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2810 J. Opt. Soc. Am. A/Vol. 22, No. 12 /December 2005 Cestelli Guidi et al.

Optical performances of SINBAD, the SynchrotronINfrared Beamline At DA�NE

Mariangela Cestelli Guidi

Laboratori Nazionali di Frascati–Istituto Nazionale di Fisica Nucleare, Via E. Fermi 40, 00044 Frascati, Italy

Massimo Piccinini

Laboratori Nazionali di Frascati–Istituto Nazionale di Fisica Nucleare, Via E. Fermi 40, 00044 Frascati, Italy,and Università degli Studi Roma Tre, Largo S. Leonardo Murialdo, 1, 00146 Roma, Italy

Augusto Marcelli

Laboratori Nazionali di Frascati–Istituto Nazionale di Fisica Nucleare, Via E. Fermi 40, 00044 Frascati, Italy

Alessandro Nucara and Paolo Calvani

Coherentia–Istituto Nazionali di Fisica della Materia, and Dipartimento di Fisica, Università di Roma“La Sapienza,” Piazzale A. Moro 2, 00185 Roma, Italy

Emilio Burattini

Università degli Studi di Verona, Facoltà di Scienze Matematiche, Fisiche e Naturali,Strada Le Grazie 15, I-37138 Verona, Italy

Received January 21, 2005; accepted May 8, 2005

SINBAD (Synchrotron Infrared Beamline At DA�NE) is the first Italian synchrotron radiation beamline op-erating in the infrared range. It collects the radiation emitted by DA�NE, an electron–positron collider de-signed to work at 0.51 GeV with a beam current I�1 A. The actual performances of the beamline, in terms ofbrilliance gain with respect to blackbodies and polarization properties, are presented and discussed. Finally,the stability of the SINBAD source, a critical issue for Fourier-transform infrared spectroscopy, is discussed.

© 2005 Optical Society of America

OCIS codes: 340.6720, 260.3060, 220.2740, 260.5430.

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. INTRODUCTIONnterest in infrared synchrotron radiation (IRSR) datesack to the mid-1980s1: currently, IRSR is widely ac-epted as an advantageous source for all spectroscopic ap-lications requiring a high signal-to-noise ratio. Indeed,ynchrotron radiation (SR) combines the advantages ofigh brilliance, natural polarization, and absence of ther-al noise, with the broadest emission band among all ra-

iation sources.2 The above-cited advantages of IRSR ap-ly to a large choice of experimental techniques andnvestigations. Among them are high-pressure studies,

icrospectroscopy in solid-state physics, earth sciencend biology, reflectance and absorption spectroscopy forurface science and chemistry, ellipsometry, and time-esolved spectroscopy. A number of IRSR beamlines areow routinely experiencing the advantages of IRSR.3,4

A beamline dedicated to IRSR was proposed in 19935

hen a new ring in Frascati was still in project and builtp in 2001 at DA�NE (Double Annular �-factory for Nicexperiments); this electron–positron collider has been de-igned to run high-energy physics experiments. The firstight at SINBAD (Synchrotron INfrared Beamline At

1084-7529/05/122810-8/$15.00 © 2

A�NE) was observed in December 2001. After a periodf tests, the beamline commissioning was successfullyompleted in 2003. Since then, several experiments, in-luding those supported by European Union programs,ave been performed successfully.In this paper we present the characteristics of the op-

ical layout of SINBAD and a comparison between the ex-ected performances and the experimental data. Theain parameters of the IRSR source, as well as the opti-

al layout of SINBAD, are briefly recalled in Section 2. Inection 3, data on the brilliance and polarization of theRSR provided by SINBAD at the sample position are pre-ented and discussed. Finally, Section 4 is devoted to theiscussion of the stability of the IRSR source in both para-itic and dedicated runs.

. DA�NE SOURCE AND THE SINBADAYOUThe DA�NE design is based on conventional technology.6

he characteristics of the ring and the main parametersave been extensively described in several reports,7–10

nd DA�NE as a source of SR has been described in Refs.

