Beamline description


General layout of FEL2 and the seed laser for users (SLU) photon delivery systems at MagneDyn. The intensity of the FEL pulses is first monitored by means of the undulator and safety hutch ionization chambers. The FEL intensity can be adjusted by using either a gas (nitrogen, neon, or helium) cell or solid-state thin film free-standing metallic filters. Then, the first deflecting mirror deviates the FEL2 pulses along the beamline. Pulse-to-pulse spectral monitoring is performed by means of a high-resolution spectrometer (TARDI). The pulses are then refocused by means of the Kirkpatrick–Baez active optics (KAOS) mirror bending system. A dedicated beamline delivers the SLU to MagneDyn, where a breadboard is assigned for the control and manipulation of the optical laser. The SLU consists of a portion of the seed laser that is delivered to the experimental stations via a low-vacuum beamline terminated by a quartz window. The main experimental chamber consists of an electromagnet coupled to an XUV polarimeter, a cryogenic sample environment, and pump–probe characterization tools and monitors. The second experimental station is equipped with an x-ray emission spectrometer for time/resolved RXES/RIXS experiments. The upstream chambers are equipped with a SLU–FEL coupling system made of in-vacuum mirrors, intensity monitors, solid-state filter wheels, and beam position monitors.
C. Svetina J. Synchrotron Rad.  (2016). 2398-105


beamline transmission

Wavelenght (nm) FEL Θrms (μrad) Geometrical acceptance (%) Total transmission (%)
43 2 53.8 68.1 68.8
32 2 40 86.9 68.3
20 2 25 99.3 65.6
10 2 30 97 75.3
7 2 15 100 50
4 2 15 100 12

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 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 2.5e12 W cm-2 and 8.2e11 W 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. 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.

Ultima modifica il Domenica, 30 Aprile 2023 13:50