Beam tails have been measured in LEP using scraping collimators and loss monistors for separated and colliding beams. Significant non-Gaussian beam tails have been observed with colliding beams for high beam-beam tune shift parameters and bunch currents

Burkhardt, H

Cornelis, C

Reichel, I

Schmickler, Hermann

Von Holtey, G

CERN Document Server

Beam tails in LEPH. Burkhardt, K. Cornelis, G. von Holtey, I. Reichel, H. Schmickler, CERN, Geneva, SwitzerlandAbstractBeam tails have been measured in LEP using scraping colli-mators and loss monitors for separated and colliding beams.Significant non-Gaussian beam tails have been observedwith colliding beams for high beam-beam tune shift param-eters and bunch currents.1 INTRODUCTIONLEP has been running for 6 years with beam energies ofabout 45.6 GeV [1]. Single beam emittances can be rathersmall: In the horizontal plane "x= 12 nm and still two ordersof magnitude less in the vertical plane. For bunch currentsof 350 A initial unperturbed tune shifts would be typically0.08. Significant increase of beam sizes is observed in bothplanes as result of the beam-beam interaction. A wigglerplaced in a region with dispersion (the emittance wiggler)is used to increase the horizontal emittance up to 36 nm i.e.reduce the initial unperturbed horizontal tune shift to about0.028. This is needed to bring beams safely in collision andto avoid flip-flop and excessive beam-beam tails with back-ground and lifetime problems during standard operation.Beam lifetimes in LEP are well understood and undernormal running conditions completely accounted for byknown physics collision processes [2]: Single Beam life-times are typically 40-50 hours due to Compton scatter-ing on black body radiation (thermal photons) and beam-gas Bremsstrahlung. In collisions, lifetimes are reduced toabout 17 hours due to ”loss” of particles in e+e  collisions.Beam-beam tune shift parameters in excess of 0.04 havebeen reached routinely in operation [3].Shorter beam lifetimes than expected and bursts of back-ground to the experiments have been observed for collidingbeams and high beam currents and have limited the maxi-mum currents that can safely be collided [4]. They are at-tributed to non-Gaussian tails in the transverse beam pro-files, generated by the beam-beam effect.2 LOSS MONITORS AND TAIL SCANSA bunch current of 350 A represents 2  1011 particlesand 100 hours lifetime correspond to 5:5  105 particleslost per second or about 50 particles per turn. A very highsensitivity to beam tails can be reached using loss moni-tors close to aperture limits [5]. Scintillators have been in-stalled in LEP at the aperture limiting collimators [6]. Re-cently, many more loss monitors using PIN-diodes basedon a design for HERA [7] were installed close to most col-limators in LEP. Figure 1 shows schematically the layoutbox containingPIN-diodesvacuum chambercollimator jawscintillatorFigure 1: The horizontal aperture limiter with its PIN-diodebeam loss monitors and scintillators (schematic).around the main horizontal aperture limiting collimator inLEP, equipped both with scintillators and PIN-diode beamloss monitors. Several measurements using these loss mon-itors have been performed in recent machine developmentsessions and are analysed [8, 9].Measurements were performed by moving one jaw of acollimator closer to the beam in steps. Beam current andbeam size measurements were recorded for each collima-tor setting. The collimators were moved closer until signif-icant lifetime reductions were observed. Lifetimes calcu-lated from beam currents for these points were used to cal-ibrate the loss monitors. This allows to give loss rates di-rectly in terms of equivalent lifetimes. The uncertainty inthe absolute calibration is estimated to be about 30%. Thereis some additional systematic error from noise and satura-tion for very high and low loss rates:The PIN-diode monitors are nearly noise-free but giveonly counts and saturate therefore at one hit per bunch pas-sage. Measurements have been linearised using Poissonstatistics [10].The scintillators use pulse height information and did notreach saturation. Due to noise, these monitors are not sen-sitive to losses corresponding to lifetimes longer than 104hours.The two different types of loss monitors are thereforerather