Guignard, Gilbert

CERN Document Server

SL/Div Reps9 mai 1995 OCR OutputGeneva, SwitzerlandPAC95 , Dallas 1-5 May 1995directly apply to single—bunch mode and can be extended to multibunch mode after adequate adjustments.boost the fraction of luminosity contained to within two percent of the centre-o -mass energy. The conclusionsratio and beam-size aspect ratio were adjusted in order to keep the average energy loss in the collisions low andpreservation of the very small vertical emittance, in the presence of strong wakefields. Simultaneously, the emittancethe increase in the ratio between the bunch intensity ande vertical beam-size for improving luminosity, and on thelinac, emphasis was put on the mirumization of the ener -spread for limiting losses in the telescope acceptance, on §libeam parameters that simultaneously satisfy the emittance requirements and the experimental conditions. ln the mainFor the Compact Linear Collider (CLIC) study, the beam dynamics has been revisited in order to search for consistentAbstractG. GuignardUPDATING OF BEAM DYNAMICS IN THE CLIC MAIN LINACW 93 Z 3CUC NmCERN-sL-95-23CE“N‘SL‘°5‘23llIllllllllllllllIllIlllllllllllllllllllll éiljCERN LIBRARIES, GENEVACERN - SL DIVISIONEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEEG(7:; ;,; OCR Outputimately by algebraic formulae which are partly deduced 5 ~ LL · h L ~ L9. 4 B 6, . wit 6,( lThe beam-bearn phenomena can be described approxII. BEAM-BEAM PHENOMEN A approximated bymeaning. When the beam is flat and T is small, 6 B can beand reviews the arguments on which it was based. where the physics constants 1-,, A, and Q have their usualthe single-bunch parameters recently carried out for CLICadded. The prwent article deals with the reoptimization of3 <>below the desirable valuu the multibunch option should be <Z * E . e E > *24 l1+(1·5T)"“l’AE o¤0,'I`2 1conditions briefly recalled above. If the luminosity remainstion of the single—bund1 parameters that includes all the-—-M. — 6¤a,(6;+&;) nl }i,·y (1+T’/3)*/2T = cxa,T 1 z 2.54—i5 r“·yN;, Lmade obvious the necmity to perform a general optimizabeing too large and unacceptable This considerationand the relative energy loss 6B due to beamstrahlungpoint, the average energy lm during collision in particularthe average number n., of emitted photons per electron,provide satisfactory physim conditions at the interactionfractional energy of the photons emitted in the collision,sion parameters coming later. However, this proved not totities: the beamstrahlung parameter 'I` proportional to thetrol of the emittance dilution, the optimization of the oolli—beam phenomena can be characterized by three basic quanin the past our attention was focused mainly on the conter’. Starting from Eqs. (1) and (2), the consequent beam(R-band) of the accelerating structurw are so strong thatinteraction point and D is the so-called ‘disruption parame(CLIC), the wakehelds associated with the high frequencyA is the ratio of the bunch length 0, to the B-function at thethe beam before collision. In the Compact Linear Colliderteract and emit photons, and the intrinsic energy spread of "" ·r¤;,,(¤; +¤;) ’ "" B;collision as a function of the energy of the leptons that in 2r,N;,0, A _ 2selected for optimizing the luminosity, its distribution atother hand, the same beam parameters must be carefully withdeflections in order to prevent ernittance dilution. On themizing perturbations such as linear coupling and wakefield = i/4 __ _ HD 1+D 1+D3ln(»/E-i—1)+2ln A] (2)to ensure beam stability and bunch coherence while mini 0.8D3 ()]of the bunch as well as its population must satisfy criteria[31of opposite charge. On the one hand, the three dimensionsH D, the behaviour of which are deduced from simulationsthe bunch when it collidw with a counter travelling bunchetition rate. The pinch effect is described by the factorspresent in the linac, and by the strong forcw that disruptNb is the number of particles per bunch and f,,,, the repcusing conditions and the waketields that are unavoidablyels in the linac is simultaneously constrained by the fo f (R) - jgIn a. linear collider, the dynamics of the beam that trav 2R3 1 +withI. INTRODUCTION‘”extended to multibunch mode after adequate adjustments. (Hoi 1*/* " (Ho.)’