LEP is now routinely filled from two elementary injector cycles, one positron and one electron. During each of these cycles, 8 bunches are accelerated in the PS accelerator, then the SPS and then injected into 4 bunches in LEP. Two SPS bunches being injected into the same LEP bucket. This mode of operation has become possible as a result of synchrotron injection. The time between successive injections into the same LEP bucket is presently 6 LEP turns. This value is optimum for the present synchrotron tune used at injection in LEP. The LEP injectors normally run with a 14.4 second cycle repetition, of which four 1.2 second elementary cycles were traditionally dedicated to LEP filling. However with the new scheme only two elementary cycles (each lasting 1.2 seconds) are needed for LEP. The time saved, amounting to one sixth of the total injector cycle time has become available for other users. During the last year these have included extra physics cycles in the PS and heavy ion and LHC machine development cycles in the SPS. No loss in performance has been observed with the accumulation rates in LEP. The scheme as put into place will be described in detail

Baudrenghien, P

Collier, Paul

CERN Document Server

EUROPEAN LABORATORY FOR NUCLEAR RESEARCHCERN - SL DIVISIONCERN-SL-96-21 OPDOUBLE BATCH INJECTION INTO LEPP. Baudrenghien, P. CollierAbstractLEP is now routinely filled from two elementary injector cycles, onepositron and one electron.  During each of these cycles, 8 bunches areaccelerated in the PS accelerator, then the SPS and then injected into 4 bunchesin LEP.  Two SPS bunches being injected into the same LEP bucket. This modeof operation has become possible as a result of synchrotron injection. The timebetween successive injections into the same LEP bucket is presently 6 LEPturns.  This value is optimum for the present synchrotron tune used at injectionin LEP. The LEP injectors normally run with a 14.4 second cycle repetition, ofwhich four 1.2 second elementary cycles were traditionally dedicated to LEPfilling. However with the new scheme only two elementary cycles (each lasting1.2 seconds) are needed for LEP. The time saved, amounting to one sixth of thetotal injector cycle time has become available for other users. During the last yearthese have included extra physics cycles in the PS and heavy ion and LHCmachine development cycles in the SPS. No loss in performance has beenobserved with the accumulation rates in LEP. The scheme as put into place willbe described in detail.Paper presented at the 5th European Particle Accelerator Conference (EPAC’96),10-14 June 1996, Sitges (Barcellona), Spain.Geneva, Switzerland26 June, 1996DOUBLE BATCH INJECTION INTO LEPPhilipe Baudrenghien & Paul Collier, CERN, Geneva, SwitzerlandABSTRACTLEP is now routinely filled from two elementaryinjector cycles, one positron and one electron.  Duringeach of these cycles, 8 bunches are accelerated in the PSaccelerator, then the SPS and then injected into 4 bunchesin LEP.  Two SPS bunches being injected into the sameLEP bucket. This mode of operation has become possibleas a result of synchrotron injection. The time betweensuccessive injections into the same LEP bucket ispresently 6 LEP turns.  This value is optimum for thepresent synchrotron tune used at injection in LEP. TheLEP injectors normally run with a 14.4 second cyclerepetition, of which four 1.2 second elementary cycleswere traditionally dedicated to LEP filling. However withthe new scheme only two elementary cycles (each lasting1.2 seconds) are needed for LEP. The time saved,amounting to one sixth of the total injector cycle timehas become available for other users. During the last yearthese have included extra physics cycles in the PS andheavy ion and LHC machine development cycles in theSPS. No loss in performance has been observed with theaccumulation rates in LEP. The scheme as put into placewill be described in detail.1. INTRODUCTIONDuring 1994, LEP operated with a pretzel scheme,colliding 8 bunches of positrons with 8 bunches ofelectrons.  The beams being separated in mid-arc by theuse of  electrostatic separators installed at the ends of eachof the 8 straight sections [1]. In this mode the mostefficient way to operate the injectors was to provide,within one elementary injector cycle, 8 equidistant SPSbunches, which could be directly transferred into 8 equallyspaced buckets in LEP. This scheme had severaladvantages for the LEP injector chain, shown in figure 1,consisting of the LEP Injector Linac, the electron-positron accumulator, the PS, and SPS.  