The SwissFEL facility and its preliminary optics beamline layout

Oberta P., Flechsig U., and Abela R.

Swiss Light Source, Paul Scherrer Institut, 5232 Villigen, Switzerland

ABSTRACT

The planned XFEL at the Paul Scherrer Institut, the SwissFEL, is a fourth generation light source. Meanwhile the first hard X – ray FEL was taken into operation, the LCLS at Stanford, USA. Two further hard XFELs are in construction. One in Hamburg, Germany and the second at Spring – 8, Japan. Thanks to the beam properties of the XFEL, these new sources promise to bring novel insights and breakthroughs in many scientific disciplines. For engineers and designers new challenges arise in terms of material choice, damage thresholds and beam property conservation. The Swiss Light Source optics group is currently working on the beamline optics design of the SwissFEL beamlines. The preliminary optics design of the two undulator beamlines which serve six experiments is under preparation. In this article a preliminary layout of the hard X – ray Aramis undulator beamline is presented. Several beamline designs have been evaluated. Beam deflection and splitting via mirrors and diamonds is presented. The SwissFEL is planned to be operational in 2016.

1. INTRODUCTION

The SwissFEL is a project within the Paul Scherrer Institut (PSI). It is a fourth generation hard X – ray synchrotron source [1]. The SwissFEL is one of four hard X – ray FEL sources build or planned in the world. Those sources are, the Linac Coherent Light Source (LCLS) in Stanford [2], the European X – FEL in Hamburg [3] and the Spring – 8 Coherent SASE Source (SCSS) in Harima [4]. Currently there are several soft X – FEL sources build and some are already in operation like the FERMI@Elettra source at the ELETTRA synchrotron in Trieste and FLASH in Hamburg. Out of the four hard X – ray FELs only LCLS is operational since April 2009. Each of the four hard XFELs will have unique operational features, Table 1. In comparison to the LCLS and the European XFEL, the SwissFEL is thanks to a low electron – beam emittance, low charge per pulse and compact undulators a more compact source. Another unique feature of the SwissFEL is, that it will use synchronized far – infrared terahertz radiation from an independent source, to switch magnetic devices and prepare excited molecules, prior to being probed with the X – ray pulse [5].

Project Start of operation

Electron beam energy

λmin Peak brilliance @ λmin Repetition rate

Number of X – ray pulses

GeV Å 1033 ph/s mm2 mrad2 0.1% BW

Hz

LCLS 2009 13.6 1.5 1 120 1

SCSS 2010 8 1.0 0.5 60 1

Eu-XFEL 2014 17.5 1.0 5 10 3250

SwissFEL 2016 5.8 1.0 0.1 - 1 100 2

Table 1 Comparison of the radiation and machine parameters of the four hard X – ray FEL sources.

In a free – electron laser, the active medium is a beam of relativistic electrons. This beam moves in vacuum through a periodic magnet array, called an undulator, forcing the electrons to follow a wiggling orbit. The wiggling orbit introduces a transverse velocity component, which allows the electrons to exchange energy with a light wave which is co – linear with the electron beam. The electrons become accelerated or decelerated, depending on the phase of the transverse electric field of the light wave. For a particular wavelength of the light beam, this exchange becomes resonant for a single electron, leading to a continuous transfer of energy from the electron to the light wave.Two SASE (Self – Amplifying Spontaneous Emission) FEL lines will be driven by the linac - a hard X – ray FEL named Aramis and a soft X – ray line named Athos. Aramis covers the wavelength range 1 – 7 Å (12.4 keV – 1.77 keV), and Athos the wavelength range 7 – 70 Å (1.77 keV – 0.177 keV), Fig. 1. The presented beamline optics layout is only of the hard

X – ray line Aramis. For Aramis, a planar undulator with a novel type of permanent magnet (dysprosium – enriched NdFeB) is planned. The magnet array is mounted inside the vacuum tank.

Figure 1.A schematic of the proposed SwissFEL facility. Seeding of the Athos beamline is accomplished with a HHG (High Harmonic Generator) seed source and the small d'Artagnan undulator. Figure taken from [1].

To reach the required parameters, the inter – magnet gap which is available for the beam is as small as 4.5 mm, but the undulator gap can be varied between 3.2 mm – 5.5 mm. The gap can be moved mechanically and serves only for alignment purposes. The use of dysprosium – enriched NdFeB magnets allows these undulators to be operated at room temperature, thus avoiding a costly liquid nitrogen cooling system, as is normally required for undulators with comparable parameters. A total of 12 undulators of this type, each 4 m long, have to be aligned in a row within tight tolerances to ensure that the Aramis FEL can reach the SASE saturation regime. The SwissFEL and the Aramis parameters are tabulated in Table 2, 3.

