Focusing KB
Beamline Description | TARDI spectrometer | Focusing KB | Electromagnet endstation | RIXS endstation | Detectors | SLU
Kirkpatrick-Baez (KB) focusing mirrors
The focusing section, named KAOS, is composed by a set of two active plane-elliptical mirrors mounted in the Kirkpatrick-Baez (KB) configuration similar to the already adopted solution. The KB geometry allows to decouple the vertical and horizontal focusing and the available substrates have a much higher surface quality compared to the ellipsoidal mirrors. The reasons why the beamline needs active mirrors are the following:
- sources: end-stations and wavefront correction. As described in the introduction FEL2 provides two photon beams: the low energy from the first stage (down to 20 nm) and the high energy from the second stage. These stages are separated of about 28.8 m meaning that for a fixed focal length the beams can’t be both focused properly.
- Two experimental end-stations: a magnet and a RIXS separated about 1 m apart. In order to move the focal spot inside the working chamber the mirrors have to be a variable focal length.
- Wavefront control: the possibility to change the mirror shape allows to control the wavefront of the photon beam.
The active mirrors have a flat substrate of fused silica with an overall dimension of: 400 mm x 40 mm x 10 mm (L × W × H): i.e. much thinner than the regular mirrors employed along the beamlines. The clear aperture will be 360 mm x 20 mm with a residual radius of curvature above 3 km within 70 mm area, below 0.5 rad µrms and 5 rad µrms for the tangential and sagittal slope errors and with a micro-roughness below 0.3 nm rms. The relaxed constrains of the residual radius of curvature is due to the fact that the mirrors are thin because they have to be deformable. Both mirrors will be coated with a single layer made of 30 nm of Gold. The mirrors are mounted on a dedicated holder custom designed in such a way that only the sides are clamped while the central part is free to be bent. By applying two unequal forces with a set of pushers over the sides it is possible to change the mirror tangential shape accordingly to the user needs. Every pusher is composed by a stepper motor coupled with a piezo-electric actuator: the former for the coarse mirror shape, the latter for the fine optimization. A schematic of the system is shown in figure.
Focal spot
The only difference between the already operative KAOS system at DIPROI or LDM and the MagneDyn KAOS is the fact that the two mirrors are not in cascade (in series) but will be 5.5 m apart, with the VDMMD in between. As a consequence the demagnification factor of the two mirrors is different with a small focus in the vertical direction and a wider one in the horizontal. In particular the horizontal KB will demagnify the source of a factor of 9.66 while this number rises up 38.97 for the vertical one. This means that the focused beam is not circular but ellipsoidal with an average ratio between the two axis of about 4. The spots at focus are expected to be 14 μm × 3.5 μm (X × V) at the electro-magnet chamber and 16 μm × 5.3 μm at RIXS.
