Foelsche, Horst W J

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Particle Accelerators, 1990, Vol. 27, pp. 7-12Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaA STRETCHER FOR THE BROOKHAVEN AGS*HORST W.J. FOELSCHEBrookhaven National Laboratory, Upton, NY 11973ABSTRACT This paper summarizes the conceptual design of a 30 GeVStretcher ring,l which is designed to increase the capacity and thequality of the experimental physics program at the AGS. In atypical 3 second operating cycle the AGS now accomplishes twofunctions: accelerating the beam to full energy and then providing aslow spill on a 30 GeV flattop. These tasks consume approximatelyequal time. The proposed Stretcher, a dc storage ring, will take upthe task of distributing the high energy beam with a continuous slowspill, making it possible for the AGS to provide beam for theprogram at more than twice the present repetition rate. Thus theaverage current delivered to the experimenters will be more thandoubled, and the duty cycle of the spill will increase from thepresent optimum of about 40% to nearly 100%. The Stretcher willcontinue the gradual evolution of the AGS toward a kaon factory. 2At present, the AGS provides about 1 pA average proton current. ABooster, now under construction,3 is expected to increase thecurrent to above 4 ~a, and the Stretcher to about 8-10 ~A, an orderof magnitude higher than now.INTRODUCTIONThe 30 GeV AGS supports a great variety of experiments in high energyand nuclear physics. Its ongoing research program has provided thestimulus for numerous proposals for future high intensity hadron facili-ties: the study of very rare processes in kaon decays, the search forgluonic states, the study of QCD with high Pt exclusive processes, andmany more. Several rare kaon decay experiments on the AGS floor atpresent, for example, can already be sensitive to effects from massscales higher than what will be directly accessible with the SS/C , andeven higher sensitivity is sought for future experiments. Moreover, theAGS accelerates polarized protons, and provides heavy ion beams forfixed target experiments. There is now a clear call for higher inten-sity proton beams at these intermediate energies, and as the intensityincreases, for improvements in the quality and duty cycle of the beamspill.At the AGS these improvements are being provided in a step by stepmanner, in response to the most urgent needs of the physics program, ina way which minimizes the disruption of the ongoing operations. ABooster is now under construction3 which will raise the injectionenergy of the AGS from 200 MeV to 1.5 GeV for protons, to increase thespace charge limited intensity of proton beams by more than a factor offour, from an average proton current of about 1 pA at present to about4 pA in the future. The Booster will also provide higher injection*Work performed under the auspices of the U.S. Department of Energy.[253]/78/[254] H. W. J. FOELSCHEenergies for the heavy ions, so that even the heaviest species can besupplied to the AGS in the fully stripped state. This capability isimportant for RHIC, the proposed new Relativistic Heavy Ion Collider atBNL. 4 Finally, the Booster will serve as an accumulator of polarizedproton pulses from the rapidly pulsing 200 MeV Linac, thus increasingpolarized beam intensities by an order of magnitude as well.When the Booster is completed, a 30 GeV Stretcher can be added tothe AGS. It will take over from the AGS the task of providing a slowbeam spill to the experiments. This will make it possible for thesynchrotron to deliver beam at more than twice its present cycling rate,thus more than doubling the average current to 8-10 pA, while the dutycycle of the slow beam spill will increase from about 40% to nearly 100%(see Fig. 1). It is equally important to the experimenter that themicroscopic structure of the slow spill be improved as well. Quitefrequently, the AGS spill now shows instantaneous intensity fluctuationsof a factor of 2 or more, so that the most sensitive experiments mustthrottle back their event rate to avoid excessive accidental coinci-dences with background events. With an improved spill structureexpected from the Stretcher, such sensitive experiments may expect toperform at more than 4 times higher rates than would be possible withthe Booster alone.AGS STRETCHER DESIGNThe proposed AGS Stretcher l is equal in circumference to the AGS(= 807.12 m). The ring is constructed in a separate tunnel, to thesouth of the AGS, in a two-fold symmetric racetrack pattern (see Fig.2). It has two 86 m long straight sections which facilitate an optimalarrangement for the injection and extraction systems. Its easterlystraight section serves for the extraction of the beam toward theexisting East Experimental Area beam switchyard, the westerly one forinjection. Beam is supplied by single turn beam transfer from the AGS.There is no rf system in this