Beam power meter for x-ray free-electron lasers is compact, durable

Researchers at the Free-Electron Laser in Hamburg have come up with a bolometric radiometer that is only 26 mm on a side, enabling easy measurement of a free-electron laser beam at many locations along the beam.

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A number of large free-electron lasers (FELs) emitting in the extreme-ultraviolet (EUV) and x-ray ranges have come online within the past decade or so, including the Free-Electron Laser in Hamburg, Germany, the SPring-8 Compact SASE Source and the SPring-8 Angstrom Compact Free-Electron Laser (SACLA; both in Hyogo, Japan), and the Linac Coherent Light Source (Menlo Park, CA). Others, including the European x-ray FEL, the SwissFEL, and the Pohang Accelerator Laboratory XFEL, are under development. These powerful light sources share something in common: the need for a beam power measurement system that is not only accurate, but can stand up to peak powers up to the gigawatt range.

Content Dam Lfw Print Articles 2017 12 1712lfw Nb F1

Researchers at the Free-Electron Laser in Hamburg have come up with just such a device-a bolometric radiometer that is only 26 mm on a side, enabling easy measurement of a FEL beam at many locations along the beam. The room-temperature device includes a vacuum-chamber housing and thermal shields—within the chamber is a 1-mm-thick gold plate inside a cylindrical copper shell that has a 5 mm aperture. The absorptance is >99.7% over the 1–60 keV photon energy range. Heaters within the chamber maintain an even temperature of about 300 K to within 40 μK. When a beam strikes the device and the temperature is maintained, the heaters require less current—it is this current that is monitored to determine beam power. Experiments of half a year's time confirmed the bolometer's efficacy and durability. Reference: T. Tanaka et al., Opt. Lett. (2017); https://doi.org/10.1364/ol.42.004776.

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