On the influence of monochromator thermal deformations on X-ray focusing

Abstract

A cooled double crystal monochromator system is used on many high heat load X-ray synchrotron radiation beamlines in order to select, by diffraction, a narrow spectrum of the beam. The wavelength of the diffracted beam is tuned by changing the angle of incidence, per the Bragg equation. Thermal deformation of the first crystal monochromatorand the potential loss of beam brillianceis often a main concern. However, if downstream beam focusing is planned, the lensing effect of the monochromator must be considered even if thermal deformations are small enough to have any imperceptible impact on intensity. In this paper we report on recent experiments at an Advanced Photon Source (APS) beamline that uses focuses compound diffractive lenses downstream of an X-ray monochromator system. Increasing the X-ray beam power by increasing the storage ring current from 100 mA to 130 mA resulted in the doubling of the focal distance, to some location outside the experimental hutch. We show quantitatively that this is be due to the well-known lensing effect of a distorted monochromator that results in the creation of a virtual source, at some distance downstream of the actual source. An analysis of the defocusing and options to mitigate are provided.

Characterization of the X-ray coherence properties of an undulator beamline at the Advanced Photon Source

“…9) in the Fresnel region indicated an apparent source distance larger than the actual distance. All of these effects are consistent with aberrations in the monochromator (Antimonov et al, 2016) arising from thermal or mounting strains of the crystals. Further work will be needed to understand and potentially improve the monochromator performance to deliver the full brightness and coherent flux.…”

“…While the vertical beam size matches the calculation at 12.0 keV, it becomes successively smaller at higher energies. One possible contribution to this effect would be an overall curvature of the monochromator crystals, which can act to focus or defocus the beam (Antimonov et al, 2016). The observed reduction in beam size at 25.75 keV would require a change in angle of 20 µrad across the vertical beam profile.…”

Focusing a round coherent beam by spatial filtering the horizontal source

“…Zhang et al vertically focused a 7.35 keV X-ray beam in 8-ID-I using the same geometry with Be lenses with R = 0.2 mm using N = 10 instead of N = 7, the best choice from optical constants and equation 11 (Zhang et al, 2016). Calculations of this effect have been discussed recently (Antimonov et al, 2016). Fig.…”

“…There is another valuable function of reducing the whitebeam slit opening in conditions where the optics cannot preserve the brightness of the source due to spatial strain induced by the absorbed power density on the surface of the optics (Antimonov et al, 2016). We reduced the total power on the first Ge(111) crystal of the double-crystal monochromator in 8-ID-I by a factor of seven with the smaller slit and were able to produce a diffraction-limited spot in the horizontal direction, and a much improved vertical focus on 8-ID-I.…”

Cryo-cooled silicon crystal monochromators: a study of power load, temperature and deformation

“…Right: thermal strain relative to 97 K (or P = 0). (Zhang et al, 2013;Antimonov et al, 2016). Power scenarios, calculated using the SPECTRA code (Tanaka & Kitamura, 2001), are summarized in Table 1.…”