/Chicago U. /ANL, APS

/Fermilab, /Northern Illinois U.

Kim, K. J.

Piot, P.

Sun, Y. E.

U., /Chicago

UNT (University of North Texas) Digital Library

arXiv:physics/0511011 v1   2 Nov 2005Photoinjector-generation of a flat electron beamwith transverse emittance ratio of 100P. Piot,1, 2, ∗ Y.-E Sun,3, † and K.-J. Kim3, 41Northern Illinois University, DeKalb IL 60115, USA2Fermi National Accelerator Laboratory, Batavia, IL 60510, USA3University of Chicago, Chicago, IL 60637, USA4Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA(Dated: May 25, 2006)The generation of a flat electron beam directly from a photoinjector is an attractive alternativeto the electron damping ring as envisioned for linear colliders. It also has potential applications tolight sources such as the generation of ultra-short x-ray pulses or Smith-Purcell free electron lasers.In this Letter, we report on the experimental generation of a flat-beam with a measured transverseemittance ratio of 100 ± 20.2 for a bunch charge of ∼ 0.5 nC; the smaller measured normalizedroot-mean-square emittance is ∼ 0.4 µm and is limited by the resolution of our experimental setup.The experimental data, obtained at the Fermilab/NICADD Photoinjector Laboratory, are comparedwith numerical simulations and the expected scaling laws.PACS numbers: 29.27.-a, 41.85.-p, 41.75.FrFlat electron beams, e.g. beams with large trans-verse emittance ratios, have been proposed in the con-text of linear colliders and some novel electron-beam-based light sources. In the case of a linear e+/e− col-lider, a flat beam at the interaction point reduces theluminosity disruption caused by beamsstrahlung [1]. Inthe case of light sources, such as the LUX project pro-posed at LBL [2], a flat beam with a smaller emittanceof 0.3 µm and emittance ratio of 50 is needed to pro-duce x-ray pulses that can be compressed to the order offemtoseconds via standard x-ray pulse compression tech-niques [3]. Another type of light source recently drawingattention is based on self-amplification of Smith-Purcellradiation [4]. Given one or two planar metal gratings, aflat beam could enhance the interaction between the elec-trons and metal grating surface, thus reducing the gainlength associated with the Smith-Purcell free-electron-laser mechanism [5, 6, 7].In the proposed International Linear Collider (ILC) theneeded flat-beam parameters (emittance ratio of 300) areforeseen to be achieved via radiation cooling in a damp-ing ring [8]. Although the required transverse emittancesfor the ILC have been demonstrated at the ATF damp-ing ring of KEK [9], ILC puts stringent requirements onthe damping ring design, and the cost of the dampingring is a significant portion of the total collider cost.Therefore alternative ways of producing flat beams di-rectly from an electron source have been explored by sev-eral groups [10]. In conjunction with the invention of alinear transformation capable of transforming an incom-ing flat beam into an angular-momentum-dominated (or“magnetized”) beam [11], a scheme which inverses this∗Electronic address: piot@fnal.gov†Electronic address: yinesun@uchicago.edu; now at Argonne Na-tional Laboratory.transformation was proposed to generate a flat beam di-rectly out of a photoinjector [12]. The method consists ofgenerating an magnetized beam by immersing the pho-tocathode in an axial magnetic field. After acceleration,the beam is transformed into a flat beam using threeskew quadrupoles [13]. This has been verified experimen-tally [14, 15, 16, 17], and transverse emittance ratios of40-50 were reported. Theoretical analysis of the conver-sion of a magnetized cylindrically-symmetric beam intoa flat beam has been presented [18, 19] and some of theassociated limitations explored [20, 21]. In the presentLetter we report on an improvement of the experimentalconditions