accelerated charge due to radiation emission, has long been the subject of extensive theoretical and experimental research. Experimental verification of a quantum, strong-field description of radiation reaction is fundamentally important, and has wide-ranging implications for astrophysics, laser-driven particle acceleration, next-generation particle colliders and inverse-Compton photon sources for medical and industrial applications. However, the difficulty of accessing regimes where strong field and quantum effects dominate inhibited previous efforts to observe quantum radiation reaction in charged particle dynamics with high significance. We report a high significance (>5σ) observation of strong-field radiation reaction on electron spectra where quantum effects are substantial. We obtain quantitative, strong evidence favouring the quantum-continuous and quantum-stochastic models over the classical model; the quantum models perform comparably. The lower electron energy losses predicted by the quantum models account for their improved performance. Model comparison was performed using a novel Bayesian framework, which has widespread utility for laser-particle collision experiments, including those utilising conventional accelerators, where some collision parameters cannot be measured directly.

Abstract We present a reinforcement learning (RL) framework for optimizing particle accelerator experiments that builds explainable physics-based constraints on agent behavior. The goal is to increase transparency and trust by letting users verify that the agent's decision-making process incorporates suitable physics. Our algorithm uses a learnable surrogate function for physical observables, such as energy, and uses them to fine-tune how actions are chosen. This surrogate can be represented by a neural network or by an interpretable sparse dictionary model. We test our algorithm on a range of particle accelerator optimization environments designed to emulate the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab. By examining the mathematical form of the learned constraint function, we are able to confirm the agent has learned to use the established physics of each environment. In addition, we find that the introduction of a physics-based surrogate enables our reinforcement learning algorithms to reliably converge for difficult high-dimensional accelerator optimization environments.

Superconducting radiofrequency (SRF) cavities are essential for high-energy particle accelerators rendering ultralow power dissipation and a high acceleration gradient. We demonstrate a superior performance in niobium-based 1.3 GHz cavities via medium-temperature (Mid-T) baking with remarkable values up to 5.5 × 1010 for the quality factor (Q0) at 16 and 32.7 MV/m for the maximum acceleration gradient (Eacc). Through correlative in situ spectroscopy, mass spectrometry, and electron and tunneling microscopy, we establish that nanoscale spatial distributions of impurities (O, C, H) directly evidenced at the metal-oxide interface govern performance enhancement. In niobium, annealing at 300 °C drives uniform oxygen doping (10-100 nm depth) via diffusion from the native oxide, while an optimized Mid-T protocol in delivering high Eacc values suppresses interfacial NbO segregation through competitive C-O interactions. For the optimized protocol, post-treatment characterization reveals up to an 8.8% increased superconducting gap and 29.7% reduced quasiparticle broadening, corroborating strain-mediated defect-impurity interactions at proximity layers. These results provide a Mid-T baking recipe to simultaneously enhance Q0 and maximum Eacc in SRF cavities via competitive impurity interactions at the metal-oxide interface.

The development of modern particle accelerators such as FCC-ee requires improved energy efficiency. On the SRF cavity side, the intermetallic compound [Formula: see text]is a promising alternative to niobium: its higher critical temperature (18.3 K) results into a BCS surface resistance at 4.5 K comparable to the one of Nb at 2 K, potentially allowing improved performance and reduced cryogenic costs while maintaining operation at 4.5 K. However, its brittleness makes bulk machining impractical, restricting its application to thin-film coatings. This study presents [Formula: see text]thin films deposited on copper substrates via DCMS using a single stoichiometric target. The optimization of the deposition parameters via the evaluation of the critical temperature, morphology, elemental composition and crystalline structure of the films is outlined. A niobium buffer layer is implemented to prevent copper-tin interdiffusion, and plays a key role in the film quality. The results demonstrate [Formula: see text]films deposited at ≤[Formula: see text] on copper substrates pre-coated with a 30µm niobium buffer layer which exhibit a critical temperature ≥ 17 K. The RF test of a film deposited via the same recipe on a bulk Nb QPR sample yielded an RF surface resistance of 23 nΩ at 4.5K, 20mT and 400MHz. These findings open the way to a scalable approach to high-performance [Formula: see text]/Cu cavities.

