Insertion device

The photon source of the FinEstBeaMS beamline is the Elliptically Polarizing Undulator (EPU). FinEstBeaMS EPU has 26 full periods and period lenght is 95.2 mm. The device is based on the APPLE II design with permanent magnets. FinEstBeaMS EPU enables to use light with linear and circular polarization.

Beamline optics

The beamline design is based on collimating plane grating monochromator. The first optical element in the beamline is a toroidal mirror (M1). M1 collimates the beam both vertically and horizontally. The monochromator consists of a plane mirror (M2) and two plane gratings (PG1 and PG2). The dispersed radiation from the grating is focused by the toroidal focusing mirror (M3) at the exit slit that resides 6 m away from the focusing mirror. The refocusing for the FinEstBeaMS gas-phase branchline or FinEstBeaMS solid state branchline is accomplished by a single mirror (M4, ellipsoidal) deflecting the sideways and keeping the beam in the horizontal direction. The most aberration-free imaging of the source into the exit slit is achieved by having both the horizontal and the vertical focusing. The demagnification in case of sold state end-station (gas-phase end-station) the entrance arm length is 6.5 m (7.475 m) and the exit arm length is 2.5 m (2.875 m), producing about 100 micron beam horizontally at the sample plane (see Figure below). The refocusing mirror is demagnifying vertically by the factor of 2.6. Relatively large exit slit values can be used to increase the flux, while the vertical beam size at the sample still remains within 100 microns.

The monochromator will be equipped with one plane mirror and two plain gratings (92 l/mm and 600 l/mm). The low density grating is designed to work mostly in very low energies, below some tens of eVs, whereas the medium density high efficient grating will cover the energy range from 17 eV up to 1000 eV.

Figure below depicts the estimated flux at an experiment for a low energy grating and a wide energy range grating for R=5000. The calculated curve is based on the calculated reflectivities of the mirrors and on the grating efficiency at this energy range – a conservative assumption about the acceptance of the undulator radiation was adapted.


Figure. On the left flux of FinEstBeaMS at endstation when resolving power is 5000. On the right spot size at experiment when slit is 50 microns.