The tight transverse emittance budget for the bright beams foreseen for the LHC era demands that all sources of emittance blow-up in the injector chain are reduced to a minimum. A critical region is the transfer between the PS Booster (PSB) and the 26 GeV PS. The four rings of the PSB run with RF harmonic one, and for the LHC beam the PS will be filled with eight bunches originating from two consecutive PSB cycles. Thus, each bunch will be different and has to be individually treated. The present recombination scheme introduces an important difference in lattice parameters between the bunches from different rings. The difference between the bunches would, if left uncorrected, result in a substantial emittance blow-up. Several possible improvements of the recombination stage have been studied, including magnet shims, correction quadrupoles and an RF quadrupole magnet. To complement the theoretical studies, the contribution of mismatch and missteering to the emittance blow-up have been measured using a LHC-type beam, measuring the emittance in the PS with a wire-grid and fast wire-scanners. Results of the calculation and the measurements will be discussed and a strategy to minimise the blow-up will be indicated

Jansson, A

Lindroos, M

Martini, M

Schindl, Karlheinz

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN – PS DIVISIONCERN/PS 98-058 (OP)STUDY OF EMITTANCE BLOW-UP SOURCES BETWEEN THE PS BOOSTER ANDTHE 26 GeV PSA. Jansson, M. Lindroos, M. Martini, K. SchindlThe tight transverse emittance budget for the bright beams foreseen for the LHC era demands that all sources ofemittance blow-up in the injector chain are reduced to a minimum. A critical region is the transfer between the PSBooster (PSB) and the 26 GeV PS. The four rings of the PSB run with RF harmonic one, and for the LHC beam the PSwill be filled with eight bunches originating from two consecutive PSB cycles. Thus, each bunch will be different andhas to be individually treated. The present recombination scheme introduces an important difference in latticeparameters between the bunches from different rings. The difference between the bunches would, if left uncorrected,result in a substantial emittance blow-up. Several possible improvements of the recombination stage have been studied,including magnet shims, correction quadrupoles and an RF quadrupole magnet. To complement the theoretical studies,the contribution of mismatch and missteering to the emittance blow-up have been measured using a LHC-type beam,measuring the emittance in the PS with a wire-grid and fast wire-scanners. Results of the calculation and themeasurements will be discussed and a strategy to minimise the blow-up will be indicated.6th European Particle Conference (EPAC'98), June 22-26, 1998, Stockholm, SwedenGeneva, Switzerland20 November 1998STUDY OF EMITTANCE BLOW-UP SOURCES BETWEENTHE PS BOOSTER AND THE 26 GEV PSA. Jansson , M. Lindroos, M. Martini, K.Schindl, CERN, Geneva, SwitzerlandAbstractThe tight transverse emittance budget for the brightbeams foreseen for the LHC era demands that all sourcesof emittance blow-up in the injector chain are reduced toa minimum. A critical region is the transfer between thePS Booster (PSB) and the 26 GeV PS. The four rings ofthe PSB run with RF harmonic one, and for the LHCbeam the PS will be filled with eight bunches originatingfrom two consecutive PSB cycles. Thus, each bunch willbe different and has to be individually treated. Thepresent recombination scheme introduces an importantdifference in lattice parameters between the bunches fromdifferent rings. The difference between the buncheswould, if left uncorrected, result in a substantial emittanceblow-up. Several possible improvements of therecombination stage have been studied, including magnetshims, correction quadrupoles and an RF quadrupolemagnet. To complement the theoretical studies, thecontribution of mismatch and missteering to theemittance blow-up have been measured using a LHC-typebeam, measuring the emittance in the PS with a wire-gridand fast wire-scanners. Results of the calculation and themeasurements will be discussed and a strategy tominimise the blow-up will be indicated.1  INTRODUCTIONThe production of the relatively low intensity, but verybright beam for LHC will put high demands on efficientemittance conservation in the injector chain. The injectormachines were originally, to a large extent, designed forhigh intensity beams meaning that some sources forpossible emittance blow-up were either ignored orconsidered irrelevant. A systematic review of all suchpossible sources