Abstract High‐energy particles are prevalent in space, and magnetic reconnection is one of the physical processes responsible for their generation. The heating and acceleration of ions and electrons typically occur respectively in the ion diffusion region and the electron diffusion region. In addition, the reconnection front is also a region for particle heating and acceleration. However, the spatial distribution of energetic particles in the reconnection region is less understood. In this work, we present in situ observations by the Cluster spacecraft that capture the spatial distributions of energetic ions and electrons in the magnetotail reconnection region. Our results show that high‐energy electrons are preferably observed around the separatrix region, while high‐energy ions are observed throughout the outflow region. To verify this spatial difference, we also conduct fully kinetic particle‐in‐cell (PIC) simulations, which correspond well with the observations. Our results improve the understanding of the generation and the spatial distribution of high‐energy particles in space.

This research proposes an innovative method that proton irradiation technology for defect control in practical engineering YBCO tapes,to improve the critical current density of YBCO high-temperature superconducting tapes in high magnetic fields.Based on the material irradiation terminal of a 4.5 MV electrostatic accelerator at Peking University, systematic irradiation experiments were conducted using 3 MeV proton beams on YBCO superconducting tapes at different fluence rates, successfully constructing high-density, low-dimensional controllable artificial pinning centers in the high superconducting tapes. This defect engineering significantly suppresses the flux creep phenomenon and enhances the pinning effect by creating low-energy pinning sites for flux lines, thereby significantly weakening the inhibitory effect of external magnetic fields on critical current (Ic). Comparative analysis of superconducting tapes before and after irradiation, including superconducting transition temperature, superconducting critical performance, and critical current density on magnetic field dependence.As the irradiation dose increases, high-density point defects (vacancies, interstitial atoms, etc.) and a small number of vacancy clusters are implanted inside the superconducting tape, resulting in a corresponding decrease in the superconducting phase. Therefore, as the dose increases, the orderliness of the superconducting phase in the superconducting tape decreases sharply, leading to a gradual widening of the superconducting transition temperature zone. By measuring the hysteresis loops of samples irradiated with different doses of protons and calculating the critical current density Jc based on the Bean model, the experimental data show that under irradiation conditions with a fluence rate of 8×10<sup>16</sup> P/cm<sup>2</sup>, the critical current of the sample under extreme operating conditions of 4.2K@6.5T achieved an 8-fold breakthrough improvement. Meanwhile, the maximum improvement factors in critical current density at 20K@5T and 30K@4T were also 5.5 times and 4.8 times, respectively. The logarithmic curve was fitted using the Jc ∝ B– α power exponent model to obtain the power parameterα values of 0.276, 0.361, and 0.397 for the variation of critical current density with magnetic field at three temperature ranges of 4.2K, 20K, and 30K, respectively. This indicates that the superconducting tape irradiated with protons will form more effective strong pinning centers at lower temperatures, reducing the dependence of the critical current density of the superconducting tape on the magnetic field.This performance breakthrough significantly enhances the application potential of high superconducting tapes in low-temperature and high magnetic fields environments, especially in frontier fields such as particle accelerators and fusion reactors, where there is an urgent demand for high-performance superconducting magnets. The study confirms that proton irradiation technology can achieve efficient optimization of critical performance through defect engineering without altering the existing preparation process of YBCO tapes, providing a highly feasible and process-compatible technical path for practical performance control of superconducting materials.

Doubly-Periodic Processing in Particle Accelerators and Fusion Reactors

Laser-driven particle acceleration and related laser-matter interaction experiments require an ultrashort pulse laser with high temporal contrast. Here, we presented a plasma mirror (PM) temporal contrast enhancement system implemented at the SG-II 5PW laser facility, with a comprehensive investigation of spatiotemporal properties and physical applications. Key performance parameters of a PM were successfully obtained through single-shot online measurement by combining a spatiotemporally overlapped chirped pulse method. At a 45° incidence angle, the plasma reflectivity reached 84% for S-polarization and 48% for P-polarization, while the focal spot maintained excellent quality and the temporal contrast was improved by two orders of magnitude. The PM system was further applied in proton acceleration experiments under both polarization configurations. Supported by corresponding physical diagnostics, a significant reduction in optimum target thickness from 8 to 0.8μm was achieved-clear evidence of effective pre-pulse suppression. Additionally, the PM and target installation were evaluated using a triple laser-damaged imaging method, based on the analysis of the three PM damage spots.