Parameter Value

Overall length 713 m

Max. electron beam energy 5.8 GeV

Electron gun 3 GHz RF gun

Repetition rate 100 Hz

Period length λU 15 mm

K 1.2

Pulse energy 200 μJ

Polarization planar

Pulse duration (σ) 25.4 fs

Table 2SwissFEL fix parameters of the Aramis undulator.

Parameter Max. value Min. value

Wavelength 1 Å (12.4 keV) 7 Å (1.77 keV)

Electron beam energy 5.8 GeV 2.1 GeV

Bandwidth 0.034 % 0.144 %

Peak power 2.0 GW 1.9 GW

Beam size (σ) 22.2 μm 37.9 μm

Beam divergence (σ) 1.1 μrad 5.7 μrad

Table 3SwissFEL variable parameters of the Aramis undulator.

2. OPTICAL LAYOUT

In the next paragraphs we will present the preliminary optical layout of the hard X – ray undulator beamline called Aramis. An optical design of a beamline is strong dependent of its operational energy range and the scientific case of the proposed beamline. The main purpose of X – ray optics is to redirect and shape the X – ray beam according to the users and experimental needs. The choice of optical components have to be done very carefully and they should to the maximum possible degree respect and preserve the beams properties like spectral characteristics (brilliance, harmonics), polarization, beam geometry, pulse structure and wavefront preservation. The Aramis beamline is a hard X – ray beamline, operated within and energy range of 1.77 keV to 12.4 keV. Based on the working energy range the choice of optical components fells on reflecting mirrors and traditionally used crystal optics like silicon, germanium and diamond.Compared to a third generation synchrotron source, the optical components of the XFELs have to cope with several new problems. One of these problems is beam separation. The XFEL as a source is an array of undulators structured in a line. If one wants to build several experimental stations, the only way is, to build them also in a line. This represents a problem of how to separate physically the beam from its geometrical path to create enough space for experimental end – stations. This problem is partially solved with reflecting mirrors. But the separation only with mirrors is not enough, because the reflecting angles in the hard X – ray regime are small, in the order of milliradians (tenths of degrees). Therefore another concept was used, the so – called Large Offset Monochromator (LOM). The LOM is a double crystal monochromator (DCM) with a large offset reaching up to several tens of centimeters. This concept is also used at the LCLS, where the offset is 0.6 m and the PETRA III, where they have an offset of 0.5 m and 1.25 m [6].The second problem is the damage of the optics due to the high spatial energy densities of the sources pulse structure. The damage can be done either by a single – shot (pulse) or a single – photon. The most important process in the case of the single – shot damage is the process of melting. The single – photon damage is accompanied by photo – ionization processes [7]. The processes involved in the surface damage are dependent on the pulse energy and duration, Fig.2, [8].

Figure 2.Basic processes during the interaction of X – rays with matter depending on the characteristic time and energy scale. Figure taken from [8].

There have been several studies investigating the damage problem [9 – 18]. The most important results obtained in these studies was the elaboration of a theory and understanding of the thermal and non – thermal processes occuring during the X – ray matter interaction and the determination of the damage thresholds for different materials. Especially the second point is of high interest for beamline designers. The choice of a coating material in the case of the SwissFEL for the hard X – ray branch will be either carbon (C) or silicon carbide (SiC).The XFELs will push the brilliance of synchrotron sources of several orders of magnitude higher. Therefore the reflectivity

properties of mirrors should not degrade the source brilliance. A reasonable lower limit is 90%, the reflecting angle should be chosen so, that over the whole energy range the reflectivity is ≥ 90%, [19]. To fulfill this requirements one has to use very shallow reflecting angles. In Fig. 3a and 3b are the reflectivity plots versus incident angle for a SiC and a C coated mirror. As one can see from the plots, to cover the whole energy range with a ≥ 90% reflectivity, one has to use a 0.17º (2.96 mrad) grazing angle for the SiC coating and a 0.14º (2.44 mrad) grazing angle for the C coated mirror.

Figure 3.(a) reflectivity plot of an SiC coated mirror substrate at various grazing angles, (b) reflectivity plot of an C coated mirror substrate at various grazing angles.