005 Optical Society of America

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Cestelli Guidi et al. Vol. 22, No. 12 /December 2005 /J. Opt. Soc. Am. A 2811

1 and 12. We report in Table 1 the parameters of the col--ider of significance for the IRSR source. Detailed and up-ated information can be found in Ref. 10.The storage ring lattice of the achromats is a four-

eriod modified Chasman–Green type with a 1.8 T con-entional wiggler magnet inside.13 The electron-beamifetime is shortened to achieve the highest luminosity foruclear experiments, and topping-up injections from anccumulator ring are regularly performed every 25 min.he top-up operation is also useful for SR experiments as

t provides an actual infinite beam lifetime. Unavoidableeam instabilities during the injections are not too harm-ul for most infrared spectroscopy applications, which re-uire high current values and short acquisition times.his procedure is still unusual on dedicated SR storageings, but DA�NE has been designed to optimize a refill-ng procedure, which usually takes less than 3 min forach ring. During the refilling procedure the radiation istill available, but particle beam instabilities prevent ef-ective data acquisition (see Section 4).

The angular dimension of the source is 20�30 �HV� mrad. The maximum vertical acceptance 2� is lim-

ted to 30 mrad because of the presence of a fast shutterlaced in the front end. The distribution of the emitted ra-iation, sampled with a step ��=2 mrad along the verti-al direction in the far-infrared region, is shown in Fig. 1.

Table 1. Relevant Parameters of the DA�NEElectron Ring

Parameter Mean Reference Value

article beam energy 0.51 GeVax e− current �2 Aending radius 1.4 mending magnet field 1.2 Tritical energy 208 eVynchrotron radiation loss per turn 9.3 keVverage refilling time 3 minverage time between injections 30 minumber of electron bunches 100adial emittance 0.8 mm·mradnergy spread 0.04%

ig. 1. Angular vertical distribution of the IRSR emission, aseasured at 200 cm−1 through a slit that selects ��=2 mrad in

he vertical plane and a 20 mrad angle in the horizontal plane.he curve is a guide to the eye.

he horizontal collecting angle is limited to 20 mrad, aalue that optimizes the brilliance of the infrared sourceccording to ray-tracing simulations.14

The electron-beam emittance of DA�NE affects bothhe intrinsic divergence and the source size. Indeed, bothhe intrinsic size ��xy� and the divergence ���� of the SRource significantly increase from the near-infrared (at �1 �m, �x=2 mm, �y=0.3 mm, and ��=10 mrad) up to

he far-infrared region (at �=100 �m, �x=2 mm, �y0.7 mm, and ��=100 mrad). SINBAD was designed toxtract infrared radiation from a bending magnet of thelectron ring. Starting from the characteristics of DA�NEs a SR source, the original design of SINBAD was simu-ated using the SHADOW ray-tracing routine.15 Theiffraction-limited source size contribution, not negligiblen the infrared domain, has been included ad hoc in theimulation.16

The SINBAD optical layout is sketched in Fig. 2; theesign results from technical considerations as well asrom the need to circumvent the installations in theA�NE hall and to use the available experimental hall.long �25 m there are six gold-coated mirrors,16 whoseptical parameters are reported in Table 2. The beamlineront end is �3 m long, so the extraction mirror (M1) islaced 4.5 m apart from the source, which is selected at1.175° from the origin of the bending magnet. Four inde-endent, remotely controlled slits are placed at 2.25 mrom the source, selecting both the flux and the polariza-ion of the collected radiation. Mirror M1 deflects it by 55°n the horizontal plane toward an ellipsoidal mirror (M2),laced 0.7 m apart from M1. The radiation reaches M2nder an incidence angle of 40°, and it is focused at 2.38

from the M2 pole. The presence of an intermediate fo-us originated by mirror M2 is essential in our design toptimize the transfer of IRSR to the experimental area.therwise the residual divergence of the radiation beamropagating along the 25 m long pipe would producetrong optical aberrations on the final image.

SHADOW simulation identified the best layout forINBAD as that made of four parabolic off-axis mirrors.17

owever, both technical and economical considerationsuggested a combination of two plane and two toroidalirrors (see Table 2), which still ensured acceptable ab-

rrations in the final spot. As a consequence, M3 is a tor-id placed 0.8 m apart from the focus of the ellipsoid,hich deflects the radiation by 90° in the vertical plane.

ig. 2. Optical layout of SINBAD. The parameters of the sixirrors are summarized in Table 2. DW, diamond window.

