Commissioning of a hard x-ray Hartmann wavefront sensor at SwissFEL

Abstract

Hartmann wavefront sensors are robust, achromatic and need nothing in the beam but the sensor itself, which makes them attractive as permanent diagnostics on X-ray beamlines. Their absolute accuracy is set by the reference wave used for calibration, which conventionally requires a dedicated pinhole set-up or beamtime at a synchrotron. We report the commissioning of a commercial hard X-ray Hartmann sensor as an online diagnostic at the SwissFEL free-electron laser. The sensor was calibrated in situ using an existing beamline slit about 100 m upstream as the reference source: the residual after calibration is below 5 pm RMS (λ/20 at 12.4 keV), and a calibration data set is recorded in minutes. Operated at 10 keV on a sample diffractometer, the sensor showed that a bendable Kirkpatrick–Baez mirror pair was not flat at its nominal zero settings (3.39 nm peak-to-valley, mainly astigmatism) and guided a live correction to 0.44 nm. It resolved a 20 mm change in focal distance, reproduced a directly imaged beam size to within 8%, and showed that a visible scratch on a monochromator crystal has no measurable effect on the wavefront. The calibration method transfers to other beamlines with a suitable slit far enough upstream.