The reflectivity requirement has an direct impact on the mirrors length. The lower the incident angles, the higher reflectivities we get and the longer the mirrors will become. Fig. 4 shows the dependence of the mirror length on the incident angle. The source size and the beam divergence has also an impact on the mirror length. From Table 3 we can see that the beam size and divergence is relatively small, but on the other hand the optical components are placed several tens of meters behind the source.As was mentioned before, X – ray optical components have to preserve beam properties like the shape of the wavefront. This requires extreme good surface quality, with a surface roughness not exceeding 0.5 nm (rms) and slope errors low as 0.1 μrad (rms), [20]. Achieving such surface quality is more difficult for longer mirrors than for shorter ones.

Figure 4.Mirror length versus grazing angle of a mirror placed 50 m behind the Aramis undulator for 4σ acceptance at 7 Å.

The preliminary optical layout of the Aramis undulator beamline is shown in Fig.5. In the next four sections we will describe in detail the used optical elements in the pre – experimental hall area and in the three experimental halls.

Figure 5.Preliminary optical layout of the hard X – ray Aramis undulator beamline.

2.1 Pre – experimental hall optics

The pre – experimental hall area is the space between the Aramis undulator and the wall of the first experimental hall, Fig.5. In this area, the first two deflecting mirrors (M1 and M2) are situated. Mirrors M1 and M2 are two plane mirrors situated 50 m and 53 m, respectively behind the Aramis undulator, Table 4. The suggested coating materials are SiC and C. The primary function of mirrors M1 and M2 is to deflect the undulator beam to the experimental hutches. On those two mirrors the maximum heat load will be deposited. Earlier studies, [21], showed that a distance of 50 m behind the Aramis undulator is sufficient to minimize the possibility of damage. After the two mirrors a beam stop (BS) is situated. The outer wall of the first experimental station is 47 m behind mirror M2. To ensure the switching between the left and right beam branch, either mirror M1 or both mirrors have to be movable in the perpendicular direction of the beam. Because of the low beam total deviation angle, the reflected beam from mirror M1 will propagate through the chamber of mirror M2. Therefore both vacuum chambers will be connected and share one vacuum unit. The combination of a small source size, small source divergence and a relatively short undulator – mirror distance allows us to accept the whole FEL footprint. The grazing angle of 0.17º (3 mrad) allows a separation of the two beam branches by 68 cm at the wall of the first experimental hutch. The right beam branch is 33 cm from the middle of the experimental hutch and the left branch is 35 cm from the middle of the experimental hutch.

Parameter M1 M2

Shape plane plane

Position (m) 50 53

Geom. surface size (mm) 375 × 20 375 × 20

Bulk material Si Si

Coating SiC/C SiC/C

Coating thickness (nm) 50 50

Grazing angle (º) 0.17/0.14 0.17/0.14

Direction of ref. left right

Table 4Parameter overview of mirror M1 and M2.

Mirror M1 will be exposed to the highest radiation power. This could lead to heat load. Calculations have shown, [21], that the average heat load of the SwissFEL is negligible compared with a storage ring heat load. The FEL puls energy (W) is 0.2 mJ, together with a 100 Hz repetition rate (f) the average power (Pave) is,

Pave = W × f = 0.2 mW. (1)

The spontaneous radiation is of the order of 0.2 mJ with an opening angle of 0.1 mrad. After a distance of 50 m, at the position of the first mirror, the spatial power density will be also negligible. Bremsstrahlung radiation is emitted when a fast moving charged particle is decelerated in the Coulomb field of the atoms. The electron beam of the SwissFEL traveling at least 100 m inside a beam pipe will create Bremsstrahlung on the residual gas in the vacuum. It is supposed that stainless steel vacuum chambers with Titanium – Nitride (TiN) inner coating will be used. In this particular technology the predominant residual gas is CO [22]. In [23] it was shown that the amount of high energy photons (≥ 300 keV) in the Bremsstrahlung is about 0.6%.In the pre – experimental hall area is also optics situated which is assigned to the second and third experimental hall. It is a LOM and a focusing toroidal mirror. More about these components will be written in the next sections.

2.2 Experimental hall 1

The first experimental hutch is situated 100 m behind the Aramis undulator and 47 m after mirror M2. The dimensions are 14.85 m × 6.6 m (L × W). The first experimental hutch will use the left beam branch deflected by mirror M1. As an optical system a Double Diamond Crystal Monochromator (DDCM) is proposed. The DDCM is a LOM system placed 2 m behind the experimental hutch wall. The DDCM can be operated as in the Laue so in the Bragg diffraction regime. One can even use a combination of Laue and Bragg diffraction, Fig. 6. Because of the similar lattice spacing (only a difference of 2.95%) between diamond and germanium, one can also use a germanium crystal in combination with a diamond crystal, forming a DCM. The distance of the two crystals (offset) is proposed to be 0.6 m, like in the case of the LCLS.

