APS undulator radiation—first resultsZ. Cai, R. J. Dejus, P. Den Hartog, Y. Feng, E. Gluskin, D. Haeffner, P. Ilinski, B. Lai, D. Legnini, E. R. Moog, S.Shastri, E. Trakhtenberg, I. Vasserman, and W. Yun Citation: Review of Scientific Instruments 67, 3348 (1996); doi: 10.1063/1.1147262 View online: http://dx.doi.org/10.1063/1.1147262 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/67/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Recent results in photoionization of atoms and ions using undulator radiation AIP Conf. Proc. 576, 711 (2001); 10.1063/1.1395407 High accuracy momentum compaction measurement for the APS storage ring with undulator radiation AIP Conf. Proc. 546, 234 (2000); 10.1063/1.1342591 First results of the circularly polarized undulator beamline at BESSY II AIP Conf. Proc. 521, 134 (2000); 10.1063/1.1291773 Advantages of using a mirror as the first optical component for APS undulator beamlines Rev. Sci. Instrum. 67, 3353 (1996); 10.1063/1.1147398 Undulator radiation Am. J. Phys. 64, 662 (1996); 10.1119/1.18264

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Rev. Sci. Instrum. 67 (9), September 1996 © 1996 American Institute of Physics 1

APS undulator radiation - first results

Z. Cai, R.J. Dejus, P. Den Hartog, Y. Feng, E. Gluskin, D. Haeffner, P. Ilinski, B. Lai,D. Legnini, E.R. Moog, S. Shastri, E. Trakhtenberg, I. Vasserman, and W. YunAdvanced Photon Source, Argonne National Laboratory, Argonne, IL 60439

(Presented on 19 October 1995)

The first undulator radiation has been extracted from the Advanced Photon Source (APS). The resultsfrom the characterization of this radiation are very satisfactory. With the undulator set at a gap of15.8 mm (K=1.61), harmonics as high as the 17th were observed using a crystal spectrometer. Theangular distribution of the third-harmonic radiation was measured, and the source was imaged using azone plate to determine the particle beam emittance. The horizontal beam emittance was found to be6.9 + 1.0 nm-rad, and the vertical emittance coupling was found to be less than 3%. The absolutespectral flux was measured over a wide range of photon energies, and it agrees remarkably well withthe theoretical calculations based on the measured undulator magnetic field profile and the measuredbeam emittance. These results indicate that both the emittance of the electron beam and the undulatormagnetic field quality exceed the original specifications. © 1996 American Institute of Physics.

I. INTRODUCTION

The 7-GeV Advanced Photon Source (APS) at ArgonneNational Laboratory is a third-generation synchrotron-radiation source, designed to provide x-ray beams of highbrilliance using undulators.1 It is well known that, at agiven current and energy of the particle beam in the storagering, the brilliance of the x-ray beam from an undulator relieson a low emittance of the particle beam and a high quality ofthe undulator.2 As the APS begins operation withundulators, measurements of the particle beam emittance andthe spectral flux of undulator beams are an essential part ofthe commissioning process.

Characterization of the radiation from the first installedUndulator A has been carried out during the commissioningof the 1-ID beamline at the APS. This paper reports theresults of absolute spectral-flux measurements of theundulator radiation and the results of emittance measurementsof the stored electron beam using the undulator radiation. Themeasured absolute spectral flux agrees quantitatively with atheoretical calculation of the undulator brilliance based on themagnetic measurements of the undulator. Also, the emittancederived from the measured beam parameters indicates that theemittance of the stored electron beam has well exceeded theinitial design specifications.

II. UNDULATOR A

Undulator A is a planar insertion device capable ofgenerating high intensity x-rays in the spectral range from2.9 keV to 60 keV by using the first, third, and fifthharmonics of radiation. It has a 3.3-cm period and is of ahybrid design (consisting of Nd-Fe-B magnets with vanadiumpermendur poles) with 72 periods and is 2.4 m in length.The device has been optimized for the APS so that thevariation in brilliance is small when tuning from oneharmonic energy to the next.3 This was achieved bymaximizing the magnetic field at a given gap and by allowinga smaller minimum gap when installed in the storage ring.The characteristics of the Undulator A are listed in Table I.

