We describe a possible layout and parameters of a polarized positron source for CLIC, where the positrons are produced from polarized gamma rays created by Compton scattering of a 1.3-GeV electron beam off a YAG laser. This scheme is very energy effective using high finesse laser cavities in conjunction with an electron storage ring. We point out the differences with respect to a similar system proposed for the ILC

Araki, S

Artru, X

Braun, H

Bulyak, E

Chehab, R

Chevallier, M

Fukuda, M

Gao, J

Gladkikh, P

Guiducci, S

Higashi, Y

Hirano, K

Hirose, T

Honda, Y

Korostelev, M S

Kurihara, Y

Kuriki, M

Mönig, K

Okugi, T

Omori, T

Raimondi, P

Rinolfi, Louis

Sakaue, K

Sato, H

Schulte, Daniel

Soskov, V

Strakhovenko, V M

Takahashi, T

Takano, M

Taniguchi, T

Terunuma, N

Tsunemi, A

Urakawa, J

Variola, A

Washio, M

Zimmermann, Frank

Zomer, F

CERN Document Server

EU contract number RII3-CT-2003-506395 CARE-Note-2006-024-ELAN                   Acknowledgements We acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003-506395)  -  - 1CLIC POLARIZED POSITRON SOURCE BASED ON LASER COMPTONSCATTERINGF. Zimmermann, H. Braun, M. Korostelev, L. Rinolfi, D. Schulte, CERN, Geneva, Switzerland;E. Bulyak, P. Gladkikh, NSC/KIPT, Kharkov, Ukraine; S. Araki, Y. Higashi, Y. Honda, Y. Kurihara,M. Kuriki, T. Okugi, T. Omori, T. Taniguchi, N. Terunuma, J. Urakawa, KEK, Ibaraki, Japan;K. Moenig, DESY, Zeuthen, Germany; A. Variola, F. Zomer, LAL, Orsay, France; X. Artru,R. Chehab, M. Chevallier, IPN, Lyon, France; V. Strakhovenko, BINP, Novosibirsk, Russia;J. Gao, IHEP, Beijing, China; S. Guiducci, P. Raimondi, INFN/LNF, Frascati, Italy;V. Soskov, LPI, Moscow, Russia; M. Fukuda, K. Hirano, M. Takano, NIRS, Chiba, Japan; T. Hirose,K. Sakaue, M. Washio, Waseda U., Tokyo, Japan; T. Takahashi, H. Sato, Hiroshima U., H., Japan;A. Tsunemi, Sumitomo Heavy Industries Ltd., Tokyo, Japan (“POSIPOL collaboration”)AbstractWe describe a possible layout and parameters of a po-larized positron source for CLIC, where the positrons areproduced from polarized gamma rays created by Comptonscattering of a 1.3-GeV electron beam off a YAG laser.This scheme is very energy effective using high finesselaser cavities in conjunction with an electron storage ring.We point out the differences with respect to a similar sys-tem proposed for the ILC.INTRODUCTIONAt Snowmass 2005, a polarized-positron source basedon laser-Compton scattering was proposed for the ILC [1].Polarized photons generated by laser-electron scatteringare here converted into polarized positrons via pair produc-tion in a target. Two novel features distinguish the “new”ILC Compton source from its single-pass predecessor de-veloped for the JLC in the 1990’s [2], namely photon stack-ing using high-finesse optical cavities in a Compton stor-age ring and positron stacking in a damping ring, both ofwhich relax the laser requirements. The case of a Comptonsource is bolstered by recent experimental results from theKEK/ATF, where the production of    polarized positronsper bunch with   	   polarizationhas been demonstrated [3], as well as by an improved un-derstanding of compact Compton storage rings [4, 5]. Inthis paper we discuss how the ILC Compton scheme canbe adapted, and scaled down, to CLIC.ILC COMPTON SCHEMEThe ILC Compton source [1] comprises a Compton ringwith 30 optical cavities, which are either coupled in a daisychain or powered by separate lasers. Various types of laserscan be used. In Ref. [1], YAG or CO2 lasers were consid-ered, which require a different beam energy of 1.3 GeVor 4.1 GeV, respectively, and a different ring circumfer-ence of 277 m or 649 m. During 100 or 50 Compton-ring turns    “bunch-lets” of about     polar-ized positrons are produced from polarized photons gen-erated