Source characteristics

The ESRF machine is routinely operated at 6 GeV electron energy with a maximum current of 200 mA. The horizontal source emittance is 3.8 10^-9 m.rad and the coupling is usually 1%. The X-ray microscopy beamline is installed on a 4.8 m long low beta straight section the parameters of which are given in table 1. The choice of a low beta section (small source but larger divergence, see table 2) was driven by the following considerations. Firstly, the small horizontal source allows the zone plate to be used in a diffraction limited regime even for long focal lengths. Secondly, the scanning X-ray microscope benefits from higher coherent brilliance produced by such a source (at the ESRF low beta sections have a better phase-space matching than the high beta sections). Furthermore the larger horizontal divergence offers the possibility to split the beam for parallel operation of the two branch-lines and limits the heat load on the first optical components by spreading the power over a larger surface. However, although the low emittance of the beam is best suited for the operation of the SXM, it complicates the numerical aperture matching between the illumination and objective zone-plate for the full-field microscope. 

The straight section is equipped with 3 different insertion devices (see table 3). As shown in figure 2 the use of two linear undulators, a U42 and a W80, allows the full energy range to be covered while maintaining the total power to manageable levels (always below 700 W through an aperture of 5x5 mm² placed at 28 m from the source) without compromising the available flux. At low energy (large wavelength) the lower brilliance is largely compensated by an increase of the lateral coherence. The helical undulator produces 100% circularly polarised light between 3.4 and 6.6 keV. However, the degree of circular polarisation remains higher than 80% for energies below 3.4 keV. Furthermore most of the outgoing power produced by a helical undulator is concentrated in the fundamental peak and the absence of harmonics is an appreciable attribute for specific experiments.