Ultrafast Electron Diffraction/Microscopy (UED/UEM) are powerful tools for directly observing ultrafast dynamic processes at the atomic level. The quality of the electron beam is crucial for image resolution, but the space charge effects can degrade parameters such as energy spread and emittance, reducing the spatiotemporal resolution. By reducing the charge per electron bunch and increasing the emission frequency, the space charge effect can be effectively suppressed, ensuring a high signal-to-noise ratio in the images. Superconducting Radio Frequency (SRF) photocathode guns can operate in continuous wave (CW) mode and generate highly stable and bright electron beams, making them promising electron sources for the next generation of UED/UEM. This paper aims to optimize the design of a 1.4-cell SRF gun using Nb3Sn for UED/UEM. The focus is on minimizing thermal losses in the cavity to enable efficient conduction cooling and ensure stable operation at 4K in the superconducting state. Furthermore, beam dynamics analysis of the electron beam inside the cavity is performed to assess beam quality for different charges and bunch sizes. This enables us to achieve a high-quality electron beam that meets the design requirements.
A 1.4-cell photocathode RF gun was developed for the MeV-UED to mitigate the space charge effect during electron emission through a higher acceleration gradient. However, this advancement introduces the risk of field-emitted dark current, leading to a degradation in the quality of the ultrafast electron beam. This paper investigates dark current emission within critical regions of the RF gun cavity. The results show that dark current electrons from the cathode and cathode edge escape from the electron gun, resulting in increased image background noise. The study examines the temporal characteristics of the dark current, including waveform in relation to the emission phase. Additionally, different collimator apertures are analyzed for their suppressive effect on the dark current, aiming to minimize its impact on the ultrafast electron beam.
Ultrafast electron diffraction using photocathode microwave electron guns is a powerful tool for investigating ultrafast science. To improve the spatial and temporal resolution of diffraction, it is crucial to enhance the quality of the electron beam, particularly the initial quality of the electron beam emitted from the photocathode that is influenced by the driving laser. To meet the strict requirements, the performance parameters of the femtosecond laser transmission system play a significant role. In this paper, we analyze the impact of femtosecond laser system parameters on diffraction resolution and investigate the primary indicators of the femtosecond laser system. We conducted experiments to measure the primary parameters of the laser, including pointing stability, beam diameter, pulse width, and pulse energy. Based on the experimental results and considering the complexity of engineering implementation, we proposed an optical scheme for the femtosecond laser transmission path to satisfy the requirements of the ultrafast electron diffraction device for further improving the diffraction resolution. This research aims to provide valuable insights into optimizing the femtosecond laser system for ultrafast electron diffraction experiments.
Mega-electron-Volt Ultrafast Electron Microscope (MeV UEM) has become a promising tool to real-time observe ultrafast dynamics at the atomic scale, where a magnetic objective lens system is critical to manipulating the high-energy beam to achieve point-to-point imaging. However, the upper limit of spatial resolution is mainly determined by the high-order chromatic aberration resulting from the electron energy spread and the imaging lens system. A magnetic lens system based on the Russian Quadruplet (RQ) is being studied to improve the degree of symmetry and further reduce the aberration. The beam optics design and multi-target optimization are finished to achieve a good spatial resolution of point-to-point imaging. This paper will introduce the theoretical deviation and design results of our first-stage imaging lens system, and second-order beam optics is optimized further to improve the resolution.
Streak cameras based on THz-driven split-ring resonator (SRR) are recently proposed to achieve electron bunchlengthmeasurement with femtosecond resolution due to the available GV/m level streaking field. However, to apply the SRRtothe streaking experiment, the SRR needs to have a relatively large gap to accommodate the beamto traverse. Alargergap leads to higher electromagnetic power radiation, which requires high exciting THz power to compensate powerradiation to achieve a strong streaking field. The maximum stored energy in the gap is determined by the availableexciting THz power. If a single THz pulse drives the SRR, the achievable streaking field is not enough for highresolution because of the radiation diluting the stored energy. This paper proposes a novel method to illuminate theSRRwith multipulse, which can accumulate the energy stored in the gap to compensate the electromagnetic radiationuntil saturation and consequently enhance the resonance to a much higher peak field. We explore the effects of drivingpulseswith various intervals and obtain an optimal field enhancement factor up to 47 with the THz field strength of 1MV/m. The particle tracking simulation indicates that the multipulse-driven method can achieve the temporal resolution of 0.4fswith the central frequencies of SRR at 0.3 THz.
Bunch trains consisting of ultrashort picosecond-spaced microbunches have potential applications in generating pulsed, tunable, narrow-band radiation sources in the THz region via coherent Smith-Purcell radiation (cSPr). However, the electrons in each microbunch experience longitudinal space-charge field, blurring the periodicity of the bunch train. There has been an increasing interest in manipulating each microbunch individually, and therefore significantly improving radiation intensity and bandwidth. The commonly used RF cavities (with nanosecond working period) cannot match the picosecond bunch spacing and, fail to compress each bunch individually. This paper proposes a novel method to simultaneously compress each microbunch in a picosecond-spaced bunch train using a THz-driven resonator with a customizable working frequency. A multi-pulse drives the THz-driven resonator to compensate for the field decay in the THz-driven resonator and preserve the well-defined periodicity of the bunch train. We demonstrate a resonating field with an amplitude fluctuation within±20%, which can be utilized to compress up to ten microbunches simultaneously.
An Ultrafast Electron Diffraction (UED) based on an RF photocathode electron gun has the advantage of producing MeV relativistic probing electron beams, which can maintain a high time resolution of ~100 fs while keeping more electrons to improve the S/R ratio of the image. However, the jitter of driving RF power in the electron gun between pulse to pulse has an indispensable impact on the electron energy stability leading to the Time of Flight (ToF) jitter, which creates asynchronization between the pump laser and the probing electron worsening the time resolution. To stabilize the beam energy to the designed value 3 MeV and reduce the ToF jitter further, we propose controlling the electron energy based on an energy spectrometer directly. An electron spectrometer based on a C-type dipole is being designed to achieve high energy resolution. This paper will introduce the design of the energy spectrometer, and particle tracking is implemented to demonstrate the feasibility of the design.
The advances in electron accelerator science and technology continue to reach shorter bunch lengths, even down to femtosecond, paving a way to generate coherent Smith-Purcell radiation naturally, taken as one of the most promising THz sources. In order to design a high power and broadly tunable THz radiation source, we make theoretical and numerical analysis of the characteristic of coherent Smith-Purcell radiation, which demonstrates good agreement between them. In the paper, we also present the comparison of spectra of coherent Smith-Purcell produced from the interaction of a single bunch and a train of microbunches.
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