in 30 laser-electron collisions. Next, a 100-Hz 5-GV s.c. linac accelerates these positrons to 5 GeV. On 10successive damping-ring turns, positron bunch-lets are in-jected 10 times into each of 2800 ILC damping-ring rfbuckets. Then the injected positron emittance is dampedby synchrotron radiation for 10 ms. The entire process isrepeated 9 times, always leaving a 10-ms damping time be-tween sets of 10 consecutive injections. After 90 ms theaccumulation is completed. The ILC damping ring nowstores the full-intensity positron bunches for 100 further msbefore extraction to the main linac. A major challenge inthe ILC Compton-ring design are the 30 laser-beam inter-action points (IPs), which result in a large energy spread,reducing the photon yield, and which likely compromisethe dynamic aperture. In case the 30 cavities are coupled, anovel multi-chamber feedback is needed.Table 1: CLIC and 3-km ILC damping ring parameters.variable CLIC ILCenergy 2.424 GeV 5 GeVcircumference 360 m 3230 mbunch population       # bunches per train 110 280gap (missing bunches)   80# trains per pulse 2 10bunch spacing 0.533 ns 2.8 nshor. normalized emittance 600 nm 5 mvert. normalized emittance 5–10 nm 20 nmrf frequency 1.875 GHz 650 MHzrepetition rate 150 Hz 5 HzILC-CLIC DIFFERENCESThe positron sources for ILC and CLIC must provide thebunches required in the respective damping rings. FromTable 1, we infer the main differences between ILC andCLIC: (1) The CLIC bunch charge is almost 10 times lowerand the number of bunches per pulse about 20 times smallerthan for the ILC. (2) The bunch spacing for CLIC is about6 times shorter. (3) The CLIC repetition rate is higher by afactor 30.As a consequence of the first point, the number of lasercavities in the CLIC Compton ring can be reduced, ideallyto a single one, a case which will soon be tested experimen-tally in the ATF damping ring.CLIC SCHEMEFigure 1 displays a schematic of the polarized positronsource proposed for CLIC. Its main components are a com-pact Compton ring with a single optical cavity, a photontarget and positron collection system, a 2.4-GeV 150-Hzn.c. linac, and the 2.424-GeV pre-damping ring used foraccumulation. Table 2 compares preliminary parameters ofthe CLIC source with those of the ILC.For simplicity, we here consider only the case of a YAGlaser. Due to its 10 times shorter wavelength  , for theYAG laser the injection linac is 3 times shorter and theCompton-ring energy 3 times lower than for a CO2 laser.Table 2: YAG-laser Compton-source parameters.parameter CLIC ILCCompton ring energy 1.3 GeV 1.3 GeVC.-ring circumference 42 m 277 mrf frequency 1.875 GHz 650 MHzbunch spacing 0.16 m 0.923 mnumber of bunches 220 280bunch population        no. of optical cavities 1 30total ’s/bunch/turn        selected ’s/bunch/turn         pol. e+/bunch/turn         injections/bunch 300 100total # injections         # e+/pulse        # e+/second         # Compton-ring turns 300 100Compton-pulse duration 42 s 90 spause between cycles 6.1 ms 9.9 ms1.3 GeV linac, 150 HzCompton ring1.3 GeV storage ring, C=42 m, 140 ns/turn2x110 bunchesNb=6.2x10 102.424 GeV Pre-Damping & Accumulator Ring:300 turns of Compton ring makes220 bunches with 3x10 9 e+/ bunch; then 6.1 ms for dampingafter stacking and damping, extract beam and inject intodamping ring, 220 bunches2.4 GeV linac150 Hz1 YAG Laser Pulse Stacking Cavity, 590 mJg (23-29 MeV)6.9x10 8/turn/bunch9.8x10 6 pol. e+/turn/bunchFigure 1: Schematic view of CLIC Compton source.COMPTON RINGThe Compton ring can be designed as a racetreack withfour straight sections [1, 4, 5]. Two of these accommo-date wigglers, counteracting the effect of the Compton col-lisions by enhanced longitudinal damping, one houses anrf system which restores the energy lost in Compton col-lisions