Abstract
This paper reports novel measurements of x-ray optical radiation on an absolute scale from the intense and ultra-short radiation generated in the soft x-ray regime of a free electron laser. We give a brief description of the detection principle for radiation measurements which was specifically adapted for this photon energy range. We present data characterizing the soft x-ray instrument at the Linac Coherent Light Source (LCLS) with respect to the radiant power output and transmission by using an absolute detector temporarily placed at the downstream end of the instrument. This provides an estimation of the reflectivity of all x-ray optical elements in the beamline and provides the absolute photon number per bandwidth per pulse. This parameter is important for many experiments that need to understand the trade-offs between high energy resolution and high flux, such as experiments focused on studying materials via resonant processes. Furthermore, the results are compared with the LCLS diagnostic gas detectors to test the limits of linearity, and observations are reported on radiation contamination from spontaneous undulator radiation and higher harmonic content.
Highlights
An impressive progress has been achieved in the development of powerful, generation short-wavelength sources in the last few years, such as the self-amplified spontaneousemission (SASE) free-electron laser (FEL) [1], enabling new investigations of photon-matter interactions on nanometer length and femtosecond time scales with ultra-high brightness
This paper reports novel measurements of x-ray optical radiation on an absolute scale from the intense and ultra-short radiation generated in the soft x-ray regime of a free electron laser
In order to ensure the accuracy of different detection schemes, a successful comparison was performed at SPring-8 Compact SASE Source (SCSS) between a new stand-alone upgrade of the free-electron laser in Hamburg (FLASH)-gas-monitor detectors (GMDs), the so-called Transfer-GMD, and the AIST cryogenic radiometer at the wavelengths of 51 nm, 56 nm, and 61 nm, revealing that radiant power values obtained with the two different detectors agree to within 2.6%, with their combined relative standard uncertainty being 4.5% [13]
Summary
An impressive progress has been achieved in the development of powerful, generation short-wavelength sources in the last few years, such as the self-amplified spontaneousemission (SASE) free-electron laser (FEL) [1], enabling new investigations of photon-matter interactions on nanometer length and femtosecond time scales with ultra-high brightness. In spite of the progress in the development of these novel laser facilities, the characterization of FEL beam parameters such as the absolute photon flux, an important and fundamental quantity for many experiments, is a challenge Existing methods such as calorimetry have been tested at an FEL before [7], but these intercept the beam and generally lack the required pulse resolution. Using a gas-based transmissive measurement method mitigates these limitations and has the additional advantages that the wavefront of the coherent beam is preserved and that the gas target does not degrade with time as previously observed with solid state absorbers or beam splitters It has a larger dynamic range, and can be placed in front of or close to the sample and measure the number of photons per pulse directly at the sample location. This provides an estimate of the photon number per bandwidth
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