The focusing mirrors for the new LCLS soft x-ray (SXR) experimental hutches are tangential pre-shaped mirrors mounted in a Kirkpatrick Baez configuration. The mirrors are prefigured with an elliptical profile, coinciding with the longest working focal distance. The mirrors are equipped with benders to enable focusing of the beam at different experimental stations and to work out of focus with an uniform beam. To add complexity to the system, the mirrors are also water-cooled and need to fit in a very tight space, due to real estate limitation.
For ensuring that the mirror profile is maintained at its sub-nm quality after the assembly of the mirror into its cooling and mechanical system, these mirrors need to undergo an extensive optics metrology study. The vertical and horizontal KB mirrors are first checked for twist error due to the mounting of the mirror substrate to its mechanics. This is measured with grazing incidence Fizeau interferometry. Then the mounted mirror needs to be shimmed to correct for any errors that may be caused by gluing of the mirror. This step requires a sequence of shimming and metrology measurement and must be repeated until the mirror shape is satisfactory.
In addition, the mirror bender response function must be well-characterized and documented for the commissioning as well as operation of these mirrors in the experimental hutches. The response function can be attained by measuring the mirror profile using the instruments available in the LCLS Optics Metrology Laboratory and the stitching techniques developed at LCLS. The mirrors are scheduled to be installed in the new SXR beamline in spring 2020. Metrology data and initial commissioning results proving the performance of these wavefront preserving optics will be presented in this report.
An ongoing collaboration among four US Department of Energy (DOE) National Laboratories has demonstrated key technology prototypes and software modeling tools required for new high-coherent flux beamline optical systems. New free electron laser (FEL) and diffraction-limited storage ring (DLSR) light sources demand wavefront preservation from source to sample to achieve and maintain optimal performance. Fine wavefront control was achieved using a novel, roomtemperature cooled mirror system called REAL (resistive element adjustable length) that combines cooling with applied, spatially variable auxiliary heating. Single-grating shearing interferometry (also called Talbot interferometry) and Hartmann wavefront sensors were developed and used for optical characterization and alignment on several beamlines, across a range of photon energies. Demonstrations of non-invasive hard x-ray wavefront sensing were performed using a thin diamond single-crystal as a beamsplitter.
The x-ray free electron laser facility at SLAC National Accelerator Laboratory (named as LCLS) will be upgraded to LCLS-II in the near future. The high repetition rate light source makes the x-ray optics or components exposed to trillions of pulses over years of operation. Material fatigue properties of x-ray optics are essentially important for their lift-time prediction, optics optimization and opto-mechanical design. In this work, the fatigue properties of typical x-ray optics materials such as single-crystal silicon are experimentally measured by using laser pulses. The laser source can have an average power of 50 W at wavelength of 1.03 μm and repetition rate of 0.928 MHz with pulse duration of ~230 fs. The SHG crystal is used to generate 515 nm laser beam for the test to get an equivalent absorption length to soft x-rays. The maximum single-pulse energy is more than 16 μJ. The numbers of pulses that the optics can survive are measured for different pulse energies (fluences). The definition of the damage of x-ray optics is the significant reduction of reflectivity, which is premonitory of damage, and much more stringent than the ablation threshold.
Nine bendable mirrors will be installed as part of the upgrade to Linac Coherent Light Source. To achieve the target performance, accurate elliptical shapes must be generated with these focusing mirrors to an accuracy in the order of 104 to 105. We briefly summarize the developmental work including surface metrology via stitching and actuator characterization as well as fitting algorithm to achieve shape control of a KB developmental prototype. The height error of the centerline shape generated by the current system is in the order of 3 nm for a one meter long silicon mirror. The most important limiting factor is metrology due to environmental control.
With the onset of high power XFELs and diffraction limited storage rings, there is a growing demand to maintain sub nanometer mirror figures even under high heat load. This is a difficult issue as the optimum cooling design for an optic is highly dependent on the power footprint on the mirror, which can be highly dynamic. Resistive Element Adjustable Length (REAL) cooling can be utilized to change the cooling parameters during an experiment to adapt for changing beam parameters. A case study of the new soft x-ray monochromator for the LCLS L2SI program is presented that utilizes this new capability to allow the beam to translate across the mirror for different operation modes, greatly simplifying the monochromator mechanics. Metrology of a prototype mirror will also be presented.
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