Room-temperature calorimeter for x-ray free-electron lasers

Area of Science:

• Physics
• Metrology
• Optical Engineering

Background:

• Accurate radiant power measurement is crucial for characterizing advanced light sources like X-ray free-electron lasers (XFELs).
• Existing primary standards, such as cryogenic radiometers, have limitations in measurement range and operational conditions.
• The need for a robust, room-temperature alternative for absolute radiant power determination is evident.

Purpose of the Study:

• To develop and validate a room-temperature calorimeter for absolute radiant power measurements.
• To establish traceability to the International System Units (SI) for XFEL radiant power measurements.
• To demonstrate performance exceeding current primary standards in specific parameters.

Main Methods:

• An electrical substitution radiometer operating at room temperature was designed and constructed.
• The principle of equivalence between electrical and radiant heating was employed for calibration.
• Offline electrical power measurements and indirect comparison with a cryogenic radiometer using synchrotron radiation were performed.

Main Results:

• The room-temperature calorimeter successfully measured external powers up to 6.9 mW, exceeding the typical range of cryogenic radiometers (∼4 mW).
• Measurement uncertainties were evaluated to be less than 1.0%, suitable for XFEL applications.
• Indirect comparison showed agreement with a cryogenic radiometer within 0.6%, validating the absolute radiant power measurements.

Conclusions:

• The developed room-temperature calorimeter provides a viable and traceable method for absolute radiant power measurements of XFELs.
• This instrument offers a higher upper measurement limit compared to cryogenic radiometers.
• The low uncertainties and validated performance make it a valuable tool for characterizing high-power light sources.