Inverse Compton scattering (ICS) gamma ray sources are capable of producing quasi-monochromatic, continuously tunable, high-brightness, precisely polarization-controllable, and ultrashort gamma ray pulses in the energy range from tens of keV to several MeV or even higher. The energy spectrum measurement of an ICS source not only serves as a key indicator of the device operating status but also provides foundational information. In this paper, we proposed a novel method of using nuclear resonance fluorescence (NRF) as a probe for spectrum measurements. By utilizing the continuous tunability of an ICS source, NRF photons can be excited at different points across the spectrum. The shape of the energy spectrum can then be effectively scanned and reconstructed by recording the relative NRF yields at different energy points. The feasibility of the proposed method was validated by Geant4 simulations of measuring NRF photon emission from 56Fe irradiated by an ICS source. We utilized the 845 and 3449 keV NRF photons of 56Fe to measure a Gaussian spectrum and a segmented Gaussian spectrum, which correspond to ICS spectra under different collection angles. The simulation results showed high precision for quasi-monochromatic gamma ray spectrum measurements, with a normalized root mean square error of less than 5%. To maintain a sufficient signal-to-noise ratio during the measurement, the energy resolution of detectors is suggested to be less than 1% of the energy being measured. Given an energy tuning precision of ΔE, the minimum measurable width of the energy spectrum, in terms of standard deviation, can reach 0.85 ΔE. Further experiments are needed to validate its practical feasibility.

MeV ultrafast electron diffraction (UED) is a widely used technique for ultrafast structural dynamics studies of matter in numerous areas. The development of the laser wakefield accelerator (LWFA) shows great potential for an advanced all-optical electron source based on LWFA in UED applications. Here we experimentally demonstrated that an LWFA-based device with a miniaturized permanent magnet beamline can generate and manipulate electron beams suitable for UED. During beam transport, the LWFA electron beams with intrinsically short duration undergo temporal stretching owing to the energy spread and are subsequently compressed by the following double-bend achromat. The optimized double-bend achromat can make the beamline isochronous such that the arrival time jitter induced by the shot-to-shot energy fluctuation can be eliminated, and allow the advantage of the natural laser-beam synchronization for LWFAs to emerge. With the energy filtering, the beam energy spread can be reduced to 3% (full-width at half-maximum), while a sufficient amount of charge (11.9 fC) per bunch for diffraction is retained. Using a laser-driven terahertz deflector, the beam length and arrival time jitter measured at the sample location are approximately 49.6 fs (root mean square (r.m.s.)) and 4.7 fs (r.m.s.), respectively, resulting in a temporal resolution of ~49.8 fs. Comprehensive start-to-end simulations indicate the potential of reducing the bunch length to ~10 fs (r.m.s.) with a lower energy spread of around 1.6%. Clear single-shot and multi-shot diffraction patterns of single-crystalline gold samples are obtained, and the derived lattice constant agrees well with the actual value. Our proof-of-principle experiments open the door to the detection of ultrafast structural dynamics using MeV LWFA beams, and pave the way for UED applications with sub-10 fs temporal resolution. Researchers demonstrate that a laser wakefield accelerator-based device with a miniaturized permanent magnet beamline can generate and manipulate electron beams suitable for ultrafast electron diffraction.

A very-high-frequency (VHF) photocathode electron gun serves as an electron source capable of generating high-repetition-rate, high-brightness electron bunches. It operates in continuous-wave (CW) mode with a 100% microwave duty cycle, a configuration where multipactor emerges as a critical operational challenge requiring meticulous mitigation. The exponential amplification of secondary electrons within the electron gun may cause power dissipation, cavity material degradation, and beam quality deterioration, ultimately compromising the performance and operational lifetime of the VHF gun. The analysis of electron trajectories inside the cavity reveals that the energy dispersion and angular distribution of emitted electrons significantly affect their subsequent motion. Although the multipactor behavior is nonresonant, multiple resonance modes can still be identified from the statistical distribution of particle flight times. This study examines the influence of radio frequency (RF) cavity geometry on multipactor behavior, with the underlying mechanisms elucidated through statistical analysis of particle trajectories. The effect of the secondary electron yield of copper on multipactor is also discussed. Finally, an optimized RF cavity profile exhibiting very low multipactor intensity is presented, providing a promising approach to achieving stable VHF electron gun operation.

