After the completion of the antiproton programme, the Low-Energy Antiproton Ring (LEAR) will be able to serve as an accumulator ring for heavy ions in the injector chain of the Large Hadron Collider (LHC). For injection and accumulation, a scheme is proposed, which exploits both the longitudinal and the transverse acceptances of LEAR. Compared to a classical multiturn injection, a factor 3 to 5 in intensity is gained for the set of parameters pertaining to this case; furthermore emittances, which are more favorable for electron cooling, result. The principle is outlined and computer simulations are presented

Carli, Christian

Maury, S

Möhl, D

CERN Document Server

COMBINED LONGITUDINAL AND TRANSVERSE MULTITURNINJECTION IN A HEAVY ION ACCUMULATORCh. Carli, S. Maury, D. Mo¨hl, CERN/PS, CH 1211 Geneva 23, SwitzerlandAbstractAfter the completion of the antiproton programme, theLow-Energy Antiproton Ring (LEAR) will be able to serveas an accumulator ring for heavy ions in the injector chainof the Large Hadron Collider (LHC). For injection andaccumulation, a scheme is proposed, which exploits boththe longitudinal and the transverse acceptances of LEAR.Compared to a classical multiturn injection, a factor 3 to5 in intensity is gained for the set of parameters pertainingto this case; furthermore emittances, which are more favor-able for electron cooling, result. The principle is outlinedand computer simulations are presented.1 INTRODUCTIONTo prepare intense bunches of ions for injection into theLHC, it has been proposed [1] to cool and accumulateheavy ions in LEAR or a similar ring constructed for thispurpose. Particles coming from the linac are injected bymultiturn injection and cooled and stacked by electroncooling; the process is repeated until the required intensityis reached. In order to increase the number of injected parti-cles and to obtain a particle distribution more favorable forelectron cooling, an injection scheme has been proposed[2], which exploits both the longitudinal and the transverseacceptances.Computer simulations to investigate this new techniqueof combined longitudinal and transverse injection forLEAR have been performed. Some results will be pre-sented here. A more detailed description of the simulationscan be found in [3]. A summary of recent experiments inview of ion accumulation is given in [4].2 PRINCIPLEThe principle of combined longitudinal and transverse mul-titurn injection is best explained with the help of Fig. 1. Asfor convential multiturn injection, a local deformation ofthe closed-orbit (the”bump”) is created and decreases dur-ing the injection of the linac pulse. The linac energy isramped in such a manner, that at the injection septum theclosed-orbit corresponding to the instantaneous linac en-ergy remains fixed, or changes only by a small amount.For conventional transverse multiturn injection, the linacenergy is constant and the closed-orbit moves away fromthe septum as the bump decreases. Thus a large emit-tance of the circulating beam results whereas the momen-tum spread is ideally the same as that of the incomingbeam. With the combined injection (which has some sim-ilarities with the ”painting” used in H  injection [5]) bothlongitudinal and transverse phase space are exploited andtransverse emittance can be traded for momentum spread.horiz.positionSeptumorbit deformation of injected beamtimeorbit bumporbit shift D   due to rampδ iFigure 1: Horizontal positions for an example of combinedlongitudinal and transverse multiturn injection. The closed-orbit bump decreases during the injection of the pulse asfor conventional multiturn injection. The momentum of theincoming beam is ramped p = p0(1 + i(t)), which leadsto an additional time dependent orbit shift Di(t) (whereD is the dispersion) for the incoming beam. The bumpdecrease and the energy ramp are adjusted, such that theinstantaneous closed-orbit of the injected particles remainsfixed.3 LEAR LATTICESWith the fixed geometry of LEAR different ”machines” canbe obtained varying the currents in the quadrupole families.For the injection a small horizontal  function and a largedispersion are necessary; whereas for the electron coolingwe expect vanishingly small dispersion and intermediate in both transverse phase spaces to be advantageous. InFig. 2, the Twiss functions are shown for two of the ma-chines used for the simulations; (a) shows standard LEARand (b) the machine best for combined injection, but with functions a little too large for electron cooling. A third lat-tice used for the simulations has smaller -functions at thecooler, but leads to a situation less suitable for the injection.4 SIMULATIONSThe injection process is simulated with a Monte-Carlomethod. Each particle is described by its coordinates in thetwo transverse phase spaces, the relative momentum devia-tion and the time. First a Gaussian beam with particle coor-dinates corresponding to given Twiss parameters is gener-0 5 10 15 20s/m01020β    ,D/m30InjectionCoolera)0 5 10 15 20s/m01020β    ,D/m30InjectionCoolerb)Figure 2: Lattice functions (the solid and dashed lines rep-resent the horizontal and vertical -function respectively,the dot-dashed line is for the dispersion) for one fourth ofthe machine extending from the center of the injection tothe center of the electron cooling straight section for (a)standard LEAR and (b) a lattice improved for combinedmultiturn injection. The lattice functions in the rest of theaccelerator are obtained by mirroring and repetition.ated at the injection septum with a random number genera-tor. The injected particles are tracked through the accelera-tor consisting of the lattice magnets (dipoles, quadrupoles),closed-orbit bumpers, aperture limitations and the injectionseptum. Particles hitting an aperture limitation or the sep-tum or falling outside an acceptance of 50m at the end ofthe injection are