complementary.3 BEAM-BEAM TAILSFigures 2 and 3 show tail distributions as measured intwo machine development sessions. The machine condi-0 4 8 12 16 20 24 28 32 36 400 20 40 60 80 100 120 140Q'=10Q'=7separatedbeams10/6/95 using scint.5σ  core24 σ collimatorsetting forphysicsLifetime poor0.111010210310410510-510-410-310-210-1110Losses from Scraping in 1/hours Lifetime from Scraping in hoursCollimator Setting in σ using measured emittancesCollimator Setting in σ using nominal emittancesFigure 2: Measured beam tails in the vertical plane.0 2 4 6 8 10 12 14 16 18 200 2.5 5 7.5 10 12.5 15 17.5 20Q'=5Q'=7Q'=1010/6/95 using scint.0.111010210310410510-510-410-310-210-1110Losses from Scraping in 1/hours Lifetime from Scraping in hoursCollimator Setting in σ using measured emittancesCollimator Setting in σ using nominal emittances5σ  core12 σ collimatorsetting forphysicsLifetime poorFigure 3: Measured beam tails in the horizontal plane.tions were close to standard physics operation with stan-dard pre-collision tunes set to Qx= 90:3 and Qy= 76:2and Qs= 0:065. The horizontal emittance is measured us-ing a synchrotron radiation monitor (BEUV). The verticalbeam size is calculated using luminosity information. Thespecific conditions were:1. Fill 2364 on 4-9-94, 4+4 bunches, about 360 A perbunch, tails measured using PIN-diode loss monitors;emittances were "x= 40 nm and "y= 0.36 nm, beam-beam tune shift parameters x= 0.025 and y= 0.042 .Results are shown as solid lines for different settings ofchromaticity (Q’ between +5 and +10). In the verticalplane tails were also measured with separated beams.2. Fill 2724 on 10-6-95, 4+4 bunches, about 250 A perbunch, losses measured with scintillators; beams col-liding, using ’95 optics with vertical separation on ei-ther side of the interaction regions for bunch trains.Rather unequal beam sizes, about 60 % higher e emittances in both planes. e  beam parameters were"x= 44 nm and "y= 0.4 nm, with beam-beam tuneshifts of x= 0.025 and y= 0.035 . Measurementswere done for one machine setting with Q’=10 in bothplanes.The results are shown as broken lines. At the end ofthe second machine development scraping in the ver-tical plane was continued down to very low electronlifetimes in the order of minutes.We distinguish between nominal and measured emit-tances. The nominal emittances are 45 nm for the horizontaland 4.5 nm for the vertical plane. Nominal emittances arevery useful to compare collimator settings for various opticsand positions around the ring. The values of 45 and 4.5 nmhave been used in LEP for the last couple of years and stillreflect quite well the maximum beam sizes observed in op-eration.The collimator settings in figures 2 and 3 are given inunits of  of nominal beam sizes. In addition, a second x-scale is shown on top of the figures using the measured beamsizes (measured beam sizes of all shown measurements wereequal within errors of about 10%). The expected Gaussiancore is illustrated as shaded area to 5  using the measuredemittances. The theoretical lifetime for a Gaussian beamand collimators at 5  is below one minute.At this stage of measurements and analysis we found in-dications for: Non-Gaussian transverse tails in both planes. Far tailsand losses corresponding to lifetimes of the order of103 hours were also recorded without collidingbeams. Very substantial non-Gaussian beams are present forcolliding beams in the vertical plane. High chromatic-ities can increase vertical tails.4 BEAM LIFETIME FROMSCATTERING PROCESSESLifetimes for single beams, separated beams and low-current collidingbeams in LEP are well explained by knownphysics collision processes Compton scattering on black-body radiation photons Beam-Gas scattering Beam-Beam BremsstrahlungThese collision processes result mainly in off-energy parti-cles. The scattering angles are rather negligible comparedto the beam divergence. If the energy loss exceeds the sta-ble bucket height (about 1% for LEP1) these particles willbe lost, typically by hitting an aperture limit in a dispersionregion.5 SIMULATION OF BEAM TAILSThe Compton scattering and Beam-Gas scattering can oc-cur anywhere around the ring. Even if the scattering anglesare neglected, the combination of energy loss and significantdispersion, as present in the horizontal plane in the arcs