and in the wigglers, and the last one contains theCompton collision point with optical cavity.Parameters for the Compton collision point are summa-rized in Table 3. The maximum back-scattered photon en-ergy is 29 MeV. Photons between 23.2 and 29 MeV are se-lected for polarized positron generation. Laser pulse deple-tion from Compton scattering is negligible compared withlosses on the optical mirrors. Also nonlinear Compton ef-fects can be ignored. Transverse emittance growth due toeither the ponderomotive force or quantum diffusion in thelaser field is small too.Table 3: Parameters of Compton collision.parameter valuee- bunch length at Compton IP 5 mme- rms hor./vert. beam size 25, 5 me- beam energy 1.3 GeVe- bunch charge 10 nClaser type YAGlaser photon energy 1.164 eVrms laser radius 5 mlaser pulse energy 592 mJ# laser cavities 1crossing angle   Æphotons in cavity pulse    polarized ’s/bunch/turn    positron yield e+/ 0.014effective e+ yield/     A simulation of the turn-by-turn photon yield per elec-tron which considered an optimized 1st order momentumcompaction      , a zero second order ,and strong wiggler damping, predicts an average total pho-ton yield per electron of about 0.0447, which is close to theideal value expected for a longitudinally point-like bunch[5]. The yield stays almost constant as a function of time,unlike for the ILC [1, 5]. The positron yield per collisionand per Compton-ring electron is estimated by multiplyingthe simulated total photon yield with a factor 0.248 (about25% of the photons are selected) [1] and with a factor 0.014for the approximate positron yield per photon [1].The electron-beam energy spread induced in a Comptoncollision is [6]   , with thephoton energy,   	   the aver-age energy loss,  the laser pulse energy, 	the classi-cal electron radius, and the laser Rayleigh length. TheYAG laser requires a large momentum acceptance of 7–8% in the Compton-ring, which may be difficult to achieve.Possible remedies include decreasing the turn number andincreasing the number of electron bunches (and ring cir-cumference), introducing additional wigglers, or using aCO2 laser with higher  . Many other improvementsconsidered for the more demanding ILC conditions, suchas rf phase manipulation, low & nonlinear momentumcompaction factor, pulsed momentum compaction lattice,and strong rf focusing with minimum bunch length at theCompton IPs [5, 7], could also be applied at CLIC.LASER AND OPTICAL CAVITYThe laser system, sketched in Fig. 2, consists of threestages. The mode-locked laser oscillator produces seedpulses with about 170 nJ energy. The solid-state amplifierprovides a gain by about a factor 3500 via chirped pulseamplification (CPA). For comparison, the existing ampli-fier of the ATF rf-gun laser achieves a factor 10000. Thefinal enhancement by a factor 1000 to about 600 mJ pulseenergy is accomplished by stacking in a high-finesse op-tical cavity. The optical cavities at the ATF have demon-strated enhancement factors of 300 (pulsed laser wire) and1000 (cw laser wire).Several alternatives exist: (1) increasing the laser pulseenergy and decreasing the optical-cavity quality factor; 2)replacing the YAG laser by a 0.21 mJ/pulse CO2 laser; (3)continuous mode operation (with fiber laser?) at 50 MHzand 10 J/pulse combined with a higher cw optical-cavityquality factor of       [8]; (4) feedback on the laser(LAL scheme) and/or on the optical cavity (KEK scheme).laseroscillatorsolid stateamplifierstackingcavity600 mJ,factor 1000enhancement600   J, µgain 3500170 nJ,10 ps FWFigure 2: Schematic layout of laser system, including pulseenergy and gain factors [9].At the intersection of optical cavity and beam pipe, thelatter features two elongated holes for the laser beam. Theminimum cavity length is estimated as      