Bunch compressor for the SPring-8 compact SASE source (SCSS) project

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    The Center for High Energy Physics, Daegu 702-701, Republic of Korea

    Keywords: Bunch compressor; SASE-FEL; CSR; CSR microbunching instability

    wavelength radiation. During the Phase-II stage(20052007), three additional C-band main linear

    current can be obtained by compressing the bunchlength. After upgrading the injector system, a 4 ps

    ARTICLE IN PRESSaccelerators will be added to generate a wave- (FW) long bunch will be directly supplied to thebunch compressor at the Phase-II stage. And byretuning the operation conditions of the bunchcompressor, our required high peak current of2 kA can be obtained using the same bunch

    *Corresponding author.

    E-mail address: yujong.kim@desy.de (Yujong Kim).

    0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved.doi:10.10161. Introduction

    The SPring-8 compact SASE source (SCSS)project is divided into two phase stages accordingto its installed components and the radiationwavelength of the SASE source [1,2]. During thePhase-I stage (20012005), an injector, a C-bandmain linear accelerator, and a bunch compressor(BC) will be installed to generate a 40 nm

    length radiation of about 3:6 nm: Detail designbeam parameters for the two stages are summar-ized in Table 1 of Ref. [2]. At the Phase-I stage, apeak current of about 500 A is required to saturatethe SASE mode within a 13:5 m long undulator.However, no present injector technology candirectly supply the required high-quality electronbeam. Since the peak current is inversely propor-tionate to the bunch length, the required peakAbstract

    At the SPring-8 compact SASE source (SCSS), a high-quality beam with high peak current and low emittance should

    be supplied to saturate the SASE mode within a 22:5 m long undulator. In this paper, we describe the design concepts ofan SCSS bunch compressor and the manner in which the CSR-induced emittance growth can be reduced in the bunch


    r 2004 Elsevier B.V. All rights reserved.

    PACS: 29.17.+w; 41.85.Lc; 41.60.Ap; 41.60.CrNuclear Instruments and Methods in Physic

    Bunch compressor for the S(SCSS

    Yujong Kima,*, T. Shintakeb, H. KitamaDeutsches Elektronen-Synchrotro

    bRIKEN Harima Institute, ScKEK, High Energy Accelerator Resea

    d/j.nima.2004.04.124search A 528 (2004) 421426

    ng-8 compact SASE sourceroject

    ab, H. Matsumotoc, D. Sond, Y. Kimd

    SY, D-22603 Hamburg, Germany

    -8, Hyogo 679-5148, Japan

    rganization, Ibaraki 305-0801, Japan

  • compressor. In this paper, we describe the designconcepts of the SCSS bunch compressor.

    2. The principle of a bunch compressor

    When electrons in a bunch pass through amagnetic chicane, their traveling path lengths ortimes are changed according to their energies orenergy spreads, which implies a bunch lengthchange [2,3]. In a bunch compressor, the long-itudinal coordinate relation to the second order is

    3. Design concepts of bunch compressor

    When the bunch length is compressed in abunch compressor, the bunch length may becomesmaller than the radiation wavelength. In this case,CSR can be generated. Since CSR from the tailelectrons can overtake the head electrons aftertraveling the overtaking length, the head electronswill be accelerated by CSR, and the tail electronswill be decelerated due to their own CSR loss. Theelectrons will be transversely kicked at the nonzerodispersion region due to the CSR-induced corre-


    1.4 m


    d Electr

    6.8 m

    Yujong Kim et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 421426422Unit-I C-band Linac, 300 MeVX-band Correction Cavity, 70 MeVgiven by

    dzf dzi R56dE=Ei T566dE=E2i 1



    LBE 23 T566 2

    where dzf dzi is the longitudinal position devia-tion from the bunch center after (before) thechicane, dE=Ei is the relative energy deviationbefore the chicane, which is supplied by aprecompressing linac, R56 is the momentumcompaction factor, which is supplied by thechicane, T566 is the second-order momentumcompaction factor due to the second-order disper-sion in the chicane, yB is the bending angle in theradian, DL is the drift space between the rstdipole and the second dipole in the chicane, andLB is the effective length of the dipole in thechicane. Here, we assume that the chicane consistsof four rectangular dipoles, and the head electronshave positive dz and negative dE=E:


    HeaQD QF QD BM

    1.3 m

    L = 0.2 m

    L = 2.3 m

    BFig. 1. Layout of the SCSlated energy spread along the bunch. It must benoted that the projected emittance can be diluteddue to CSR in the bunch compressor while theslice emittance dilution is sufciently small [3]. Wehave reduced the emittance dilution due to CSR inthe BC using the following methods:First, after considering the slice emittance

    growth in a double-chicane due to the CSRmicrobunching instability, the slice emittancegrowth in a wiggler-combined single chicane dueto the spontaneous radiation in the wiggler, andthe projected and slice emittance growths in an S-chicane due to the residual dispersion and CSRmicrobunching instability, we chose a normalsingle chicane with four rectangular magnets forour bunch compressor, as shown in Fig. 1 [3].Second, we have reduced CSR by a weak

    strength chicane or small R56 [3]. According toEqs. (1) and (2), this is possible by choosing asomewhat large dE=Ei; a small bending angle,and a somewhat large drift space between the rstand second dipoles. Optimized parameters of the




    ons QDQFQF QD QF

    0.18 m

    2.9 m

    11.0 mS bunch compressor.


    ethodTable 1

    Parameters of the SCSS BC in Phase-I and Phase-II stages


    Beam energy E (MeV)

    Initial bunch length Dzi (mm)Final bunch length Dzf (mm)Initial projected relative energy spread sd (%)Initial slice rms relative energy spread sds 105Initial maximum relative energy deviation dE=Ei 10

    2Beam phase at the C-band linac fc (deg)Momentum compaction factor R56 (mm)

    Second-order momentum compaction factor jT566j (mm)Effective dipole length LB (m)

    Drift length between rst and second dipoles DL (m)Dipole bending angle yB (mrad)Dipole magnetic eld jBj (T)Maximum horizontal dispersion Zx;max (m)Initial slice emittance before the BC ens mmInitial projected emittance before the BC en mmChange in slice emittance Dens=ens (%)Change in projected emittance Den=en (%)Change in energy spread due to CSR Dsd;CSR 104Change in energy spread due to ISR Dsd;ISR 108

    Yujong Kim et al. / Nuclear Instruments and MSCSS BC for the Phase-I and Phase-II stages aresummarized in Table 1, where ens en is the slice(projected) transverse normalized rms emittanceand sds sd is the slice or local (projected) rmsrelative energy spread [2]. Since electrons sepa-rated by more than one cooperation length Bmmwill not interact with each other, we should focuson the slice parameters to predict FEL perfor-mance [3]. Although the normal operational R56can be reduced further by choosing higher sd; wehave chosen sdB2% to keep the chromaticity-induced projected emittance dilution in linacswithin 5%.Third, we have reduced CSR further by instal-

    ling a higher harmonic X-band correction cavitybefore the BC to compensate the nonlinearities inthe longitudinal phase space. The X-band correc-tion cavity compensates the second-ordernonlinearities due to the RF curvature of the C-band linac, the second-order path dependence onparticle energy in the chicane T566; and the short-range wakeelds in linacs, as shown in Figs. 1 and2 [3,4]. Here, the residual weak nonlinearity afterthe X-band correction cavity is to compensate T566Phase-I Phase-II

    230 218

    2.4 1.2

    0.6 0.15

    2.14 1.98

    B5 B53.6 3.5

    12.5 24

    24.8 15.0

    37.2 22.5

    0.2 0.2

    2.3 2.3

    71 56

    0.27 0.20

    0.18 0.14

    1.50 1.50

    1.58 1.54

    0.04 0.18

    2.3 63.2

    2.9 8.7

    6.2 3.8

    s in Physics Research A 528 (2004) 421426 423of the chicane. Since the beam energy at the SCSSBC is sufciently high, the nonlinearity due to thespace charge force is negligible. If those nonlinea-rities are not properly compensated, then a localcharge concentration or spike is generated duringthe compression process. This local charge con-centration or spike amplies CSR effects in thebunch compressor [2,4]. By compensating thenonlinearities with the X-band correction cavity,the nal obtainable minimum bunch length can bealso reduced further [2]. However, beams aredecelerated by about 70 MeV in the X-bandcorrection cavity to compensate the nonlinearities[2,4]. The projected emittance dilution due to thetransverse short-range wakeeld in the X-bandcorrection cavity is within 7% if its misalignmentis smaller than 50 mm [4].Fourth, the projected emittance dilution due to

    CSR can be reduced further by forcing the beamwaist close to the fourth dipole, where a-function 0:5b0 is zero, and the b-function is minimum[2,3]. The optimized b-functions are shown in Fig. 3.Fifth, although the nal bunch length at the

    Phase-II stage is four times smaller than that at the





    ethodFig. 2. ELEGANT simulation results of the longitudinal phase s

    green, and blue lines indicate the longitudinal phase space distrib

    after the BC, respectively. The negative s jdzj indicates the bYujong Kim et al. / Nuclear Instruments and M424Phase-I stage, we can control CSR effects byreducing the chicane strength or R56 to 15 mm:This R56 can be obtained by maintaining sd at theBC at about 2% and by supplying 4 ps (FW) longbunch from the injector. We can reduce the energyspread of 2% to the design value by the long-itudinal short-range wakeelds in three additionalC-band main linacs and by operating those linacsclose to the RF crest.