TABLE I. Undulator A parameters and specifications.

Parameter Value

magnet material Nd-Fe-Bpole material Vanadium permendurperiod length 3.30 cmnumber of periods 72length 2.4 mminimum gap 10.5 mmminimum range of gap taper 0 to 2 mraddeflection parameter, Keffa 2.76maximum field, Beffa 0.896 Tfirst harmonic energyb 2.9 keVrms peak magnet field errorc < 0.5%rms phase errord < 8°

a Measured at the minimum gap of 10.5 mm.

b Predicted at the minimum gap of 10.5 mm.

c Specified at a gap of 11.5 mm.

d Specified at a gap of 11.5 mm, actual device < 5.4° atall gaps.

The undulator was designed, built, and tuned by STIOptronics. The original magnetic field quality specifica-tionswere: a first field integral that varies by no more than100 G-cm vertically and 50 G-cm horizontally through theentire range of gaps; horizontal and vertical second fieldintegrals that vary by no more than 105 G-cm2 through theentire range of gaps; and a phase error of less than 8° at allgaps. The undulator, as delivered by STI, more than metthese demanding requirements. However, in order to furtherreduce the effect the undulator has on the closed orbit of theelectron beam in the storage ring, the gap dependence of thefield integrals was reduced by additional fine tuning of theundulator's magnetic field at the APS. Final magneticmeasurements found first field integral variations of 38 G-cmin the vertical direction and 21 G-cm in the horizontal, secondfield integral variations of 6.2×104 G-cm2 in the verticaldirection and 1.1×104 G-cm2 in the horizontal, and a phase

This article should be cited as Rev. Sci. Instrum. 67, 3348 (1996).

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2 © 1996 American Institute of Physics Rev. Sci. Instrum. 67 (9), September 1996

error less than 5.4°. As a result, the closed orbit displace-ments in both the vertical and horizontal directions weremeasured to be less than 0.5% of the beam size over the gaprange from 15.7 to 40 mm. The tune shift was measured tobe as small as 100 Hz, almost the same as one would expectfrom an ideal device.

III. ABSOLUTE SPECTRAL FLUX MEASUREMENT

A gas-scattering spectrometer and a crystal spectrometerwere used to measure the absolute spectral flux of theundulator radiation. The gas-scattering spectrometer measuresthe x-rays scattered at 90° from a well-defined helium gasvolume at a controlled pressure.4 The spectrum of thescattered radiation is recorded using an energy-dispersiveSi(Li) detector and a calibrated multichannel analyzer (MCA).By using scattering from a gas, this method makes absolutemeasurements possible at a high stored beam current. Theparameters of the scattering geometry, such as the detectorsolid angle and the scattering volume, can be varied toaccommodate a wide range of beam current and to get anappropriate count rate of the detector. Alternatively, thecrystal spectrometer can measure the spectral flux to higherenergies with better resolution than is possible with the gas-scattering spectrometer.5

The experimental setup is schematically shown in Fig. 1.Upstream from the two spectrometers, a water-cooled conicalpinhole with an exit diameter of 0.8 mm was used to removea substantial fraction of the undulator power from the beam.Vertical and horizontal water-cooled slits made of 1.4-mm-thick molybdenum were located at 32.8 m from the source,50 mm downstream from the pinhole. The slit size, whichdefined the angular acceptance of the incident beam, was foundto be 240 µm horizontally and 122 µm vertically, bymeasuring the size of the x-ray beam after the slits. In orderto collimate the bremsstrahlung radiation, a 100-mm-thicktungsten block with a 5-mm-diameter hole was placeddownstream from the slits. The pinhole, slits, and thetungsten collimator were placed in a helium enclosure andwere carefully shielded with lead.

