The time structure of the CLIC (Compact Linear Collider) drive beam is obtained by the combination of electron bunch trains in rings using RF deflectors [1]. The rings must be isochronous, in order to preserve the bunch length and separation during the combination process (4-5 turns). A first isochronicity test has been performed in the CERN EPA (Electron Positron Accumulator) ring. The calculated isochronous lattice can be obtained by changing the strength of existing quadrupole families without hardware modifications. Measurements of the synchrotron frequency and of the beam's time structure have been made for both the normal and the isochronous lattices. Streak camera measurements of the bunch length have been used to tune the lattice around the isochronous point. The bunch length increases rapidly over a few turns in the normal case, while no appreciable bunch lengthening is observed over 50 turns in the isochronous case. A quantitative evaluation of the momentum compaction is obtained by measuring the bunch separation in a train when close to, and far from, the isochronous condition. Plans for future tests in the EPA ring are also outline

Corsini, R

Potier, J P

Rinolfi, Louis

Risselada, Thys

Potier, JP

Rinolfi, L

Risselada, T

CERN Document Server

Isochronous optics and related measurements in EPA

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN-PS DIVISIONCERN/PS 2000-033 (AE)CLIC-Note 440ISOCHRONOUS OPTICS AND RELATED MEASUREMENTS IN EPAR. Corsini, J.P. Potier, L. Rinolfi, T. RisseladaAbstractThe time structure of the CLIC (Compact Linear Collider) drive beam is obtained by the combination of electron bunchtrains in rings using RF deflectors [1]. The rings must be isochronous, in order to preserve the bunch length andseparation during the combination process (4-5 turns). A first isochronicity test has been performed in the CERN EPA(Electron Positron Accumulator) ring. The calculated isochronous lattice can be obtained by changing the strength ofexisting quadrupole families without hardware modifications. Measurements of the synchrotron frequency and of thebeam's time structure have been made for both the normal and the isochronous lattices. Streak camera measurements ofthe bunch length have been used to tune the lattice around the isochronous point. The bunch length increases rapidlyover a few turns in the normal case, while no appreciable bunch lengthening is observed over 50 turns in the isochronouscase. A quantitative evaluation of the momentum compaction is obtained by measuring the bunch separation in a trainwhen close to, and far from, the isochronous condition. Plans for future tests in the EPA ring are also outlined.7th European Particle Accelerator Conference, 26th-30th June 2000Vienna, AustriaGeneva, Switzerland18 July 2000ISOCHRONOUS OPTICS AND RELATED MEASUREMENTS IN EPAR. Corsini, J.P. Potier, L. Rinolfi, T. Risselada, CERN, Geneva, SwitzerlandAbstractThe time structure of the CLIC (Compact LinearCollider) drive beam is obtained by the combination ofelectron bunch trains in rings using RF deflectors [1]. Therings must be isochronous, in order to preserve the bunchlength and separation during the combination process (4-5 turns). A first isochronicity test has been performed inthe CERN EPA (Electron Positron Accumulator) ring.The calculated isochronous lattice can be obtained bychanging the strength of existing quadrupole familieswithout hardware modifications. Measurements of thesynchrotron frequency and of the beam's time structurehave been made for both the normal and the isochronouslattices. Streak camera measurements of the bunch lengthhave been used to tune the lattice around the isochronouspoint. The bunch length increases rapidly over a fewturns in the normal case, while no appreciable bunchlengthening is observed over 50 turns in the isochronouscase. A quantitative evaluation of the momentumcompaction is obtained by measuring the bunchseparation in a train when close to, and far from, theisochronous condition. Plans for future tests in the EPAring are also outlined.1  LATTICEThe dispersion in the normal EPA lattice, used for LEPoperation, is shown in Fig. 1. The dispersion is zero in thestraight sections, and positive (up to 2.2 m) in the arcs.The momentum compaction is D = 0.034. To first order,the momentum compaction is linked to the dispersion DXand the bending radius in the magnets by the expression:ds1 ‡ UD XDCwhere the integral is over the ring circumference C. Inorder to reduce D to zero, DX has to be forced to negativevalues in some of the bends. This can be obtained byincreasing the strength of quadrupoles located