In the framework of the CLIC (Compact Linear Collider) RF power source studies, the scheme of electron pulse compression and bunch frequency mulitiplication, using injection by RF deflectors into an isochronous ring, will be tested, at low charge, during the preliminary phase of the new CLIC Test Facility (CTF3) at CERN. In this paper, we describe the beam dynamics studies made in order to assess the feasibility of the bunch combination experiment, as well as the related beam measurements performed on the LEP Pre-Injector complex (LPI) before its transformation into CTF

Corsini, R

Ferrari, A

Rinolfi, Louis

Risselada, Thys

Royer, P

Tecker, F A

CERN Document Server

BEAM DYNAMICS FOR THE PRELIMINARY PHASEOF THE NEW CLIC TEST FACILITY (CTF3)R. Corsini, A. Ferrari, L. Rinolfi, T. Risselada, P. Royer, F. TeckerCERN, Geneva, SwitzerlandAbstractIn the framework of the CLIC (Compact Linear Collider)RF power source studies, the scheme of electron pulsecompression and bunch frequency multiplication, using in-jection by RF deflectors into an isochronous ring, will betested, at low charge, during the preliminary phase of thenew CLIC Test Facility (CTF3) at CERN. In this paper,we describe the beam dynamics studies made in order toassess the feasibility of the bunch combination experiment,as well as the related beam measurements performed on theLEP Pre-Injector complex (LPI) before its transformationinto CTF3.1 INTRODUCTIONThe new CLIC Test Facility (CTF3) [1, 2] is being builtat CERN in the LPI area. The aim of this facility is todemonstrate the technical feasibility of the CLIC RF powersource by generating a 30 GHz RF pulse with the CLICnominal peak power and pulse length [3]. The scheme useselectron pulse compression and bunch frequency multipli-cation based on the combination of electron bunch trainsin isochronous rings using RF deflectors for injection. Theconstruction of CTF3 is scheduled in stages over five years.As a preliminary phase, to begin in September 2001, itis planned to test the bunch combination scheme at lowcharge [4], making use of the existing LPI components.In the following, the beam dynamics studies for this pre-liminary phase of CTF3 [5] are described and the relatedbeam measurements performed on the LPI installation arereported.2 BEAM DYNAMICS STUDIESThe general layout of the CTF3 preliminary phase isshown in Figure 1. A pulse train of up to five pulses isproduced at the source in the front-end, is then acceleratedin a linac and injected into an isochronous ring. Each pulseis composed of 20 electron bunches, spaced by 10 cm (cor-responding to the 3 GHz acceleration frequency). The dis-tance between the pulses corresponds to the ring circum-ference, so that the pulses can be combined turn after turnat injection, by interleaving their bunches using RF deflec-tors. A single pulse with up to five times the initial pulsecharge and up to five times the bunch repetition frequencywill thus be obtained after combination.2.1 The Front-EndSince the former LEP Injector Linac (LIL) thermionicgun is not adapted to the CTF3 requirements, a collabo-ration with the “ Laboratoire de l’Acce´le´rateur Line´aire”(LAL, Orsay) has been established to build a newthermionic gun similar to the one used in the CLIO ma-chine [6]. This triode gun produces a train of up to sevenpulses with a repetition rate of 50 Hz. The nominal lengthof each pulse is 6.6 ns FWHM, and the pulses are spacedby 420 ns, corresponding to the ring circumference. Thegun operates with a nominal peak current of 1 A. At theexit of the gun, the LIL bunching system, composed ofa single-cell pre-buncher and a standing wave buncher, ispowered independently by a 30 MW klystron and producesa 3 GHz bunched beam at 4 MeV. Each pulse then con-sists of 20 bunches, each bunch having a charge of 0.1 nCand a length of about 10 ps FWHM. The transverse match-ing section between the front-end and the linac is made ofone solenoid and one quadrupole triplet, with independentpower supplies.2.2 The LinacSome modifications of the former linac are mandatory:the first part of LIL is dismantled, eight of the sixteen ac-celerating structures are removed, the existing bunchingsystem with the new gun is moved downstream, and a re-arrangement of the remaining components is required. Thepulse train is then accelerated to the reference energy of350 MeV, using eight travelling wave accelerating struc-tures which are powered in groups of four by two 45 MWklystrons. The whole electron pulse train is accelerated us-ing a single RF pulse. The beam parameters have beenchosen in order to minimize the energy spread generatedby beam-loading in the LIL structures, while still keepinga charge per bunch high enough to give a good resolutionfor the measurements of the beam time structure with astreak camera. Because of beam-loading, the total energyspread is about 6 MeV, which is within the ring full accep-tance (±1%). It can nevertheless be reduced by a factor oftwo by delayed RF filling of the