Helium gas is used in the gas-scattering spectrometerbecause of its small coherent-scattering cross section forenergies above 10 keV at the 90-degree scattering angle. Italso minimizes the energy broadening due to the electron-momentum distribution (Compton profile). Thus themeasured data are easier to interpret. A Si(Li) detector wasmounted vertically 222 mm from the beam. A 1-mm-thicktungsten slit of size 3.0 mm was placed in the scatteringpath, 31.5 mm from the incident beam, to define the lengthof the scattering volume. The efficiency of the detector(EG&G ORTEC SLP 06165PS) was determined by calcu-lating the fraction of x-rays absorbed in a 3.2-mm-thickSi(Li) sensitive volume. The radial distribution of theefficiency across its active surface was effectively eliminatedby the use of a 3-mm diaphragm in front of the detector. Theabsorption due to the detector's beryllium window and frontaldead layers and the escape peak corrections were not taken into

FIG. 1. Illustration of the experimental setup for absolutespectral-flux measurements.

account because they are negligible for energies above 6 keV.Detector efficiency was calibrated at 6 keV. The energy rangeof the spectrometer is from 5 to 45 keV.

The crystal spectrometer employs a Si(111) crystal and anion chamber detector filled with nitrogen gas. The singlereflection geometry makes spectral measurements quiteinsensitive to thermal distortion of the crystal in the sensethat the integrated reflectivity of the crystal can be calculatedby the dynamical diffraction theory even in the presence of themoderate thermal bump. The efficiency of the ion chamberwas determined by calculating the fraction of x-rays absorbedin a 100-mm active path. The silicon crystal was charac-terized using surface topography and x-ray diffraction and wasfound to be a perfect crystal.

The measurements were performed with an electron beam of7.0-GeV energy and about 1-mA current, and the resultsshown in this report were obtained at a undulator gap of15.8 mm (K=1.61). Figure 2 shows the on-axis spectralbrilliance of the undulator from 4 to 40 keV derived from theabsolute spectral flux measured using the gas-scatteringmethod and the measured beam emittance (discussed later).The first six harmonics are clearly visible. The measuredspectrum is a convolution of the incident spectrum with theenergy broadening due to the detector response function, theCompton profile, and the finite detector acceptance angle.Deconvolution to obtain the initial undulator spectrum islimited by the noise present in the data. In order to evaluatethe performance of the undulator, the initial spectrum wascalculated using the measured magnetic field of the undulatorat the same gap, the measured electron beam emittance, andan electron beam energy spread of 0.1% (design value). Tocompare with the experimental results, the calculatedspectrum was further convolved with the broadening functionmentioned earlier. The resulting spectral brilliance is shownin Fig. 2. As is clearly seen from the figure, themeasurements are in remarkably good agreement with thecalculation. The small peaks located at 8.16 keV and9.9 keV are due to copper flourescence from the conicalpinhole.

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Rev. Sci. Instrum. 67 (9), September 1996 © 1996 American Institute of Physics 3

FIG. 2. Measured on-axis spectral brilliance (solid line), usingthe gas-scattering spectrometer, and calculated on-axis spectralbrilliance (dotted line) of the undulator radiation from a 7-GeVelectron beam at a gap of 15.8 mm (K=1.61). The calculationincluded the measured magnetic field of the undulator, themeasured electron beam emittance (εx=6.9 nm-rad, εy=0.2 nm-rad), and the design value for the electron beam energy spread(0.1%). The calculated spectral brilliance was further convolvedwith the energy broadening functions mentioned in the text. Themeasured spectrum has been corrected for the Compton shift.

FIG. 3. Measured on-axis spectral brilliance (solid line), usingthe crystal spectrometer, and calculated on-axis spectralbrilliance (dotted line) of the undulator radiation from a 7-GeVelectron beam at a gap of 15.8 mm (K=1.61). The calculationincluded the measured magnetic field of the undulator, themeasured electron beam emittance (εx=6.9 nm-rad, εy=0.2 nm-rad), and the design value for the electron beam energy spread(0.1%).

