Cristallina beamline

The Cristallina beamline branch delivers the X-ray beam generated by the hard X-ray ARAMIS undulator beamline to the Cristallina experimental hutch, serving either one of the Cristallina-MX or Cristallina-Q experimental stations. The beamline covers the full ARAMIS photon energy range and the layout is optimized to enable horizontal beam at the sample position with adjustable beam size down to 1 um. The beamline operates in pink beam mode, with the natural FEL bandwith, or in monochromatic mode, exploiting a double Si(111) channel cut monochromator (2.7 - 14.5 keV) that can be inserted on the beam path.

The straight X-ray beam from the ARAMIS undulator line is directed onto the Cristallina branch by a pair of horizontal offset mirrors placed in the optical hutch that also serve for te suppression of the third harmonic. In the experimental hutch, the beam size at the experiment can be tuned vertically and horizontally by a bendable KB-mirror system. In the standard beamline configuration, the beam at the sample is on a horizontal trajectory, which is made possible by a vertical offset mirror positioned right upstream of the vertical KB-mirror to compensate the beam deflection. 

In addition the the shot-by-shot beam diagnostic devices built in the common front-end section, additional beam intensity and position monitors are installed along the Cristallina branch. Furthermore, movable destructive screens to inspect the beam shape and trajectory are placed in strategic positions for beam alignment purposes and to confirm beam path clearance.

Two horizontal offset Si-mirrors to direct the FEL beam onto the Cristallina beamline branch are placed at 66.5 and 84.5 m from the end of the ARAMIS undulator line. The first one is retractable to enable clear beam path on the Alvra and Bernina branches. The incidence angle on the mirror surfaces is 3 mrad, which results in a cutoff at about 12.0 keV and 19.5 keV for the SiC- and Mo-coatings, respectively.

The beam at the experiment position is shaped by a system of three mirrors:

• Vertical offset mirror: 151.7 m from the end of the ARAMIS undulator line, 3.60 m upstream of the main experimental position; at the standard 4 mrad incidence angle, photon energy cutoff at about 9.0 and 14.7 keV for the SiC- and Mo-coatings, respectively.
• Vertical bendable KB-mirror:  152.9 m from the end of the ARAMIS undulator line, 2.35 upstream of the main experimental position, corresponding to the optimal distance for vertical focusing, giving vertical focus size of smaller than 1 um; the tunable curvature enables adjusting the vertical beam size at custom position along the beam trajectory; at the standard 4 mrad incidence angle, photon energy cutoff at about 14.7 keV Mo-coating, optionally reflection from uncoated Si possible. 
• Horizontal bendable KB-mirror:  153.7 m from the end of the ARAMIS undulator line, 1.60 upstream of the main experimental position, corresponding to the optimal distance for horizontal focusing, giving horizontal focus size of smaller than 1 um; the tunable curvature enables adjusting the horizontal beam size at custom position along the beam trajectory; at the standard 4 mrad incidence angle, photon energy cutoff at about 14.7 keV Mo-coating, optionally reflection from uncoated Si possible. 

In the standard configuration, all three mirrors are inserted on the beam path with 4 mrad incidence, which enables horizontal beam through the experimental setup. The incidence angle can be optionally adjusted between 2.5 and 8 mrad for optimized 3rd harmonic suppression at a chosen photon energy.

All mirrors can be retracted, enabling different alternative configurations, in particular:

• KB-only: vertical offset mirror retracted, beam with upwards trajectory, focusing as in the standard configuration
• Direct beam: all mirrors retracted, no focusing / beam size tuning

The beamline UHV section ends with a 100 um thick diamond window on the beam trajectory at the exit of the KB chamber. In standard configuration, the beam trajectory continues through a low-vacuum section (1.5 e-2 mbar) terminated by a Kapton exit window. Optionally, the post-diamond window section can be connected directly to a vacuum experiment chamber.

The critical online photon diagnostics devices are:

• Two beam position and intensity monitors relying on backscattering from a thin target (either SiN 30 um,  SiN 200 um, 50 um CVD) collected by four diodes, placed at 112.7 m and 149.4 m from the ARAMIS undulator end.
• An intensity monitor based on vertical scattering from a custom thin target collected by a 0.5M Jungfrau detector, placed after the beamline diamond exit window and right downstream preceded by a four-blade slit system that prevents diffuse scattering from the KB-mirrors to reach the sample.