counted as lost. In simulations of a purelytransverse process, the transverse acceptance had to be in-creased to 150m to permit injection of a sufficient numberof turns.For the beam coming from the linac the followingcharacteristics (corresponding to one standard deviation)were taken from [6] :emittance : 2:1 mrelative momentum spread : 0:2 10 3In addition to the (instantaneous) spread, the mean mo-mentum of the linac beam changes with time; this ramphas been limited to  < 6 10 3. Further it is requiredthat the distance from the center of the stack to thevacuum chamber must be at least 2.5 times the standarddeviation of the stack. This leads to the following phasespace volumes (2.5 rms) reserved for the stack (volumesexpected for 1 rms taken from [1]) :horizontal emittance : (2:5)2  10 m = 62:5 mvertical emittance : (2:5)2  5 m = 31:2 mrel. momentum spread : 2:5  0:5 10 3 = 1:25 10 3where the contributions due to momentum spread anddue to horizontal betatron oscillations have to be addedquadratically.For the model of the accelerator, the real apertures avail-able were followed as close as possible, after removing10 mm in order to account for closed-orbit distortions upto 5 mm.To optimize the injection efficiency all parameters of theincoming beam (position, angle, Twiss parameters, mo-mentum) and the position of the injection septum are var-ied. The mean momentum of the stack is such that it is nearthe inner wall of the vacuum chamber, whereas the injec-tion is from outside.βe-cool largeβe-cool intermediatestandard LEARstandard LEARpurely transversenineff5 10 15 20 25 30 35510152025Figure 3: Number of ’efficient’ turns neffaccepted per in-jection as a function of the number of injected turns niforoptimizations of combined multiturn injection. The crossesare for standard LEAR; triangles and squares are for injec-tion into improved lattices with large and intermediate beta-tron functions at the electron cooler. For comparision, effi-ciencies for a purely transverse process and standard LEARmachine are given as a dashed line. In the latter case the fullacceptance (150m) of LEAR is used whereas in all othercases the emittance was limited to 50 m to permit fastercooling.In Fig. 3 the number of turns accepted as a function ofinjected turns is plotted for various conditions. Phase-spaceplots for injection of 20 turns into the standard LEAR ma-chine and into the machine best suited for combined injec-tion (but expected to be less appropriate for electron cool-ing) are given in Figs. 4 and 5. For the incoming beamthree phase space ellipses with small momentum deviationswere overlapped; they separate after injection due to thedispersion. One notes that this separation is more importantfor the improved lattice because the normalized Dispersiond=p at the injection septum is increased; this is a conse-quence of improving the lattice for combined injection witha limited linac energy ramp.For comparision, a plot of the horizontal phase spacefor the purely transverse process and the standard LEARFigure 4: Plots of the horizontal phase space for the com-bined longitudinal transverse multiturn injection into thestandard LEAR machine. 20 turns are injected; the threeplots show the particles after 2, 5 and 20 turns.Figure 5: Plots of the horizontal phase space for the com-bined longitudinal transverse multiturn injection into theimproved lattice with large  at the cooler. 20 turns are in-jected; the three plots show the particles after 2, 5 and 20turns.machine is given in Fig. 6. One notes, that particles areinjected only in a ring-shaped region of the phase spacearound the stack.Figure 6: Plots of the horizontal phase space for purelytransverse multiturn injection into the standard LEAR ma-chine. 10 turns are injected; the two plots show the particlesafter 2 and 10 turns.5 CONCLUSIONSComparing the number of turns that can be injected with agiven efficiency, one notes that a factor of 3 to 5 is gainedwith combined injection as compared to the purely trans-verse process (in the standard LEAR machine); anotherfactor of 2 can be gained if the lattice is modified. In ad-dition a smaller emittance of the injected beam, more fa-vorable for electron cooling, is possible with the combinedinjection.6 REFERENCES[1] P. Lefe`vre and D. Mo¨hl, A low Energy Accumulation Ringof Ions for LHC, CERN/PS 93-62 and LHC Note 259.[2] S. Maury and D. Mo¨hl, Combined Longitudinal and Trans-verse Multiturn Injection into a Heavy Ion AccumulatorRing, internal note (CERN) PS/AR/Note 94-12.[3] Ch. Carli, Simulation of Multiturn Injection into LEARin View of Ion Accumulation, internal note (CERN)PS/DI/Note 96-24.[4] J. Bosser, J. Broere, C. Carli, M. Chanel, C. Hill,R. Ley, A. Lombardi, R. Maccaferri, S. Maury, D. Mo¨hl,G. Molinari, H. Mulder, E. Tanke, G. Tranquille,M. Vretenar, Recent Lead Ion Storage Tests on LEAR,this conference.[5] D. Raparia, C.W. Planner, G.H. Mackenzie andJ.R. Richardson, Efficient Capture in an Accumulator,of 20000 Turns of Beam Injected from TRUMF, Proc. ofPAC 1985 (Vancouver), IEEE Trans. Nucl. Sci. NS-32, p2456.[6] N. Angert, M.P. Bourgarel, E. Brouzet, R. Cappi,D. Dekkers, J. Evans, G. Gelato, H. Haseroth, C.E. Hill,G. Hutter, J. Knott, H. Kugler, A. Lombardi, H. Lustig,E. Malwitz, F. Nitsch, G. Parisi, A. Pisent, U. Raich,U. Ratzinger, L. Riccati, A. Schempp, K. Schindl,H. Scho¨nauer, P. Teˆtu, H.H Umsta¨tter, M. van Rooij,D. Warner and M. Weiss, CERN Heavy-Ion Facility DesignReport, CERN 93-01.andN. Angerth, W. Bleuel, Y. Bylinsky, H. Gaiser, H. Haseroth,G. Hutter, H. Kugler, E. Malwitz, R. Popescu, M. Rau, U.Ratzinger, R. Scrivens, E. Tanke, D. Warner, The IH Linacof the CERN Lead Injector, proceedings of the 1994 inter-national linac conference, Tsukuba, Japan

Combined longitudinal and transverse multiturn injection in a heavy ion accumulator

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