XMaS Beamline Description

The X-ray Source

The ESRF storage ring circulates electrons at an energy of 6 GeV and a current of 200 mA. It consists of: an 844 m circumference storage ring fed from a 300 m circumference 6 GeV booster synchrotron functioning at 10 Hz, and a LINAC pre-injector. The ESRF synchrotron was initially conceived to provide only insertion device source points, undulators and multipole wigglers. During the design evolution it was realised that the dipole magnets could provide excellent additional sources, albeit with less intensity, but without compromising their more powerful neighbours. The dipoles have been designed to give a fringing field having a flat intermediate step at 0.4 T, half of the full field, 0.8 T. This produces a critical energy in the photon beam of 9.8 KeV, half that for sources on the 0.8 T section. The XMaS beamline uses a 0.4 T section as its source point.

The Beamline

The XMaS beam line has been designed to perform single crystal diffraction over an energy range of 2.5 keV to 15 keV. The beamline is sited on the soft end of dipole 28 (critical energy 9.8 keV) and the beamline layout is shown below.

The optics, shown schematically below, consists of a double-crystal monochromator followed by a toroidal mirror. The monochromator comprises two plane silicon crystals, currently silicon (111), mounted in a crystal cage which maintains a constant 20 mm offset for the exit beam. The first crystal is water cooled and absorbs most of the incident synchrotron radiation power. The mirror, uncooled, is made from single crystal silicon and has a sectored cylindrical cross-section of 116 mm radius (sagittal). The small cylindrical curvature required tangentially, 5.5 km, is produced by a pneumatically actuated bending mechanism. The two curvatures thus form the toroidal surface that focuses the beam to a small spot onto the sample surface. To enhance the mirror reflectivity the surface has been coated with a thin layer of Rhodium. The maximum flux collected comes from a fan of radiation of 3.1 milliradians horizontal width and 0.2 milliradians vertical height. The grazing angle of reflection is 4.5 milliradians. For more facts and figures see Summary of Beamline Technical Specifications.

Operating Modes

Energy Range

2.5 keV - 15 KeV continuously tunable (focussed monochromatic mode)