has therefore been initiated. The resultspresented here concern the passage of the beam from the1 GeV PS Booster (PSB) to the 26 GeV PS.2  THEORETICAL STUDIESThe sources of emittance blow-up during the beamtransfer can be separated in two categories; dynamicsources, which originates from fluctuations in powersupplies and magnet imperfections, and static sources thatare properties of the nominal optics.2.1  Dynamic sources of blow-upThe dynamic blow-up sources related to the beamtransfer PSB-PS are mainly missteering and betatronmismatch. These are treated within the framework of theAutomated Beam Steering (ABS) [1] project and havebeen found to be manageable.2.2  Static sources of blow-up The PSB consist of four vertically stacked acceleratorsfrom which beam can be ejected in sequence andrecombined at the level of the PS during transfer. Thevertical recombination scheme introduces a horizontalmismatch in the Twiss values due to edge effects in therectangular vertical bending magnets.  A theoretical studyof this process has already been published [2] and inTable 1 the results expressed as Twiss parameters at acommon reference point are given. For the sake ofconvenience this point was chosen at the first SEM-gridin the PSB measurement line, a line to which the beamcan be deviated after the vertical recombination iscompleted.  Table 1: Theoretical lattice parameter values at the firstSEM-grid of the PSB measurement lineHorizontalBeam from ring 1 2 3 4βTwiss 4.96 5.46 6.46 6.14αTwiss 1.47 1.72 2.06 1.77Relative mismatch 38% 28% 0% 15%VerticalBeam from ring 1 2 3 4βTwiss 5.21 5.27 5.27 5.21αTwiss 1.62 1.62 1.63 1.63Relative mismatch 1.6% 0.7% 0% 2.1%3 EXPERIMENTAL STUDIESThe measurements were performed partly in the PSBmeasurement line and partly in the PS using the availableinstrumentation. The use of the SEM-grids in themeasurement line was motivated by the fact that they canbe used without interfering with the production beam. Forstudies of relative differences between the four PSB ringsthe further transfer and injection of the beam into the PSis irrelevant.3.1 Difference between PSB ringsThe Twiss values of the recombined beam at the firstSEM-grid in the PSB have been measured, in Table 2 theresults for the horizontal plane is shown.Mismatchvectors with resp. to ring 3-0.3-0.2-0.1 0 0.1 0.2 0.3-0.3-0.2-0.100.10.20.3Theoretical124-0.4 -0.2 0 0.2 0.4-0.4-0.200.20.4Measured124Figure 1: The theoretical and measured mismatchvectors at the first SEM-grid in the PSB measurementline. Table 2: Measured horizontal lattice parameter valuesat the first SEM-grid of the PSB measurement lineHorizontalBeam from ring 1 2 3 4βTwiss 4.03 3.90 5.75 5.71αTwiss 1.19 1.63 1.75 1.59The average measured values proved to be within 30% ofthe theoretical values. However, more interesting is thefact that plotting the experimental and measuredmismatch vectors in the same diagram, Figure 1, showsthe expected symmetry between the four rings.Vector addition of the mismatch vector of ring 2 and 4gives both for the theoretical and experimental values themismatch vector of ring 1, a symmetry that origins fromthe symmetry in the used recombination scheme, seefigure 2.3.2 Dispersion mismatchMeasurements have been performed to determine thepresent dispersion mismatch between the transportedbeam and  the PS lattice. If the momentum spread is σp, adispersion mismatch blows up the rms emittance by [3]It is important to note that the absolute blow-up isindependent of the beam emittance, while beingproportional to the square of the momentum spread. Thismeans that dispersion mismatch is important for smallbeams with large momentum spread, such as the LHCbeam which will have ε=1.1 µm and σP=1.25 10-3. Themeasured numerical value of the expression inparentheses was found to be 0.4-0.5m in the case of thePSB-PS beam transfer. Using the values for the LHCbeam, this predicts a rms blow-up due to dispersion of15-25%, which clearly indicates that dispersion matchingis important. Therefore, a change of optics is necessary inorder to eliminate the dispersion mismatch.3.3 Measured emittance blow-up in the PS ringTo validate the theoretical formulae for blow-up due tovarious kinds of misadaptation, a series of measurementshave been performed.In the first series, the influence of missteering wasstudied. The measured values, plotted in Figure 3, shownrather good agreement with the theoretical predictionIn the second series of measurements, the emittanceincrease due to betatron mismatch was investigated. Herethe results (shown in Figure 4) was less obvious tointerpret, since a betatron mismatch created by a changein a quadrupole in the transfer line necessarily isaccompanied by a dispersion mismatch, if the dispersionis non-zero at the quadrupole as in this case.  