MagneDyn: the future beamline for ultrafastmagnetodynamical studies at FERMI

Cristian Svetinaa,c, Martina Dell’Angelaa, Nicola Mahnea, Marco Malvestutoa, FulvioParmigiania,d, Lorenzo Raimondia, Marco Zangrandoa,b

aSincrotrone Trieste SCpA, S.S. 14 km 163.5 in Area Science Park, 34149 Trieste, Italy;bIOM-CNR, Laboratorio TASC, S.S. 14 km 163.5 in Area Science Park, 34149 Trieste, Italy;cUniversity of Trieste, Graduate School of Nanotechnology, Piazzale Europa 1, 34127 Trieste,

Italy;dDepartment of Physics, University of Trieste, Via A. Valerio 2, 34127 Trieste, Italy

ABSTRACT

The future beamline Magneto Dynamics (MagneDyn) will be devoted to study the electronic states and thelocal magnetic properties of excited and transient states of complex systems by means of the time-resolved X-rayabsorption spectroscopy (TR-XAS) technique. The beamline will use the high energy source at FERMI coveringthe wavelength range from 60 nm down to 1.3 nm. An on-line photon energy spectrometer will allow to measurethe spectrum with high resolution while delivering most of the beam to the end-stations. Downstream the beamwill be possibly split and delayed, by means of a delay line, and then focused with a set of active KB mirrors.These mirrors will be able to focus the radiation in one of the two MagneDyn experimental chambers: the electro-magnet end-station and the Resonant Inelastic X-ray Scattering (RIXS) end-station. After an introduction ofMagneDyn scientific case, we will discuss the layout showing the expected performances of the beamline.

Keywords: MagneDyn, Deformable active KB, On-line photon energy spectrometer, Time-resolved X-ray ab-sorption spectroscopy (TR-XAS), Resonant Inelastic X-ray Scattering (RIXS), Free Electron Laser (FEL)

1. INTRODUCTION

The Italian FERMI@Elettra is the first seeded Free Electron Laser (FEL) user facility. It is composed of twoindependent undulator chains: the low energy (FEL1) and the high energy (FEL2) branches. Both sourcesare based on the principle of the High-Gain Harmonic Generation (HGHG)1 providing highly intense almosttransform limited ultra-short pulses.2,3 FEL1 is a single stage cascade FEL that emits in the wavelength rangefrom 100 nm down to 20 nm (15 - 62 eV), while FEL24 is based on a double stage cascade scheme where theradiation emitted by a first stage is used to seed the second stage in order to reach shorter wavelengths from 20nm down to 4 nm (62 - 310 eV).

In Table 1 the machine parameters achieved during the commissioning for both FEL1 and FEL2 are reported.

2. MAGNEDYN SCIENTIFIC CASE

Manipulation of magnetic order with light is an important topic intensively studied in modern magnetism.In particular, the strongly non-equilibrium conditions developed in magnetic materials following excitation byintense femtosecond laser pulses represent a subject that has attracted continuous growing interest over the lasttwo decades.5 However, due to this strong non-equilibrium, the conventional description of magnetic phenomenain terms of thermodynamics is no longer valid. As a result, the ultrafast channels for transferring energy andangular momentum between photons, electrons, spins, and phonons remain elusive and a subject of debate. TheMagnedyn beamline will constitute a groundbreaking tool in the field of ultrafast magneto-dynamical studies byoffering a combination of pump-probe optical and x-ray spectroscopies.6 It will be possible to perform studies

Further author information: (Send correspondence to C.Svetina)C.Svetina: E-mail: cristian.svetina@elettra.trieste.it, Telephone: +39 040 375 8832

Table 1. FERMI@Elettra parameters for FEL1 and FEL2. The achieved ”in-commissioning” values, updated to mid-2013,for some parameters of FEL2 are reported.

