Tigner, M

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Particle Accelerators1975, Vol. 6, pp. 211-212© Gordon and Breach Science Publishers Ltd.Printed in the United KingdomIMPROVED METHOD FOR FILLING AN ELECTRONSTORAGE RING FROM A SYNCHROTRONtM. TIGNERCornell University, Ithaca, N.Y.14853, USA(Received January 31, 1975)By switching bunches back and forth between the storage ring and an injyctor synchrotron of circumference differentfrom that of the storage ring, many bunches circulating in the storage ring can be combined rapidly into a single bunch.Such a technique can decrease the filling time for the storage ring significantly. An example is given in which it is estimatedthat 100 bunches circulating in a storage ring the size of Cornell synchrotron can be combined into a single bunch in1.7 sec.INTRODUCTIONIn electron storage rings the luminosity at highenergy is usually limited by the amount of rf poweravailable. Thus to maximize the luminosity onemust arrange that all of the charge be contained inone bunch circulating in each direction. In loadinga storage ring operating in this mode from asynchrotron one is at a disadvantage because,while the charge circulating in the synchrotronis only one to two orders of magnitude less thanthat required in the storage ring (electrons only),this charge is divided up into hundreds of bunchesonly one l of which can be put directly into thedesired bunch in the storage ring. In other wordsthe effective transfer efficiency is very low.2 Thesituation is doubly bad for positrons since thecharge circulating in the storage ring is reducedby the conversion efficiency from e- to e+ .3This situation can be ameliorated if an econom-ical method can be found by which a significantfraction of the total charge accelerated in thesynchrotron can be combined into one or a fewbunches in the storage ring. In principle a lowfrequency rf system using adiabatic and radiationdamping of the energy oscillations in the syn-chrotron combined with transverse phase-spacestacking in the storage ring can accomplish thistask. Such a scheme is proposed for EPIC.4Alternatively one may use an intermediatestorage ring between the synchrotron and the mainstorage ring. In this case one accumulates charge int Work supported by the National Science Foundation.211many bunches in the auxiliary storage ring andthen stacks them in transverse phase space intoone or a few bunches in the main storage ring.Such a scheme is proposed for PETRA. 5THE METHODIt is our purpose here to suggest how this desiredbunch combination might be carried out veryrapidly without the use of an auxiliary rf systemor storage ring. We assume that the injectorsynchrotron is capable of accelerating its beamto such an energy that the damping time forbetatron oscillations in the storage ring (SR) iscomparable with or smaller than the cycle timein the synchrotron (SYN). While the principlecan be applied in a number of ways only one isrecounted here.Suppose that the SR and its injector SYN aredifferent in circumference by a carefully selectedamount, that amount to be specified momentarily.Suppose further that the· SYN is filled with andaccelerates bunches spaced in time by the combinedrise and fall times of a fast kicker system capableof ejecting bunches from the storage ring. Havingbeen accelerated in the SYN this multibunchbeam is then extracted in a single turn and injectedinto the SR using a fast beam bump and septumin the SR. This process is repeated many times in theusual way, the beam in the SR being built up bystacking in transverse phase space. When the totalcharge stored in this multibunch beam is equalto the amount required in the single bunch theinjector of the SYN is switched off, the SYN212 M. TIGNERitself continuing to cycle. The beam in the SR isnow ready to be compressed. Focus attention onone of the bunches in the SR and call this bunchthe accreting bunch (AB). Assume for illustrativepurposes that the circumference of the SR is largerthan that of the SYN.6 Now fire a fast extractionsystem in the SR which ejects the bunch followingthe AB but leaves the remaining bunches un-disturbed. This bunch is reinjected into the SYNand circulates therein. The circumferences of thetwo rings are chosen different by an integral numberof rf wavelengths, that number being a submultiple,n, of the separation of the bunches. Since the cir-cumferences are different the rf buckets in the tworings slide past each other in phase, those in theSYN gaining on those in the SR. After n revolutionsin the SYN the bunch in question arrives at theinjection-ejection point in phase conjunction withthe AB and is reinjected into the SR and stackedtogether with the AB in transverse phase space, thetwo merging through radiation damping. The nextfollowing bunch is now fast ejected from the SRand injected into the SR. After 2n revolutions thisbunch is now in phase conjunction with the ABand can be reinserted into the SR and stackedtogether with the AB. This process continues untilall of the bunches circulating the SRhave beencombined into the AB. Since the AB itself is notejected and reinjected successively the net transferefficiency is applied to each bunch separately andthe total charge in the AB after accretion is thecharge at the beginning of compression multipliedby the net transfer efficiency. This efficiency isthe product of the SR and SYN injection andejection efficiencies and the transport- efficienciesand can be of the order of 80 percent without undueexpense.The transfer and combining process is bestcarried out at constant energy near the top of thesynchrotron cycle. If desired the energies of the tworings could be adjusted during the transfer andcombination process so that the bunch beingmanipulated always enters and leaves the syn-chrotron at exactly the correct energy but this isprobably not necessary.For example if a storage ring of size comparablewith the Cornell synchrotron (circumference =750 m) were built and their gross radii were differentby 60 cm or more the time required for the bucketsin the two rings to slip past each other by one fullcircumference would be 0.5 msec or less. Duringthis time the change in synchronous energy of thesynchrotron would be only 0.25 percent, themagnet excitation being a fully biased 60 Hz sinewave. Such a variation is well within the aperturesinvolved.To continue with this example it is estimated thatkicker coils appropriate for ejecting the SRbunches could be switched on and off in a periodof perhaps 25 nsec, certainly in 50 nsec.This means that in a machine the size of theCornell synchrotron (2.5 Jlsec revolution time)50 to 100 bunches could be combined into one.If one bunch is accreted per synchrotron cyclethis means that the total compression could beaccomplished in i to Ii seconds and the filling timecould be reduced by a factor of 50 to 100 as com-pared to the situation in which only one bunch persynchrotron cycle is injected into the storage ring.REFERENCES1. If the rf wavelength in the storage ring is greater than that inthe synchrotron a few bunches might be accommodated.2. The same problem affiicts injection from a high energy linacbut can generally be overcome by increasing the instan-taneous beam power in the linac.3. The exact ratio of e+ to e- current in the synchrotrondepends upon. the details of the injector linac and the mannerin which the linac beam is modulated and converted intopositrons.4. Rutherford Laboratory Report RL-74-100, September 1974.5. G. A. Voss, private communication concerning plans for ahigh energy e+ e- storage ring at DESY.6. The same principle can be applied if the reverse is true. Someefficiency may be sacrificed.

Improved Method For Filling An Electron Storage Ring From A synchrotron

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