Nuclear resonance fluorescence (NRF) is a promising technique for non-destructive testing (NDT) of isotopic compositions, where a nucleus is excited by absorbing a specific quantum of energy and then de-excites by emitting one or more gamma rays. Recent advancements in gamma-ray sources, particularly inverse Compton scattering (ICS) sources, have enhanced the application of NRF in isotopic analysis. This paper presents the design and optimization of an isotope identification system based on ICS sources and NRF, and provides a quantitative analysis of the system’s detection capabilities. A Monte Carlo simulation code based on Geant4 is developed to model and optimize the system. The optimized system can resolve a 1

Quantum efficiency (QE) is a key property of photocathodes, and its uniformity is essential for producing high-brightness electron beams. Cathode imaging provides an in-situ and real-time approach for QE mapping, but in RF guns, a high charge per bunch is often needed to obtain a sufficient signal-to-noise ratio. Under such conditions, space charge effects can significantly degrade the imaging resolution and may even make point-to-point cathode imaging ineffective. In this paper, we propose a novel cathode imaging method based on a magnetized electron beam. Its feasibility is examined through theoretical analysis and beam dynamics simulations. The results show that the proposed method enables point-to-point cathode imaging in the ten picocoulomb charge regime. For a 10 pC, 3ps beam with a cathode magnetic field of 1200 Gauss, simulations indicate an imaging resolution of 11 um, representing nearly an order-of-magnitude improvement over the non-magnetized beam method.

This paper reports two versions of terahertz (THz)-driven nanotip field-emission electron guns: single-layer reflective guns (SLRGs) and double-layer reflective guns (DLRGs). SLRGs employ a reflective structure to superimpose the initial and subsequent half-cycles of the THz electric field, enhancing the field amplitude and acceleration efficiency. Experimental results demonstrate its superior acceleration efficiency over a single-layer non-reflective gun (SLNRG) with identical input, providing direct validation of the concept. Theoretically, the single-feed SLRG can match the dual-feed SLNRG's acceleration efficiency while offering significantly simpler synchronization. DLRGs comprise two independent reflective structures, each with individual THz injection. By precisely scanning the delay between the two incident THz beams, we demonstrate THz-driven cascaded electron acceleration, which represents a direct experimental demonstration and a pioneering step of cascaded acceleration in THz-driven electron sources. The experimental results of DLRGs align closely with the results of electron dynamics predicted by simulations, establishing the foundation for developing multi-layer high-acceleration-efficiency THz-driven high-energy electron guns. The ability to manipulate the THz for each layer individually holds promising potential for improving the performance of THz electron guns.

The quasi-monochromatic, continuously energy-tunable, and high-brightness gamma rays that are produced by an inverse Compton scattering (ICS) light source provide an ideal probe for gamma-ray imaging. However, owing to the influence of the intrinsic energy-angle correlation spectrum of this type of light source, monochromatic computed tomography (CT), especially in the gamma-ray energy region, can only be realized in a low-efficiency manner, similar to first-generation CT. A dual-energy scan scheme with a large imaging field of view (FOV) was developed in this study to improve the imaging efficiency. The effectiveness of this scheme was demonstrated based on the beam parameters of a typical ICS light source using Monte Carlo simulations. By leveraging the principle of basis material decomposition, the influence of the energy-angle correlation spectrum on CT reconstruction was corrected, and a monochromatic CT image of the imaging object was accurately reconstructed. Furthermore, the electron density and effective atomic number of the imaging object could be obtained simultaneously.

