ALBA is a third generation 3-GeV synchrotron radiation facility built near Barcelona servicing worldwide academic and industrial users since 2012. It currently hosts eight operating beamlines, including one dedicated to Macromolecular Crystallography (MX), BL13-XALOC. Current expansion plans of the facility include further 4 beamlines, one of them being a specific microfocus MX beamline, BL06-XAIRA, now in design stage.
The scientific case for XAIRA includes two aims: 1) to provide a full beam with a size of 3×1 μm2 FWHM (h×v) and a flux of >3×1012 ph/s (250 mA in storage ring) at 1 Å wavelength (12.4 keV); and 2) to reach a wide range of energies, 4-14 keV, to support MX experiments at long wavelengths exploiting the anomalous signal of the metals naturally occurring in proteins (native phasing), which is enhanced in the case of small crystals.
To match these requirements, the design of the beamline foresees a powerful photon source, a 2.3-m long in-vacuum undulator device, and the optics system prioritizes a high beam spatial stability. A channel-cut monochromator (CCM) will be installed, followed by a vertically focusing mirror long enough to accept the variations of the beam height introduced by the monochromator. This way, thanks to the large demagnification factor, the parasitic excursion of the beam at the sample position will be kept within only 60 µm for the whole energy range, which greatly simplifies the alignment of downstream elements. A high flux operation mode will also be possible, at the expense of energy resolution, by using a multilayer monochromator. To minimise the loss of flux at the low energy limit, the air path will be reduced between the sample and the detector by means of a He cone, and the whole beamline will only include two vacuum windows, upstream and downstream the sample.
The beam size will be adjustable to the users needs, producing larger sizes by defocusing the beam, or closing the horizontal secondary source slits to obtain a smaller spot size at sample, down to 1×1μm2. A high precision air bearing goniometer with vertical rotation axis will provide a stable alignment of micron-sized samples to the small beam.
With a state-of-the-art pixel array detector and automated sample mounting and data analysis, XAIRA will allow rapid sample screening and data collection of crystals, and it will support a broad range of advanced diffraction experiments using micron-sized beam, from raster scanning to serial crystallography.