Within oneinjector supercycle, only 8 bunches of positrons can beproduced.  If these are taken on one elementary cycle thenthe other cycles become free for other users. The use ofonly one electron and one positron cycle also simplifiesoperation considerably.  In 1995, however, LEP moved to bunch train operation.In this scheme four equidistant trains of up to 4 bunchesper beam are accumulated.  The bunch spacing within thetrain was chosen to be 247 m s [2].  With this scheme it isnot possible to inject into more than one bunch of eachtrain within the same elementary injector cycle.  In orderto maintain the 8 bunch mode in the injectors, it wastherefore necessary to design a new scheme which wouldallow to inject two bunches from the injectors into onebucket in LEP.  As both injections would be within thesame elementary cycle, the time between them would bein the sub-millisecond, rather than the second range.  Asthe transverse damping time in LEP at injection is of theorder of 400 ms, the beam from the first injection wouldhave no time to damp out the injection oscillations beforethe second batch arrived.  For betatron injection thiswould have almost certainly resulted in the loss of thefirst batch as the second was injected.   A means ofavoiding this problem was found once LEP moved to theuse of synchrotron injection.*LEPSPSPSLPIe-e +p Pb ionselectronspositronsL3ALEPH OPALDELPHIEPA LILFigure 1 : The LEP Injector Chain of Accelerators2. MULTI-BATCH SYNCHROTRONINJECTIONSynchrotron injection has been routinely used in LEPnow for more than one year [3].  It has several advantagesover the more normal betatron injection; including flattrajectories in the straight sections of the machine and afactor of two faster damping of injection oscillations.  Inaddition this method of injecting beam into the machineallows several injections into the same RF bucket over aperiod of a few turns.Injection in synchrotron phase space relies on theinjection point being in a place where the dispersion (inthe plane of the injection) is non-zero. The circulatingbeam is brought close to the septum with an AC bumpand the incoming beam is injected with a negative offsetin energy such that the product of the energy offset andthe dispersion matches the distance to the circulatingbeam. The sign of the energy offset is determined bywhether the beam is injected on the inside, or the outsideof the machine. The injected beam then follows thenatural closed orbit that a circulating particle would havewith the same energy offset.  As the beam is injected off-energy, it will perform synchrotron oscillations at thesynchrotron tune (Qs) and slowly damp into thecirculating beam.The synchrotron tune is always very much lower than thebetatron tunes.  In LEP the horizontal betatron tune isnominally 90.28, whereas the synchrotron tune is 0.085.The latter corresponds to about 12 LEP turns to completea single synchrotron oscillation.  6 turns after theinjection, the recently injected beam will have an energyoffset of opposite sign. It will thus be further away fromthe septum and will not be perturbed by a secondinjection. This is illustrated in Figure 2 for the case ofthe injection of two batches.  Using this method up tofour batches can be injected, separated in time by _ of thesynchrotron period in the receiving machine.  EfEfRF Bucketcirculating beamcirculating beamTurn N+0.5/Qs; Injection 2Turn N; Injection 1Figure 2: Multiple Injections into the Same RF Bucket.The situation for the double batch scheme implemented inLEP is shown schematically in figure 2.  The secondinjection has to be  timed to take place an odd multiple of_Qs after the first. The bunch pattern in the injectors fixesthe minimum time between batches at 6 LEP turns (seesection 3).  Extra time between batches can be added, tocope with changing values of Qs in LEP by insertingmultiples of 7 LEP turns (27 SPS turns), given by thecircumference ratio between the two machines.  Thebunch to bucket configuration repeats exactly with thisperiod.The Qs in LEP strongly affects the delay betweensuccessive batch injections. Figure 3 shows the variousbatch delays as a function of Qs in LEP.  The verticalscale corresponds to the angular rotation of the first batchin phase space in the time between successive injections.For optimum injection with the double batch scheme,cosine(q ) should be close to -1.00.510.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16QsCoq) 61 32 02 73 4TurnsFigure 3 : The Choice of the Second Batch Delay isDetermined by the Qs in the Receiving Machine.For the present configuration in LEP, the six turn delay isused.  