Figure 6Possible diffraction schemes of the DDCM.

The schematic layout of experimental hutch 1 is shown in Fig.7. The right beam branch deflected from mirror M2 is propagating 68 cm from the the first diamond crystal. Thanks to the large offset, the DDCM allows a large separation of the diffracted beam from the right beam branch and makes space for a potential experimental end – station. Furthermore thanks to a high thermal conductivity and a low expansion coefficient the diamond crystal does not suffer from lattice distortion introduced by heat load, [24]. Diamond crystals are also very transparent, which offers a possibility so split the left beam branch into two beams. One will be diffracted within the DDCM and used as the operation beam in the first experimental hutch. Similar diamond applications for beamline designs have been already realized in the past, [25 – 26].

Figure 7.Optical layout of the experimental hall 1.

The usable energy range of a DDCM begins at 7 keV. This range is restricted due to the low transmission of diamond at lower energies and the low diffraction efficiency below 7 keV. The forward transmitted beam can be either stopped by a beam stop or used in the second experimental hutch. This beam could be used in an energy range from > 4 keV (T > 40%). In Fig.8 is the plotted absorption vs. energy plot for a diamond crystal of four different thicknesses (50 μm, 100 μm, 150 μm and 200μm). This allows a simultaneous operation of two experimental stations. From Fig.8 one can see how much radiation will be absorbed by a 100 μm thick crystal. The rest, the transmitted radiation will be partially diffracted and partially forward diffracted. In Table 5 are the diffraction efficiencies of four different thick diamond crystals. For example, the absorption of a 100 μm thick diamond crystal at 8 keV is ~10%, Fig.8. From the 90% left, 69% will be diffracted and 31% will propagate through the crystal.

Figure 8.Absorption vs. energy of a diamond crystal with various thicknesses.

2.3 Experimental hall 2

The second experimental hutch is situated 61.85 m from the second mirror M2 and 114.85 m from the Aramis undulator. The second experimental hutch will use the right beam branch reflected from mirror M2, Fig. 9. As mentioned in the previous subsection, there is also the possibility to use a second beam. The forward diffracted beam from the first diamond crystal from the DDCM. This beam would cover an energy between 4 – 12 keV and would be 45.2 cm from the central point of the second experimental hutch. The operational beam reflected from mirror M2 is 43.2 cm from the central point of the experimental hutch.

Figure 9Optical layout of the experimental hall 2.

The primary beam reflected from the second mirror M2 will propagate through the LOM situated between the second mirror M2 and the first experimental hutch. By making the mechanical design of the first crystal movable in the perpendicular direction to the impinging beam, we allow the beam to propagate to the second experimental hall. The second experimental hall would accept the raw FEL beam from the Aramis undulator, exploiting his unique properties. The only optical element interfering with the FEL beam would be for security reasons mirror M2. An additional focusing system could be installed in the second experimental hutch. One can think either of a KB system, Fresnel zone plates (FZP) or compound refractive lenses (CRL). Because of the high ratio between the image and object distance (~125:1) these optical systems could be used for nanometer focal spot sizes.

2.3.1 Nanometer focusing

The second experimental hutch could be used for nanometer focusing. The long object distance to the source and the short image distances created by the proposed focusing optics (FZP,CRL,KB) make the optical layout of the second experimental hutch a perfect candidate for nanometer focusing. There are several optical features and approaches which one has to consider. Slope error approach. The slope error is a critical point for present nanometer focusing devices. If we propose a mirror with a slope error (σs) of 0.1 μrad and a focusing distance (f) of 1 m, than the slope error contributions to the focal spot are as follows: δy = 2 × 2.35 × f × σs. (2)

From equation (2) the slope error contributions to the focal spot are 500 nm for a focusing distance of 1 m.

Magnification approach. The source is 125 m away from the walls of the second experimental hutch. Which gives us a demagnification of 125:1 for a 1 m focusing distance. The source size is 37.9 μm (rms) @ 7Å (1.77 keV), which corresponds to a spot of 89 μm FWHM. For a demagnification factor of 125:1 this gives us 0.7 μm FWHM.

