Preliminary characterization of a plasma-polished channel cut for future monochromator applications at European XFEL

The generation of intense X-ray beams is achieved at synchrotrons and free-electron lasers using magnetic undulators and relativistic accelerated electron beams. Although this process has some similarities with the one occurring in an optical laser, it is based on the single pass of electrons in the undulators so it still has a relatively large energy bandwidth. The advantage of course is the high tunability and the possibility to emit beams at the soft and hard X-ray wavelengths. If a highly monochromatic beam is needed, a monochromator setup is necessary to narrow down the bandwidth. In the hard X-ray range, silicon crystal monochromators are widely used for this purpose, at 3rd generation synchrotron sources and free-electron lasers. In the second case, however, such devices require higher angle stability and overall reliability. The main reason is the longer distance between monochromators and the final interaction point, which is amplifying any small vibration or drift of the crystals. On top of that, such devices are placed in radiation-controlled areas, which are difficult to access in case of failure. Another strict requirement is the quality of the crystals, not only in the crystalline structure but also in the optical polishing and roughness: the high degree of coherence of the beam that is produced at European XFEL in particular requires a good optical quality to avoid scattering and wavefront errors. Usually, the crystals are also cryocooled in case of MHz machines as European XFEL, to increase transmission and to be less sensitive to heat load. One of the classical ways to adhere to these high-quality standards is to combine two separate crystals in a configuration called “artificial channel-cut”, so that they can be prepared with the best possible optical quality. This concept allows us to have a very well polished and high quality crystal surface, but with the drawback of complexity in mounting and aligning. This is particularly severe at European XFEL, where crystals can be damaged by the powerful beam and need to be replaced and realigned, with potentially loss of beamtime that is precious for the intended experiments. Recently, some monolithic channel-cuts became available, polished up to a superior quality surface than in the past due to a method called PCVM (Plasma Chemical Vaporization Machining) [1]. We received two of these crystals, and we present here some preliminary characterisation of them.