A Photon Beam Diffusor for the ATHOS Beamlines at SwissFEL.

SwissFEL is a free electron laser at the Paul Scherrer Institut in Switzerland, which became operational by the end of 2017. Initially, it will host two undulators ARAMIS and ATHOS. The former is operational and generating tender to hard x-rays (2 to 12 keV), whilst the latter is in the process of being installed and will provide soft x-rays ranging from 0.2 to 2 keV. Each undulator will eventually serve three different endstations. Recent Genesis [1] simulations have shown that the ATHOS branch can operate in a high-power mode, which will increase the photon pulse energy by one order of magnitude to 8.1 mJ at 250 eV. This will lead to peak beam intensities inside the frontend, which will damage any normal incidence absorber. To overcome this issue a photon beam diffusor is proposed in this paper. At the core of this instrument is a bent rod, which is positioned at a grazing angle relative to the incident beam, so that it acts like a defocussing mirror, reflecting most of the incident power and increasing the divergence of the reflected beam. Due to the increased divergence, the beam diameter is increased by a factor of 10 after propagating over a few meters, thus reducing the peak intensity by a factor of 100 to a level, which can be safely absorbed by a normal incidence absorber. [ABSTRACT FROM AUTHOR]