    4. Investigation of CSR wakeeld effects

    We have investigated CSR wakeeld effects inthe bunch compressor for the SCSS Phase-I andPhase-II stages using ELEGANT and TraFiC4

    codes. As shown in Figs. 2 and 4, the nal bunchlength and peak current are 0:6 mm and about500 A at the Phase-I stage, respectively, and those

    Fig. 3. b-functions at the SCSS BC Phase-II.of the Phase-II stage are 0:15 mm and about 2 kA;respectively. Since the SCSS injector has a beamdeector, a beam chopper, and an energy lter, wehave assumed that the initial linear density beforethe BC is a somewhat at-top shape, as shown inFig. 4 [1]. With the help of the X-band correctioncavity, the linear densities are fairly symmetricaround the bunch center, and there is no strongspike or local charge concentration after the BC,as shown in Fig. 4. The CSR induced correlatedenergy spread along the bunch is increased as thebunch length is compressed, as shown in Fig. 5.Here, the head electrons with negative s areaccelerated by CSR, and the tail electrons aredecelerated by their CSR loss. At the Phase-IIstage, there is one small special spike around thetail region in the energy change, as shown in Fig. 5

    distribution at Phase-I (left) and Phase-II (right) stages. The red,

    after the C-band linac, after the X-band correction cavity, and


    s in Physics Research A 528 (2004) 421426(right). This is due to the uncompensated higherorder nonlinearities such as the third order term inthe longitudinal short-range wakeelds and thethird order term in the momentum compaction.Since the longitudinal space charge force and

    the intrabeam scattering increase the slice energyspread in the linac, and sds in the BC is increasedfurther by the incoherent synchrotron radiation(ISR), CSR, and the longitudinal space chargeforce, the estimated gain of the CSR microbunch-ing instability is less than 4, as shown in Fig. 6[5,6]. Therefore, the CSR microbunching instabil-ity in the SCSS BC can be prevented to a certainextent.After the SCSS BC, the nal slice (projected)

    normalized rms emittances at the Phase-I and

  • Phase-II stages are about 1:501 mm 1:6 mm and1:503 mm 2:5 mm; respectively, all of which arewithin our design values as summarized in Table 1of Ref. [2].

    5. Summary

    We have designed a single chicane bunch com-pressor for the SCSS project. By compensating


    2.0 1.0 0.0 1.0 2.00








    s [mm]2.0 1.0 0.0 1.0 2.00








    s [mm]Fig. 4. ELEGANT simulation results of the linear density at Phase-I (left) and Phase-II (right) stages. The red and green lines indicate

    the linear density at the ends of the rst and last dipoles, respectively.

    2.0 1.0 0.0 1.0 2.04.0





    s [mm]


    03 ]


    03 ]

    2.0 1.0 0.0 1.0 2.012.0





    s [mm]Fig. 5. ELEGANT simulation results of the energy change Dg due to CSR at Phase-I (left) and Phase-II (right) stages. The red andgreen lines indicate Dg at the ends of the rst and last dipoles, respectively.













    C Ph

    Yujong Kim et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 421426 42500 40 80 120 160

    [m]Fig. 6. Gain of the CSR microbunching instability at the SCSS Bgreen, blue, and magenta lines are 9:5 106; 1:25 105; 2:5 10 [m]

    00 40 80 120 160

    ase-I (left) and Phase-II (right) stages. Here, sds indicated by red,

    5; and 5:0 105; respectively.

  • nonlinearities in the longitudinal phase space withthe X-band correction cavity, using the weakstrength chicane under a somewhat large energyspread condition, and by optimizing Twiss para-meters around the bunch compressor, we canreduce the CSR-induced emittance growth and themicrobunching instability in the bunch compressor.


    The authors sincerely thank M. Borland, Ph.Piot, Z. Huang, and P. Emma for their usefuldiscussions and recommendations.


    [1] T. Shintake, et al., Proceedings of LINAC 2002, Gyeongju,

    Korea, 2002, pp. 526530.

    [2] Yujong Kim, et al., Proceedings of LINAC 2002, Gyeongju,

    Korea, 2002, pp. 100102;

    Yujong Kim, et al., Proceedings of EPAC 2002, Paris,

    France, 2002, pp. 808810.

    [3] LCLS Conceptual Design Report, SLAC-R-593, 2002.

    [4] P. Emma, LCLS-TN-01-1, 2001.

    [5] Yujong Kim, et al., Proceedings of PAC 2003, Portland,

    USA, 2003.

    [6] Z. Huang, et al., Phys. Rev. ST Accel. Beams 5 (2002)



    Yujong Kim et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 421426426

    Bunch compressor for the SPring-8 compact SASE source (SCSS) projectIntroductionThe principle of a bunch compressorDesign concepts of bunch compressorInvestigation of CSR wakefield effectsSummaryAcknowledgementsReferences


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