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2812 J. Opt. Soc. Am. A/Vol. 22, No. 12 /December 2005 Cestelli Guidi et al.

he IRSR beam leaving M3 propagates almost as a planeave in both the horizontal and the vertical plane, which,

hrough plane mirrors M4 and M5, is transferred to thexperimental area. At the end of the beamline, a toroidalirror (M6) deflects the radiation by 60° and focuses it at

.5 m from its pole on the pupil of the interferometer. Thisatter is occupied by a chemical vapor deposition wedgediamond window, with a wedge angle of 1.1°, which sepa-ates the different vacuum regimes of the beamline and ofhe interferometer. All mirror adjustments are remotelyontrolled and seven cameras monitor the visible lightlong the SINBAD path. The final spot is affected byoma aberrations mainly in the horizontal plane producedy the toroids M3 and M6.18 The visible spot at the en-rance of the interferometer has an area of �2 mm2, andts divergence well matches the f-number of 4 of the inter-erometer. This latter is a Bruker Equinox 55, suitablyodified to work under vacuum, which covers a spectral

ange from 10 through 15,000 cm−1 with a maximumpectral resolution of 0.5 cm−1. A port of the interferom-ter hosts a Bruker IRscope I microscope, operating in theear- and mid-IR range.

. BRILLIANCE AND POLARIZATION OFINBAD RADIATIONhe most noteworthy figure of merit for an IRSR beam-

ine is its advantage in brilliance with respect to a con-entional source [actual brilliance ratio (ABR)].19 Previ-us theoretical evaluations of the ABR at SINBAD werebtained from standard SR emission formulas correctedor the actual size of the SR source.11,12,14 Once the beam-ine layout was chosen, a more realistic ABRcal was evalu-ted at a circular aperture of diameter d placed at theample site by taking into account the transmittance ofhe optical system. This transmittance was then com-ared with that of a conventional source.ABRcal can be directly compared with experimental

ata.16 To do this, we measured versus frequency the in-ensity of the IRSR transmitted through a pinhole placedt the sample site of the interferometer, ISR� ,d�, andhat transmitted by a conventional source through theame aperture, Ic� ,d�. We thus obtained

ABRexp�,d� =ISR�,d�

Ic�,d�. �1�

n the range between 50 and 600 cm−1 the conventionalource was a mercury lamp, in the mid-infrared

Table 2. SINBAD

MirrorDistancea

(cm)Incidence Angle

(deg)

M1 450 27.5M2b 70 40M3 80 45M4 50 45M5 312 72.5M6 1300 30

aThe distance in the second column is referred to the sfollowing mirrors.

bThe M2 optical parameters are the semimajor and sem

600–5000 cm−1� a globar. All IRSR data and calculationsere normalized to a beam current of 1 A. Figure 3 showsBRexp� ,d� in the far-infrared region for several valuesf d. Data were collected by using a silicon bolometer op-rating at 4.2 K. As shown in Fig. 3, the advantage withespect to a mercury lamp increases below 200 cm−1 andeaches its maximum value at the smallest aperture �d300 �m�. Below 100 cm−1 an intensity cutoff is clearlyetected, which is due to diffraction effects as interferenceringes appear in the spectra. ABRexp� ,d� would furtherncrease in the very-far-infrared region (below 100 cm−1)f, in addition to the conventional one-particle SR emis-ion, coherent emission from the electron bunch could bexploited under peculiar conditions.20–26 In the inset ofig. 3 we note that the integrated intensity of IRSR doesot scale as the pinhole area for apertures of the order ofmm.ABRexp� ,d�, as measured in the mid-infrared range

or several values of d, is reported in Fig. 4. As for anyRSR source, the ABR advantage in this spectral region isot comparable with that achieved in the far infrared be-ause a blackbody at 1500 K has its maximum efficiencyetween 2000 and 3000 cm−1, while around 2000 cm−1

ig. 3. Intensity ratio ABRexp between the SR of SINBAD and aercury lamp, as measured in the far infrared at the sample po-

ition through circular apertures of different diameters d. Allurves have been normalized to a circulating current of 1 A. Inhe inset the intensity I0, as integrated over the spectral rangehown in the figure, is reported versus d for both the IRSRcircles) and the conventional source (squares).

