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.
Ultrafast Electron Diffraction (UED) is an indispensable tool that enables the study of ultrafast dynamics on an atomic/molecular scale. Ultrashort high brightness electron beams are needed to capture the critical ultrafast events, particularly for studying the irreversible biochemical processes in the single-shot mode. However, the Coulomb interactions in the space-charge dominated electron beam limit attainable beam length and dilute beam quality during its propagation. The beam emittance increases significantly during propagation due to the severe space charge effect (SCE) because of low energy. It is essential to understand the emittance evolution behavior in detail during its passage for improving the UED performance further. The multi-slit method is selected to eliminate the SCE influence on the measurement by a low sampling rate of the electrons, making it possible to diagnose the emittance. However, the insufficient samplings create challenges in reconstructing the original beam information. This paper introduces an algorithm that can precisely reproduce beam parameters from severely under-sampled data.
New accelerator technologies such as laser wakefield accelerators (LWFA) or dielectric laser accelerators (DLA) have pushed the electron bunch length down to femtosecond or sub femtosecond regime. These ultrashort electron bunches find many applications, e.g. seeding for free-electron lasers (FEL), ultrafast electron diffraction (UED) and coherent Smith-Purcell radiation (cSPr) sources etc. The characterization of such ultrashort bunches is becoming a challenging task, especially at low energy regime due to the space charge effects. Usually, the streak cameras based on RF cavities are used to obtain accurate bunch length. However, the phase jitter between the incident beam and the electromagnetic field phase in the cavity set a resolution limit. A bunch length diagnostic based on a self-emission THz driven split-ring resonator (SRR) is proposed to reach the sub-picosecond (ps) or femtosecond (fs) resolution. Since the coherent SmithPurcell radiation from the incident electron beam produces the driving THz pulse, it can essentially eliminate the time jitter between the incident beam and the deflection THz field in the SRR gap. Besides, this THz pulse frequency can be tunable to easily match the SRR resonance frequency. In this paper, we describe the mechanism of the THz generation method and present the simulation results of the novel bunch length measurement based on a THz-driven SRR. The results show that this novel method can successfully measure the bunch length with the temporal resolution of 2-10 fs.
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.
The orientation of molecules is essential to study molecular angle-differential properties such as ionization and scattering cross-sections in material physics and chemistry. Ultrafast electron diffraction (UED) facilities offer effective ways to explore the ultrafast dynamics of orientated molecules. Generally, the orientation of molecules is generated by a strong dc-field. However, the presence of a strong field may influence detection outcome. Field-free orientation of molecules is preferable, avoiding the disadvantages of traditional dc-field excitation. This paper proposes a practical and versatile method for field-free molecular orientation using the co-rotating two-color circularly polarized ultrafast laser pulses, and the orientation of the molecules can be controlled by the relative phase of the two-color laser fields. We also performed our simulation in CO molecules with the Born-Oppenheimer and rigid rotor approximations, and the light-molecule interaction Hamiltonian is given by the low-order perturbation theory.
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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