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A New Monochromator for Sirius Beamlines

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A new high-dynamics DCM is under development at the LNLS.


Aiming at an inter-crystal stability of a few tens of nrad (even during the Bragg angle motion for flyscans) and considering the limitations of the current DCM implementations, several aspects of the DCM engineering are being revisited. In order to achieve a highly repeatable dynamic system, with a servocontrol bandwidth in the range of 200 Hz, solutions are proposed for a few topics, including: actuators and guides, metrology and feedback, $latex \rm LN_{2}$ indirect cooling, crystal clamping, thermal management and shielding. In the recent years it has become clear to the synchrotron community that the stability performance of X-Ray Double Crystal Monochromators would turn out to be one of the main bottlenecks in the overall performance of many X-rays beamlines, particularly for the new generation of machines, the so called Diffraction Limit Storage Rings (DLSR). This is because the instabilities in the DCM affect the position and/or the effective size or of the virtual source, and, consequently, the spot size and/or the position of the beam at the sample. It is thus imperative that the virtual source instability is kept within a small fraction of the source size.

The instability between the two crystals is even more critical because its effects on the virtual source scales with the lever-arm between the monochromator and the source. In the DLSR, the sources are expected to have only a few microns, whereas the typical distances between the source and the DCM is of tens of meters. Due to that, the stability between crystals is required to be within a few nrad. This scenario has motivated numerous special forums in conferences and even dedicated workshops, as the ESRF DCM Workshop in 2014. Since then, an effort has been made by suppliers and the synchrotron engineering community, trying to upgrade the existing systems and come up with new solutions. This work, which is the outcome of such effort at the Brazilian Synchrotron for Sirius beamlines, presents the advanced conceptual design of a DCM with high servo bandwidth closed-loop control. Sirius is in construction phase and the beginning of machine commissioning is planned for mid-2018.

GUIDELINES

  • The first rule in this development was to restrict the number of degrees of freedom . Therefore, the crystal cage presents only pitch, roll and gap adjustments, and all of them are associated to the 2nd crystal subsystem. The more degrees of freedom, the harder to keep the stiffness and robustness of the system, and the more error sources.
  • Second, it was believed that the required stability numbers could not be achieved without a high bandwidth closed loop servo control, through which disturbances can be attenuated to acceptable levels. In this DCM the target is a closed-loop control bandwidth of at least 200 Hz, for which force actuators and a reaction mass are used for pitch, roll and gap adjustments.
  • This high controllability of the system also imposes strict requirements on the feedback system, particularly on resolution, stability and acquisition rates. With respect to that, some uncertainty lies on what can be available at the beamlines: intensity monitors may not be sensitive enough for nrad variation; beam position monitors (BPMs) may not be sensitive, accurate and/or fast enough (kHz range); due to the high coherence properties of the new machines, some beamlines may not accept any interference of windows or foils in the be beam path. Thus, it has been developed an internal interferometric metrology architecture, integrating the 1st and 2nd crystals with nanometric performance at kHz.

 

Finally, the cooling induced vibrations have been considered as the main risk item, so that alternatives to the cooling technology have been investigated, in order to minimize disturbances and mechanical constrains.

To the best of our knowledge, the main innovation in this project is in the way the degrees of freedom of CRYS2 are guided, actuated and controlled [1]. Firstly, all the guiding is performed by folded leaf-springs. By this choice, lubrication, friction effects and vibrations from recirculating spheres or rollers can be mostly eliminated.

Fig 1. Schematic drawing for the 1st and 2nd crystals submodules, according to the  high-dynamics concepts.

Fig 2: DCM assembly: (a) Overall view.  (b) Downstream internal view of the rotary system.

The high-dynamics concepts that are applied here are proven technologies in high-end lithography machines, meaning that, although there is some innovation within the synchrotron instrumentation community, it comes under calculated risks, based on extensive and continuously updated analyses. Thus, with a realistic closed-loop bandwidth of about 200 Hz, the high stability target has been confidently pursed and a functioning prototype should be ready for commissioning in mid-2017.

Source:

[1] R. R. Geraldes, R. M. Caliari, G. B. Z. L. Moreno, L. Sanfelici, M. Saveri Silva, N. M. Souza Neto, H. C. N. Tolentino, H. Westfahl Jr., LNLS, CNPEM, Campinas, Brazil, T. A. M. Ruijl, R. M. Schneider, MI Partners, Eindhoven, Netherlands. The New High Dynamics Double Crystal Monochromator for Sirius. Presented at MEDSI 2016, Barcelona, Spain, Sep. 2016. (to be published).

[2] R. R. Geraldes, Patent Pending, Jan-2016.