ical Parameters

gureMajor Radius

(cm)Minor Radius

(cm)

epsoidal 379 269idal 226 113eeidal 116 87

cation for M1 and to the previous optical element for the

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Cestelli Guidi et al. Vol. 22, No. 12 /December 2005 /J. Opt. Soc. Am. A 2813

here is the absorption of the diamond window. The aber-ations of the optical system also play a role. In particu-ar, the mercury cadmium telluride detector in the inter-erometer (with an area of 1 mm2) is enlightened by anff-axis ellipsoid with a larger demagnification that en-ances the aberrations of the IRSR spot, an effect that be-omes negligible only when small apertures are used,ince they act as spatial filters for the radiation. The ab-rration effect becomes negligible in the far-infraredange since a Winston cone is used to enlighten the detec-or in this spectral region. Therefore use of different de-ectors and different optics for the mid- and far-infraredange explains why ABRexp� ,d� data in Figs. 3 and 4 doot superimpose in the same spectral region.In Fig. 5 we compare ABRexp� ,d� at two different en-

rgies, namely, 1=200 cm−1 and 2=5000 cm−1, with theBRcal� ,d� reported in a previous paper.16 As one canee, at SINBAD, the experimental and theoretical dataersus the dimension of the circular aperture d are in aood agreement with the former ones, providing resultshat are even better than predicted by the simulation. Its worth noticing that the simulation is based on a conser-ative approach, as it provides a lower limiting value forhe actual ABR.

The ABR advantage obtained in the far-infrared regions particularly helpful for experiments with diamond an-il cells (DACs). Indeed, high-pressure experiments withDAC require samples of very small size. In Fig. 6 we

ompare the intensity of the IRSR transmitted through a00 �m wide and 5 �m thick film of manganese oxide,oused in a DAC, with that transmitted by the sameample with a conventional source. The IRSR intensityhrough the diamonds and the film is more than 20 timesigher than the intensify of the mercury lamp. Thereforehe absorption spectrum obtained by IRSR exhibits theetails of three well-defined phonon lines, while even

ig. 4. Intensity ratio ABRexp between the SR of SINBAD and alobar, as measured in the mid-infrared at the sample positionhrough circular apertures of different diameters d. All curvesave been normalized to a circulating current of 1 A. In the insethe intensity, integrated over the spectral range shown in the fig-re, is reported versus d for both the IRSR (circles) and the con-entional source (squares).

hese lines are hardly seen in the spectrum collected withhe Hg lamp.

A second figure of merit to be mentioned is the polar-zation degree. SR exhibits in the whole spectral range aigh degree of both linear and circular polarization. Theadiation emitted in the plane of the orbit by a relativisticarticle is linearly polarized, with the electric field A� ly-ng in the plane of the particle’s orbit. When SR is col-ected within a vertical angle �, the off-axis electric fieldas a component A orthogonal to A� that provides theadiation with elliptical polarization of opposite helicityor ��0 or ��0. In the limit of a large off-axis angle (de-ending on the wavelength), the observed radiation tendso become circularly polarized.

The polarization properties of a light source are gener-lly described in terms of the polarization degree, or byeans of the Stokes coefficients.27 The linear polarization

egree � can be easily measured, being defined as

ig. 6. Intensity transmitted through a La0.8Ca0.2MnO2.92ample housed in a DAC. Spectra are collected with IRSR (con-inuous curve) and Hg lamp (dashed curve). The arrows indicatehe phonon absorptions.

ig. 5. ABRexp (circles) is reported versus the aperture diameterand compared in the mid-infrared and the far infrared with theBRcal (dashed curves). The theoretical data are from Ref. 16.olid curves are a guide to the eye.

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2814 J. Opt. Soc. Am. A/Vol. 22, No. 12 /December 2005 Cestelli Guidi et al.