in a regionwhere DX z 0. In the EPA ring it is not possible to do sowhile keeping DX = 0 in the straight sections, without re-cabling the existing quadrupole families. Therefore acompromise solution (shown also in Fig. 1) has beenfound [2], in which the dispersion oscillates in the straightsections. The injection line is, in this case, badly matchedto the ring and the larger beam envelopes give rise tobeam losses in the first few turns. The two centralquadrupoles in the arcs (QFL family) contribute most tothe change in D, while the other quadrupoles in the arcsare used to trim the tune values. The strength ofquadrupoles in the straight sections has been keptunchanged in order to keep the injection bump closed. Figure 1: Dispersion in EPA different optics (half ringshown).2 SYNCHROTRON FREQUENCYMEASUREMENTSIn order to estimate the values of D, the synchrotronfrequency fS was measured for different optics,corresponding to calculated values of D ranging from0.034 to –0.01. The measurement requires a stored ebeam and the use of the RF cavity, and therefore excludesD values too close to zero. Unfortunately, all attempts tomeasure fS for negative D values failed. Fig. 2 shows thatthe measured values do not coincide with  the theoreticalones (solid line) for small values of D, pointing to animprecision of the theoretical model. A similarobservation [3] was reported in an earlier experiment in1992. The reason for this discrepancy is not yetunderstood. Figure 2: Calculated and measured synchrotronfrequencies.3 STREAK CAMERA MEASUREMENTS3.1 Experimental apparatusIn order to measure the bunch length over many turnsin the EPA, the synchrotron light emitted [4] has beenobserved with a streak camera. The source of thesynchrotron light is located 192.5 mm upstream of thecentre of the third dipole of the last arc. The dispersion atthis place is 1.5 m for the normal optics and 1.7 m for theisochronous optics. The source length is limited by theoptical aperture to about 50 mm. The light can be dividedusing a movable beam splitter in order to monitor itsintensity on-line with a diode, while using the streakcamera, and to set up the proper timing.3.2 MeasurementsThe beam was injected axially, and lost on the septumafter the desired number of turns by firing the secondinjection kicker. The pulse was composed of severalbunches, spaced by 333 ps, corresponding to the 3 GHzaccelerating frequency. The total intensity of the pulse,measured at injection, was 2.4 u 1010 electrons. The beamenergy was 500 MeV, and the FWHH energy spread was1.8 %. Streak camera images were recorded for differentturns (up to the 40th for the isochronous optics). In Fig. 3the streak camera image of the pulse is shown. Theabscissa is time (about 5 ns full scale), while the ordinatecorresponds to horizontal particle position. The bunchesin the pulse are visible (about ten can be counted). TheFWHH pulse length was about 4 ns. The light source islocated in a dispersive region, therefore the horizontalbeam position and size depend on the beam energy andmomentum spread, respectively. This is evident in theimage, where the momentum-time correlation due tobeam loading along the pulse is clearly visible. In Fig. 4and Fig. 5, corresponding to a faster sweep-time (2.25ps/pixel), the time profile of the individual bunches canbe seen. The FWHH bunch length at injection, evaluatedusing a Gaussian fit, was about 60 ps.tx ('p/p) Figure 3: Streak camera image of the electron pulse.0 200 400 600 800 100001000200030004000t (ps)I (a.u.) 11I (arbitrary units)Figure 4: Pulse profile in time with normal optics at the1st turn (bottom) and 10th turn (top). The loss of structure(due to the bunch length increase) and the decrease inbunch-to-bunch spacing are clearly visible.0 200 400 600 800 100001000200030004000t (ps)I (a.u.) I (arbitrary units)11Figure 5: Pulse profile in time at the 1st turn (bottom) and20th turn (top), for the isochronous optics. The timestructure is virtually unchanged.The bunch structure inside the pulse was almost lostafter 10 turns in the normal machine. Also the distancebetween bunches decreased in the normal machine, asexpected, due to the bunch-to-bunch energy variationcaused by beam-loading along the pulse. The machineoptics was then changed and put close to isochronicity,and the exact tuning found by changing the current in theQFL quadrupole family, while looking at the streakcamera image of the bunches. The optimum value foundfor the QFL current was 88.5 A, to be compared to thetheoretical value of 84.5 A, obtained from the MADmodel simulations. In this case the bunch length and thebunch distance were essentially