Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 126–129

Insertion devices at the Swiss Light Source (phase I)

T. Schmidt*, G. Ingold , A. Imhof, B.D. Patterson, L. Patthey, C. Quitmann,C. Schulze-Briese, R. Abela

Paul Scherrer Institut, Swiss Light Source, WSLA 122, CH-5232 Villigen PSI, Switzerland

Abstract

The insertion devices under construction for phase I of the Swiss Light Source (SLS) are described. Five undulatorsand one wiggler will be installed in four straight sections of the third generation 2:4 GeV SLS storage ring, underconstruction at the Paul Scherrer Institute. To provide undulator radiation in the energy range from 10 eV to 18 keV,

both long period and short period, small gap undulators will be installed. # 2001 Elsevier Science B.V. All rightsreserved.

PACS: 41.85.L; 41.60

Keywords: Undulator; Circular polarized; Small gap; Small period

1. Circular polarized IDs for soft X-rays

For soft X-rays, both the SIS (‘‘Surfaces/Interfaces: Spectroscopy’’, 10–800 eV) and theSIM beamline (‘‘Surfaces=Interfaces: Micro-scopy’’, 95 eV–2 keV) use elliptical twin-undula-tors to produce two parallel displaced photonbeams with opposite linear or circular polarization(Fig. 1). The same ‘‘footprint’’ of the optics in thebeamline is used while transporting the twooppositely polarized, parallel displaced beamsonto the sample [1]. A chopper at an intermediatehorizontal focus will be used for fast helicityswitching with a frequency up to 1 kHz. A 7-

magnet chicane system, identical for both the SISand SIM beamlines, will be used to separate thesource points or match the phase between theundulators.

The SIS twin-undulators (UE212,lu ¼ 212 mm), to be installed in a long straightsection (10:6 m), are electromagnetic DC devices,whose basic magnetic design is adapted fromELETTRA [2] and which are currently underconstruction at BINP, Novosibirsk. The 4:55 mlong solid iron vertical pole structure is self-supporting and is fixed with 7 ribs at the supportstructure. The horizontal poles are fixed at the I-beams using aluminum spacers to minimize thecrosstalk between the field components (Fig. 2).To achieve high straightness (10 mm), the upperand lower beams will be machined simultaneouslyon a grinding machine. According to FEMcalculations at PSI, a gap parallelism of 10 mm

*Corresponding author. Tel.: +41-56-310-4592(-2665); fax:

+41-56-310-3151.

E-mail addresses: thomas.schmidt@psi.ch (T. Schmidt),

gerhard.ingold@psi.ch (G. Ingold).

0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 2 3 7 - 6

can be reached. The vertical poles are powered by145� 28 A turns (horizontal 120� 20 A turns)resulting in peak fields of #By ¼ 0:4 T; #Bx ¼ 0:1 Tand effective fields of %By ¼ 0:35 T; %Bx ¼ 0:09 T.The vertical poles are driven with two separatecircuits to allow a quasiperiodic field variation inan electromagnetic undulator for the first time.The periodicity is hard wired but the amplitude

difference can be varied to optimize for differentenergies (Fig. 3). In combination with a NIMmonochromator [3], the higher harmonics aresuppressed to a level of 10�3 in the energy range10–30 eV. An overview of the insertion devicesinstalled in phase I is given in Table 1.

The SIM twin-undulators (UE56, lu ¼ 56 mm),built in collaboration with BESSY [4] and to beinstalled in a medium straight section (6:2 m) areof the APPLE II type. Besides circular polariza-tion, these devices will also produce rotable linearpolarized light by shifting the magnet arrays inopposite direction [5]. Analytical expressions forthe APPLE II fields and the performance of theantisymmetric mode are summarized in [6].NdFeB magnets (VACODYM 633) have beenchosen with Br ¼ 1:28 T for transverse pressedblocks with vertical magnetization, Br ¼ 1:24 Tfor the axial pressed horizontal blocks andHcj517:0 kOe). The inhomogeneities (North–South effect) of �1% and �2:5% for the blockshave been taken into account wire measurementsof every single magnet at BESSY.