Themeasured effect is therefore a combination of the two.However, when this is taken into account, the datafollowes the theoretical prediction quite well.4 POSSIBLE IMPROVEMENTS4.1 PSB recombination schemeThe betatron mismatch from the recombination schemecan only be reduced if additional hardware is added to therecombination line. These additions could include: i)magnets shims for vertical bending magnets, ii)individual quadrupoles in the four PSB lines or iii) a setof RF quadrupoles in the PSB-PS transfer line. Thedifferent possibilities have all been theoretically studied.The addition of shims proved effective but unrealistic [4]( )222 ''22DDDDp ∆⋅+∆∆⋅+∆⋅=∆ βαγσεFigure 3: The measured emittance blow-up versus theinjection steering error. The line is the theoreticalpredictionBT4.BVT10BT4.SMV10BT1.BVT10BT1.SMV20BT.BVT20BT.SMV201234Figure 2: The PSB recombination schemedue to the complicated design of the compact verticaldipoles. The addition of a single quadrupole in ring 2after the recombination of ring 1 and 2 can reduce themismatch considerably, see table 2. The focal length ofthis quadrupole should be 125 m. Constrains on theavailable space in the recombination line leave this as theonly possible additional quadrupole. The possible, butmaybe not necessary, use of two RF quadrupoles in thefollowing transfer line could probably render themismatch entirely negligible. Table 2: The theoretical mismatch values afterrecombination using one additional quadrupole in ring 2.HorizontalBeam from ring 1 2 3 4βTwiss 6.18 6.91 6.86 6.14αTwiss 1.80 2.10 2.06 1.77Relative mismatch 13% 2.4% 0% 15%VerticalBeam from ring 1 2 3 4βTwiss 5.39 5.45 5.27 5.21αTwiss 1.72 1.72 1.63 1.63Relative mismatch 6.0% 5.1% 0% 2.1%4.2 Dispersion at PS injectionThe dispersion mismatch can be reduced by modifyingthe optics of the transfer line. The magnet positions in theline are basically fixed due to lack of space, which meansthat the possible changes in transfer line optics areconstrained to changes in quadrupole gradients. With the10 presently used quadrupoles, it is not possible to matchthe lattice parameters and the dispersion to the PS at thesame time. However, the line has an eleventh quadrupolewhich has not been used up to now because of itsproblematic position inside a shielding wall. With thisextra degree of freedom, it is theoretically possible tomatch the dispersion and the Twiss parameterssimultaneously [5]. Test of this matched optics has sofargiven better, but not yet perfect, results.5 CONCLUSSIONSThe theoretical study of possible sources of mismatchbetween the PSB and the PS points towards severalpossibly critical points: the used recombination scheme,the dispersion mismatch and the possible drift in steeringand betatron parameters at PS injection. The mostimportant of them being the dispersion mismatch. Themeasurements performed shows that all these effects canbe observed as a measurable emittance blow-up of abeam similar to the beam for the LHC.  A number ofpossible improvements have been investigated and it hasbeen shown that the emittance blow-up can be keptwithin the limits of the foreseen emittance budget for theLHC beam. However, it is doubtful whether that can beachieved without the addition of some hardware in thePSB recombination line.REFERENCES[1] M. Lindroos, M. Arruat, B. Autin, R. Cappi, L.Giulicchi, A. Jansson, A. Lombardi, M. Martini, M.Gourber-Pace, K. Schindl, O. Tungesvik, D.Williams, “Automated Emittance Preservation in thePS Complex”, PAC’97, Vancouver, May 1997.[2] A. Jansson, M.Lindroos, “Mismatch between thePSB and PS due to the present vertical recombinationscheme”, Particle Accelerators 58 (1997) p.201[3] D.A.Edwards, M.J.Syphers,”An introduction to thephysics of high energy accelerators”, John Wiley &Sons, New York[4] A. Jansson, “A simple way of reducing the PSB-CPSmismatch due to the vertical recombination schemeby using shims”, CERN note: PS/DI/Note 97-14(Tech.) [5] M.Martini, T.Risselada, K.Schindl, “Measuremnt ofBetatron and Dispersion matching between Boosterand PS”, Cern note: PS/PA Note 91-1340302010 10 20 30 40dI +Amps/255075100125150175200s + /Figure 4: The measured emittance blow-up versus the aquadrupole current. The parabolic curve shows thetheoretical prediction.

Study of emittance blow-up sources between the PS booster and the 26 GeV PS

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