unchanged over at least20 turns. A further measurement was also taken after upto 50 turns. At this point, beam losses started to limit thelight intensity at the streak camera, and affect itsresolution. In spite of that, it was possible to evaluate thebunch length, still unchanged. The bunch-to-bunchdistance, evaluated off-line afterwards for the 40th turn,was only slightly reduced (by about 7 ps). In order tomake sure that the observed constant bunch length wasnot due to losses, a different kind of measurement wastaken. The QFL current was changed by –1 A and + 2 Aaround the estimated isochronous condition. Streakcamera images of the beam were taken.D > 0 - QFL = 87.5 AD = 0 - QFL = 88.5 AD < 0 - QFL = 90.5 AI (arbitrary units)0 500 100000.51t (ps)0 500 100000.51 0 500 100000.51 Figure 6: Streak camera images and time profiles fordifferent settings of the QFL quadrupole family.The results are shown in Fig. 6, and demonstrateclearly the transition from a positive to a negative valueof D. From top to bottom, the bunch length reaches aminimum, then increases again while time-momentumcorrelation along each bunch changes sign.4   ANALYSIS OF MEASUREMENTSWhen D  is close to zero, the measured variation inbunch length is too close to the resolution limit to obtaina reliable evaluation of D. In addition, the increase ofbunch length should be quadratic rather than linear, anddepends both on the initial time-momentum correlationwithin each bunch and on its charge distribution. Thenon-zero momentum compaction of the injection line('L/L / 'p/p), that cannot be tuned to be isochronous, isan added complication. On the other hand, the variationin bunch-to-bunch distance is both relatively large andlinear with the number of turns:   0tNppcCNt '' ' Dwhere N is the number of turns, C the ring circumference,c the speed of light and 'p/p the bunch-to-bunch energyvariation. The bunch distance is easier to use than thebunch length for comparison with the normal case (seeFig. 4), where the bunches are mixed after a few turns,while the distance between bunch centres is stillmeasurable. We know that 't = 333 ps (corresponding tothe RF period) at the end of the linac.At injection, 't is reduced by the momentum compactionof the transfer line. It can be evaluated using a linear fit tothe data taken with the normal optics, obtaining 't(0) =320 ps. In Fig. 7, the measured bunch spacing is plottedas a function of the number of turns, for both the normaland the isochronous case. A linear fit of the measureddata is also shown.0 10 20 30 40100200300No of turnsBunch spacing (ps)  Figure 7: Bunch-to-bunch spacing as a function of thenumber of turns for the normal (squares) and theisochronous optics (circles).The fit can be used to evaluate the momentumcompaction in the isochronous case, equal to the ratio ofthe slopes, times the normal momentum compaction:NNIImmDD  Assuming DN = 0.034, which has been confirmed bythe synchrotron frequency measurement, one gets DI = 2.3u 10-4 for the isochronous case. The data spread for theisochronous machine (Fig. 7) is due to re-tuning of theQFL current while the measurements were taken.Therefore the estimated value of D is rather an averagethan a precise value. Nevertheless, such evaluation isvalid for the last data point taken, where the sensitivity ishigher.5   CONCLUSIONSThe preliminary tests on EPA have demonstrated thepossibility of obtaining and measuring values of D assmall as 2.3 u 10-4, close to the goal of the future CLICtest facility, CTF3 (|D | d r 10-4). We hope to performfurther measurements this year, in order to reach such avalue and check the higher orders of momentumcompaction, and their compensation by sextupole tuning[2]. As a preliminary stage of CTF3, in the year 2001, weplan to modify the EPA quadrupole families and theinjection line. This would allow us to obtain anisochronous lattice with DX = 0 in the straight sectionsand a good matching of the line to the ring. The next stepwill be a first demonstration, at low charge, of theprinciple of beam combination with RF deflectors.REFERENCES[1] H. Braun and 16 co-authors, “The CLIC RF PowerSource-A Novel Scheme of Two Beam Accelerationfor er Linear Colliders”, CERN 99-06 (1999).[2] T. Risselada, “An isochronous optics for EPA”,CERN PS/LP Note 99-02, (1999).[3] L. Rivkin, Private communication[4] S. Battisti, J.F. Bottollier-Depois, B. Frammery,E. Marcarini, “The synchrotron radiation measure-ments in EPA”, CERN/PS 87-72 (LPI), (1987).

Isochronous Optics and Related Measurements in EPA

Isochronous Optics and Related Measurements in EPA

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