2. Minigap IDs for hard X-rays

For hard X-rays, a hybrid in-vacuum undulatorwill be installed for the PX-beamline (ProteinCrystallography, 5–18 keV) and a small gaphybrid wiggler for the MS beamline (MaterialsScience, 5–40 keV), both in a short straight section(3:4 m). In a first step, an existing U24 in-vacuumhybrid undulator (lu ¼ 24 mm) will be installedwithin a SPring-8=SLS collaboration. Later thisdevice will be replaced by a new U17 in-vacuumhybrid undulator (lu ¼ 17 mm).

The U24 in-vacuum undulator is based on anexisting 1:5 m long prototype undulator built forSPring-8 and has been upgraded (modified baseframe, new magnet arrays and new flexible tapertransitions) for SLS. Operating at gaps of 8–10 mm, this device is optimal for the beamlinecommissioning using high brilliant photons up to12 keV as shown in Fig. 4. The specification forthe first field integral (I1450 G-cm forx ¼ �20 mm) and the phase error (2:58 rms) hasbeen achieved for the gap range 6–50 mm [7].

Fig. 1. For the polarization switching a parallel displacement

of the electron beam in the two undulators in series will be used,

which provides a full overlap of the photons on all optical

elements in the beamline but the foci, where a fast chopper will

be placed. The sample will be placed in a blurred beam slightly

out of focus.

Fig. 2. Lower half of the electromagnetic elliptical undulator

UE212. The 4:55 m long I-beams with the vertical poles are

machined in one piece.

T. Schmidt et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 126–129 127

Hybrid type magnet arrays with NdFeB magnetblocks (Neomax-35EH) are used. The new to bebuilt U17 in-vacuum undulator will replace theU24 device after about one year of operation.Operating on the 9th and 11th harmonic in the gaprange 4–5 mm, photons with more than one orderof magnitude higher brightness at energies of 12–18 keV will be provided. Because of possibleradiation damage [8], it is planned to use Sm2

Co17 magnets with less remanent field but highercoercivity. For a gap parallelism below 2 mm,which is needed to control the phase error within2:58� 0:48 rms, the U17 gap drive will operatewith two separate motors and four linear encoders

Fig. 3. Calculated single electron on axis flux density for the UE212 in quasiperiodic mode. The integer harmonics are shifted to

rational harmonics.

Table 1

Insertion devices for SLS in phase I

UE212 UE56 U24 U17 W61

Type elm twin elliptical APPLE II twin elliptical Hybrid in vacuum Hybrid in vacuum Hybrid wiggler

Gap magn=vac 19=16 16=11 8=8 4=4 7:5=5lU ðmmÞ 212 56 24 17 61

No. of per 2� 21 2� 32 57 117 30

By=Bx ðTÞ 0:4=0:1 0:83=0:6 0:67=– 1:0=– 2:0=–Ky=Kx 7:9=2:0 4:3=3:1 1:5=– 1:6=– 11:4=–E ðkeVÞ 0.01–0.8 0.09–2 5–18 7–18 5–40

Harmonics 1–5 1–5 3–11 3–11

Fig. 4. Comparison of brilliance for the U24 and U17 (dashed

line) at 10 gap. The tuning range of the U17 is not suitable for

commissioning.

T. Schmidt et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 126–129128

for a direct gap reading on either side of theelectron beam axis. For alignment the device willbe installed on movable supports which have beendeveloped for an active 5-axis alignment of theSLS storage ring girders.

The W61 hybrid wiggler (lw ¼ 61 mm) for theMS beamline is designed for maximum flux densityup to 40 keV, with a total radiated power notexceeding 8 kW. This can be accomplished by atriangular rather than trapezoidal field shape, toachieve a high peak field of #B ¼ 2 T (effective field%B ¼ 1:6 T), at short periods (and therefore asmaller K-value). Side magnets are used toenhance the flatness of the magnetic field in thetransverse direction. The W61 will ultimately beoperated at a magnetic gap of 7:5 mm by usingsmall gap, fixed aperture (5 mm) aluminumvacuum chambers built at APS [9]. For thecomissioning of the small gap IDs, additionalvacuum chambers with 12 and 8 mm vacuum stayclear aperture will be used. The wiggler, currentlyunder construction at DANFYSIK, will be oper-ated with 2 stepper motors and linear encoders forthe gap reading.

Acknowledgements

The authors would like to thank ELETTRA,BESSY, SPring-8, APS and ESRF for theircollaboration and fruitful discussions.

References

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T. Schmidt et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 126–129 129