ofLEP, will generate significant betatron oscillations and re-sult in horizontal tails.To study this quantitatively, we interfaced a simulationof the Compton-scattering on thermal photons [11] with thedetailed tracking code DIMAD [12]. First results indicate,that the far horizontal tails observed in LEP can be largelyexplained by the scattering on thermal photons.Extra losses, not explained by the three scattering pro-cesses, are seen for colliding beams and high currents. Theyare generally accompanied by background spikes observedin the experiments and have limited the maximum usefulbunch currents at LEP1. Emittance blow-up is observedfor colliding beams in LEP in both planes. A wiggler isused to artificially increase the horizontal beam sizes and toavoid flip-flop and excessive tails. With the observation ofvery substantial vertical tails and the rather lower, constantlevel of horizontal tails observed so far, we expect that thecurrent limitation in collisions with extra losses and back-ground spikes can be attributed to vertical tails.We have started to use results and computer codes ofbeam-beam simulations [13, 14, 15] with the aim to get abetter understanding of the processes and main machine pa-rameters involved in generating vertical tails.6 SUMMARYAt beam energies of about 45.6 GeV, the maximum usefulcurrent per bunch in collisions in LEP has been limited bybackground spikes as observed in the experiments and by re-duction in beam lifetime. This can be explained by scrapinginto non-gaussian tails generated by the beam-beam effect.We have quantitatively measured horizontal and verticalnon-gaussian tails using scraping collimators and loss mon-itors. Substantial non-gaussian tails and a dependence onmachine parameters like chromaticity has been observed inthe vertical plane.We use tracking programs and started to included scatter-ing processes. First results indicate that the far horizontaltails observed in LEP are mainly due to the Compton scat-tering of beam particles on thermal photons.7 REFERENCES[1] G. Arduini et al. LEP1 operation, 1989-1995. these proceed-ings.[2] H. Burkhardt and R. Kleiss. Beam Lifetimes in LEP. InV. Suller and Ch. Petit-Jean-Genaz, editors, Proceedingsof the 4th European Particle Accelerator Conference, vol-ume II, page 1353 ff., 1994.[3] R. Bailey et al. Operational Procedures to obtain high Beam-Beam Tune Shifts in LEP Operation. In Proceedings of thParticle Accelerator Conference, 1995. and CERN-SL-95-49 (OP).[4] H. Burkhardt. What is the maximum current we can collideat 45 GeV. In J. Poole, editor, Proceedings of the 5th Work-shop on LEP Performance, page 131 ff., 1995.[5] S. Zholents. Experimental Programme. In J. Poole, edi-tor, Proceedings of the 2nd Workshop on LEP Performance,page 387 ff., 1992.[6] K. Cornelis et al. Commissioning of a Scintillator at theAperture Collimators for the Study of Beam Tails. SL-MDNote 60, CERN, 1992.[7] W. Bialowons, F. Ridoutt, and K. Wittenburg. ElectronBeam Loss Monitors for HERA. In Proceedings of the 4thEuropean Particle Accelerator Conference, 1994.[8] H. Burkhardt et al. Scrapers and Tails. SL-MD Note 175,CERN, 1995.[9] I. Reichel. RWTH Aachen (Germany) and CERN, work fora Ph.D. thesis on transverse beam profiles in LEP.[10] H. Burkhardt and I. Reichel. Correction of the LEP beamloss monitors for known saturation effects. SL Note 96-26(OP), CERN, March 1996.[11] H. Burkhardt. Monte Carlo Simulation of Beam Particlesand Thermal Photons. SL Note 93-73 (OP), CERN, July1993.[12] R. V. Servranckx, K. L. Brown, L. Schachinger, and D. Dou-glas. Users guide to the program dimad. SLAC REPORT285 UC-28 (A), SLAC, May 1985.[13] S. Myers. Simulations of the beam-beam effect for e+e storage rings. In Nuclear Instruments and Methods 211,pages 263–282. North-Holland Publishing Company, 1983.[14] E. Keil. Nonlinear coherent effects between two strongbunched beams colliding in a storage ring. In Nuclear Instru-ments and Methods 188, pages 9–14. North-Holland Pub-lishing Company, 1981.[15] T. Chen, J. Irwin, and R. Siemann. Simulation of the Beam-Beam Lifetime for LEP. In V. Suller and Ch. Petit-Jean-Genaz, editors, Proceedings of the 4th European ParticleAccelerator Conference, volume II, page 1356 ff., 1994.

Beam Tails in LEP

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