This gives a maximum for values of Qs around0.085.  In the future higher values of Qs will be used toachieve higher accumulated intensities in LEP.  For thisscheme, larger delays will be necessary.  For example,with a Qs of 0.13, the delay between the first and secondbatches should be 27 turns.There is a forbidden region in Qs for the present scheme.This is for values in the range 0.14 < Qs < 0.15. Thisrange corresponds to the region where one synchrotronperiod is around 7 LEP turns.  Generating an odd multipleof _Qs is not possible here by adding multiples of  7turns.3.   MODIFICATIONS TO THEINJECTOR CHAINThe new injection scheme called for modifications in theRF systems of both the PS and the SPS.  Only the SPSwill be presented here.  The re-phasing used in the PS waspresented in the original  LEP design report[4]. In theSPS, three RF systems are used for lepton capture andacceleration:• Long lepton bunches from the PS are captured andaccelerated during the first part of the cycle by a 100MHz system consisting of 4 cavities.• Bunches are then accelerated by adding a 200 MHzsystem, consisting of 21 cavities.• In the middle of the accelerating ramp, four   superconducting RF cavities operating at the LEPfrequency (352.2 MHz) are used to supply sufficientadditional voltage to reach top energy (22 GeV).We wish to inject 8 SPS bunches into 4 equi-spaced  LEPbuckets.  Keeping the harmonic number of the SPS RF at352.25 MHz unchanged (h1 = 8120), and given the ratioof the LEP and SPS radii (R2/R1 = 27/7), one can showthat only 7 SPS buckets can be used for injection into agiven LEP bucket [5]. We thus have 7 families of SPSbuckets, each family containing 4 equi-spaced buckets andthe families being rotated, one with respect to the other,by a multiple of 1/7th of an SPS turn. Any pair offamilies can be chosen, but a more uniform choice for thesecond family is the one resulting in a pattern of 8bunches in the SPS spaced by 3/28th, 4/28th, 3/28th of anSPS turn, as shown in figure 4.Figure 4 : The Arrangement of the 8 bunches in the SPS.Capture and acceleration of this bunch pattern implies thatthe 3 lepton RF systems operate with harmonic numbersthat are all divisible by 28.  With the frequencies in usefor the scheme in use until 1994, this condition was notsatisfied by the 100 MHz and the 200 MHz systems(respective harmonic numbers h1 = 2312 and h1 = 4616).The new parameters are given in table 1. The 200 MHzsystem could easily be operated at the new frequency, bychanging the temperature of the water cooling circuits forthe cavities.  The 100 MHz cavities had to be squeezed by7 mm to reduce their resonant frequency by 650 kHz.Figure 5 shows the kicker pulses injecting the 8 SPSbunches into the 4 LEP buckets.  The time betweensuccessive injections into one LEP bucket is set to 6turns.  As explained in section 2, this corresponds to arotation of 183.6o (with Qs = 0.085), very close to theideal value of 180o.SPS RF (at 352 MHz)Harmonic Number352.209  MHzh1 = 8120 (=23.5.7.29)SPS RF (at 200 MHz)Harmonic Number200.395  MHzh1 = 4620 (=22.3.5.7.11)SPS RF (at 100 MHz)Harmonic Number99.590  MHzh1 = 2296 (=23.7.41)Table 1 : The New SPS RF Parameters.4.   CONCLUSIONSThe double batch injection scheme has been usedoperationally throughout 1995.  It has worked well withno reduction in the accumulation rate of beam in LEP.  Infact, with only one injector cycle for each particle typebeing used, more effort could be put into keeping thesecycles optimised. This has resulted in a higher beamtransport efficiency to LEP.The availability of spare time in the injector supercycleshas allowed extra machine development time to beallocated in the SPS.  During 1995 an estimated 42 daysof available beam time were used for machinedevelopment purposes using the time liberated by the newLEP injection scheme.  This time is set to growsignificantly in the coming years.      5.   REFERENCES[1] Jowett J., ‘Pretzel Operation’, Proceedings, 3rd LEPPerformance Workshop, Chamonix, 1993.[2] Herr W., ‘First Experience with Bunch Trains’, Proc.,5th LEP Performance Workshop, Chamonix, 1995.[3] Collier P ., ‘Synchrotron Phase Space Injection intoLEP’, Proceedings, PAC-95 Conference, Dallas,[4] Hubner K., ‘Bunch Transfer in the LEP InjectorChain’, LEP Note 411, October 1982.[5] Baudrenghien P. & Linnecar T.P.R.,‘SPS BunchPattern for LEP Bunch Trains’,SL-Note/94-76(RF),1994Figure 5 : Kicker Pulses Transferring 8 SPS Bunches into 4 LEP Buckets

Double batch injection into LEP

Abstract

Similar works

Full text

Available Versions

CERN Document Server