and methods that led to a measured transverseemittance ratio of approximately 100.The flat-beam experiment was carried out atthe Fermilab/NICADD [30] Photoinjector Laboratory(FNPL) [22]; see Fig. 1 for the layout. In brief, electronbunches with variable charge (Q ≤20 nC) are generatedvia photoemission from a cesium telluride photocathodelocated at the back plate of a 1+1/2 cell radio-frequency(rf) cavity operating at 1.3 GHz (the “rf gun”). Thebeam is then accelerated in a 1.3 GHz superconducting rfcavity (the booster cavity) [23] to approximately 16 MeV.The rf gun is surrounded by three solenoidal lenses thatare designed to control the beam transverse emittance.For flat-beam production the first solenoidal lens (L1) isturned off, and the two others (L2 and L3) are tuned toprovide the desired magnetic field on the photocathodealong with the proper focusing. The beam is therebyproduced in the presence of a significant axial magneticfield and has an average angular momentum given by〈L〉 = eB0σ2c , where e is the electron charge, B0 the ax-ial magnetic field on the photocathode surface, and σcthe root-mean-square (rms) transverse size of the drive-laser spot on the photocathode. The transformation ofthe magnetized beam into a flat beam occurs downstreamof the booster cavity. Three skew quadrupoles (S1, S2,and S3 in Fig. 1) provide a net torque on the beamFERMILAB-PUB-06-330-AD2spectrometerQ1S3X5X4X3booster cavitygun andsolenoidsS1 S2 Q2Q3Q4X6 X7 X8L1L2L3D5023513503770rf-gunphotocathodeFIG. 1: Overview of the Fermilab/NICADD photoinjector. “X” refer to diagnostics stations (beam viewers, and/or slitlocation), “L” to the solenoidal lenses, “Q” to quadrupoles and “S” to the skew quadrupoles. All distances are in mm, withD=800 (or 1850 for the data presented in Fig. 3).thereby removing its initial angular momentum [24, 25].The skew quadrupoles are henceforth referred to as the“transformer”. Given the incoming 4 × 4 beam covari-ance matrix Σ0, the quadrupole strengths are set to pro-vide the proper transport matrix M so that the covari-ance matrix at the exit of the transformer, Σ = MΣ0M̃(where the upper tilde denote the transpose), is block-diagonal. An analytical solution for the quadrupole set-tings was derived under the thin-lens approximation forthe quadrupoles [16] . This solution is used as a startingpoint for a simplex minimization algorithm that searchesthe quadrupole settings to minimize the figure-of-meritχ2 = Σ213+Σ214+Σ223+Σ224, where Σij is the (ij)th elementof matrix Σ. Upon proper tuning of the transformer, theexpected normalized flat-beam emittances, ε±n , are givenby [18, 19]ε±n =√(εun)2 + (βγL)2 ± (βγL)βγL≫εun−→{ε+n ≃ 2βγLε−n ≃(εun)22βγL,(1)where εun = βγεu is the normalized uncorrelated emit-tance of the magnetized beam prior to the transformer,β = v/c, γ is the Lorentz factor, L = 〈L〉/2pz, and pz isthe longitudinal momentum. Note that ε+n ε−n = (εun)2.The flat-beam emittances are measured using the slitmethod [26]. A movable single-slit assembly (either ver-tical or horizontal), located at position X7 (see Fig. 1),is used to sample the beam in one direction. The slitassembly consists of a ∼ 50 µm slit made of a 3 mmthick tungsten block. The beamlet passing through theslit is observed after a drift of distance D, at the lo-cation X8. Given the measured horizontal beam sizeat X7, σX7x , and horizontal rms size of the beamlet atX8 when a vertical slit is inserted at X7, σX8,hx , thehorizontal emittance is then computed as the productεxn = γσX7x σX8,hx /D. Similarly the vertical emittanceis measured as εyn = γσX7y σX8,vy /D where σX8,vy is thevertical rms size of the beamlet at X8 when a horizon-tal slit is inserted at X7. The beam viewer at loca-tions X7 is an optical transition radiation (OTR) foil,while at X8 it is a yttrium aluminum garnet (YAG)screen. The measured rms