Steady-State Microbunching(SSMB) light source is a highly potential new type of light source, which combines the advantages of synchrotron radiation and FEL. It possesses the characteristic of both high repeat frequency and high peak power. In order to build such a light source, we are trying to design a 20ns, 0.7A burst mode injector to produce high brightness electron beam, and we have already designed a S band RF gun for this injector.

High-quality electron beams are critical for imaging experiments that provide detailed microscopic structural insights into materials. The Very-High-Frequency electron gun, capable of operating in continuous and pulsed modes, is a preferable option. In this paper, we employ a tungsten tip with an apex radius of curvature approximately 100nm as a cold field emission cathode in the VHF gun and measure the beam charge, transverse emittance, and energy spread to characterize the beam quality. In preliminary experiments, we have achieved a normalized transverse emittance of 54.01nm·rad. With an electron gun power of 37kW, we obtained astrongbeamcurrentofapproximately 6µA, which remained stable and continuous for several hours in a single experiment. By using an aperture to block electrons with large divergence angles and adjusting the solenoid’s focusing strength, we propose to converge the target-energy electron beam onto the aperture, increasing its transmission rate and optimizing the energy spread. Prior to optimization, the energy spread was approximately 3.57% at 536keV when using a 20µm diameter aperture.

VHF CW photocathode electron guns suffer from severe multipactor effects that significantly degrade their operational performance. In this work, the RF cavity geometry was optimized via CST simulations to suppress multipactor, employing a three-phase strategy with fixed key structural parameters. Compared with the SHINE gun baseline, the optimized cavity achieves a much lower multipactor growth rate $\alpha$ while preserving excellent high-performance RF characteristics. This demonstrates that geometry optimization is an effective approach to mitigate multipactor in VHF CW photocathode electron guns.

The generation of high-brightness electron beams with ultrashort duration and high temporal stability is essential for ultrafast electron diffraction (UED), which has become a powerful tool for ultrafast structural dynamics studies with high spatiotemporal resolution. However, simultaneously achieving sub-10 fs bunch duration and synchronization with pump laser remains challenging in practice. Here we report an upgrade of the FORTRESS (Facility Of Relativistic Time-Resolved Electron Sources and Scattering) beamline at Tsinghua University. By utilizing an α-magnet-based scheme and a 1.4-cell S-band photocathode gun, with optimized control of the time-of-flight (TOF)-momentum correlation and the time-momentum correlation in longitudinal phase space mainly governed by the initial chirp, a bunch duration of 3.47 fs with 4.34 fs TOF jitter characterized by THz streaking has been achieved, corresponding to a temporal precision of ~5 fs. The beamline layout, beam dynamics simulation, diagnostic methods and pump-probe UED experiments with high-performance results will be discussed.

We propose a novel MeV UED beamline design based on a normal-conducting very-high-frequency (VHF) electron gun that generates a high-quality electron beam at a megahertz (MHz) repetition rate. Beam compression and time-of-flight (TOF) control are achieved using an alpha-magnet. Simulations show compression of the 4.1 fC electron beam to a duration of 4.7 fs RMS, with a TOF jitter of 1.9 fs RMS, alongside normalized transverse emittances of 5.6 nm·rad in the x-direction and 5.2 nm·rad in the y-direction. This design substantially enhances both the signal-to-noise ratio and temporal resolution of the UED beamline, establishing a robust foundation for the future development of UED facilities.

To meet the energy compensation requirements of Steady-State Micro-Bunching (SSMB), a 20 MHz RF cavity with extreme axial compactness is essential. This paper presents an innovative triple-folded quarter-wave cavity design that overcomes the size limitations of conventional structures. Based on cascaded transmission line theory, we established an analytical model to optimize the cavity’s shunt impedance and tuning range. A triple-folded 20 MHz prototype was designed and validated via CST simulations, showing that the axial length is compressed to 1.25 m (approximately 1/3 the length of a standard QWC) while achieving a high shunt impedance of 558 kΩ with excellent thermal stability. This compact design offers a high-efficiency solution for low-frequency RF systems.

Steady-State Microbunching(SSMB) light source is a highly potential new type of light source, which combines the advantages of synchrotron radiation and FEL. It possesses the characteristic of both high repeat frequency and high peak power. In order to build such a light source, we are trying to design a 20ns, 0.7A burst mode injector to produce high brightness electron beam, and we have already designed a S band RF gun for this injector.