Parameter FEL1 FEL2

Electron beam energy (GeV) 0.9–1.5 0.9–1.5

Bunch charge (nC) 0.5–0.8 0.5–0.8

Peak current (A) 500–800 500–800

Repetition rate (Hz) 10 10

Wavelength (nm) 100–20 20–4

Pulse length FWHM (fs) 30–100 ≤100

Bandwidth rms (meV) 20–40 20–40

Central wavelength fluctuation within BandWidth within BandWidth

Energy/pulse (µJ) >100 100

of time resolved X-ray Magnetic Circular Dichroism (tr-XMCD), X-ray Magnetic Linear Dichroism (tr-XMLD),Resonant Inelastic X-ray Scattering (tr-RIXS) and Magneto Optical Kerr Effect (tr-MOKE) as a function ofsample temperature (from 300K down to 4K) and in high magnetic fields (few Tesla). Such experimentaltechniques, pioneered in X-ray slicing experiments7 and in HHG table top setups,8 will benefit from the usage ofa FEL source with variable (circular, linear) polarization. In particular, with respect to X-ray slicing experiments,the high photon flux across the full soft X-ray range available at FERMI will allow (for tr-RIXS) or speed up(for the other techniques) data acquisition.

3. BEAMLINE LAYOUT AND OPTICAL COMPONENTS

After the emission from the FEL2 second stage the radiation propagates through the PADReS9 front-end, thebeam defining apertures, the gas attenuator and the on-line diagnostics such as the intensity monitors and thebeam position monitors, reaching the safety hutch. After the first dedicated plane mirror (PM2aMD) the beamis delivered the on-line photon energy spectrometer which is designed to collect a small fraction of the incomingbeam for measuring the energy distribution while transmitting most of the beam unperturbed to the end-station.After the grating the beam impinges on a bendable plane elliptical mirror (KBHMD) working in the horizontalplane which is the first of the two mirrors composing the KB focusing section. The radiation then propagatesuntil a vertical deflecting plane mirror (VDMMD) that will rise the beam height with respect to its originalvalue. After 4.5 meter the beam is then focused in the vertical direction by the second bendable plane-ellipticalmirror (KBVMD) inside the experimental chambers. In figure 1 a sketch of the MagneDyn beamline with allthe expected optical component is shown. The 14 meter free space between the spectrometer and the firstfocusing mirrors will be probably filled with a wavefront split and delay line similar to the already installedauto-correlator. At its entrance the photon beam will be split by means of a sharp edged plane mirror. Changingthe optical path of the reflected and transmitted beams, before their recombination, will allow to delay one withrespect to the other. This will allow to perform pump and probe experiments with the FEL radiation using boththe fundamental and/or the third harmonics. The typical delays will be about 10 picoseconds, reaching up tonanoseconds with the aid of additional delay lines employing multi-layer based mirrors.

3.1 On-line photon energy spectrometer

After the first plane mirror the photon beam impinges over the gratings composing the on-line spectrometer. Theworking scheme is similar to the already installed PRESTO10 and will employ two plane Variable Line Spacing(VLS) gratings respectively for the low and high energy range. The spectrometer will work at a constant grazingangle of 2 degrees in order to deliver the beam to the following components of the beamline and the diffractedradiation is focused into a movable detector mounted on a X–Y stage as shown in 2.

Figure 1. Layout of the FERMI experimental hall with the future MagneDyn beamline at the bottom. The photon beamemitted by the high energy source FEL2 is delivered to the beamline by means of a dedicated plane mirror (PM2aMD)inside the safety hutch. The radiation is then transported to the experimental stations by means of a dedicated systemof plane and focusing active KB mirrors

Figure 2. Schematic of the working principle of the already installed on-line spectrometer PRESTO and the futureMagneDyn spectrometer. The incoming radiation impinges over the plane VLS grating at a constant incident angle andis diffracted to a movable detector placed at focus.

The low energy grating, carbon coated, will cover the wavelength range from 60.5 nm down to 6.75 nm(from 20.5 eV to 185 eV) while the high energy grating, gold coated, will cover the wavelength range from 9.6nm down to 1.05 nm (from 130 eV to 1180 eV) by using both the first and second orders of diffraction. Thegrating profiles have been designed in order to keep the diffracted efficiency below 2% in order to maximize thebeamline transmission. The expected efficiencies have been simulated by means of the code REFLEC12 for boththe diffracted and transmitted beams. The spot size and the resolving power at the detector have been simulatedwith Shadow-code13 using the Rayleigh criteria. Thanks to the high number of illuminated grooves it has beenpossible to reach a theoretical resolving power around 105 over almost the whole working range. In table 2 thegrating parameters and the expected performances are summarized. The movable detector is expected to be aYAG screen (in vacuum) coupled with a visible CCD camera (in air).