beam size, σmeas, is af-fected by the resolution of the diagnostics σres and spu-rious dispersion η introduced, e.g., by steering dipolesrequired to keep the beam centered along the beamlineaxis: σmeas =√σ2 + σ2res + (ησδ)2, where σδ is the rmsfractional momentum spread of the beam. The measure-ment method used to report emittances in the followingwas numerically benchmarked [25]. The resolution of thebeam size measurement system which includes the opticalcomponents and a charged coupled device (CCD) camerawas characterized for various operating points [25]. Forall the quoted measurements of transverse beam sizes,we quadratically subtract the smallest measured resolu-tion (σres = 35 µm). The unavoidable contribution fromspurious dispersion (discussed later) results in an overes-timated value for the smaller flat-beam emittance. Hencethe emittance ratio reported hereafter is underestimated.For the flat-beam experiment reported in this Letter,the nominal operating parameters for the photoinjectorare reported in Table I. The rf-gun and booster-cavitysettings are kept the same during the experiment whilethe drive-laser spot size on the photocathode and thesolenoid currents are adjusted for the different sets ofmeasurements.TABLE I: Nominal settings for the photocathode drive laser,rf-gun and booster cavity during the flat-beam experiment.parameter value unitlaser injection phase 25 degreerms laser spot size on cathode 0.75 − 1 mmrms laser pulse duration (Gaussian) ∼3 psbunch charge 0.5 nCaccelerating gradient on cathode 32 MV/maxial magnetic field on cathode 400 − 900 Gaussbooster cavity peak electric field 23 MV/mGiven the experimental conditions, numerical simu-lations are performed with the tracking program As-tra [27]. Using the simulation outputs of the beam prop-erties at the entrance of the transformer, the aforemen-tioned simplex minimization algorithm is used to deter-mine the skew quadrupole settings needed to transformthe magnetized round beam into a flat beam. In the ex-periment, the quadrupole settings are then empiricallyfine-tuned to insure the x-y correlation on the beam hasbeen removed downstream of the transformer. This isachieved by observing the beam transverse image on the3viewers downstream of the transformer: upon removalof the angular momentum, the beam should remain flatand upright. In Table II we compare, for two cases of rmsdrive-laser spot sizes (σc=0.76 mm and σc =0.97 mm),the final quadrupole currents used in the experimentwith the initial values obtained numerically. Most ofthe quadrupole currents agree with predicted values, thelarger discrepancies observed for the settings of the lastquadrupole reflect a looser tolerance on this quadrupolesetting [21].TABLE II: Comparison of the experimental skew quadrupolecurrents with the numerical predictions for different laser spotsizes σc. Ii is the current of the skew quadrupole Si.quadrupole σc = 0.79 mm σc = 0.97 mmcurrent experiment simulation experiment simulationI1(A) -1.92 -2.03 -1.97 -1.98I2(A) 2.40 2.57 2.56 2.58I3(A) -2.99 -4.01 -4.55 -5.08For the transverse emittance measurements, the beamimages on the different viewers are taken for a single-bunch beam. In Figure 2, we present the set of experi-mental images, along with their respective simulated im-ages, needed to infer the two transverse flat-beam emit-tances. Several shots of each of the particular imagesare taken and analyzed to obtain the rms beam sizes.The results are then averaged and a statistical error isattributed to the mean. Given the uncertainty of themeasurement method the systematic errors are estimatedfrom error propagation. The rms beam sizes are esti-mated on 95% of the total integrated image intensity. InTable III, we gather the measured and simulated param-eters for the case of σc = 0.97 mm. The smaller ofTABLE III: Measured and