Research on the methods of coordinate transfer of dual-laser-tracker system in the confined space of particle accelerator

Abstract Quasi-separatrix layers (QSLs) at the Sun are created in regions where channels of open magnetic flux have footpoints near regions of large-scale closed magnetic flux. These regions show rapid changes in curvature and field strength. Numerical simulations of a relaxed coronal magnetic field and solar wind using the Magnetohydrodynamic Algorithm outside a Sphere model coupled to the Energetic Particle Radiation Environment Module model indicate common sources of energetic particles over broad longitudinal distributions in the background solar wind. These regions accelerate energetic particles from QSLs and current sheets. Here, we develop an analytical framework to describe the acceleration of energetic particles due to the magnetic field changes within and near separatrix layers. The reduced field strength near the separatrix layer drives magnetic field magnitude changes that accelerate energetic particles in the presence of plasma flow along the structure. Separatrix layers are prone to magnetic reconnection, creating fluctuations in the field that propagate out from the Sun, and release material previously contained within closed magnetic field structures, which are often rich in heavy ions and 3 He ions. Thus, we present a model of energetic particles accelerated from separatrix layers in the corona. Our results provide a plausible source for seed populations near the Sun.

This article aims to describe a novel workflow designed for generating a new family of minimal surface-based lattice structures with improved performance in terms of material budget compared to the well-known cells like Gyroid and Schwartz. The implemented method is based on the iterative resolution of a dynamic model, where proper forces are applied to generate minimal surface lattices, considering the boundary conditions and the constraint configurations. The novelty of the approach is given by the ability to create a minimal surface without resolving the partial differential equation and without knowing the exact minimal surface generative function. The starting geometry used for the lattice generation is the hypercube, parametrized to create different lattice configurations. Creating five different starting geometries and two constraint configurations, ten different lattice cells were created. For the comparison, a representative parameter of the material budget has been introduced and used to define the two best cells. The material budget is crucial for particle accelerator components, sensors, and detectors. These cells have been compared with Gyroid and Schwartz of the same thickness and bounding box, highlighting improvements of a factor of 2.3 and 1.7, respectively, in terms of material budget. The same cells have also been 3D-printed and tested under compression, and the obtained force–displacement curves were compared with those from a finite element analysis, demonstrating good agreement in the elastic region.

Increasing the service life of parts by forming protective and restorative coatings through cold spraying (CS) is a tough scientific and technical challenge. The object of the study is the process of accelerating powder particles in a supersonic rotating nozzle for CS. For CS, it is difficult, and sometimes even impossible, to form coatings on internal and hard-to-reach surfaces. In the practice of using the technology, this is considered one of the most problematic places, which limits the capabilities of the technology. This paper focused on improving the CS process by developing a new supersonic rotating nozzle for coating deposition on internal and hard-to-reach surfaces of parts, establishing the regularities of the trajectory of motion and acceleration of powder particles in it. During the study, classical methods of computational gas dynamics were used, including methods for investigating two-phase flows. Experimental verification of the modeling results was performed by the pneumatic method of determining the Mach number using a Pitot-Prandtl tube. Numerical modeling of CS processes was performed for two designed rotating nozzles – two-channel and three-channel. The values of the maximum velocity of aluminum powder particles with a diameter of 10 μm at an air stagnation pressure of 4.0 MPa and stagnation temperature of 550°C were obtained: 558 m/s for the two-channel nozzle, and 585 m/s for the three-channel one, which is sufficient for adhesion of particles to the substrate. A three-channel nozzle was chosen for manufacturing and experimental testing. The difference between the experimental and calculated values of the Mach number at the nozzle outlet did not exceed 10%. The presence of two additional nozzles, located in the main channel and directed at an angle to the main flow direction, ensures the rotation of the flow with particles from the initial direction at an angle of approximately 75 degrees, which satisfies the requirements for forming CS coatings.

