Wavefront sensing at a high-repetition-rate X-ray free-electron laser

Area of Science:

• Physics
• Optics
• Materials Science

Background:

• Wavefront characterization is crucial for diagnosing limitations at X-ray free-electron lasers (XFELs).
• High repetition rates in XFELs pose challenges for traditional wavefront metrology due to increased thermal load and data throughput.
• Existing methods struggle with the demands of next-generation XFEL facilities.

Purpose of the Study:

• To develop a fast, robust, and sensor-free wavefront metrology technique for next-generation XFELs.
• To quantify local wavefront fluctuations within pulse trains at the European XFEL.
• To identify the sources of wavefront errors in high-repetition-rate XFEL operations.

Main Methods:

• Utilizing X-ray speckle generated by beamline optics for wavefront metrology.
• Applying a statistical formulation of X-ray speckle tracking.
• Analyzing shot-to-shot wavefront fluctuations and their statistical signatures.

Main Results:

• Demonstrated X-ray speckle as an effective method for wavefront metrology.
• Quantified wavefront fluctuations as predominantly planar phase tilts.
• Identified periodic wavefront errors linked to mechanical oscillations and intra-train fluctuations correlated with electron bunch trajectory instabilities.

Conclusions:

• Established a practical framework for high-repetition-rate wavefront characterization at XFELs.
• The X-ray speckle approach offers a robust solution for advanced XFEL diagnostics.
• Understanding wavefront error sources is key to optimizing XFEL performance.