simulated flat-beam parameters forσc = 0.97 mm. Both systematic and statistical (in brackets)errorbars are included.parameter experiment simulation unitσX7x 0.088±0.01 (±0.01) 0.058 mmσX7y 0.63±0.01 (±0.01) 0.77 mmσX8,vx 0.12±0.01 (±0.01) 0.11 mmσX8,hy 1.68±0.09 (±0.01) 1.50 mmεxn 0.41±0.06 (±0.02) 0.27 µmεyn 41.1±2.5 (±0.54) 53 µmεyn/εxn 100.2±20.2 (±5.2) 196 −the flat beam emittance is εxn = 0.41 ± 0.06 µm; this isless than half of the expected thermal emittance due tothe photoemission process of the cesium telluride mate-rial. From [28, 29], we infer the thermal emittance to beεth = 0.99 ± 0.10 µm given σc = 0.97 ± 0.05 mm.To gain more insight into the round-to-flat-beam trans-formation, we compare the expected flat-beam emit-tances, εn± in Eq. (1), given the incoming magnetizedx (mm)y (mm)−2 0 2−404X7x (mm)y (mm)−2 0 2−404X8 Hslitx (mm)y (mm)−2 0 2−404X8 Vslitx (mm)y (mm)−2 0 2 −40 4 x (mm)y (mm)−2 0 2 −40 4 x (mm)y (mm)−2 0 2 −40 4 X7 X8 VslitX8 HslitFIG. 2: Top three images are taken with digital cameras:beam at X7, horizontal and vertical slit images at X8. Bot-tom three are the corresponding beam profiles from Astrasimulations. These images are associated with the flat-beampresented in Table III.TABLE IV: Parameters measured from the angular-momentum-dominated round beam and the correspondingflat beam.parameters round-beam flat-beam simulationβγL 25.6±2.6 − 26.3εun 5.1±0.9 − 3.8ε+n 53.8±5.4a 41.0±2.5 53ε−n 0.49±0.22a 0.41±0.06 0.27√ε+n ε−n 5.1±0.9 4.1±0.8 3.8aexpected value given the measured round beam parameters.ε n+ (µm)βγ L (µm)5 10 15 20 25 301020304050607080FIG. 3: Larger one of the flat beam emittances (ε+n ) versusβγL. A linear regression (solid line) of the experimental data(circle) is compared with the theoretical dependence (dashedline). The shaded area represents the 95% confidence boundsassociated with the linear regression.4beam parameters, with the measured flat-beam emit-tances downstream of the transformer. The uncorrelatedemittance of the magnetized beam εun is measured usingthe slit technique from the beam image at X3 and thecorresponding slit images at X5. L has been obtainedwith the two different methods detailed in [17]. The re-sulting measurements for the case σc = 0.97 mm aresummarized in Table IV: within the experimental errorswe observed that the measured four-dimensional (4-D)emittance ε4D ≡√εxnεyn is conserved during the round-to-flat-beam transformation. We note a ∼25% discrep-ancy for the measured larger flat-beam emittance, com-pared to the simulation and the value predicted from theround beam parameters. This is probably due to imper-fectly optimized settings for the transformer. We finallyreport the dependence of ε+n versus L. The value of Lwas varied either by changing B0 or σc. As expected ε+nis linearly dependent on L, and a linear regression givesε+n = (1.78 ± 0.26)L; see Fig. 3. The slope is in agree-ment with the theoretically expected slope value of 2 inthe limit L ≫ βγεun; see Eq. (1).In summary we generated and characterized a highlyasymmetric beam in a photoinjector. The lower limit forthe best measured emittance ratio of ∼100 is limited byour experimental set-up: the fact that the transformationoccurs at low energy along with σδ ≃ 0.25% made ourmeasurement sensitive to spurious dispersion. Simula-tions based on steering dipole settings used to correct thebeam orbit indicate that the thereby introduced disper-sion could result in an overestimation of the smaller flat-beam emittance by a factor up to 2. Spurious dispersionaccounts for most of the discrepancy between numericalsimulations and measurements. The experiment is lim-ited to low charge in order to avoid space charge to sig-nificantly impact the beam dynamics in the transformerat 16 MeV. Nonetheless our measurements support thepotential flat-beam injector designs either for