Context. Numerous classical novae have been observed to emit γ -rays ( E > 100 MeV) detected by the Fermi-LAT. The prevailing hypothesis attributes this emission to the interaction of accelerated particles within shocks in the nova ejecta. However, the lack of non-thermal X-ray detection coincident with the γ -rays remains a challenge to this theory. Aims. We aim to constrain the γ -ray production mechanism by combining optical and X-ray data with a detailed analysis of the Fermi-LAT observations for two classical novae, V1723 Sco 2024 and V6598 Sgr 2023. Methods. We performed similar analyses of the Fermi-LAT data for both novae to determine the duration, localization, and spectral properties of the γ -ray emission. These results were compared with optical data from the AAVSO database and X-ray observations from NuSTAR, available for V1723 Sco 2024 only, to infer the nature of the accelerated particles. Finally, we used a physical emission model to extract key parameters related to particle acceleration. Results. V1723 Sco 2024 was found to be a very bright γ -ray source with an emission duration of 15 days allowing us to constrain the spectral index and the total energy of accelerated protons. Despite early NuSTAR observations, no non-thermal X-ray emission was detected simultaneously with the γ -rays. However, unexpected γ -ray and thermal hard X-ray emission were observed more than 40 days after the nova outburst, suggesting that particle acceleration can occur even several weeks post-eruption. V6598 Sgr 2023, on the other hand, was detected by the Fermi-LAT at a significance level of 4 σ over just two days, one of the shortest γ -ray emission durations ever recorded, coinciding with a rapid decline in optical brightness. Finally, the high ratio of γ -ray to optical luminosities and γ -ray to X-ray luminosities for both novae, as well as the curvature of the γ -ray spectrum of V1723 Sco below 500 MeV, are all more consistent with the hadronic than the leptonic scenario for γ -ray generation in novae.

Abstract Turbulence is a ubiquitous process that transfers energy across many spatial and temporal scales, thereby influencing particle transport and heating. Recent progress has improved our understanding of the anisotropy of turbulence with respect to the mean magnetic field; however, its exact form and implications for magnetic topology and energy transfer remain unclear. In this study, we investigate the nature of magnetic anisotropy in compressible magnetohydrodynamic turbulence within low- β solar wind using measurements from the Cluster spacecraft. By decomposing small-amplitude fluctuations into Alfvén and compressible modes, we reveal that magnetic anisotropy is largely mode dependent: Alfvénic fluctuations are broadly distributed in propagation angle, whereas compressible fluctuations are concentrated near the quasi-parallel (slab) direction, a feature closely linked to collisionless damping of compressible modes. For β → 0, compressible modes become dominant within the slab component at smaller scales. These findings advance our understanding of magnetic anisotropy in solar wind turbulence and offer a new perspective on the three-dimensional turbulence cascade, with broad implications for particle transport, acceleration, and magnetic reconnection.

Abstract The Parker Solar Probe Integrated Science Investigation of the Sun (IS⊙IS) instrument suite measured a variety of suprathermal and energetic particle events during orbits 18 and 19. We provide an overview of key features of the observations to provide guidance critical to making progress on complicated, integrated data sets like those provided by IS⊙IS. In this work, we analyze and describe observations of particle acceleration signatures associated with coronal mass ejection (CME)–driven shocks and solar flares from 2023 November to 2024 March as measured by the IS⊙IS/Energetic Particle Instrument-Low Energy and Energetic Particle Instrument-High Energy particle detectors. We present energy spectra for protons through Fe ions from ∼10 keV nuc −1 to >10 MeV nuc −1 , abundance ratios, and time series analyses for seven solar energetic particle (SEP) events with respect to the magnetic field and plasma context provided by the FIELDS and Solar Wind Electrons Alphas and Protons instruments, respectively. For SEP events in orbits 18 and 19, we find that acceleration driven by multiple CMEs in succession have larger variability in 4 He/H and Fe/O ratios than singular CMEs, that flare-associated SEP events preferentially accelerate higher mass-to-charge ratio particles, and that shock upstream transients may be present in CME-driven interplanetary shocks.