structures, or by dumpingthe first two pulses out of seven in the train. Indeed, thefrequency multiplication test requires at most five pulses.The transverse matching to the injection line is done at theend of the linac with five independent quadrupoles. Thismatching section is instrumented to provide a full charac-terization of the beam, with time-resolved measurements ofthe transverse parameters and of the beam energy spectrumusing Transition-Cherenkov monitors.0-7803-7191-7/01/$10.00 ©2001 IEEE. 3777Proceedings of the 2001 Particle Accelerator Conference, ChicagoFigure 1: Layout of the CTF3 preliminary phase complex.2.3 The Injection LineThe transfer line from the linac to the ring must beisochronous and achromatic in both planes (in addition tothe main horizontal bending magnets, the line contains twovertical dipoles to match the heights of the linac and of thering). Isochronicity is mandatory in order to preserve thebunch length from the linac to the ring. This is essential forthe combination process with the RF deflectors, for whichshort bunches (≤ 15 ps FWHM) are needed. To achievethe isochronicity condition and the transverse matching tothe new ring lattice, two new quadrupoles are added andthe positions and gradients of the existing quadrupoles aremodified. The overall geometry of the line is preservedbut all these quadrupoles need independent power supplies.This configuration allows a quasi-isochronous operationwith |R56| ≤ 10−2 m where the R56 factor is the stan-dard linear dependence of the path length with the energyspread. The second order in isochronicity is small and nosextupole corrections are needed in the transfer line.2.4 The RingThe lattice of the existing Electron-Positron Accumula-tor (EPA) ring is modified to make the ring isochronous (i.ewith a zero momentum compaction factor), thus preserv-ing the bunch length and spacing during the combinationprocess (up to five turns). In the new isochronous lattice,the dispersion vanishes in the long straight sections. Thisnew optics requires the displacement of four quadrupolesand the decoupling of three of the six existing quadrupolefamilies. Although the symmetry with respect to the cen-tre of each of the four arcs is abandoned, the symmetryof the ring with respect to the centre of each of the fourstraight sections between the arcs is preserved. By choos-ing appropriate integer tune values, the required gradientsare kept at a moderate level, and the new isochronous op-tics can be achieved with the present magnets. In order tobetter match the arc optics to the straight section optics, onequadrupole in each arc is displaced by 1.85 m towards thelong straight section. The natural chromaticities of the newisochronous lattice are comparable with those of the for-mer EPA lattice. Equally, the off-momentum path lengthscan be controlled using the chromaticity sextupoles (orga-nized in three families) to within 0.5 mm per turn across themomentum range of the bunches, while keeping the chro-maticities small. Tracking over 1000 turns shows horizon-tal and vertical dynamic apertures around 15σ across themomentum range of the incoming beam. Combination fac-tors of three, four and five will be tested in the preliminaryphase of CTF3. With a given ring layout, the different com-bination factors can be explored by changing the frequencyof both the linac and the RF deflectors by ±150 kHz. Inorder to follow the change of the RF frequency the ac-celerating structures and the deflectors are tuned in oper-ation by varying their temperature by ± 3◦ C [7]. The ringcircumference is fixed to an average value, correspondingto a combination factor four. This requires a reduction of17 mm of the ring circumference (whose nominal value is40 π m, as recently confirmed [7]). Apart from the standardring instrumentation, the main diagnostic tool is the streakcamera which is used to observe the beam structure and toprovide the demonstration of the bunch combination.2.5 The Injection using RF DeflectorsIn order to achieve bunch frequency multiplication, thebunch trains are injected into the isochronous ring usingtwo RF deflectors. The deflectors are located in the ringwith a betatron phase advance of π between them, so thatthey produce a time-dependent closed bump of the refer-ence orbit, allowing the interleaving of three to five bunchtrains. The RF deflectors are travelling wave, iris-loadedstructures for which the fundamental mode is a deflectingmode, with a phase advance of π/2 per cell and a negativegroup velocity. They are powered by one of the existing30 MW klystrons, and a power of about 7 MW per cavityis needed to obtain the nominal deflecting angle of 4.5 mradat 350 MeV. Figure 2 shows the variation of the transversekick for the injected and the circulating bunches. The cor-responding beam envelope simulations are shown in Fig-ure 3. The fast injection kickers are kept in the ring toallow conventional single-turn injection, for checking thering optics prior to the installation of the