proposedlight source such as LUX or envisioned Terahertz radia-tion sources based on Smith-Purcell effect. Our resultsalso open a possible path for the production of flat e−-beam for the ILC, where the main challenge is to alsoachieve a 4-D emittance ε4D ∼ 0.3 µm for Q = 3.2 nC.This value is one order of magnitude lower than what ourphotoinjector can presently produce at Q = 0.5 nC.We wish to acknowledge C. L. Bohn, D. Edwards, andH. Edwards, for encouragements, fruitful discussions andcomments. We are thankful to J. Li and R. Tikhoplavfor improving the photocathode laser.[1] K. Yokoya and P. Chen, in Proceedings of the 1989 Parti-cle Accelerator Conference, Chicago, IL, 1989, edited byF. Bennett and J. Kepta (IEEE, New York, NY, 1989),pp. 1438-1440.[2] W. Barry et. al, Lawrence Berkeley National Lab. ReportNo. LBNL-51766, 2002.[3] A. Zholents, P. Heimann, M. Zolotorev and J. Byrd,Nucl. Instrum. Methods A 425, 385-389 (1999).[4] S. J. Smith and E. M. Purcell, Phys. Rev. 92, 1069(1953).[5] K.-J. Kim and S.-B. Song, Nucl. Instrum. Methods A475 158-163 (2001); V. Kumar and K.-J. Kim submittedto Phys. Rev. E[6] H. L. Andrews and C. A. Brau, Phys. Rev. ST Accel.Beams 7, 070701 (2004).[7] Y. Zhang, Ya. Derbenev and R. Li, Nucl. Instrum. Meth-ods A 507 459-463 (2003).[8] http://lcdev.kek.jp/ILCWS/.[9] Y. Honda, et al., Phys. Rev. Lett. 92, 054802 (2004).[10] J. B. Rosenzweig, E. Colby, G. Jackson and T. Nicol, inProceedings of the 1993 Particle accelerator Conference,Washington DC, edited by S. T. Corneliussen (IEEE, Pis-cataway, NJ, 1993), pp. 3021-3023; G. Fiorentini, C. Pa-gani, and L. Serafini, in Proceedings of the 1995 Particleaccelerator Conference, Dallas, TX, edited by L. T. Gen-nar and R. H. Siemann (IEEE, Piscataway, NJ, 1995), pp.973-975.[11] Ya. Derbenev, University of Michigan Report No. UM-HE-98-04, 1998.[12] R. Brinkmann, Y. Derbenev and K. Flöttmann, DESYReport No. TESLA 99-09, 1999; Phys. Rev. ST Accel.Beams 4, 053501 (2001).[13] A. Burov, V. Danilov, FERMILAB-TM-2043 (1998).[14] D. Edwards et al., in Proceedings of the XX InternationalLinac Conference, Monterey, CA, pp. 122-124 (2000).[15] D. Edwards et al., in Proceedings of the 2001 ParticleAccelerator Conference, Chicago, IL (IEEE, Piscataway,NJ, 2001), pp. 73-75.[16] E. Thrane et al., in Proceedings of the XXI InternationalLinac Conference, Gyeongju, Korea (Pohang AcceleratorLaboratory, Pohang, Korea, 2002), pp. 308-310.[17] Y.-E Sun, et al., Phys. Rev. ST Accel. Beams 7, 123501(2004).[18] A. Burov, S. Nagaitsev and Ya. Derbenev, Phys. Rev. E66, 016503 (2002).[19] K.-J. Kim, Phys. Rev. ST Accel. Beams 6, 104002 (2003).[20] Y.-E Sun, K.-J. Kim, and P. Piot, in Proceedings of theXXII International Linac Conference, Lubeck, Germany,pp. 150-152 (2004)[21] Y.-E Sun, K.-J. Kim, and P. Piot, to appear in Pro-ceedings of the 2005 Particle Accelerator Conference,Knoxville, TN; see also preprint fermilab-conf-05-158-AD(2005).[22] http://nicadd.niu.edu/fnpl.[23] B. Aune et al., Phys. Rev. ST Accel. Beams 3, 092001(2000).[24] C. L. Bohn, private discussions.[25] Y.-E Sun, Ph.D. thesis, University of Chicago, 2005.[26] C. Lejeune and J. Aubert, Adv. Electron. Electron Phys.Suppl. A 13, 159 (1980).[27] K. Flöttmann, “Astra: A Space Charge Tracking Algo-rithm,” available at http://www.desy.de/∼mpyflo.[28] V. V. Miltchev, et al., in Proceedings of the 2004 FELConference, Trieste, Italy, pp. 399-402.[29] K. Flöttmann, DESY Report No. TESLA-FEL-1997-01,1997.5[30] NICADD is an acronym for Northern Illinois Center forAccelerator and Detector Development.

Photoinjector-generation of a flat electron beam with transverse emittance ratio of 100

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Photoinjector-generation of a flat electron beam with transverse emittance ratio of 100

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