To meet the energy compensation requirements of Steady-State Micro-Bunching (SSMB), a 20 MHz RF cavity with extreme axial compactness is essential. This paper presents an innovative triple-folded quarter-wave cavity design that overcomes the size limitations of conventional structures. Based on cascaded transmission line theory, we established an analytical model to optimize the cavity’s shunt impedance and tuning range. A triple-folded 20 MHz prototype was designed and validated via CST simulations, showing that the axial length is compressed to 1.25 m (approximately 1/3 the length of a standard QWC) while achieving a high shunt impedance of 558 kΩ with excellent thermal stability. This compact design offers a high-efficiency solution for low-frequency RF systems.

VHF CW photocathode electron guns suffer from severe multipactor effects that significantly degrade their operational performance. In this work, the RF cavity geometry was optimized via CST simulations to suppress multipactor, employing a three-phase strategy with fixed key structural parameters. Compared with the SHINE gun baseline, the optimized cavity achieves a much lower multipactor growth rate $\alpha$ while preserving excellent high-performance RF characteristics. This demonstrates that geometry optimization is an effective approach to mitigate multipactor in VHF CW photocathode electron guns.

High-quality electron beams are critical for imaging experiments that provide detailed microscopic structural insights into materials. The Very-High-Frequency electron gun, capable of operating in continuous and pulsed modes, is a preferable option. In this paper, we employ a tungsten tip with an apex radius of curvature approximately 100nm as a cold field emission cathode in the VHF gun and measure the beam charge, transverse emittance, and energy spread to characterize the beam quality. In preliminary experiments, we have achieved a normalized transverse emittance of 54.01nm·rad. With an electron gun power of 37kW, we obtained astrongbeamcurrentofapproximately 6µA, which remained stable and continuous for several hours in a single experiment. By using an aperture to block electrons with large divergence angles and adjusting the solenoid’s focusing strength, we propose to converge the target-energy electron beam onto the aperture, increasing its transmission rate and optimizing the energy spread. Prior to optimization, the energy spread was approximately 3.57% at 536keV when using a 20µm diameter aperture.

Multipactor in very-high-frequency (VHF) continuous-wave (CW) electron guns threatens stability, with non-resonant behavior and secondary electron yield (SEY) as key factors. This study clarifies non-resonant multipactor arises from weak local electric fields and disrupted electron-field resonance. It compares SEY models and adjusts the Furman model to match practical conditions. Combined with the optimized cavity, the adjusted SEY model reduced $\alpha$ to near-zero, eliminating multipactor. Particle analysis also showed multipactor concentrates on the anode side. This work provides a comprehensive solution for VHF gun multipactor risks.

The generation of high-brightness electron beams with ultrashort duration and high temporal stability is essential for ultrafast electron diffraction (UED), which has become a powerful tool for ultrafast structural dynamics studies with high spatiotemporal resolution. However, simultaneously achieving sub-10 fs bunch duration and synchronization with pump laser remains challenging in practice. Here we report an upgrade of the FORTRESS (Facility Of Relativistic Time-Resolved Electron Sources and Scattering) beamline at Tsinghua University. By utilizing an α-magnet-based scheme and a 1.4-cell S-band photocathode gun, with optimized control of the time-of-flight (TOF)-momentum correlation and the time-momentum correlation in longitudinal phase space mainly governed by the initial chirp, a bunch duration of 3.47 fs with 4.34 fs TOF jitter characterized by THz streaking has been achieved, corresponding to a temporal precision of ~5 fs. The beamline layout, beam dynamics simulation, diagnostic methods and pump-probe UED experiments with high-performance results will be discussed.

We propose a novel MeV UED beamline design based on a normal-conducting very-high-frequency (VHF) electron gun that generates a high-quality electron beam at a megahertz (MHz) repetition rate. Beam compression and time-of-flight (TOF) control are achieved using an alpha-magnet. Simulations show compression of the 4.1 fC electron beam to a duration of 4.7 fs RMS, with a TOF jitter of 1.9 fs RMS, alongside normalized transverse emittances of 5.6 nm·rad in the x-direction and 5.2 nm·rad in the y-direction. This design substantially enhances both the signal-to-noise ratio and temporal resolution of the UED beamline, establishing a robust foundation for the future development of UED facilities.