m,where    mm is the size of the optical mirror, and   Æ the laser-beam collision angle. This implies that,for CLIC, at least 3 or 4 pulses are stored in the same cavity.Mode-locked lasers operating at frequencies as high asthe CLIC bunch frequency of 1.875 GHz are not yet avail-able. However, a lower-frequency laser can feed a singleoptical cavity whose length is tailored such that succes-sively fed pulses are properly interleaved and the laser fre-quency is multiplied, as is illustrated in Fig. 3.laser mode-lock period1st laser bunch injected nth laser bunch injectedfraction of lasermode-lock period~0.533 nsFigure 3: Schematic of laser frequency multiplication.POSITRON STACKINGThe accumulation ring used for positron stacking musthave a large longitudinal and transverse acceptance, as wellas provide fast damping. The only economical solution forthe ILC is to use one or several of the 3 or 6-km longmain damping rings for stacking, as the ILC accumula-tor ring should have at least the same circumference asthe main damping ring. By constrast, at CLIC the pre-damping ring can be used for accumulation. The minimumrequired circumference for accommodating 220 positronbunches is only 42 m. In addition, this ring can be opti-mized for accumulation efficiency fully independently ofany damping-ring constraints. The former 2-GeV 200-mNLC pre-damping ring optics [10] is a good candidate. Itfeatures a 10-fold symmetric double-bend achromat struc-ture, wiggler damping equal to twice the arc damping, 0.4-m bunch spacing, 2-ms longitudinal damping time, 100–150 Hz repetition rate, and a dynamic aperture of 0.2 m-radfor      .OUTLOOKThe Compton ring design is challenging due to the highcurrent. Further optimization of ring circumference, bunchspacing, and bunch charge will likely be required.An ongoing R&D programme at the KEK/ATF ad-dresses the design and fabrication of laser-pulse stack-ing cavities with high enhancement factor and small spotsize, the design of an IP with minimum collision angle,and the installation of laser-pulse stacking cavities in theATF damping ring, culminating in X-ray generation. Aparallel proposal to the European Union’s 7th frameworkprogramme includes technological R&D on high-powerhigh-repetition rate lasers and optical cavities, a designstudy of the Compton ring, collection system, and stack-ing schemes, as well as experiments at ATF and DAFNE.Over the last decade, the output power of cw double-cladfiber lasers with diffraction limited beam quality has in-creased by a factor 400 [7, 11]. The demonstrated value of2 kW is close to the power needed for the CLIC source (if asingle laser is used). Extrapolating past evolution, an 8-kWcw fiber laser should become available around 2008/2009.ACKNOWLEDGEMENTSWe thank Z. Huang for helpful discussions. This work issupported by the Commission of the European Communi-ties under the 6th Framework Programme “Structuring theEuropean Research Area”, contract RII3-CT-2003-506395.REFERENCES[1] S. Araki et al, KEK-Preprint 2005-60, CLIC Note 639 (2005).[2] T. Omori, KEK Preprint 99-188 (1999).[3] T. Omori et al, Phys. Rev. Lett. 96, 114801 (2006).[4] E. Bulyak, P. Gladkikh, V. Skomorokhov, arXiv, p. 5physics/0505204v1 (2005).[5] E. Bulyak et al, this conference, WEPCH097 (2006).[6] Z. Huang, R.D. Ruth, Phys. Rev.Lett. 80, 976 (1998).[7] Posipol 2006 workshop http://www.cern.ch/posipol2006[8] A. Variola and F. Zomer at [7].[9] J. Urakawa, POSIPOL meeting, KEK, 04.11.2005.[10] A. Wolski, EPAC’02, p. 521 (2002); A. Wolski, I. Reichel,EPAC’04, 827 (2004).[11] A. Tu¨nnermann et al, J. Phys. B 38, S681 (2005).

CLIC Polarized Positron Source Based on Laser Compton Scattering

CLIC Polarized Positron Source Based on Laser Compton Scattering

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