The Very Compact Inverse Compton Scattering Gamma-ray Source (VIGAS) is a gamma-ray facility under construction at Tsinghua University. It has the ability to produce more than 106 quasi-monoenergetic gamma photons per pulse within 10 ps. Due to ultra-short pulse length, conventional detectors and methods cannot directly measure the energy spectrum of the VIGAS. In this study, we employ the Compton scattering method to reduce the photon flux and collect the scattered photons in a specific direction using high-purity germanium (HPGe) detectors. The central energy and energy spread of the incident gamma rays can be determined by analyzing the spectrum of the scattered photons. To correct for the Doppler broadening effect during the Compton scattering process, the error transfer formula method is developed. Monte Carlo simulations show that the energy spectrum of the VIGAS can be reconstructed accurately by error transfer formula method, with a central energy accuracy better than 0.1% and energy spread accuracy better than 3%. A proof-of-principle experiment conducted at the Shanghai Laser Electron Gamma Source (SLEGS) validates the feasibility of the Compton scattering-based reconstruction method for energy spectrum measurements.

Photocathodes are extensively utilized in electron injectors for accelerator-based applications, with their performance substantially impacting electron beam quality. Cesium telluride is a commonly used semiconductor material for photocathodes, conventionally fabricated through sequential or co-deposition methods. In this study, we propose a new multilayer deposition technique in which tellurium and cesium are alternately deposited. This approach enables the precise determination of the ideal tellurium-to-cesium ratio, offering an effective alternative to sequential and co-deposition methods. Our experimental results demonstrate that cathodes prepared using multilayer deposition exhibit high quantum efficiency and reproducibility.

With the development of high-brightness electron beams and chirped pulse amplification technology, inverse Compton scattering (ICS) X/ γ -rays are characterized by compactness, quasi-monochromaticity, continuous energy tunability, and high photon energy. The application of ultrafast laser technology has improved the temporal resolution, brightness, and spectral control capabilities of X/ γ -rays. In this study, we present the design and implementation of a laser system for the very compact inverse Compton scattering γ -ray source. The laser system consisted of a photoinjector driving laser system and a scattering laser system. The photoinjector driving laser system produced ultraviolet pulses with pulse energy, pulse width, and repetition rate of 0.58 mJ, 7.2 ps (FWHM), and 10 Hz, respectively, at a central wavelength of 267 nm, which illuminated a photocathode to generate a high-quality electron beam. The ICS laser system produced two alternative ultrashort laser pulses with central wavelengths of 800 nm and 400 nm, which interacted with the electron beam. An intense second-harmonic (SH) laser with 0.5 J pulse energy was achieved experimentally by passing a terawatt Ti:sapphire laser pulse through a 0.59 mm potassium dihydrogen phosphate (KDP) crystal. A uniform SH laser focal intensity distribution was obtained via wavefront correction using a deformable mirror.

Six X-band accelerating structures with identical radio frequency (rf) designs have been developed and fabricated for the Very Compact Inverse Compton Scattering Gamma-Ray Source (VIGAS) project at Tsinghua University, representing the first large-scale deployment of such devices with consistent rf performance in China. This article presents the tuning, conditioning, and high-power performance results of these structures, with emphasis on in-depth analysis of conditioning processes and rf breakdown phenomena. The six structures achieved an accelerating gradient of 80 MV/m with a breakdown rate of no more than 1×10^{−4} per pulse after systematic conditioning. Our study reveals several critical aspects of rf breakdown behavior in X-band high-gradient accelerating structures. Key observations include the localization of breakdown events in regions of high electric field, particularly in cells adjacent to the input coupler; a power-law dependence of the normalized breakdown rate on cumulative pulse count, with notable variation in fitting parameters across different structures; and a two-exponential statistical distribution characterizing breakdown intervals. These findings offer important implications for optimizing conditioning strategies for high-gradient rf structures. Furthermore, the analysis of performance variations among nominally identical structures uncovers underlying physical mechanisms governing breakdown, establishing a useful benchmark for the development of future compact accelerator facilities.

Understanding the atomic-scale structure of liquids is fundamental to solution phase chemistry, yet its direct characterization remains a challenge. Mega-electron-volt liquid-phase electron scattering (MeV-LES) has emerged as a powerful laboratory-scale technique, but the sample thickness often surpasses the penetration depth of the MeV electron beam, which introduces multiple scattering effects that hamper the scattering data quality. Here, we employ a state-of-the-art MeV-LES platform to systematically investigate the structures of thirteen chemically diverse solvents, providing a crucial benchmark library of atomic correlations. Our high-resolution data reveal a significant sample-dependent discrepancy between experimental results and simulations. By comparing with molecular dynamics simulation results, we demonstrate that this discrepancy originates from multiple scattering and is strictly governed by the solventu2019s intrinsic short-range structural ordering. While simple liquids like water exhibit short-range order only within 8 u00C5, many organic solvents possess persistent order extending 15u201320 u00C5. Such an extended order generates a very sharp first liquid peak in the reciprocal space, thereby rendering the solvent highly susceptible to multiple scattering artifacts. These advances provide a key perspective in understanding liquid-phase electron scattering data, paving the way for accurate analyses of complex molecular systems.