�L =I� − I

I� + I

, �2�

here I�, �A�,�2. Since we are interested in the actualegree of polarization available for experiments, we mea-ured �L at the sample site of the interferometer, thus in-luding any possible loss of polarization due to the beam-ine optics. With the acceptance angle set at its maximumalue �20�30 mrad�, both linear and circular polarizedight are collected. However, as the degree of circular po-arization is antisymmetric with respect to �, there is noircular polarized radiation when a symmetric emissionith respect to the plane of the orbit is collected.The experimental �L measured in the far-infrared re-

ion for the maximum acceptance angle is reported in Fig.. We note that �L is high and, as expected, increaseshen the solid angle is reduced by the remotely controlled

lits placed on the front end. Data also show that, in theery far infrared (below 200 cm−1), the linear polarizationeems to increase, possibly due to the effect of diffraction.n the inset of Fig. 7 we compare the �L measured for ver-ical angles of 30 and 10 mrad, symmetrical with respecto the orbit plane, with the values provided by ray-tracingimulations. A reasonable agreement is obtained.

. REPRODUCIBILITY OF THE EMISSION INHE INFRARED DOMAIN

n experiments on samples of small size, the advantage ofRSR in terms of brilliance may be limited by the pres-nce of instabilities of the source that affect the reproduc-bility of the data. These instabilities may concern thelectron-beam position, its transverse dimension, and itsngular deviation with respect to the standard orbit.28

nlike the natural decrease of the beam current, to whichhe collected interferograms can be easily normalized byn automatic procedure,29 those instabilities randomly af-ect the interferogram baseline, if active on short times,nd the reproducibility of spectroscopic data if effective ononger time scales. Beam instabilities of both types haveeen clearly detected at SINBAD and are basically re-ated to DA�NE operations, a low energy e+/e− colliderhat is continuously optimized for the high-energy par-icle experiments. Currently, IRSR experiments run only

ig. 7. Linear polarization degree �L of IRSR at the sample siten the far-infrared region. The inset shows a comparison betweenay-tracing simulation results and experimental data.

arasitically. Therefore most reproducibility data pre-ented next refer to parasitic runs during high-energy ex-eriments, even if data taken in a limited number of dedi-ated runs are also presented and discussed.

In a first approximation one can assume that the stan-ard deviation � of the experimental data, when repeat-dly taken in the same (apparent) conditions, is given by

�2 = �02 + �2�t�, �3�

here �0 is a limiting value fixed by the optical, elec-ronic, and mechanic performances of the interferometer,nd �2�t� is a time-dependent term accounting for the un-ertainty induced by the dynamics of the photon beam. Inddition to the decrease of the current I�t�, two terms con-ribute to the particle (and photon) beam change in time:unch length oscillations and displacements of the beams a whole. Therefore we can assume that

�2�t� =A

�I�t��+ �d�t�2 + �b�t�2, �4�

here A is a constant, �I�t�� is the time-averaged value ofhe current in the acquisition time, �d�t� accounts for thencertainty due to displacements of the beam orbit, andb�t� accounts for the bunch dynamics. At SINBAD, theain contributions to �d�t� and �b�t� in Eq. (4) are on a

ime scale of a few minutes at least, much larger than thecquisition time of a single interferogram.30 Therefore itsaseline is not affected and the reproducibility can betudied by comparing interferograms taken with delays �f several minutes at least from each other. To determinehe short- and long-term reproducibility of our data, weompare interferograms taken in the same run betweenwo injections in the electron ring (i.e., with a delaymaller than the refilling repetition time of �25 min) andn different injections (delay much larger than this timenterval), respectively. The reproducibility of the collectedpectra is determined by

ig. 8. Ratio between two spectra taken with a delay � of (a) 2in and (b) 20 min in the mid-infrared region.

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Cestelli Guidi et al. Vol. 22, No. 12 /December 2005 /J. Opt. Soc. Am. A 2815

