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Beamline Description


Beamline Description | TARDI spectrometerFocusing KB | Electromagnet endstation | RIXS endstation Detectors | SLU


Overview




In order to maximize collection efficiency, the beamline was designed to convey the highest possible flux density at the focus.


Beam parameters:

  • Repetition rate: 10 - 50 Hz
  • Photon energy range: 4 - 60 nm (1st FEL-2 harm and 1st stage of FEL-2), 
  • polarization: LV, LH, CL, CR
  • FEL-2 pulse intensity: up to 15-20 μJ@100 eV 
  • KB system parameters: focus size down to ~10 μm 

BEAMLINE LAYOUT AND OPTICAL COMPONENTS 


Unlike the other FERMI beamlines, MagneDyn is fed by the high energy source FEL2 only. For this reason a brand new branch of PADReS has been implemented in order to measure and manipulate the photon beam down to the two planned end-stations. The layout is hereafter described (shown in figure below) while the main optical components will be discussed in the next sections in detail.The beamline starts inside the safety hutch, a restricted access area at the beginning of the experimental hall, by means of a gold-coated plane mirror, named PM2aMD located 41.4 m downstream the last undulator. The beam will then be reflected to the on-line photon energy spectrometer (TARDI), about 17 m downstream, where a small fraction of the radiation is diffracted by one of the two diffraction gratings employed to measure on-line the spectral content. After the spectrometer the beam propagates freely for 14 m until the first element of the KAOS (Kirkpatrik-Baez Active Optics System) designed to focus the FEL light in the horizontal direction. After 1.2 m the beam is reflected by another plane mirror devoted to rise the beam vertically, named Vertical Deflecting Mirror (VDMMD). About 4.3 m after the VDMMD the beam is finally reflected by the second element of KAOS and focused in the vertical direction inside one of the experimental endstations. The number of mirrors/gratings employed by the beamline is five (six considering the two gratings) and all of them work at 2 degree of grazing incidence in order to maximize the beamline transmission keeping the dimension of the substrates reasonable. The total length of the beamline, from the last undulator to the focal spot inside the first experimental vacuum chamber, is around 80 m.

 Plane mirrors
As described in the previous section the beamline employs two plane mirrors. The dimension of the PM2aMD substrate (made of Silicon) has been chosen to be 350 mm x 60 mm x 70 mm (L × W × H) with a clear aperture of 300 mm x 30 mm in order to collect the whole incoming beam. The second mirror, VDMMD, placed 32.2 m downstream PM2aMD, has a larger substrate (made of Silicon too) since the divergent photon beam enlarges along the beamline: 410 mm x 40 mm x 50 mm (L × W × H) with an optical surface of 390 mm x 20 mm. 
For both mirrors the tangential and sagittal radii of curvature will be more than 30 km in order to prevent unwanted focusing effects. The surface quality is within the standards of the other mirrors employed at the other FERMI beamlines with a tangential slope error below 0.5 µrad rms and 5.0 µrad rms for the sagittal. These values guarantee the preservation of the wavefront and limit the possible aberrations of the photon beam. The substrates have a gold coating (30 nm thick) with a surface roughness below 0.3 nm rms in order to avoid scattering effects of the reflected light that can’t be focused properly. 

 

Optics

Bulk material Bulk dimensions (mm3) Roughness rms (nm) Tangential radius (km) Sagittal radius (km) Optical area (mm2) Tangential slope error rms (µrad) Sagittal slope error rms (µrad) Shape error P-to-V (nm) Coating
PM2a_MD Silicon 350x60x70 < 0.3 > 30 > 30 300x30 < 0.5 < 5 < 10 § Au
KBM_H_MD Glass-based 400x40x10 < 0.3 > 1 * > 3 * 360x20 < 0.5 * < 5 * < 5 * Au
VDM_MD Silicon 410x40x50 < 0.3 > 30 > 30 390x20 < 0.5 < 5 < 10 § Au
KBM_V_MD Glass-based 400x40x10 < 0.3 > 1 * > 3 * 360x20 < 0.5 * < 5 * < 5 * Au

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.

Transmittivity


Ultima modifica il Lunedì, 18 Novembre 2019 10:32