High average current and high-brightness electron bunch trains enable a broad range of accelerator-based facilities to explore scientific frontiers with higher resolution and efficiency. The beam loading effect induced by high beam currents strongly influences the beam dynamics and is a critical challenge for normal-conducting linacs operating in burst mode. Effective compensation methods are essential for enhancing beam quality. Traditional compensation techniques often require the addition of extra compensation structures or dedicated power supply components, which increases both the cost and complexity of the accelerator system. In this paper, we propose a novel beam loading compensation method by employing chicanes, which are commonly utilized in accelerator-based facilities, to adjust the spacing between bunches for optimal phase tuning during acceleration. Simulation results for an electron injector design show a root mean square energy spread of less than 2×10^{−4} and a transverse normalized emittance below 0.5 mm mrad for a 1 A bunch train.

Radiotherapy with nanoparticles has been widely investigated, showing an energy dependence performance. A Compact radiotherapy facility providing energy-tuneable X-rays is highly demanded. Inverse Compton Scattering (ICS) sources can offer quasi-monochromatic and continuous energy-tuneable X-rays, enabling optimization of radiation enhancement effect for various nanomaterials. This study simulated enhanced radiotherapy with an ICS source using the Geant4 at microscale. The radiation enhancement effect of a typical nanomaterial, i.e., gold nanoparticles (GNPs), was evaluated and compared with the conventional X-ray tube. An improved model of tumors was built to evaluate the microscopic dose enhancement factor (DEF) of different nanoparticles, including GNPs and hafnium oxide nanoparticles. DEF of nanoparticles showed an energy dependence and the largest DEF of GNPs was 17.89 at 36 keV with monochromatic beams. The ICS source displayed a similar performance in terms of DEF (17.77 at 38 keV), which was higher than the 80 kV X-ray tube (12.80), due to the narrow bandwidth. The improved simulation method showed that the DEF was 22.8 for GNPs at 30 keV and 9.5 for hafnium oxide nanoparticles at 25 keV. The implementation of the ICS source, with a higher DEF at optimal energy, is feasible for enhanced radiotherapy. The continuous energy-tuneable property makes it a promising equipment for the application of enhanced radiotherapy with different nanomaterials.

Field emission under ultra-fast intense terahertz fields provides a promising approach for generating electron bunches with ultrashort pulse duration and high charge densities. It is generally believed that the field emission current described by traditional field emission theory increases dramatically with the applied electric field. However, we conducted extensive field emission experiments using quasi-single-cycle strong-field terahertz radiation at various energy levels and different temperatures and observed an intriguing phenomenon where the emitted charge reached saturation. A novel model is proposed to interpret this phenomenon, which considers the contribution of surface valence electrons and the dynamic replenishment of free electrons from the bulk to the surface. The experimentally observed convex relationship between the emitted charge and terahertz energy is consistent with the model prediction, unlike the concave relationship derived from the traditional field emission formula. In addition, another observed counter-intuitive phenomenon, the inverse correlation between the cathode temperature and saturated emission charge, is also well interpreted by the model. This work offers comprehensive insights into field emission dynamics under ultra-fast intense fields, paving the way for generating electron bunches with unprecedented temporal resolution.

