Evidence for microbunching "sidebands" in a saturated free-electron laser using coherent optical transition radiation

Area of Science:

• Physics
• Accelerator Physics
• Quantum Optics

Background:

• Electron beams in free-electron lasers (FELs) exhibit microbunching, leading to coherent radiation emission.
• Understanding coherent optical transition radiation (COTR) is crucial for characterizing electron beam properties and FEL performance.

Purpose of the Study:

• To measure z-dependent coherent optical transition radiation (COTR) in a high-gain self-amplified spontaneous emission free-electron laser (SASE FEL).
• To investigate COTR spectral evolution, intensity growth, and angular distribution, particularly after saturation.
• To explore evidence for transverse spectral dependencies and the role of the electron beam core in microbunching.

Main Methods:

• Measurements of COTR spectra and intensity were performed at varying positions (z-dependence) within a SASE FEL.
• Experiments were conducted at high gain regimes (>10^4), including the saturation regime.
• Analysis included spectral width, intensity growth, angular distribution, and transverse spectral dependencies.

Main Results:

• The first measurements of z-dependent COTR from microbunched electron beams in a high-gain SASE FEL are reported.
• COTR spectra transitioned from simple to complex forms (5% spectral width) after FEL saturation.
• Intensity growth, angular distribution, and transverse spectral dependencies were observed, indicating an effective beam core involvement in microbunching.

Conclusions:

• COTR measurements provide valuable insights into electron beam microbunching dynamics in high-gain FELs.
• Saturation significantly alters COTR spectral characteristics, highlighting nonlinear effects.
• The findings suggest a specific region of the electron beam core is primarily responsible for microbunching and coherent radiation generation.