helicity switching of circularly polarized undulator radiation by local orbit bumps

Nuclear Instruments and Methods in Physics Research A 498 (2003) 496–502

Helicity switching of circularly polarized undulator radiationby local orbit bumps

T. Haraa,b,*, K. Shirasawac, M. Takeuchib, T. Seikeb, Y. Saitod,T. Murob, H. Kitamuraa,b

aSPring-8/RIKEN, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148, JapanbSPring-8/JASRI, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan

cDepartment of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, JapandSPring-8/JAERI, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148, Japan

Received 4 October 2002; received in revised form 10 December 2002; accepted 13 December 2002

Abstract

Helicity switching of circularly polarized undulator radiation provides a valuable tool in synchrotron radiation

experiments, particularly in the study of circular dichroism. At SPring-8, we have developed a helicity switching system

by means of selecting radiation from two helical undulators. Right and left circular photon beams are alternately

supplied to the beamline using electron beam orbit bumps. Instead of reversing the magnetic field applied to a sample,

helicity switched light was used in magnetic circular dichroism measurements and the measuring time was reduced by 13:

r 2003 Elsevier Science B.V. All rights reserved.

PACS: 41.60.Ap; 07.85.Qe; 07.55.�w; 78.20.Ls

Keywords: Undulator; Helical undulator; Circular polarization; Helicity switching; Circular dichroism; Magnetic circular dichroism

1. Introduction

Variation of polarized light is one of theimportant features of synchrotron radiationsources. Since polarization of undulator radiationis determined by the electron orbit projected onthe transverse plane, a variety of undulatordesigns, such as helical, elliptical, vertical andfigure-8 undulators, have been developed in order

to generate various polarization states. On theother hand, experiments like circular dichroismmeasurements detect signal difference between twopolarized states, and demands for periodicalalternation of polarization, particularly right andleft circular polarization (RCP and LCP), arerecently increasing.

In a certain photon energy range, phaseretarders are available and they can be used tochange polarization easily and rapidly [1,2]. Inmost of photon energies in soft X-rays, however,retarders have not been developed yet andpolarization should be changed at radiationsources.

*Corresponding author. SPring-8/RIKEN, 1-1-1 Kouto,

Mikazuki-cho, Sayo-gun, Hyogo 679-5148, Japan. Tel.: +81-

791-58-2809; fax: +81-791-58-2810.

E-mail address: [email protected] (T. Hara).

0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0168-9002(02)02145-9

2. Helicity switching of undulator radiation

A synchrotron radiation source based on astorage ring is a multi-user facility and stability ofelectron beam orbit is an important issue for users.Recent third generation synchrotron radiationsources, such as SPring-8, achieve source stabilityof several microns [3]. In terms of cohabitationwith other users, electron orbit disturbance causedby the helicity switching system should be reducedto the same order.

There are mainly two ways to change helicity ofcircular polarization of undulator radiation. One ischanging the helicity of magnetic field in a helicalundulator by mechanical shift of magnet arrays incase of permanent magnet undulators [4], or byreversing polarity of coil current in case ofelectrical magnet undulators [5,6]. Since right andleft circular photon beams are emitted from thesame undulator, photon sources of two polarized

states are ideally identical. However, the eddycurrent in a vacuum chamber due to radicalreverse of undulator field disturbs the stability ofelectron beam orbit, and a feedback system may benecessary to compensate.

The second way is to use two undulators, eachbeing set as providing right and left circularlypolarized radiation. Right and left circular photonbeams are emitted at different angles or positions,and a rotating chopper installed at beamline opensa gate to two light beams alternately [7,8]. Sincethe chopper is independent from the electronbeam, there is no effect on the photon sourcestability and switching frequency can be increased.However as shown in Fig. 1(a) and (b), right andleft circular light beams have no overlap in phasespace. Therefore two photon sources are consid-ered as completely different sources.

At a SPring-8 soft X-ray beamline (BL25SU),we have developed a new switching method [9].

Fig. 1. Phase space distribution of two photon beams calculated in the middle of two undulators. Photon distributions (1� sboundary) of upstream (solid line) and downstream (dotted line) undulators are shown. Two photon beams from two undulators are

emitted at (a) different horizontal angles (750 mrad), (b) different horizontal positions (7500mm), and (c) on the same axis. In all

cases, vertical phase space distributions are partially overlapped as shown in (d). Parameters of SPring-8 BL25SU and 1 nm radiation

are assumed in the figure.