R�,�� =I2�,��

I1�,0�, �5�

here I2 and I1 are two spectra taken with a delay � of aew minutes, one with respect to the other, in the sameun between two refillings of electrons. In this way wehould obtain the so-called 100% line. Once we take intoccount the reproducibility of the measuring apparatusith conventional sources (1.000±0.005 in the mid-

nfrared and 1.00±0.01 in the far infrared), deviationsrom that ideal line measure the lack of reproducibilityue to beam instabilities. The results for �=2 min and 20in are shown in Fig. 8 for the mid-infrared and in Fig. 9

or the far infrared. Data for �=20 min were taken beforend after a topping-up procedure, with exactly the sameurrent in the ring, but in both cases with no positrons inhe other ring. The positron current was indeed found totrongly reduce the reproducibility of the IRSR extractedrom the electron ring. We note that, surprisingly, R� ,��or �=2 min is systematically larger than one in the far-nfrared. This effect can be better seen by selecting, withnarrow slit in front of the bending magnet, a small ver-

ical angle of emittance ��=2 mrad, symmetric with re-pect to the orbital plane. In this way, by reducing the ap-rture, the space fluctuations of the electron beam becomeegligible. The evolution in time, while the circulatingurrent I is decreasing, is shown in Figs. 10 and 11 for thear and mid-infrared, respectively. Again, in both regionshe intensity of the spectra steadily increases as I is re-uced. This might be attributed to a squeezing of the elec-ron bunches as they lose electrons, which leads to an en-ancement of the brilliance in the far-infrared region.ndeed, the brilliance is proportional to the charge den-ity of the electron beam, roughly expressed by the ratio/�x�y, where N is the number of electrons per bunch

nd �x,�y is the beam cross section, which decreases as aunction of the current. In the insets of Figs. 10 and 11,

ig. 9. Ratio between two spectra taken with a delay � of (a) 2in and (b) 20 min in the far-infrared region.

� ,�� is also reported for the selected angle of 2 mrad.emarkably, in the far infrared (Fig. 10) the spectralhape is not affected by density fluctuations, while in theid-infrared above 2000 cm−1 (Fig. 11), all spectra are su-

erimposed, and the squeezing effect on the beam doesot seem to be effective.

. CONCLUSIONhe main figures of merit of the beamline SINBAD haveeen reported in the present paper. The high current andhe low energy of DA�NE make it an ideal source forRSR, even if limitations arise from the present need toork in parasitic mode during high-energy physics ex-eriments. SINBAD works from the visible down to the

ig. 11. Mid-infrared spectra collected through ��=2 mradhile the current is decreasing in the electron ring. In the inset

he ratio between the spectrum collected at 800 and 1500 mA ishown.

ig. 10. Far-infrared spectra collected through a vertical angle�=2 mrad while the current is decreasing in the electron ring.

n the inset the ratio between the spectrum collected at 800 and500 mA is shown.

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2816 J. Opt. Soc. Am. A/Vol. 22, No. 12 /December 2005 Cestelli Guidi et al.

erahertz region, and the gain in the delivered intensityith respect to a blackbody reaches its maximum value

by a factor of 40) in the far-infrared domain for a sampleize of �300 �m. These characteristics open excellent op-ortunities for spectroscopic studies where high brilliances required and make SINBAD attractive for experimentsn the low-energy domain.

In the mid-infrared range, where the gain in brilliances not as favorable as at longer wavelengths, the SRource can be usefully employed for infrared microspec-roscopy. A more detailed discussion on the exploitation ofR in the mid-infrared region and the solutions will beiscussed in a forthcoming paper.Polarization data show that at SINBAD the SR beam

n the far-infrared region is strongly linearly polarized,ven more than predicted by ray-tracing simulations.

Finally, the short-term reproducibility of the interfero-rams is definitely good, while the long-term one is notatisfactory, due to frequent injections, when working inarasitic runs. Nevertheless, in the first year of SINBADperation, many experiments have been performed suc-essfully during parasitic shifts, which have exploitedINBAD’s high brilliance and low noise in the far

nfrared.31–36

CKNOWLEDGMENTShe authors acknowledge all those who since the earlyays have contributed to the design, the installation, andhe commissioning of SINBAD at DA�NE. Among theme mention D. Cannavò, G. Cinque, A. Grilli, E. Pace, A.aco, and M. Sanchez del Rio. We acknowledge the Elec-

ronic Pool of the Laboratori Nazionali di Frascati forechnical support and A. Mondini, P. Morini, and D. Salif Bruker Optics. This work has been partially supportedy the European Union Transnational Access to Researchnfrastructure contract HPRI-CT-1990-00088 and EU I3ontract RII3-CT-2004-506078.

Corresponding author’s e-mail address isariangela.cestelli@lnf.infn.it.

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