Figure 3 shows the on-axis undulator spectral brillianceobtained using the crystal spectrometer, together with theresult of a similar calculation. This calculation, however,does not include convolutions with the energy broadeningfunctions, which are either negligible or not applied in thismeasurement. The contributions from higher order reflectionsof the crystal were removed from the measured spectrum.Once again, a good agreement is obtained between measuredand calculated spectra. Note that the peak brilliance in Fig. 2

is lower than that in Fig. 3, a consequence of the largerinstrumental broadening associated with the gas-scatteringspectrometer. Fig. 4 shows the spectrum of the undulatorradiation from 40 to 110 keV measured using the crystalspectrometer. Higher order harmonics up to the 17th areclearly observed.

The accuracy of the brilliance measurement depends on theuncertainties associated with the flux and theemittance measurements. The uncertainty in the fluxmeasurement arises from uncertainties of the x-raytransmission of the filters and the air path, the detectorefficiency, the entrance slit size, and the beam current. Forthe measurement using the gas-scattering spectrometer, it alsoinvolves the uncertainties of the helium gas density, thedetector collecting angle, and the photon counting statistics.The uncertainties of the measurement at 6.1 keV (firstharmonic) are listed in Table II. The total rms uncertainty isfound to be less than 19% for both methods.

FIG. 4. Measured on-axis spectral flux of the undulator radiation,using the crystal spectrometer, from a 7-GeV electron beam at agap of 15.8 mm (K=1.61). Undulator harmonics up to the 17thare clearly visible.

TABLE II. The rms uncertainties in percentage of the undulatorradiation brilliance measurement at the first harmonic (6.1 keV).

Terms

CrystalSpectrometer

Gas-Scattering

Spectrometer

Thickness of windows & air path 6.1 2.5Detector efficiency 2.3 2.9Entrance slit size 2.2 2.2Emittance 17.3 17.3Electron beam current 0.3 0.3Crystal energy bandwidth 2.8Helium gas density 0.8Scattering volume and detector

solid angle2.9

Statistics 0.9Total 18.9 18.2

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4 © 1996 American Institute of Physics Rev. Sci. Instrum. 67 (9), September 1996

IV. EMITTANCE MEASUREMENTS

The particle beam emittance was determined by measuringthe x-ray beam size at a selected photon energy. Themeasured vertical beam size σm at a distance D from thesource can be approximated by

σm

2 = σy2 + (Dσ ′ y )2 + (Dσ ′ r )2 + (αy /3) 2 , (1)

where σm, is the rms size of the x-ray beam at a distance Dfrom the source, σy and σy′ are, respectively, the particlebeam size and the particle beam divergence, and σr′ is theundulator radiation opening angle. The parameter ayrepresents the scanning slit width and ay / 3 is an approxi-mation of a square function by a Gaussian function. Giventhe betatron function, βy, at the center of the straight section,the particle beam size and divergence are then only functionsof the emittance:

σy = εyβy

σ ′ y = ε y / βy (2)

For βy=10 m and D=32.8 m, the photon beam size is

dominated by the particle beam divergence rather than by theparticle beam size. In order to determine the emittanceaccurately, proper wavelength selection should be made sothat σr′ < < σy′. For a single electron, σr′ can be approxi-

mated by σ ′ r ≅ λn /2L at the peak of the nth harmonic

(n-odd), where λn is the wavelength of the harmonic and L is

the length of the undulator (2.4 m). This suggests thatundulator radiation from higher harmonics should be used if

λ1 /2 L >σ ′ y . Also, a small "blue shift" of the third

harmonic peak gives a substantially narrow vertical radiationdistribution, about 3.3 µrad in our case, similar to what has

been utilized recently with "red-shifted" radiation.6 Figure 5shows the x-ray beam profile measured by scanning the wholesetup shown in Fig. 1 in the vertical direction. The crystalwas tuned to diffract 18.37-keV x-rays, where the on-axis fluxis about half of that at the peak of the third harmonic(18.30 keV). In order to determine the vertical emittance, thesingle electron radiation profile at the chosen energy was

calculated using the Bessel-function approximation7 andfurther convolved numerically with the electron beamemittance to fit the measured radiation profile. From themeasured vertical beam profile (σm = 0.179 mm), we

obtained a best fit with a vertical emittance value of εy=0.20

+ 0.04 nm-rad for a βy of 10.0 m in the straight section,

corresponding to σy = 45 µm and σy′ = 4.5 µrad.