T. Hara et al. / Nuclear Instruments and Methods in Physics Research A 498 (2003) 496–502 497

Two helical undulators and five kicker magnets areinstalled in the straight section of the beamline,and two circularly polarized photon beams fromtwo undulators are separated by means ofhorizontal electron beam orbit bumps as shownin Fig. 2. During the excitation of the orbit bump,one of the photon beams passes through a slitplaced on the beamline axis and the other isstopped at the slit. By alternate generation of twoorbit bumps (A and B in Fig. 2), the helicity ofcircular polarization at beamline is periodicallyswitched. Since right and left circular photonbeams are emitted on the same axis, the distribu-tions of two beams in horizontal phase space areapproximately the same (Fig. 1(c)). In verticalphase space, two distributions are partiallyoverlapped as shown in Fig. 1(d). In terms ofthe electron orbit stability, the disturbance on theelectron orbit due to the eddy current can beminimized by using ceramic chambers at thelocations of the kicker magnets.

3. Kicker magnets

The scheme of the helicity switching by means ofhorizontal electron beam orbit bumps is shown inFig. 2. Five kicker magnets are placed symmetri-cally with respect to the center of the straightsection in addition to two helical undulators.The electron orbit bumps are designed so as thephoton beams from two undulators are separated

horizontally by 200 mrad, which is sufficiently largecompared with the angular divergence of funda-mental radiation.

Two triangular bumps, A and B in Fig. 2,should be accurately closed inside the straightsection. Otherwise, angular or position errors ofthe orbit bumps propagate around the storage ringand affect the photon source stability of otherbeamlines. Thus, fine adjustment of the kicker fieldis indispensable.

In Fig. 2, kicker magnets 1–3 and 3–5 are usedto generate the bumps A and B, respectively. Notethat the kicker magnet in middle (kicker 3) hasdouble coils. These two groups of three kickers areequipped with independent power supplies. Threekicker magnets are connected in series in order toassure synchronized field generation between threekickers. The kicker magnet has a C-shaped corewith a 40mm magnetic gap as shown in Fig. 3. Thecore dimensions of all kicker magnets are madeidentical. The magnetic field strength of the kickeris adjusted by making the number of coil turnsproportional to the kick angle of each kicker.

As core material, soft ferrite (SUMITOMOSPECIAL METALS, CXC-H3E4) was first testedin a prototype because of its good high frequencycharacteristics. Since available size of soft ferriteblocks is limited, the C-shaped core was assembledby glued small blocks. However as increasing thecoil current, partial saturation of the core wasobserved at the average field in the entire core farbelow the saturation field of the material. This was

Fig. 2. Electron beam orbit bumps A (broken line) and B (dotted line) generated by five kicker magnets (top view). When orbit bump

A (B) is generated, left (right) circular light is emitted on axis and supplied to the beamline.

T. Hara et al. / Nuclear Instruments and Methods in Physics Research A 498 (2003) 496–502498

due to the structure of the glued blocks. Sincethere were small gaps between the blocks, themagnetic field concentrates at the corners of eachblock and saturates.

In order to improve the magnetic field linearitywith respect to the coil current, core materialwas changed to permalloy (SORYO DENSHIKAGAKU, HMC), which has higher permeabilityand saturation field compared with soft ferrite.The core was made by laminated permalloy plates(0.1mm in thickness) punched out as the C-shapecross-section to prevent partial saturation of thecore. The surfaces of the permalloy plates werecovered with insulation coating to obtain goodhigh frequency characteristics. As a result, the fieldlinearity of the permalloy kicker was significantlyimproved compared with that of the soft ferritekicker.

Main parameters of the kicker magnets arelisted in Table 1 together with the parameters of

the helical undulator and the electron beam ofSPring-8.

4. Electron beam orbit distortion

The magnetic field of the kicker magnets wasmeasured by a hole probe and a flipping coil, andthe number of coil turns was adjusted beforeinstallation in the storage ring. In case of SPring-8,the magnetic field of 1 mTm moves the electronbeam orbit by 1 mm in horizontal and 0.2 mm invertical. It is difficult to measure and adjust thefield of 1 mTm in conventional field measurements,therefore final alignment should be carried out byobserving the electron beam orbit after installa-tion. In order to adjust the vertical field of thekicker magnets, field correction plates were at-tached at both sides of the kickers as shown inFig. 3. The field correction plates are made of

Fig. 3. Kicker magnet and field correction plates. Vertical magnetic field is adjusted by changing a plate gap shown in the figure.

Horizontal field is adjusted by tilting the magnet. Unit of dimensions is mm.


silicon steel, and they change slightly the fringefield distribution of the kicker as varying the plategap. The correction plates can adjust about 1% ofthe total field integral of the kicker magnet. Thehorizontal field correction was carried out bytilting the kicker magnet using a swivel.

