High-luminosity meV-resolution single-shot hard X-ray spectrograph for cavity-based X-ray free-electron lasers

Area of Science:

• Physics
• Materials Science
• Optics

Background:

• Cavity-based X-ray free-electron lasers (CBXFELs) offer potential for highly coherent hard X-ray generation.
• CBXFELs require precise diagnostics for their narrow spectral bandwidth (∼1-50 meV) and high brilliance.
• Existing methods lack the necessary meV resolution and high luminosity for shot-to-shot CBXFEL spectral analysis.

Purpose of the Study:

• To design and test a high-luminosity, single-shot hard X-ray spectrograph for CBXFELs.
• To achieve meV spectral resolution and high luminosity for precise spectral measurements.
• To evaluate the performance of the spectrograph under operational conditions.

Main Methods:

• The spectrograph utilizes Bragg diffraction from crystals for angular dispersion of X-rays.
• It is designed to image 9.831 keV X-rays within a ∼200 meV spectral window.
• Experiments were conducted at the Advanced Photon Source (APS) beamline 1-BM.

Main Results:

• The spectrograph demonstrated a linear dispersion rate of ∼1.4 µm meV⁻¹.
• A high-fidelity spectral imaging window of ∼200 meV was achieved.
• Experimentally, a spectral resolution of ∼20 meV was obtained, slightly lower than theoretical predictions due to crystal angular instabilities.

Conclusions:

• The developed hard X-ray spectrograph is a viable diagnostic tool for CBXFELs.
• The instrument provides high luminosity and spectral resolution suitable for shot-to-shot analysis.
• Further optimization is needed to mitigate crystal angular instabilities and achieve theoretical spectral resolution.