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Nuclear Instruments and Methods in Physics Research A 528 (2004) 421–426

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doi:10.1016

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

Yujong Kima,*, T. Shintakeb, H. Kitamurab, H. Matsumotoc, D. Sond, Y. Kimd

aDeutsches Elektronen-Synchrotron DESY, D-22603 Hamburg, GermanybRIKEN Harima Institute, SPring-8, Hyogo 679-5148, Japan

cKEK, High Energy Accelerator Research Organization, Ibaraki 305-0801, JapandThe Center for High Energy Physics, Daegu 702-701, Republic of Korea

Abstract

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 of

an SCSS bunch compressor and the manner in which the CSR-induced emittance growth can be reduced in the bunch

compressor.

r 2004 Elsevier B.V. All rights reserved.

PACS: 29.17.+w; 41.85.Lc; 41.60.Ap; 41.60.Cr

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

1. 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 (2001–2005), an injector, a C-bandmain linear accelerator, and a bunch compressor(BC) will be installed to generate a 40 nmwavelength radiation. During the Phase-II stage(2005–2007), three additional C-band main linearaccelerators will be added to generate a wave-

onding author.

ddress: yujong.kim@desy.de (Yujong Kim).

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.nima.2004.04.124

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

d.

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Yujong Kim et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 421–426422

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 isgiven by

dzf ¼ dzi þ R56ðdE=EÞi þ T566ðdE=EÞ2i ð1Þ

R56E2y2BðDL þ 23

LBÞE� 23

T566 ð2Þ

where dzf ðdziÞ is the longitudinal position devia-tion from the bunch center after (before) thechicane, ðdE=EÞi 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 firstdipole 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 ðdE=EÞ:

Unit-I C-band Linac, 300 MeV

X-band Correction Cavity, –70 MeV

1.4 m

Tail Electr

Head ElectrQD QF QD BM

6.8 m1.3 m

L = 0.2 m

∆L = 2.3 m

B

Fig. 1. Layout of the SCS

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-lated 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 sufficiently 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 ðdE=EÞi; a small bending angle,and a somewhat large drift space between the firstand second dipoles. Optimized parameters of the

Head

Tail

ons

ons QDQFQF QD QF

0.18 m

2.9 m

11.0 m

S bunch compressor.

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Table 1

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

Parameter Phase-I Phase-II

Beam energy E (MeV) 230 218

Initial bunch length Dzi (mm) 2.4 1.2

Final bunch length Dzf (mm) 0.6 0.15

Initial projected relative energy spread sd (%) 2.14 1.98

Initial slice rms relative energy spread sds ð10�5Þ B5 B5

Initial maximum relative energy deviation ðdE=EÞi ð10�2Þ 3.6 3.5

Beam phase at the C-band linac fc (deg) 12.5 24

Momentum compaction factor R56 (mm) 24.8 15.0

Second-order momentum compaction factor jT566j (mm) 37.2 22.5

Effective dipole length LB (m) 0.2 0.2

Drift length between first and second dipoles DL (m) 2.3 2.3

Dipole bending angle yB (mrad) 71 56

Dipole magnetic field jBj (T) 0.27 0.20

Maximum horizontal dispersion Zx;max (m) 0.18 0.14

Initial slice emittance before the BC ens ðmmÞ 1.50 1.50

Initial projected emittance before the BC en ðmmÞ 1.58 1.54

Change in slice emittance Dens=ens (%) 0.04 0.18

Change in projected emittance Den=en (%) 2.3 63.2

Change in energy spread due to CSR Dsd;CSR ð10�4Þ 2.9 8.7

Change in energy spread due to ISR Dsd;ISR ð10�8Þ 6.2 3.8

Yujong Kim et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 421–426 423

SCSS BC for the Phase-I and Phase-II stages aresummarized in Table 1, where ens ðenÞ is the slice(projected) transverse normalized rms emittanceand sds ðsdÞ is the slice or local (projected) rmsrelative energy spread [2]. Since electrons sepa-rated by more than one cooperation length ðBmmÞwill not interact with each other, we should focuson the slice parameters to predict FEL perfor-mance [3]. Although the normal operational R56

can 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 wakefields in linacs, as shown in Figs. 1 and2 [3,4]. Here, the residual weak nonlinearity afterthe X-band correction cavity is to compensate T566

of the chicane. Since the beam energy at the SCSSBC is sufficiently 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 amplifies CSR effects in thebunch compressor [2,4]. By compensating thenonlinearities with the X-band correction cavity,the final 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 wakefield 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-functionð¼ �0:5b0Þ is zero, and the b-function is minimum[2,3]. The optimized b-functions are shown in Fig. 3.Fifth, although the final bunch length at the

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

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Fig. 3. b-functions at the SCSS BC Phase-II.

Fig. 2. ELEGANT simulation results of the longitudinal phase space distribution at Phase-I (left) and Phase-II (right) stages. The red,

green, and blue lines indicate the longitudinal phase space distribution after the C-band linac, after the X-band correction cavity, and

after the BC, respectively. The negative s ð¼ jdzjÞ indicates the bunch head.

Yujong Kim et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 421–426424

Phase-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 wakefields in three additionalC-band main linacs and by operating those linacsclose to the RF crest.

4. Investigation of CSR wakefield effects

We have investigated CSR wakefield 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 final bunchlength and peak current are 0:6 mm and about500 A at the Phase-I stage, respectively, and those

of the Phase-II stage are 0:15 mm and about 2 kA;respectively. Since the SCSS injector has a beamdeflector, a beam chopper, and an energy filter, wehave assumed that the initial linear density beforethe BC is a somewhat flat-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(right). This is due to the uncompensated higherorder nonlinearities such as the third order term inthe longitudinal short-range wakefields 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 final slice (projected)

normalized rms emittances at the Phase-I and

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Yujong Kim et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 421–426 425

Phase-II stages are about 1:501 mm ð1:6 mmÞ and1:503 mm ð2:5 mmÞ; respectively, all of which arewithin our design values as summarized in Table 1of Ref. [2].

–2.0 –1.0 0.0 1.0 2.00

200

400

600

800

Den

sity

[A

]

s [mm]

Fig. 4. ELEGANT simulation results of the linear density at Phase-I (

the linear density at the ends of the first and last dipoles, respectively

–2.0 –1.0 0.0 1.0 2.0–4.0

–2.0

0.0

2.0

4.0

s [mm]

∆γ

[10−3

]

−3

Fig. 5. ELEGANT simulation results of the energy change Dg due togreen lines indicate Dg at the ends of the first and last dipoles, respec

0

0.5

1

1.5

2

0 40 80 120 160

Gai

n

λ [µm]

Fig. 6. Gain of the CSR microbunching instability at the SCSS BC Ph

green, blue, and magenta lines are 9:5� 10�6; 1:25� 10�5; 2:5� 10�

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

800

1600

2400

3200

Den

sity

[A

]s [mm]

left) and Phase-II (right) stages. The red and green lines indicate

.

∆γ

[10

]

–2.0 –1.0 0.0 1.0 2.0–12.0

–6.0

0.0

6.0

12.0

s [mm]

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

tively.

λ [µm]

0

1.5

3

4.5

6

0 40 80 120 160

Gai

n

ase-I (left) and Phase-II (right) stages. Here, sds indicated by red,5; and 5:0� 10�5; respectively.

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Yujong Kim et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 421–426426

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.

Acknowledgements

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

References

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

Korea, 2002, pp. 526–530.

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

Korea, 2002, pp. 100–102;

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

France, 2002, pp. 808–810.

[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)

074401.