(“’— - M ·· Z M "; ` "clusions directly apply to singlebunch mode and can be41r6;6*;within two percent of the centre-0f·mass energy. The conlisions low and boost the fraction of luminosity contained to L = fl‘¢padjusted in order to keep the average energ loss in the colously, the emittance ratio and beam-size aspect ratio were aspect ratio R = 41;/er;transverse beam sizes and depend on the nominal beamemittanoe, in the presence of strong wakefields. Simultanecluding the disruption effects at collision which pinch thenosity, and on the preservation of the very small verticalrecalled hereafter, starting with the luminosity L and inintensity and the vertical beam-size for improving lurni·remains essential [4, 5]. The most important formulae areceptance, on the increase in the ratio between the bunchparameters, though verification by numerical simulationsof the energy-spread for limiting lossa in the telwoope acThey were therefore used in our search for optimized beamIn the main linac, emphasis was put on the minimizationround or Hat beams and of providing simple scaling laws.emittance requirements and the experimental conditions.they offer the advantage of giving good results for eithersistent beam parameters that simultaneously satisfy thevery reliable in the intermediate use of quasi—flat beams,dynami has been revisited in order to search for confrom numerical simulations [2, 3]. Although they are notFor the Compact Linear Collider (CLIC) study, the beamG. Guignard, CERN, 1211 Geneva. 23, SwitzerlandUPDATING OF BEAM DYNAMICS IN THE CLIC MAIN LINAClution along the linac due to wakefields, which are in turn not exceed the acceptance of the final focus (~ :i:5%0 ) [9]. OCR Outputergy, at the extraction, is perfectly symmetrical and doesas much as possible, taking into account the emittance dithe linac and iterating until the charge distribution with enthe energy spread and the ratio Ng,/\/§ has to be raisedstage. The best cuts are determined by tracking throughbunch length can hence be selected in order to minimizegitudinal cuts when the bunch is rotated by the secondof the arguments based on beam-beam phenomena. Thebe deduced from the dynamic in the linac, independently [8]; this momentum collimation then transforms into lontum collimation in the first stage of the bunch compressorpect ratio R(z 33). The other beam parameters must thenRF voltage. Such a shaping can be provided by momenc.m. energy) that determines the requirement on the aslinear increase of WL that better matches the rise of theshould start from a given value of 6 8 (say 3.5% for 500 GeVwith a sharp edge in the front so as to obtain a quasiThwe considerations indicate that a reoptimizationby the RF wave. In addition, the bunch can be shapedbest compensation of the longitudinal wakefield variationFigure 1: Variation of 5B, 'I` and L with 0,.phase cbgp and of the appropriate 0,, which ensures thevi · B, lmml all the necessary freedom for the selection of a positive RF0 0.2 0.4 0.8 0.8 is achieved with microwave quadrupoles [7]. This gives us0.0ergy spread ensuring beam stability, since BNS dampingpoint that there is no need in CLIC for a deliberate enO.1mination of the bunch length 0,. Let us recall at thisOnce the intensity is fixed, one can turn to the deter0.2Figure 2: Variation of Nb / \/Q with bunch population.0.3N. ti<>°10.4 P \ Yu5 6 7 8 9 10 110.50.62 I- *1Eqs. (4) andcan be adjusted independently, as can be seen fromthan 68. In addition, for constant 5B,Nb and 0; "·{a;€-/°' ''``` `that, according to Eq. (4), L decreases las rapidlyWiden the horizontal beam size 0;, with the advantagethe L—distribution.physics, since it corraponds to an enlarged spread ofapparent gain on 6 8 for small 0, is not welcome forcrease of 0, by a factor of three or four. Moreover, theright balance between dispersion and wakefield effects.that a signihcant gain on 6B impliw a prohibitive inenergy that is specific to CLIC (B ~ ·y°·35), for it gives thein Fig. 1 for the CLIC parameters, indeed indicatahave been naturally done with the betatron scaling withraise 0;. The complete dependence of 6 8 on 0,, shownbeen adopted for the bunch population. All the trackingsL directly, but may boost wakefield effects and thenthe kind of correction applied, this new value of 8 x 109 hasIncrease the bunch length 0,, which doa not changecurrent. Although this kind of threshold may depend onunacceptable reduction in the luminosity.which begins to blow up significantly beyond this bunchDecrease the bunch population, but this involves anNb = 8 x 109. Figure 2 shows also the emittance yey,produced a curve (Fig. 2) with a maximum at around20-25%. Looking at Eq. (4), there are three ways to reduceulations with a simple one-to-few trajectory correction [6],pinched, leading to an average energy loss 6B larger thanwhen increasing Nb. Limited investigations, based on simBoth transverse beam dimensions were therefore stronglynext step consisted in looking for the maximum of Nb / ,/Q(> 1).injection of 0.5 >< I0'7. As mentioned in Section II, theand, consequently, that the horizontal disruption was highfor an intensity of Nb = 6 x 109 and for an emittance atthe fact that the aspect ratio R was as low as ~ 112 x 10" rad·m at 250 GeV, in the presence of wakefields,The previous beam parameters of CLIC suffered fromsibility of obtaining a vertical normalized emittance qc, ofPrevious tracking in the main linac [6] indicated the pcsL ~ Nr, -. (5)5 /2 evIII. BEAM DYNAMICS IN THE MAIN LINAChourglass effect):was done for CLIC.ing 6; and by using the relation 0, = B; (minimizing theproportional to Nb. The next section describes how thisand thwe two relations can than be combined by eliminatfor emittance preservation on the one side and for accept [12] H. Braim et al., this conference.described in Sections II and III, reconcile the requirements [11] C. Fischer, this conference.The updating of the beam dynamics in the CLIC linac, [10] C. Fischer, Proc. EPAC 94, London, 1994.