machine. The polarization of the beam ispreserved during beam transfer, storage, and extraction. Constructionand commissioning of the Stretcher can proceed with minimal disruptionof the ongoing program.The Stretcher is a "distribution ring", operating continuously ata constant energy, up to 30 GeV. It will be supplied with full energybeam provided by the AGS, which will be cycling at its maximum repeti-tion frequency of about 0.6-0.8 Hz, and it will distribute this beam ina slow continuous spill, interrupted only briefly when the AGS is readyto supply a new pulse (Fig. 1). The AGS beam will arrive as a train of12 bunches, separated by 224 nsec in time, having been extracted in asingle turn over a time interval of 2.7 psec. With an inherent momen-tum spread of ± 0.3% or more imposed by the AGS the beam will lose itsbunch structure in about 20 msec. The Stretcher will then immediatelystart slow extraction toward the experimental areas, while the AGS iscycling back for the next pulse. Any beam which may remain in the ringwhen the next pulse is about to arrive, will be ejected to the beam dumpby the same fast kicker magnet which deposits the arriving beam on theinjection orbit. Overall, the injection operation will consume lessthan 5% of the entire spill time, so that the slow spill duty cycle willbe greater than 95%.The Stretcher will be available for all AGS programs which use theslowly extracted beam in the East Experimental Area: unpolarized highSTRETCHER FOR BROOKHAVEN AGS [255]/9intensity proton beams, polarized proton beams, and heavy ion beams.The fast, single turn extraction of bunched beams, traditionallyemployed for neutrino physics and for other specialized experiments,will remain available directly from the AGS in the North ExperimentalArea, as before.LATTICE AND STRAIGHT SECTION DESIGNFigure 3 shows one quarter of the ring in further detail. The 86 m longstraight sections are dispersion-free, and there are 34 separatedfunction FODO cells in the arcs, including the dispersion suppressors,with a 60 degree betatron phase advance per cell. The straight sectionsrepresent approximately one betatron wavelength each. The machineoperates near a tune of about 7.7 in both planes, in fact on the hori-zontal 7 2/3 resonance for slow extraction most of the time.The betatron functions of the lattice are illustrated in Figs.4.a,b,c for the arc cells, the dispersion suppressor, and for thestraight sections, respectively. The six quadrupoles at each end of thearcs are individually powered so as to achieve the desired match intothe straight sections with zero momentum dispersion.Each straight section contains ten quadrupoles arranged symmetri-cally around its center, and the betatron functions in the centralportion can be adjusted to optimize the performance of the extractionand injection systems. The electrostatic extraction septum is locatedin the central 10 m long free space of the easterly straight section,the injection kicker in the westerly one in the same space (Fig. 5).The natural chromaticity of the lattice is about -14. One makesuse of the chromaticity to drive particles of different momentum acrossthe resonance at different times. Sextupoles are included in thedispersion supressor sections. They can be set to make the dispersioncancellation in the straight sections independent of momentum, withoutaffecting the chromaticity in a major way. A summary of the majorStretcher parameters is given in Table 1.TABLE 1. STRETCHER PARAMETERSMomentumMagnetic RigidityCircumferencePeriodicityStraight Section LengthCycle PeriodProtons/CycleNormalized Beam EmittanceBetatron AdmittanceMomentum AcceptanceBetatron TuneNatural ChromaticityTransition Energy, GammaDipole FieldSLOW BEAM EXTRACTION30 GeV/c100 Tm807.12 m286 m1.2 sec>6 x 101 360 7( mm mrad10 7( mm rnrad± 1%7.667-146.51.26 TThe beam is extracted from the machine by exciting a third order reso-10/[256] H. W. J. FOELSCHEnance, 3v x = 23, and by slowly changing the horizontal tune to driveparticles of different momentum across the resonance as time evolves.Unstable particles will spiral outward into the electrostatic septum,which deflects them horizontally into a Lambertson septum magnet, whichin turn deflects them vertically out of the ring. The layout of theextraction straight section is illustrated in Fig. 5.The third-order resonance is established by powering a pair ofsmall sextupoles, located next to the two quadrupoles denoted as Q5 andQ3 upstream of the electrostatic septum. The betatron tune of themachine is driven by four small auxiliary quadrupoles located symmetri-cally next to each of the quadrupoles denoted as Ql, in both straightsections.The optical parameters of the straight section are flexible topermit a choice of beta values at the extraction septum. The horizontalbeta value at the extraction septum is chosen to be 250 m, subject tofurther optimization in the future. The