Owing to the challenge of devising an optical configuration to deliver a beam of 1 μm cross-section to the sample, with a small enough angular divergence to be suitable for MX, we devoted most of our attention to the beamline that addresses this aspect of the optics concept. We investigated an approach involving two-stage demagnification of the x-ray beam in the horizontal direction, and one-stage in the vertical direction. This is similar to the approach which is being pursued by the NSLS-II Submicron Resolution X-ray spectroscopy (SRX) beamline,5
although its goals, and required beam properties, are different. The Diamond 124 MX beamline has been designed according to a similar two-stage demagnification approach, albeit to achieve a beam size as small as 5 μm rather than 1 μm.6
The SPring-8 BL32XU MX beamline has been designed to achieve a beam size of 1 μm using single-stage demagnification, taking advantage of the long distances which the SPring-8 experimental floor affords to its beamlines.7
Modern synchrotron radiation sources have very small vertical source size, markedly smaller than the horizontal source size. The NSLS-II short (low-β) straight sections will have electron beam source dimensions and opening angles of 33.3 μm rms (h) × 2.9 μm rms (v) and 16.5 μrad rms (h) × 2.7 μrad rms (v) respectively.8
For an undulator radiation source installed in such straight sections, the counterpart photon beam source dimensions and opening angles are bigger owing mainly to the energy spread of the electron beam, and dependent on the photon energy and choice of undulator design and harmonic. E.g., when using the 5th
harmonic of the NSLS-II U20 undulator at 12 keV, the vertical source size increases to 7 μm rms and the vertical opening angle increases to 8 μrad rms; in the horizontal direction, there are hardly any differences in the photon beam source size and opening angle from their electron beam counterparts.8
In the presence of such a small vertical source size, achieving a 1 μm beam cross-section in the vertical direction at the sample position does not pose a great difficulty for the beamline optical system employing one-stage demagnification, provided that the focusing element has a very small figure error and the distance between the focusing element and sample position isn't large. The vertical beam divergence at the sample position can be kept reasonably small for MX in this circumstance, without sustaining much loss of flux that would arise from the introduction of a divergence-reducing aperture in the beamline.
In the horizontal direction however, the required demagnification to achieve a 1 μm beam cross-section (FWHM) at the sample position can be significant, more than 70:1. Imposing this while making use of the full horizontal opening angle would incur a horizontal beam divergence of 3 mrad at the sample position, too high for state-of-the-art MX. To reduce this to an acceptable value of 1 mrad or less requires introduction of a horizontal slit to trim down the horizontal divergence and, in so doing, the flux. Such an aperture can be placed in the front end or just before a horizontal focusing mirror.
We've chosen to investigate an approach involving two-stage focusing in the horizontal direction, wherein an upstream focusing mirror delivers a focused beam at a location somewhat upstream of the sample position. At this location, an aperture is introduced to define a secondary source, whose width can be controlled easily. A secondary focusing element, which can be a mirror or a lens, thence focuses the beam onto the sample. A conceptual schematic of this configuration is shown in . In are detailed the horizontal beam widths and angular divergences calculated at selected locations along the beamline, based on use of focusing mirrors whose slope errors are assumed to be 0.1 μrad rms; the assumed source is the NSLS-II U20 undulator installed in a low-β straight section. Mirrors of such quality are judged to be available since they need not be longer than about 0.2 to 0.3 m, even if the incidence angle of the x-ray beam striking the mirror surface is as low as 3 mrad.9
Such high-quality mirrors are crucial in ensuring delivery of a beam of 1 μm cross-section to the sample; calculations undertaken using mirrors of larger slope error consistently resulted in bigger beam cross-sections, for the 0.5 m working distance (between the final mirror and the sample) which has been assumed (it is important for background suppression in MX experiments, among other reasons, to maintain this working distance to be not much smaller than this). It is just as important that there be placed in the beam path a minimum of objects, such as windows or filters, which could disrupt the x-ray wavefront, in order for such performance to be realized.
Optical concept for two-stage horizontal focusing to deliver a 1 μm wide beam.
Variation of horizontal beam width and divergence at selected locations along the beamline, all values expressed as FWHM.
If this beamline shares a straight section sector with another beamline, each viewing separate canted undulator sources installed in the straight section, it is envisioned that the separation of the beams would be achieved through the use of a tandem pair of horizontally-deflecting mirrors installed in each beamline.2
For the beamline described above which is designed to focus the beam to 1 μm at the sample position, the first horizontal focusing mirror would be one of the tandem deflecting mirrors. This is reflected in the beamlines layout which is shown in . Calculations have been made for the horizontal separation of the two undulator beams as a function of distance from the source, assuming that each tandem pair of horizontally-deflecting mirrors imparts an angular deflection of 16 mrad (i.e. 4 mrad incidence angle assumed for each mirror), toward the outboard direction in the case of the outboard canted undulator beamline and toward the inboard direction in the case of the inboard canted undulator beamline. Taking these into consideration (assuming the deflecting mirrors are positioned in accordance with what is shown in the beamline layout), in addition to an assumed canting angle of 2 mrad separating the two undulator sources, one predicts a horizontal separation of >44 cm at the sample location in the first (upstream) experimental station (which uses the outboard undulator beam). This would host a more conventional crystallography setup, albeit highly automated since space in this station is expected to be limited. Note that use of deflecting mirrors having an incidence angle of 4 mrad would incur an upper cutoff energy for these mirrors of about 21 keV if coated with Pt or about 17 keV if coated with Pd or Rh.
Figure 2 Layout of canted undulator beamlines on NSLS-II experimental floor. The x-ray beam propagates from right to left, and thus the long downstream experimental station appears at far left, the shorter experimental station just upstream of it. Just below the (more ...)