- Figure 1: (Left) Schematic of the multilane mirror – the schematic shows only three lanes
whereas the mirror installed on VMXm has seven lanes. (Right) Measured beam-size (fwhm) at
sample position on VMXm beamline as the mirror is translated across lanes 2 to 7 and inset, the
measured beam profile for mirror lane 1.
VMXm is a new beamline at Diamond, which is designed to produce a variable size micro-focused beam for rapid data collection on large molecule biological samples in crystalline form such as proteins. The requirements for the focused beam at the sample are that the focused intensity should be high and the beam size should be switchable in a range of sizes from under 0.5 μm to over 5 μm on a time scale of less than one second to enable high-speed data collection and to minimise radiation damage of the samples. This required use of a high aperture double mirror system (Kirkpatrick-Baez) to focus the X-ray beam from the undulator source to the sample position.
The vertically focusing mirror was a novel design developed by the Optics Group to provide a variable focused beam size. The mirror has a basic elliptical shape with a glancing incidence angle of 3 mrad and provides a high demagnification to give sub-0.5 μm focal spot. The mirror was, however, split into seven lanes, running the length of the mirror. The profile of the reflecting surface in each lane was a modification of the basic elliptical shape and calculated to generate a smooth symmetrical intensity distribution at the sample position, with full width at half maximum varying in sequence from 0.4 μm (lane 1) to 8 μm (lane 7). The change in beam size was achieved by translating the mirror to illuminate a different lane with the X-ray beam. Initial testing of the multilane mirror was carried out on the Diamond Test beamline (B16)6. Following first beam, the mirror was installed on the VMXm beamline and alignment was performed using wavefront sensing. The initial measurements showed a focused beam size of 0.41 μm using lane 1 of the mirror and beam-sizes in sequence 0.97, 1.42, 2.5, 4.7, 6.7, 8.7 μm for the remaining lanes (Fig. 1).