Astrophysical observations suggest that magnetic reconnection in relativistic plasmas plays an important role in the acceleration of energetic particles. Modeling this accurately requires numerical schemes capable of addressing large scales and realistic magnetic field configurations without sacrificing the kinetic description needed to model particle acceleration self-consistently. We demonstrate the computational advantage of the relativistic semi-implicit method (RelSIM), which allows for reduced resolution while avoiding the numerical instabilities typically affecting standard explicit methods, helping to bridge the gap between macroscopic and kinetic scales. Two- and three-dimensional semi-implicit particle-in-cell simulations explore the linear tearing instability and the nonlinear development of reconnection and subsequent particle acceleration starting from a relativistic Harris equilibrium with no guide field. The simulations show that particle acceleration in the context of magnetic reconnection leads to energetic power-law spectra with cutoff energies, consistent with previous work done using explicit methods, but are obtained with a considerably reduced resolution.

High-voltage vacuum bushings are vital for particle accelerators, X-ray tubes, electron microscopes, fusion devices, and electron sources. Commercial bushings, however, are limited to ~ 100 kV, and exceeding this limit has proven to be difficult. This work reports the physics-informed design, simulation, and experimental validation of Hammerhead, a compact vacuum bushing that has been tested up to 330 kV and used reliably at 300 kV. The design utilizes a coaxial electrode configuration and a ceramic insulator that bridges the gap using a flashover-resistant geometry. Compared with the state-of-the-art, Hammerhead nearly doubles the voltage holdoff per unit of volume, does not require ultra-high vacuum conditions or sub-micron polishing, and can be manufactured in a moderate cleanroom environment. It also integrates directly with a high-voltage coaxial cable, without the need for insulating fluids or pressurized gas. Here, it is shown - over 144 h of experimental testing - that Hammerhead operates stably at 300 kV with dark current below 10 μA.

Abstract Magnetic reconnection in current layers that form intermittently in radiatively inefficient accretion flows onto black holes is a promising mechanism for particle acceleration and high-energy emission. It has been recently proposed that such layers, arising during flux eruption events, can power the rapid TeV flares observed from the core of M87. In this scenario, inverse-Compton scattering of soft radiation from the accretion flow by energetic electron–positron pairs produced near the reconnection layer was suggested as the primary emission mechanism. However, detailed calculations show that radiation from pairs alone cannot account for the GeV emission detected by the Fermi observatory. In this work, we combine analytic estimates with 3D radiative particle-in-cell simulations of pair–proton plasmas to show that the GeV emission can be naturally explained by synchrotron radiation from protons accelerated in the current sheet. Although the exact proton content of the layer is uncertain, our model remains robust across a broad range of proton-to-pair number density ratios. While protons are subdominant in number compared to pairs, our simulations demonstrate that they can be accelerated more efficiently, leading to a self-regulated steady state in which protons dominate the energy budget. Ultimately, proton synchrotron emission accounts for approximately 5%–20% of the total dissipation power. The majority is radiated as MeV photons via pair synchrotron emission, with a smaller fraction emitted as TeV photons through inverse-Compton scattering.

Abstract Stellar flares are potent drivers of atmospheric evolution on orbiting exoplanets, primarily through extreme ultraviolet (EUV) and soft X-ray irradiation. However, the contribution of hard X-rays (HXR; 3–20 keV), which penetrate deeper into planetary atmospheres, to mass loss and particle acceleration has remained poorly understood. To quantify the HXR share of the total radiative budget, we conducted quasi-simultaneous observations of the active M-dwarf AU Mic using NuSTAR, Swift, and the Einstein Probe. Our analysis detected two major flares, and we performed an empirical check by deriving a quiescent-phase soft X-ray (SXR; 0.3–3 keV) to HXR relation and then applying it to the flares. By combining this with the quiescent coronal SXR–EUV relation conversion of J. Sanz-Forcada et al, we computed the total high-energy flux (EUV + SXR + HXR) and assessed the relative role of HXR in atmospheric escape. We find that HXR accounts for only a few percent of the total radiative energy budget during both quiescent and flaring states. While a high-energy spectral tail is detected in the second flare, time-resolved spectroscopy reveals a dominant chromospheric-evaporation signature, indicating that the flare energetics are primarily thermal.