Using a similar procedure for the measured horizontal beamprofile, the horizontal emittance was fitted to be 6.9 + 1.0 nm-rad, corresponding to σx=346 µm and σx′=20 µradfor a βx value of 17.3 m. Thus, the emittance coupling(εy / εx) is less than 3%, considerably less than the designvalue of 10%.

The measurement of the source size σx and σy wasconfirmed by a separate experiment in which the particlebeam was imaged by a zone plate onto a CCD x-ray camera.A horizontally deflecting Si(111) crystal was tuned to diffractthe first harmonic of the undulator beam at 7.8 keV(gap=18.2 mm), followed by a high resolution zone plate(0.25-µm minimum linewidth) of 1-m focal length. TheCCD camera was placed at the focal plane of the zone plate tocapture the demagnified image of the source. Compared to apinhole camera setup,8 the zone plate method introducesconsiderably less diffraction broadening to the image. Fromthis experiment, we obtained σx=346 + 35 µm and σy=109(+0,-63) µm, which should be regarded as an upper bound dueto the limited spatial resolution of the CCD. In the future,we plan to supplement the CCD camera with high resolutionknife-edge scans to precisely determine the image size.

FIG. 5. Measured (open circle) and calculated (solid line) verticalbeam intensity profile of the undulator radiation at an energy of18.37 keV (slightly above the third harmonic) at a gap of 15.8mm (K=1.61). The calculation used the vertical beam emittance(εy=0.2 nm-rad) and the design value for βy=10.0 m.

VI. CONCLUSIONS

The APS Undulator A was designed to provide brilliant x-rays without perturbing storage ring operation. Initialexperience found that the device produced almost noobservable net effect on the stored beam. Also, the radiationfrom the undulator was characterized, and the resultsconfirmed the high magnetic field quality of the device. Alow emittance electron beam (smaller than the initial designgoal), high brilliance radiation, and harmonics as high as the17th were all successfully demonstrated during this initialphase of operation.

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ACKNOWLEDGMENTS

The authors would like to thank all the APS members whoparticipated in this work. We also thank STI Optronics fordelivering a high quality undulator. This research wassupported by the U.S. Department of Energy, BES-MaterialsSciences, under contract #W-31-109-ENG-38.

1 G.K. Shenoy, P.J. Viccaro, and D.M. Mills, Argonne NationalLaboratory Report, ANL-88-9 (1988).

2 K-J. Kim in Physics of Particle Accelerators, edited byR.G. Lerner (New York, 1989), Vol. 1, p. 565.

3 R.J. Dejus, B. Lai, E.R. Moog, and E. Gluskin, ArgonneNational Laboratory Report, ANL/APS/TB-17 (1994).

4 P. Ilinski, W. Yun, B. Lai, E. Gluskin, and Z. Cai, Rev. Sci.Instrum. 66 , 1907 (1995).

5 J. Als-Nielsen and K. Kjaer, Nucl. Inst. Meth. A323, 686(1992).

6 P. Heimann, D. Mossessian, A. Warwick, C. Wang, S. Marks,H. Padmore, and B. Kincaid, Rev. Sci. Instrum. 66 (2), 1885(1995).

7 R.P. Walker, Rev. Sci. Instrum. 60 (7), 1816 (1989).8 Z. Cai, B. Lai, W. Yun, E. Gluskin, D. Legnini, P. Illinski,

and G. Srajer, these proceedings.

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