3.2 Focusing section: active KB

The focusing section will be similar to the operative KAOS11 system employed on DiProI and LDM: a set oftwo bendable plane-elliptical mirrors. The main difference with respect to the already operative KBs is that thetwo focusing mirrors will be separate by a distance of 4.5 meter with a vertical deflecting mirror in between. Ofcourse this will cause the focused beam to be even less circular but for the beamline requirements this will notbe a problem. In fact, especially for the RIXS measurements, the beam will be focused mainly in the verticaldirection using a slit at entrance the experimental chamber. The presence of two possible FEL sources (firstand second stage of FEL2, about 28.8 m apart) as well as the two end-stations in series (1 meter apart) requireto adjust the tangential shape of the KB mirrors in order to keep the focus in place and to avoid unwanted

Table 2. Main parameters and expected performances for the on-line photon energy spectrometer of both the low (LE)and high (HE) energy gratings

Parameter LE HE

Groove density / gr mm−1 600 3250

Wavelength range / nm 6.75-60.5 1.05-6.75

Energy range / eV 20.5-185 130-1180

Groove profile Laminar Laminar

Groove height / nm 10 4

Groove ratio / % 0.7 0.7

Coating-thickness / nm Carbon - 50 Gold - 50

Efficiency / % 0.5-3 0.3-3

Resolving Power 6.5×104-1.7×106 7×104-1.6×106

Figure 3. Schematic of the bendable plane-elliptical focusing mirror composing the KB system. The side of the mirror areclamped and connected to stepper motors acting as pushers. The tangential shape of the mirror can be easily controlledallowing to reach the desired focal length.

aberrations. In order to perform this changes in shape we have already adopted a bendable system where thesides of the mirrors are clamped and moved by means of two stepper motors allowing to reach the desired focallengths. A schematic of the system is shown in figure 3

4. EXPECTED PERFORMANCES

The beamline has mainly been designed using the Shadow-code13 considering a geometric Gaussian source withthe measured properties of FEL2 for both the first and second stage. The spots at focus are expected to be 3.5µm × 14 µm at the electro-magnet chamber and 5.3 µm × 16 µm at RIXS (figure 4).

The geometrical losses are negligible for wavelengths below 30 nm (≤ 1%) and rise up to 10 % at 60 nm dueto the high divergence of the source. The beamline transmission has been maximized using a grazing incidentgeometry for all the mirrors (2 degrees) with a proper gold coating. The calculations have been performed usingREFLEC code12 considering the geometry of the beamline and the coating of all the five mirrors/grating. It goesfrom 75 % for the long wavelengths (7–60 nm) down to 7 % for the very short wavelengths (1–6 nm) as shownin figure 5. The degree of polarization of the radiation is expected to be mostly conserved during the photontransport because the beamline will employ only single layered mirrors working at grazing incidence where 3 of

Figure 4. Example of output from the Shadow-code when simulating the spots at focus in the electro-magnet (left) andRIXS (right) chambers for a 20 nm FEL2 radiation. The beam is clearly not circular due to the fact that the plane-ellipticalmirrors composing the KB system are 4.5 meter apart. The geometrical losses are negligible at this wavelength.

them will reflect in the horizontal direction while 2 in the vertical direction. As a consequence the vertical andhorizontal components of the electro-magnetic field will be transmitted in the same way. A slight effect mightbe observed at longer wavelengths (above 30 nm) meaning that an originally circular polarized light might havesome ellipticity at focus. However this will not be a problem since it has been demonstrated that properly tuningthe APPLE 2 undulators we are able to control the degree of polarization of the emitted radiation.14

5. CONCLUSIONS

Here we have given a description of the MagneDyn beamline main components as well as the expected perfor-mances at focus and for the on-line photon spectrometer. The opto-mechanical parts have already been designedand the installation of the beamline is foreseen to be concluded at the end of 2015.

ACKNOWLEDGMENTS

The work was supported in part by the Italian Ministry of University and Research under grants FIRB-RBAP045JF2 and FIRB-RBAP06AWK3.

Figure 5. Calculated beamline transmission of both the first and second stage of FEL2 in the case of a vertical (Red)and horizontal (Blue) linear polarized radiation. The simulation takes into account the beamline geometry as well as thecoating material of the mirrors/gratings.

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