X-ray fluorescence computed tomography (XFCT) has shown great potential in molecular biomedical imaging. However, conventional XFCT has some limitations, including sensitivity and a trade-off between spatial resolution and dose. X-ray fluorescence ghost imaging (XRF-GI) makes it possible to realize high-resolution imaging with low doses. However, the efficient implantation of XRF-GI requires a quasi-monochromatic x-ray source. The inverse Compton scattering (ICS) source could meet the requirement well. In this study, we developed a Monte Carlo simulation model of an XRF-GI system based on the Geant4 toolkit. The simulation model was composed of an ICS source, a series of Hadamard pattern-based masks as a modulating system, a single-pixel energy-resolving detector, and a phantom embedded with gold nanoparticles (GNPs) of different concentrations as the contrast agent. The image reconstructed by the total variation-regularized least squares (L-2-TV) algorithm was evaluated by four criteria, including the mean square error, the contrast-to-noise ratio, the coefficient of determination (R-2), and the limit of detection. Compared to XFCT, the reconstructed image of XRF-GI has better performance in all four criteria, and the dose of XRF-GI was halved without a decrease in resolution. It was validated that the beam properties of the ICS source fitted well with the implantation of XRF-GI for different conditions. In conclusion, the XRF-GI by an ICS source has great potential for biomedical imaging.

Free-electron–laser interactions in liquids constitute a fundamental process underpinning a broad range of ultrafast spectroscopic and diffraction techniques, including time-resolved photoelectron spectroscopy, attosecond spectroscopy, and ultrafast electron diffraction. Despite their central role, the interaction between free electrons and optical fields in a liquid environment remains largely unexplored. Here, we present a comprehensive study of laser-induced streaking (LIS) of MeV electrons in liquids using a state-of-the-art liquid-phase ultrafast electron diffraction (LUED) setup. We identify a streaking mechanism distinct from the gas phase: instead of plasma lensing, liquid-phase LIS arises from a direct electron-light interaction mediated by the liquid's dielectric response, regardless of whether the laser intensity is above or below the ionization threshold. This conclusion is supported by the dependencies of the streaking on laser incident angle, laser field strength, liquid thickness, and the resulting distortion of scattering patterns. Furthermore, we demonstrate that LIS can be completely suppressed at near-colinear electron-laser incidence, even above the ionization threshold. Our findings elucidate a previously unknown interaction mechanism and provide critical guidance for preventing scattering pattern corruption in future LUED experiments.

Electron transfer, the process of one electron relocating from one chemical entity to another, is one of the most ubiquitous elementary reactions and plays a critical role in almost all fields of chemistry. While seminal theories such as Marcus theory have provided foundational frameworks for electron transfer, they rely on postulated atomistic pictures that have never been directly observed. For instance, Marcus theory describes electron transfer is driven by solvent reorganization, which is only verified indirectly through the observation of the Marcus inverted region. Here, we present a direct atomic-resolution measurement of nuclear motion during electron transfer in the liquid phase, achieved using mega-electron-volt liquid-phase ultrafast electron diffraction. Instead of solvent reorganization, we observe that the back-electron transfer following photoionization in neat CCl4 proceeds with a ballistic cleavage of a C–Cl bond, followed by a 1.7-Å shuttling of the chlorine nucleus between donor and acceptor, accompanied by a 0.6-Å umbrella opening on both sides. These nuclear motions facilitate an ultrafast intermolecular back-electron transfer within 340 fs. Our findings demonstrate that the atomistic details of electron transfer involve complicated multidimensional nuclear motions across donor, acceptor, and solvent molecules, which goes far beyond the framework of Marcus theory.

High-frequency copper cavities in particle accelerators often experience dark current enhancement and performance degradation over prolonged operation. To address these issues, it is essential to remove copper oxide from the cavity walls while simultaneously suppressing field emission. A 13.56 MHz inductively coupled plasma platform with integrated coils was developed at Tsinghua University to assess the feasibility of in situ plasma treatment for restoring high-frequency performance. Experiments conducted on this platform focused on optimizing plasma discharge parameters and treatment protocols. The results demonstrate that the “argon/oxygen + argon/hydrogen” method effectively removes hydrocarbons and copper oxides from ultra-smooth, oxygen-free copper surfaces while passivating surface burrs. This dual-action treatment is helpful in reducing field emission, consequently lowering the dark current. It is also helpful to enhance the high-frequency performance and operation stability of the copper cavity. These findings validate the potential of in situ plasma cleaning as an effective technology for restoring and maintaining the performance of high-frequency copper cavities.