In CTF3 the beam compression of the driving beam structure between the main linac and the decelerating section is obtained with a delay loop and a combiner ring which increase the pulse current by a factor 10. The design of the combiner ring is presented. Tunable isochronicity condition, corrected up to second order, should assure preservation of the correlation in the longitudinal phase space during the compression. Path-length tuning devices are included in the combiner ring layout to compensate for orbit variations

Biscari, C

Corsini, R

D'Amico, T E

Ghigo, A

Groening, L

Guignard, Gilbert

Marcellini, Fabio

Sanelli, C

Sannibale, F

Serio, M

Sgamma, F

Vignola, G

Zobov, M

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CERN – European Organization for Nuclear ResearchEuropean Laboratory for Particle Physics CLIC Note 471CTF3: DESIGN OF DRIVING BEAM COMBINER RINGC. Biscari, A. Ghigo, F. Marcellini, C. Sanelli, F. Sannibale, M. Serio, F. Sgamma,G. Vignola, M. Zobov, INFN-LNF, Frascati, Italy;R. Corsini, T.E. D'Amico, L. Groening, G. Guignard, CERN, Geneva, SwitzerlandAbstractIn CTF3 the beam compression of the driving beam structure between the main linac and thedecelerating section is obtained with a delay loop and a combiner ring which increase the pulsecurrent by a factor 10. The design of the combiner ring is presented. Tunable isochronicitycondition, corrected up to second order, should assure preservation of the correlation in thelongitudinal phase space during the compression. Path-length tuning devices are included in thecombiner ring layout to compensate for orbit variations.Paper presented at the 7th European Particle Accelerator Conference, 26-30  June 2000, Vienna, AustriaGeneva, Switzerland22 February 2001open-2001-01822/02/2001CTF3: DESIGN OF DRIVING BEAM COMBINER RINGC. Biscari, A. Ghigo, F. Marcellini, C. Sanelli, F. Sannibale, M. Serio, F. Sgamma,G. Vignola, M. Zobov, INFN-LNF, Frascati, Italy;R. Corsini, T.E. D’Amico, L. Groening, G. Guignard, CERN, Geneva, SwitzerlandAbstractIn CTF3 the beam compression of the driving beamstructure between the main linac and the decelerating sec-tion is obtained with a delay loop and a combiner ringwhich increase the pulse current by a factor 10. The de-sign of the combiner ring is presented. Tunable iso-chronicity condition, corrected up to second order, shouldassure preservation of the correlation in the longitudinalphase space during the compression. Path-length tuningdevices are included in the combiner ring layout to com-pensate for orbit variations.1  INTRODUCTIONThe first two CLIC Test Facilities, CTF1 and CTF2have demonstrated the basic principles of rf power produc-tion by decelerating a high current electron beam. CTF3is foreseen to check the feasibility of pulse compressionin the driving beam structure [1].The facility will be installed at CERN, in the existingLIL and EPA buildings, which will be available after LEPshutdown. It will use the existing 3 GHz klystrons andmodulators for the production of the beam. The macro-pulse from the linac is 1.4 µs long and is composed by140 ns long sequences of odd and even buckets. Insideeach sequence the bunch separation is twice the rf wave-length, 20 cm, with maximum bunch charge of 2.3 nC.The first compression is done in the delay loop, whereeach sequence of odd buckets is recombined with the nearsequence of even ones. In the resulting beam structure the140ns bunch trains, in which the bunch separation hasbecome 10 cm, are followed by 140ns long voids.The second compression phase is done in the combinerring (CR), 84 m long (twice the delay loop). Each groupof successive 5 bunch trains does n-1/2 turns inside thering, with n from 1 to 4. When they are extracted thebunch separation is reduced down to 2 cm, and thedistance between successive bunch trains is four times140ns.Preservation of beam emittance and longitudinal corre-lation during the compression imposes tight requirementson the ring design. The design of the combiner ring is thesubject of this paper. A preliminary design, in which thegeneral characteristics were already discussed, is describedin [1]. The availability of magnets (dipoles andquadrupoles) of EPA and related transfer lines has sug-gested the possibility of reuse for the CR project. Thefollowing