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beamline transmission

Beamline transmission and polarization

 
Considering the presence of the various coatings employed and the incidence angles, the beamline transmission has been calculated with particular attention to the effect on the polarization. In fact, one of the key elements for studying the magnetic properties of the materials, is the presence of pure circular polarized light. Since the beamline will employ grazing incidence single layered mirrors, three of those working in the horizontal direction and two in the vertical, the degree of polarization of the radiation is expected to be mostly conserved. However, for the longer wavelengths (above 30 nm) a small effect will be present leading to a slightly elliptical polarized light when circular polarized radiation will be provided by the machine. Moreover, the presence of undulators APPLEII allow to control the polarization of the emitted radiation. In particular it can be generated with an ellipsoidal polarization in such a way that the beamline itself makes it circular at the experimental endstations.

The beamline transmission has been maximized using a grazing incident geometry for all the mirrors (2 degrees of grazing incidence) employing a proper gold coating over the substrates (but for the LE grating which is Amorphous-Carbon).. It ranges from 70 % for the long wavelengths (7–60 nm), between 10% and 20% for the range 3 nm - 6 nm and drops down to 7 % for the very short wavelengths (around 1 nm).

In addition to the mirror’s reflectivity, another important cause of loss of beamline transmission are the geometrical losses. These are due to the finite size of the mirrors as well as the presence of apertures as a function of the incoming wavelength. In fact the photon beam divergence is linearly correlated to the wavelength meaning that the longer the wavelength of the radiation, the more the transverse spot will enlarge along the beamline. Their effect will be negligible at wavelength below 20 nm where the beam is confined within the length of the mirrors. At longer wavelength, radiation emitted in the first stage, the geometrical cuts become relevant with a transmission going from 87% at 30 nm down to 45% at 60 nm.
The overall beamline transmission is shown in figure (Transmission) having taken into account both reflectivity and geometrical losses in the whole wavelength range. Considering a photon flux at the source of about 100 µJ, we expect to obtain an intensity at the sample of the order of 58 µJ at 30 nm and 20 µJ at 6 nm. These intensities will correspond to a fluence at the sample of the order of 2.5e12W cm-2 and8.2e11W cm-2 respectively. Of course the photon beam can be attenuated by means of a set of dedicated solid state filters (such as Aluminum, Zirconium, Palladium, etc.) and/or by using the PADReS gas attenuator already operative (ref GA). Moreover the beam size can be easily enlarged by acting on the KAOS mirrors shape in order to drop even more the fluence if needed.

Last Updated on Thursday, 09 July 2020 19:07