spiralling rate is 2 cm, andthe electric field is 60 kV/cm across the 2 cm gap of the electrostaticseptum. The septum is assumed to be 9 m long, curved at half the curva-ture of the extracted beam. Assuming a wire thickness of effectively0.05 mm, one expects a loss of about 0.3% of the beam on the septum.A simulation of the extraction process in horizontal phase spaceat the electrostatic septum is illustrated in Fig. 6. The septum isplaced about 6 cm to the outside of the central orbit. The illustrationshows unstable particles of several momenta leaving the stable regionnear the center along the separatrices of the third order resonance,with a spiralling rate, near the septum, of about 2 cm for every thirdturn.It is expected that a small fraction of the beam will remain inthe ring without having responded to the resonance. While a fresh pulsearrives from the AGS, whatever beam may have remained in the Stretcherwill be ejected by the injection kicker magnet onto a beam dump, on atrajectory which mirrors the injection path (Fig. 5).POLARIZATIONThe Stretcher will preserve the vertical polarization of the beam, asthe AGS does now, if the energy is chosen to avoid the various depolar-ization resonances. 5 Depolarization occurs at energies where thebeam encounters certain lattice harmonics of horizontal magnetic fieldsin resonance with the frequency of precession. Depolarization can becaused by the intrinsic focusing fields of the quadrupoles - intrinsicresonances, where G~ = nP ± V z - or by the horizontal imperfectionfields due to, for example, improperly aligned quadrupoles - imperfec-tion resonances, where G~ = k. Here G = g/2 - 1 = 1.79285 is theanomalous magnetic moment of the proton, ~ is the energy of theproton in units of its rest energy and V z the vertical betatron tune.The indices nand k are integers, and P is a periodicity of the focusinglattice.Imperfection resonance harmonics are therefore spaced about 0.523GeV apart on the energy scale, and because of the 2-fold lattice period-icity intrinsic resonances are present within all of these intervals aswell. An analysis of the strength of these various effects shows!that the most significant intrinsic resonances to be avoided are in thevicinity of 14.8 and 20.8 GeV (G~ = 34 - {v z - 2}, and = 34 + {v z - 2},reflecting the very strong 34-fold periodicity of the arc cells, and theSTRETCHER FOR BROOKHAVEN AGS [257]/11approximate transparency of the straight sections). The imperfectionresonances in this vicinity tend to be strong as well.Most resonances in the Stretcher are expected to be sufficientlyweak so that one may operate in the energy intervals between them, asone does at the AGS, avoiding the neighborhood of the strongest intrin-sic resonances, and if necessary suppressing the nearest imperfectionharmonics with a compensating set of dipole correction magnets. The AGShas reached polarized beam energies up to about 22 GeV (G1 = 43), withpolarization about 50%, using these techniques. 5ACKNOWLEDGEMENTThis paper is a summary of a Conceptual Design Report! which is thejoint effort of the following individuals: D. Barton, E. Bleser, G.Bunce, G. Cottingham, E. Dale, G. Danby, J. Feldman, H. Foelsche, E.Jablonski, J. Jackson, J. Milutinovic, S.Y. Lee, Y.Y. Lee, E. Lessard,P. Mohn, L. Ratner, A. Ruggiero, M. Schaeffer, J. Spinner, K. Welch, W.Weng, and R. Witkover, all of BNL.REFERENCES1. AGS Stretcher Conceptual Design, AGS Stretcher Task Force, (H.Foelsche, Editor), Internal Report BNL 42506 (June 1989).2. D. Lowenstein, AGS Status and Plans, these Proceedings.3. W.T. Weng, Progress of the AGS Booster Project, these Proceedings.4. H. Hahn, The Relativistic Heavy Ion Collider at Brookhaven, theseProceedings.5. For a comprehensive review of the AGS polarized proton program see:F.Z.Khiari, et al., Phys. Rev. D39, 46, (1989).AGS & STRETCHER CYCLESPROPOSED_STRETCHERBOOSTERSTRETCHERAGS CYCLEBOOSTERb)Fig. 1. AGS Cycle with and withoutStretcher.Fig. 2. AGS Stretcher Layout.12/[258] H. W. J. FOELSCHE60ARCS (m)o 10,---,-.--y----r-,-,.----.--,--r~~X [mJ~ p-:5~tSTRETCHER DISPERSION SUPPRESSORSTRETCHER STRAIGHT SECTION-10 0 SCm) 10I'r ------,1 .1' ------,1 •or 00 OF-1020/10(3x.y[m] _a)701~ 6C[f 50;- 4DLb) o I~30'::t« 20102000 20 40SCm)STRAiGHT SECTIONINSERTIONINJECTION ~ DUMP S'!1WGHT SECTIONu;w~~........., IHORfl.~~--F1lt-+-4~~~~ ~Fig. 3.DISPERSIONSUPPRESSOR1-------------------i.IIIIIII10 8£AW DUMPSLOW EXTRACrJOM STlWGBT SECTION240'xlHORIZ. 200~. I 160>.c) xQ:l.. 12080 ~4000 10 20 30 4005 Q4 QJ Q2 Q1S[m]Straight SectionsFig. 5.~8ERTSONES sarMI.__---_ SEPJUl,li).OOO3O.Oooz(I.Doo!- i).()()Ol-O.OODZ-·0.0003 - ..-- T---,---r---,-- --r---.----.---,---,----,----r-,--0.0:i -0.03 -0.01 0.01 0.03 O.O~i 0.07 0.09MBTHRFig~ 4-a f b,c BetatronFunctions.Fig. 6. Resonance Ex-traction Trajectories atElectrostatic Septum.

A Stretcher For The Brookhaven Ags

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