design is based on the use of these magnets.2 COMBINER RING LATTICEThe ring consists of four isochronous arcs, two shortsections housing the path length tuning device and twosymmetric opposite long sections for injection and extrac-tion. Quadrupole triplets around the arcs are used formatching. The layout of the ring is shown in Fig. 1 andthe main parameters are listed in table I.Figure 1: Combiner ring layout.Table I – CR main parametersEnergy (MeV) 160/350Circumference (m) 84Bending Radius (m) 1.075No. Cells 4No. Dipoles 12Dipole Field (T) 0.5/1.1No. Quadrupoles 52Max.Int.Gradient (T) 0.9/1.8No. Sextupoles 24Max.Int.Gradient (T/m) 6/13Max. beta (m) (H/V) 11/11Max. Dispersion (m) .73Betatron Tune (H/V) 8.10/3.63Chromaticity (H/V) -11/.92.1 Isochronous ArcThe arcs are triple bend achromats, with negativedispersion in the central dipole, which makes vanish theterm of the first order transport matrix relating path lengthwith energy:RD dsx56 0= ∫ =ρ(Dx is the dispersion and ρ the bending radius).1Three quadrupoles in each half arc allow tunability ofbetatron functions and tunes. Tuning the arc centralquadrupole by few percent is the simple knob, whichvaries the momentum compaction α c around zero. Figure2 shows the amount of variation of α c obtained bychanging the gradient by ±6%.0.000500.0005-0.0500.05-8 -6 -4 -2 0 2 4 6 8∆(m)K (%)Figure 2: Momentum compaction (full points) and R56(hollow points) tuning.2.2 Injection and ExtractionEqual structures are designed for the injection and ex-traction regions, which have similar requirements, thusdoubling the ring periodicity. Injection and extractionsepta are placed symmetrically (see fig.1) in the middle ofa straight section 2.7m long. The septum is placed at thesection center, in a free zone, 2.7 m long. The π phaseadvance in between the two injection kickers is created byfour quadrupoles arranged symmetrically around the sep-tum. In the extraction region this assures the necessaryπ/2 phase advance between the extraction kicker and theseptum. The space corresponding to the second kicker canbe used to house a rf cavity in the hypothesis of storingthe beam for diagnostic purposes.A critical issue is the value of the horizontal betatronfunction, βxk, at the fast rf injection kickers, affecting theamount of beam loading of the successive turns [3].Values of βxk  = 2 m and αxk = 0 correspond to minimumbeam loading. The design allows the necessary tunability.The optical functions from the middle of the arc to theinjection section are shown in Fig. 3.Figure 3: Optical functions from mid arc toinjection/extraction.2.3  Path-length tuning sectionThe tuning of the relative phase of injected and circulat-ing bunches and the compensation of orbit variations isneeded twice in the ring, to adjust independently eachbunch train. The path-length tuning devices are variablefield, one period, wiggler magnets, housed in the shortstraight sections. With respect to a magnetic chicane thewiggler offers the advantages to be more compact, with alarger tuning range, and a lower contribution to the R56term. It has two half end poles and a central pole, inde-pendently powered, with bending angles –7°, 14°, -7° andtotal length 1.6m. Changing the field by ±12% the pathlength variation is ± 1mm, which is twice the originalrequirement[1]. The dispersion function introduced by thewiggler is selfmatched and the contribution to the mo-mentum compaction, -5 10-3 per wiggler, is compensatedinside the arcs. Figure 4 shows the path length variationas a function of the wiggler field and the correspondingchange of the matching triplet gradients. The optical func-tions from the arc center to the wiggler center are shownin Fig. 5.