Toward monochromated sub-nanometer UEM and femtosecond UED

Area of Science:

• Physics
• Materials Science
• Electron Microscopy

Background:

• Ultrafast Electron Diffraction (UED) and Ultrafast Electron Microscopy (UEM) require high-resolution electron beams.
• Achieving narrow energy spread in mega-electron-volt (MeV) electron sources is crucial for improving momentum resolution and reducing shot-to-shot jitter.

Purpose of the Study:

• To present a preliminary design for a MeV monochromator with an unprecedented 10^-5 energy spread.
• To optimize the monochromator for both single-shot and accumulation modes in UED and UEM applications.
• To enable advanced capabilities for UEM and UED, including sub-nanometer resolution and femtosecond temporal resolution.

Main Methods:

• Numerical optimization of monochromator efficiency for single-shot mode, achieving 13% efficiency and 1.3 million electrons per pulse.
• Application of reverse bending magnets to minimize energy-dependent path length differences, inspired by Qi et al.
• Utilizing an achromat design to reduce electron bunch pulse length and energy-dependent timing jitter to the 10 fs level.

Main Results:

• The designed MeV monochromator achieves a 10^-5 energy spread, significantly improving UED momentum resolution and UEM stability.
• Single-shot mode optimization yielded high efficiency and electron counts per pulse.
• Accumulation mode demonstrated electron counts nearly proportional to the repetition rate due to optimized gun phase, mitigating efficiency degradation.
• The achromat design successfully reduced pulse length and timing jitter to 10 fs.
• The system allows real-time, non-destructive diagnosis of beam energy spread and divergence.
• Tunable energy scanning with 10^-5 precision enables online energy matching and flux maximization.
• Integration with a chicane allows for "two-color" double pulses with tunable femtosecond delays.

Conclusions:

• The developed MeV monochromator design represents a significant advancement for UED and UEM.
• It paves the way for constructing UEM with sub-nm resolution and UED with ten-femtosecond temporal resolution.
• The monochromator's precise energy control and diagnostic capabilities offer versatile applications in spectroscopy and advanced imaging.