Mirror length approach. Present day KB mirror systems are able to reach nm focus spot sizes. The critical point in using a KB mirror system is its length. The KB system will be located at least at a distance of 125 m from the Aramis undulator. In Fig.10 is the plotted mirror length vs. incident angle. One can see from the plot, that the mirrors for a 4σ acceptance will reach up to 1 m lengths at that position. It will be extremely difficult to polish such a long mirror to the desired slope error mentioned above.

Figure 10.KB length vs. incident angle for three different sigma acceptances at 7 Å.

2.4 Experimental hall 3

The third experimental hutch is situated 76.7 m behind the second mirror M2 and 129.7 m behind the Aramis undulator. The operational beam will be the right beam branch reflected from mirror M2, Fig. 11. The main optical system for the third experimental hutch will be a LOM situated between mirror M2 and the first experimental hutch. The used crystals in the

LOM will be silicon (Si) crystals. We proposed an offset value of 0.6 m. By shifting the beam by 0.6 m we create space for a potential experimental end – station. The working beam would be 113 cm from the middle point of the third experimental hutch. Because of the fixed lattice constant of silicon crystal, the usable energy range is limited from > 4keV.

Figure 11.Optical layout of the experimental hall 3.

If we place the toroidal mirror 8.14 m behind M2 (38.9 m before experimental hutch 1) we will reach a 1:1 demagnification. The toroidal mirror can deflect the beam either vertically or horizontally. By using a horizontal deflection we will reach even a higher beam separation and gain more space in the second experimental hutch. The parameters of mirror M3 are tabulated in Table 5. Possible coating materials for mirror M3 are gold (Au), platinum (Pt) and rhodium (Rh). To keep the overall reflectivity at or above 90% the incident angles have to be kept low. In Fig. 12a, b, c are the plotted reflectivities of those three coatings (30 nm) for three different incident angles, 0.15º, 0.2º and 0.3º (2.6 mrad, 3.5 mrad and 5.2 mrad). From the plots one can see that there are several absorption edges within the energy range covered by the Aramis undulator. The reflectivity curves for Au and Pt are almost similar. Both elements have a series of M absorption edges between 2 keV and 3.5 keV and than a L – series at the end of the working energy range. On the other hand Rh has only a L – series between 3 keV and 3.5 keV. From the point of view of the incident angles, the 0.15º angle is the most suitable one.

Figure 12.Reflectivity vs. incident angle of a (a) Au coated M3, (b) Pt coated M3 and a (c) Rh coated M3.

Parameter M3

Shape toroidal

Position (m) 60.925

Geom. surface size (mm) 400 × 30

Bulk material Si

Coating Au/Pt/Rh

Coating thickness (nm) 50

Grazing angle (º) 0.2

Direction of ref. right

r radius (km) 17.45

ρ radius (m) 212

Table 5Mirror M3 parameters.

3. SummaryThe scope of this article was to present the preliminary optics layout for the planned SwissFEL Aramis undulator beamline. The beamline will have three experimental hutches and will operate in the energy range from 1.7 – 12.4 keV (0.7 – 0.1 nm). The Aramis undulator is followed after 50 m by two mirrors (M1 and M2). The deflecting angle will be approximately 0.2º (3.5 mrad) and the main functionality of the two mirrors is to separate two individual beams to the different experimental hutches. The proposed optics layout takes advantage of the so – called large offset monochromators to separate even more the beams from each other and to create space for the experimental end – stations. The first large offset monochromator is placed in the first experimental hutch and acts the same time as a beam splitter. The diffracting crystals are diamonds. This enables the simultaneous use of two experimental hutches (1 and 2). The second large offset monochromator is placed in the pre – experimental hutch area. The working crystals are silicon crystals. A toroidal mirror (M3) is placed downstream of the LOM to focus the beam down to the third experimental station. This set – up enables a 1:1 magnification scheme. By moving the first diffracting crystal of the silicon LOM along the perpendicular axis to the impinging beam, the beam is free to propagate into the second experimental hutch. The second experimental station will be equipped with a focusing system (CRL, FZP, KB). In Table 6 is a short parameter summary of the three experimental hutches.

Exp.hutch Energy range E/ΔE Op. system Source mirror

1 7 – 12 keV 6 × 10-5 DDCM M1

2a 4 – 12 keV 6 × 10-2 DDCM M1

2b 2 – 12 keV 1 × 10-2 CRL/FZP/KB M2

3 4 – 12 keV < 1 × 10-4 LOM + M3 M2

Table 6Experimental hutch parameters

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