[9] O. Napoly, CLIC Note 227, CERN, 1994.IV. NEWLY PROPOSED PARAMETERS[8] G. Guignard, T. d’Amico, Proc. EPAC 94, 1994.[7] W. Schnell, I. Wilson, PAC91, San Francisco, 1991.the B'-values, that is B; = 0.18 mm and B; = 10 mm.[6] G. Guigiard, Proc. PAC93, Washington, 1993.B; is about 90% of a,. These last two conditions dictate[5] V. Telnov, private communication.to ~ 55. In addition, the hourglam effect is minimum when[4] K. Yokoya, KEK Report 85-9, 1985.interaction point (Section II), the B' ratio has to be equal[3] P. Chen, Proc. PAC93, Washington, 1993.(9 GeV). Now, given the required aspect ratio of 33 at the[2] K. Yokoya, P. Chen, Lect. Notes in Physics 400, 1990.which can be reached easily if ye, = 2.5 x 10"° at injection[1] G. Loew, B. Wiik, Proc. LC93, SLAC·436, 1993.qc, = 3 >< 10'6 at 250 GeV and 3.9 x 10`° at 500 GeV,the end of the linac). This explains the proposed valuw of VI. REFERENCEStion of ·ye,,.is easier when ye, is about 20 times larger (at500 GeV. Then, experience tells us that such a minimiza who introduced him the physiw of beam—beam phenomena.The author is very grateful to P. Chen and V. Telnov,yey = 1.5 x 10“' at 250 GeV and qc, = 2 x 10"' atmization of the microwave quadrupola, one can achieveV. ACKNOWLEDGEMENTSby a few percent. With such a method and a good optiof the intensity, changing simultaneously the momentummeasurements made at full intensity and, say, at a tenth Luminosity in > 98% c.m. (%) 63 68current retained. They are based on trajectory difference 2.53 4.0Repetition rate (kHz)for with the same misalignments and the higher bunch 1033 cm"? s‘1[11] indicate that even better performance can be hoped 1.0 2.2Luminosity with pinchrithms. Recent irwwtigations of beam-based corrections 3.5 7.5Energy low 63 (%)tors of 10 nm [10], using dispersion- and wake-free algo 1.35 1.53Number of photons n.,alignments of accelerating structuxw and position moni 0.075 0.179Parameter Tcontrol has been obtained while coping with r.m.s. mis Disruption parameters 0.29 x 9.7 0.22 x 7.4of ye, = 2 x 10‘7 can be considered. Such an emittanoe 0.94Hourglass factorCLIC dynamics have shown that final valuw at 250 GeV Pinched FF beam sizm (nm) 232 x 5.6 194 x 4.75tical emittance must be very small, and studies of the Nominal FF beam sizes (nm) 247 x 7.4 200 x 6tion. Because the aspect ratio must be large, the ver 10 x 0.18Final Focus B"-valuw (mm)components and on the quality of the trajectory correc (10`7 rad·m)trol of the wakefields, on the misalignments of the linac Final normalized emittances 30 x 1.5 39 x 2values of the emittances, which depend directly on the con 0.2Bunch length (mm)The next critical parameters are of course the absolute Bunch population 8 >< 109Final energy (GeV) per linac 250 500Figure 3: Relative energy distribution at the linac end.Newly proposed CLIC parametersTable 1dE/Etot·0.01 -0.005 0 0.005 0.01detail before final conclusions can be drawn.0.00dynami of a train of bunches has still to be studied inthe luminosity by a factor of 5, approximately. The beam0.05 etition rates in order to keep the power constant increasesproving the performance; using 10 bunches with lower repretained. However, it relies on multibunch mode for im0.10able luminosity of I or 2 x 10cms"‘ for the energies33 `°that CLIC can deliver with one bunch only an already valu0.1 5consumption considerations [12]. It is interesting to knownosities are also given for repetition rates dictated by power0.20 approximately 20%. As an indication, single-bunch lumiulating the collisions [4, 5] and found to agree to withiniments. All these valuw have been cross-checked by programs simthe linac is a required condition for specific physics exper now achieved for the horizontal disruption, T,n.., and 88.of ~ 2.3%¤ . Such a minimization of the energy spread in quoted in II. One can emphasize the low values that areergy separation of ~ 6%¤ and to an r.m.s. energy spread sponding to the new conditions obtained by the formulaeand a, = 0.2 mm. It corrwponds to a ‘pea.k-t0—peak’ en side. Table 1 summarizm the parameter values correFigure 3 shows the distribution obtained with my- = 12° able conditions in the physics experiments on the other

Updating of beam dynamics in the CLIC main linac

Abstract

Similar works

Full text

Available Versions

CERN Document Server