-1.5-1-0.500.511.50123456-15 -10 -5 0 5 10 15( m-2)(mm)∆ B/B (%)Figure 4: Path length (full points) as a function of wigglerfield and matching triplet strength (hollow points)Figure 5: Optical functions from mid arc to wiggler center3  ISOCHRONICITYThe path length variation with energy is given up tothe second order by:c t RppTpp∆ ∆ ∆= + +56 5662...where T566 is the element of the second-order transfer ma-trix.2The isochronicity to the first order is assured by thezero integral of the dispersion function in each arc of thering. Particles with energy deviations ±2.5% and withzero transverse initial invariant change their path lengthby up to 2.5 cm, with a clear second order dependence(see Fig. 6); even going to α c values of few 10-2, thesecond order term is still dominant. The cancellation ofthe second order term is done by sextupoles. A focusingsextupole in each arc can do the job but this solution,increasing the negative vertical chromaticity leads to iso-chronicity loss in the presence of vertical oscillations.The best solution is a compromise between the T566 can-cellation and the chromaticity correction. A possible solu-tion, which can still be improved, is the use of three sex-tupoles in each half arc, which give a chromaticity of–1, -.3 in the horizontal and vertical plane respectivelyand T566 = 0.002. Figure 6 shows the path length fordifferent ∆p/p of initially on axis particles.6a) 6b)Figure 6: Path length with energy deviation with first (6a)and second (b) order isochronicity correction.4  COHERENT SYNCHROTRONRADIATIONOne of the potentially dangerous effects for the beamemittance preservation is the coherent synchrotron radia-tion wake. Short, high charge bunches can emit coherentsynchrotron radiation (CSR) at wavelengths longer thanthe bunch length. The main effects on the beam itself areenergy loss and energy spread. An analytic evaluation ofthe effect is possible in the case of constant bunch lengthand circular orbit, with uniform bending angle. In the caseof the CR it has been shown that the effect, while beingrelevant, is still tolerable. In particular, the distortion tothe longitudinal phase space distribution does not preventthe final bunch compression. However in the combinerring the bunches do not have a constant length, nor fol-low a circular orbit. Therefore, a check has been madeusing the code TraFiC4 [4], recently tested successfullyagainst experimental results in CTF2 [5]. A full isochro-nous cell has been used in the simulation. The rms bunchlength oscillates in the cell from about 1.1 mm to1.7 mm, in good agreement with MAD calculations.Unfortunately, TraFiC4 does not yet allow the use of sex-tupoles, so that the second order momentum compactioncannot be corrected, and the final bunch length after onepass in the cell is slightly longer than the initial one(from 1.35 mm to about 1.4 mm). In spite of that, a firstevaluation of the longitudinal wake has been obtained byplotting the difference between initial and final energy ofthe macroparticles against their initial position in thebunch (see Fig. 7). The average energy loss and themacroparticle energy loss are in reasonable agreementwith the analytical evaluation of the wake.Figure 7: Rms bunch length evolution in one CTF3 arc cell(top) and average energy loss along the cell (bottom), asevaluated by TraFiC4.5  EXTRACTION KICKERA pulsed kicker which extracts the entire bunch trainhas been designed. The flat top duration is slightly longerthan the entire CR bunch train length (143 ns) in order todeflect all the bunches with the same angle. The rise timeis shorter than the distance of the kicker from the injec-tion septum (~60 ns) to allow the injection and extractionof the last linac pulse in the same turn. The kicker isbased on stripline pair design where each electrode, 50 cmlong, forms with the vacuum chamber a transmission lineof 50Ω characteristic impedance. The kicker has been de-signed using HFSS electromagnetic 3D code, and a proto-type has been built to measure the efficiency and to studyand damp the high order mode trapped in the structure.REFERENCES[1] CLIC Study Team, "Proposals for Future CLIC Studies and a NewCLIC Test Facility (CTF3)" - CERN/PS 99-047 (LP) and CLICNote 402 (1999).[2] H.H.Braun et al.,’The CLIC RF Power Source’ – CERN 99-06[3] S.Gallo et al., "Studies on the RF Deflectors for CTF3", thisConference.[4] A. Kabel, M. Dohlus, T. Limberg, “Using TraFiC4 to Calculateand Minimize Emittance Growth due to Coherent SynchrotronRadiation”, to be published in NIM A.[5] H.H. Braun, R. Corsini, L. Groening, F. Zhou, A. Kabel, T.O.Raubenheimer, T. Limberg, “Emittance Growth and Energy Lossdue to Coherent Synchrotron Radiation in the Bunch Compressorof the CLIC Test Facility (CTF 2), this Conference.3

CTF3: Design of Driving Beam Combiner Ring

CTF3: Design of Driving Beam Combiner Ring

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