RF deflectors.Another kicker remains to extract the circulating beam toa dump located in the former positron injection line.3778Proceedings of the 2001 Particle Accelerator Conference, ChicagoFigure 2: Kick amplitudes for the injected and the first twocirculating bunches for a combination factor of five. Theextension corresponds to ±2σ of a Gaussian distributionwith a length of 10 ps FWHM.3 RELATED BEAM MEASUREMENTSRecent isochronicity tests were performed in the EPAring to demonstrate the capability to control the momen-tum compaction factor to the level required for the prelimi-nary phase of CTF3. The tests were made with a charge of2.5 nC per pulse and with an energy close to the nominalone (308 MeV instead of 350 MeV) [7]. The isochronouslattice was obtained by changing the strength of the exist-ing quadrupole families. However, the dispersion did notvanish in the long straight sections and the transfer line wasbadly matched to the ring, causing beam losses in the firstfew turns. Nevertheless, streak camera images showed thebunch structure unchanged up to the 40 th turn in the ring,compared to only a few turns with the nominal optics. Af-ter one turn in the ring, the measured bunch length is 33 psFWHM, due to bunch lengthening in the non-isochronoustransfer line, compared to 66 ps FWHM with the nominaloptics in the ring. These values are fully consistent witha bunch length of 7 ps FWHM at the end of the linac, aspreviously measured [8].In order to validate a new model of the existing linac,which will be used during the commissioning of the pre-liminary phase of CTF3, quadrupole scans were performedwith different acceleration conditions to characterize theTwiss parameters and the energy gains along the linac [7].Trajectory analysis allowed a better understanding of theenergy gains in the structures. The simulations are now invery good agreement with the measurements for the hori-zontal plane, but a small discrepancy remains to be under-stood in the vertical plane, though not critical.The exploration of the whole range of combination fac-tors relies on the fact that the LIL accelerating structurescan be tuned in operation by varying their temperature, inorder to follow the change of the RF frequency. Recentmeasurements [7] have shown that the dependence of theresonant frequency of the cavities on the temperature is aspredicted by theory and that no measurable effects are ob-served on the beam when following the frequency changewith the temperature.Figure 3: Beam envelopes of the bunches of Figure 2. Fromtop to bottom: injected bunch, circulating bunches withphase 2π5 and4π5 . The lines are the limits of the vacuumchamber and the rectangle stands for the injection septum.4 CONCLUSIONThe beam dynamics studies described in this paper takeinto account analytical and numerical calculations, as wellas results of a series of beam measurements made on theLPI installation. This analysis allowed us to fix the lay-out and the optics of the new CLIC Test Facility CTF3 forits preliminary phase. In addition, the required hardwaremodifications of the former LPI complex were identified.These modifications have begun, and the commissioning isexpected to begin in September 2001, prior to the bunchcombination tests.5 REFERENCES[1] G. Guignard (Ed.), “A 3 TeV e+/e− Linear Collider Basedon CLIC Technology”, CERN 2000-008 (2000).[2] CLIC Study Team, “Proposals for Future CLIC Studies anda New CLIC Test Facility (CTF3)”, CERN/PS 99-047 (LP)and CLIC Note 402 (1999).[3] R. Corsini (Ed.), “The CLIC RF power source: a novelscheme of two beam acceleration for electron-positron lin-ear colliders”, CERN 99-06 (1999).[4] R. Corsini, A. Ferrari, J.P. Potier, L. Rinolfi, T. Risse-lada, P. Royer, “A Low Charge Demonstration of ElectronPulse Compression for the CLIC RF Power Source”, Proc.20th International Linear Accelerator Conference, Mon-terey, USA, 2000.[5] R. Corsini, A. Ferrari, L. Rinolfi, T. Risselada, P. Royer,F. Tecker, “Beam Dynamics for the CTF3 PreliminaryPhase”, CLIC Note 470 (2001).[6] J.C. Bourdon, R. Belbeoch, M. Bernard, P. Brunet,B. Leblond, M. Omeich, E. Plouviez, J. Rodier, “Commis-sioning the CLIO injection system”, Nucl. Instr. Meth. inPhysics Research A 304, 322-328 (1991).[7] R. Corsini, B. Dupuy, A. Ferrari, L. Rinolfi, T. Risselada,P. Royer, F. Tecker, “LIL Lattice Parameters, LIL EnergyGains, LIL Temperature vs RF Frequency, EPA Circum-ference and EPA Isochronicity Measurements”, CTF3 note2000-18 (MD).[8] R. Corsini, A. Ferrari, L. Rinolfi, T. Risselada, P. Royer,F. Tecker, “LIL Bunch Length and Lattice Parameters Mea-surements in March 2000”, CTF3 note 2000-09 (MD).3779Proceedings of the 2001 Particle Accelerator Conference, Chicago

Beam Dynamics for the Preliminary Phase of the New CLIC Test Facility (CTF3)

Rinolfi, L

Risselada, T

Tecker, F

Beam Dynamics for the Preliminary Phase of the New CLIC Test Facility (CTF3)

Abstract

Similar works

Full text

Available Versions

CERN Document Server

CERN Document Server