After optimizing the positions of the correctionplates and swivel, electron beam orbit distortionwas measured at several beamlines considering thephase of the beta function. Although vacuumchambers at the kicker magnets are made ofceramic, small horizontal electron beam orbitdistortion of about 10 mm was observed, that wasproportional to the derivative of the kicker field.Since the correction plates do not counteract thisorbit error, air-core coils were used to cancel theeddy current effect by applying an excitationcurrent pattern determined from the measuredelectron beam orbit distortion.

Fig. 4 shows typical photon beam stability ofother beamlines measured with X-ray beamposition monitors installed at 20m away fromthe photon source. The kicker excitation pattern

was 1Hz trapezoidal as shown in Fig. 5, and thehelicity of the radiation was changed every 0.5 s.The effect of the orbit bumps was almost invisible

Table 1

Principal parameters of the helical undulator, kicker magnet and electron beam of SPring-8

Pure permanent magnet device, out of vacuum type

Helical undulator

Type

Length 1.5m� 2

Number of periods 12� 2

Periodic length 120mm

Minimum gap 20mm

Kmax Kx;Ky ¼ 6:4E1st 0.12–4.5 keV

Kicker magnet

Type C-shape

Core material 0.1mm laminated permalloy

Number of coil turns 182 (kickers 1, 5), 236 (kickers 2, 4), 54� 2 (kicker 3)

Maximum current 20A

Maximum field 0.15T (kickers 2 and 4)

Effective length of magnetic field 180mm

Electron beam

Energy 8GeV

Horizontal beam size 380mmVertical beam size o10mmBeta functions at the center of straight section bx ¼23.4m, bx ¼14.4m

Maximum beam current 100mA

Fig. 4. Horizontal (solid line, top) and vertical (dotted line,

bottom) photon beam positions measured at 20m away from

the undulator of BL39XU. During the helicity switching ‘‘ON’’

(left side of the spike in the figure), electron orbit bumps were

generated by 1Hz trapezoidal pattern. The spike of the

horizontal beam position at 4.2 s was intentionally added to

indicate the time of switched off.


under the background drift of the electron beam.With keeping the helicity switching ‘‘ON’’, thephoton energy (undulator gap) was scanned overthe range between 0.2 and 2 keV (30–70mm) andno significant disturbance on the electron orbitwas observed. In case of the sinusoidal excitationpattern of the kickers, the orbit distortion was alsosuccessfully canceled up to the frequency of 10Hz.Within this range of switching rate, cohabitationwith most of synchrotron radiation experiments isensured. When the switching frequency exceeds10Hz, it becomes difficult to precisely cancel theeddy current errors under the current setup of thesystem.

5. Magnetic circular dichroism measurements

The magnetic circular dichroism (MCD) spectraat 2p absorption edge of Fe metal were measuredto prove performance of the helicity switching.The excitation pattern of the orbit bumps wastrapezoidal and the helicity of the undulatorradiation was changed by 1Hz. After a 0.2 stransient time, helicity was kept for 0.3 s as shownin Fig. 5. Two absorption spectra with right andleft circularly polarized light were measured atroom temperature by scanning a monochromator.The measured spectra and their difference (MCDsignal) are shown in Fig. 6. Compared with aconventional method [10], which reverses themagnetic field applied to a sample, the measuringtime was reduced to less than 1

3:

Fig. 7 shows the photoabsorption at 2p3/2absorption peak as a function of time. Thephotoabsorption intensity changes according to

the alternated helicity of circular light, and itconfirms stable alternation of two polarized states.

6. Conclusions

The helicity switching system of circular un-dulator radiation by means of electron beam orbitbumps was established. The disturbance on theelectron beam orbit was reduced to negligible levelcompared to the electron beam size without usingany feedback system. The MCD spectra of Femetal were measured using the helicity switchedcircular light instead of reversing the magnetic field

Fig. 5. 1Hz trapezoidal excitation pattern of electron orbit bumps. When the bump is ‘‘ON’’, electron orbit deviates from the beamline

axis.

Fig. 6. Spectra of Fe 2p photoabsorption measured with right

and left circular light (dotted lines). Since photon sources of two

circular states are not identical, the signals are normalized with

incident flux, which is typically different by a few %. Solid line

at bottom shows MCD signal.


at a sample. As a result, the measuring time wasreduced by 1

3: We are planning to improve the

resolution of MCD signals combining the helicityswitched radiation and lock-in techniques. Thehelicity switched circular light will also be animportant tool for the study of natural circulardichroism.

Acknowledgements

The authors thank T. Terakawa of SoryoDenshi Kagaku Corp. for the development ofthe permalloy kicker magnets. We thank also

H. Aoyagi and the staff of the ID group andBL25SU for the achievement of this work.

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Fig. 7. Variation of photoabsorption as a function of time.

Photon energy was fixed at Fe 2p3/2 peak. Helicity of light was

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pattern.


helicity switching of circularly polarized undulator radiation by local orbit bumps

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