Mirror systems for SwissFEL, from concept to commissioning with x-rays

Abstract

The two A\uf772\uf761\uf76d\uf769\uf773 beamlines at SwissFEL are currently under commissioning, first pilot experiments have already been carried out. In total we have 10 mirror systems, 6 offset mirrors and 4 mirrors in KB [1] configuration. We use bilayer mirror coatings to handle the single shot damage challenge at FELs. The optics has to keep the coherence of the FEL pulses. We use flat mirrors with figure errors on the single nm level in two-actuator mechanical bender systems. The vertically deflecting KB mirrors have a polished-in gravity sag compensation. The performance of the mirror systems has been verified with X-rays. INTRODUCTION SwissFEL the new hard X-ray free electron laser (FEL) facility at the Paul Scherrer Institut in Switzerland [2] started X-ray optics commissioning with two beamlines at one undulator called A\uf772\uf761\uf76d\uf769\uf773 in summer 2017. In the meantime all the optic and diagnostic components are in operation. So far photon energies up to 3 keV with a repetition rate of 25 Hz have been realized (status begin of June 2018). The conditioning of the linear accelerator is still ongoing in order to reach the design parameters (12 keV, 100 Hz) and start regular user operation in the beginning of 2019. The optical layout of the two beamlines called A\uf76c\uf776\uf772\uf761 and B\uf765\uf772\uf76e\uf769\uf76e\uf761 is shown in Fig. 1. About 64 m downstream the undulator there are two retractable horizontally deflecting offset mirrors to guide the beam into A\uf76c\uf776\uf772\uf761 followed by a double crystal monochromator (DCM) with a maximum Bragg angle of 80 deg., a pair of vertically deflecting mirrors and the KB [1] system. The layout of B\uf765\uf772\uf76e\uf769\uf76e\uf761 is very similar but the vertically deflecting mirrors are upstream the DCM. Both beamlines allow pink beam and monochromatic operation i.e. with– and without the DCM. In the following we will focus on the mirror systems, present the concept, realization and first commissioning results with X-rays. MIRROR SYSTEMS Coatings Single shot damage is a major concern during the optics design for FEL beamlines, see for example [3]. The typical outcome are low-Z coating materials, small grazing angles and long mirrors; in our case: B4C, 3 mrad and 650 mm optical surface. With 3 mrad and a thick B4C coating the reflectivity would drop rapidly at about 10 keV. To extend the useful photon energy range to higher photon energies and keep the damage under control we choose a bilayer of thin B4C on top of SiC and Mo respectively. Concerning damage a SiC monolayer would be safe while the Mo layer has a higher damage risk and may be used just as an option with reduced fluence (3rd harmonic). The reflectivity prediction Proceedings of the 13th International Conference on Synchrotron Radiation Instrumentation – SRI2018 AIP Conf. Proc. 2054, 060001-1–060001-5; https://doi.org/10.1063/1.5084632 Published by AIP Publishing. 978-0-7354-1782-3/$30.00 060001-1 64.5 76.5 92.5 95.8 98 105 108 109.2 123.1 124 125.5 142 distance from end of undulator (m) offset mirrors mono, HIOS mirrors ALVRA KB