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Beam-Beam Effects at the Tevatron: Observations and Solutions

This presentation discusses the experience with beam-beam effects at the Tevatron, including incoherent beam-beam at injection/acceleration/squeeze, collisions, emittance blowup, and beam-beam compensation. The speaker also discusses the observations of coherent beam-beam modes and various solutions to address these effects.

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Beam-Beam Effects at the Tevatron: Observations and Solutions

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  1. Experience with Beam-Beam Effects at the Tevatron Y. Alexahin (Fermilab APC) • Incoherent beam-beam @ injection/acceleration/squeeze • Incoherent beam-beam @ collisions - emittance blowup @ initiate collisions - lifetime in colliding beams • Beam-beam compensation • Observations of coherent beam-beam modes Instabilities & Coherent Effects group meeting CERN, 02/16/2011

  2. 2 Tevatron Run II Parameters 36 bunches in each beam are grouped in 3 trains by 12 bunches. Each bunch experience head-on collisions at 2 detectors (B0, D0) and 70 LR interactions Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  3. 3 Run II Initial Separator Arrangement CDF (B0) @ injection/acceleration/squeeze only 2 separators – B17H and C17V were used @ collisions separators formed closed 3-bumps between IPs in each plane @ step 13 of the squeeze (out of 17 initially) the transition from injection to collision helix took place B17H B11V B11H C17V A49V A49H C49H A17V C49V D0 D11H D11V D48H Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  4. 4 Typical Early Run II “Comfort Plot” Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  5. 5 Early Run II Debacle Impact of beam-beam effect on peak luminosity (V. Shiltsev) (design total proton intensity = 1013 = 10000e9) Record Run I average initial lumi = 27e30 Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  6. 6 Long Laundry List Increase beam separation - use more separators (including new ones) Reduce chromaticity - Landau damping octupoles to stabilize coherent oscillations - transverse feedback (helped at some point but was abandoned) - feed-down octupoles to lower pbar chromaticity even more Reduce beam emittance - eliminate mismatch between the machines - e-cooling in RR - helped immensely Optimize beam “cogging” (determines IP azimuthal positions) Better control tunes & chromaticity (compensation of the drift caused by the persistent current decay @ 150) Remove unused C0 Lambertsons (the tightest aperture restriction) and cover F0 Lambertsons with foil to reduce impedance Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  7. 7 Figure of Merit for Beam Separation Separation in respective (betatron) sigmas or separation in maximal sigmas? • both can be quite misleading*, but Sr was found to better reflect the helix properties. • Still, it was necessary to calculate beam-beam tuneshifts and RDTs to make a choice. • *) using full sigmas (with dispersion contribution) does not make more sense Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  8. 8 “Radial” Beam Separation - “radial” separation (reference emittance 15 mmmrad) - Jan 2002 helix ~ 2005 helix “5-star” helix had to be reduced due to aperture restrictions and was finally replaced with design using 7 separators Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  9. 9 Squeeze Sequence 13 Beam intensities through the squeeze: left - store 1074 (03/13/02), right – store 3101 (12/17/03). Note higher proton intensity in store 3101, 8.51012, compared to 61012 in store 1074. Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  10. 10 Losses @ 150 GeV vs Chromaticity (data mining by V. Shiltsev) With introduction of octupoles in Jan. 2005 the chromaticity for protons and pbars was lowered to 3/0. Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  11. 11 Effect of Octupoles on Pbar Losses @ 150 GeV Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  12. 12 Pbar Efficiency @ 150 GeV Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  13. 13 Effect of Octupoles on Proton Losses @ Pbar Injection Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  14. 14 Particle Losses at Different Stages Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  15. 15 More Comforting “Comfort Plot” Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  16. 16 Incoherent BB Effects @ Collisions • Manifestations: • Pbar (and sometimes proton) emittance blowup at the start of HEP • Proton and (to lesser degree) pbar non-luminous losses • Problems: • Not enough room for pbar tunes between 5th and 12th order resonances • Large BB-induced split in chromaticity: Chpbar - Chproton ~ 7 • Large emittance ratio proton/pbar (good for pbars, not for protons) • Insufficient separation at the nearest parasitics Calculated tune distribution of protons (orange) and pbars (blue) in collision and measured with 1.7GHz Schottky bunch-by-bunch pbar tunes (yellow) in store 3867,07/28/2004 Both calculations and measurements (!) ignore coupling Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  17. 2 17 Nearest Parasitic IPs (u) (d) Beam separation and optics functions @ nearest parasitics (ideal optics): upstream PIPs – not seen by 1st proton and last pbar bunches in trains downstream PIPs – not seen by last proton and 1st pbar bunches in trains Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  18. 18 Beam-Beam Pbar Tuneshifts 1.7 GHz Schottky 07/27/2004 Qh Qv “PACMAN” bunches have much lower tunes – do not see 5th order Tunes go down with time due to proton emittance growth (mostly by IBS) Calculations satisfactorily reproduce measurements Analytics Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  19. 19 Beam-Beam Chromaticity: Theory • Comes from: • head-on interactions owing to -function modulation measured chromatic functions were initially up to 600 (A.Valishev): • long-range interactions owing mainly to modulation of the beam separation dx,y Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  20. 20 chromaticty bunch # LR chromaticty bunch # Beam-Beam Chromaticity: Measurements vs. Calculations Antiproton chromaticity measured with 1.7GHz Schottky monitor(store 3678, 07/28/04). Bare lattice chromaticity: Cx=10.5, Cy=11.5 on the pbar helix Cx=12.5, Cy=10.5 on the proton helix Calculated long-range contribution for antiprotons with small betatron amplitudes • The bb-chromaticity is huge and difficult to compensate since it: • varies from bunch to bunch • depends on betatron amplitudes Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  21. 21 “Scallops” in Pbar Emittance hN ( mm mrad) vN ( mm mrad) Pbar emittance growth over first 10’ of HEP (store 3456 04/29/04) - clearly the work of the 5th order resonances Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  22. 22 Diffusion due to Beam-Beam Resonances: Theory bunch # 11 bunch # 12 ay (sigmas) ay (sigmas) ax (sigmas) ax (sigmas) Analytically calculated 5th order resonance widths in the plane of betatron amplitudes (magenta - 5Qx, red - 5Qy) for on-momentum pbars at Qx=.577, Qy=.582 At synchrotron amplitude p =2p ~ 1.3e-4 the synchrotron satellites overlap  dynamical chaos for bunch #11 (but not for bunch #12) Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  23. 23 ay 4x+8y 6x+6y 12y 2x+10y ax Diffusion due to 12th Order Resonances: a Puzzle Observations indicate a strong effect of the 12th order resonances on the lifetime and even on pbar emittance. But calculations (left) show that they are too weak to produce dynamical chaos. Amplitude beat on 12th order resonances and their synchrotron satellites at Qx=.585, Qy=.575 Possible explanation: cooperative effect of resonances and external noise (“multiplicative diffusion enhancement”- D.Neuffer, A.Riddiford, A.Ruggiero, 1980) The mechanism - loss of phase correlation between subsequent crossings of a resonance in the course of synchrotron oscillations - was first discussed by P. Sturrock while at CERN in 1958) Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  24. 24 Pbar & Luminosity Losses vs. Separation Pbar NL losses at the beginning of 35 stores (March–April 2005) at indicated values of the helix size. Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  25. 25 What Was Attempted to Reduce Pbar Blowup & Losses • Increased beam separation with new separators making 4-bumps instead of 3-bumps • Reduced pbar emittance (e-cooling in RR, matching) • Compensation of pbar BB tuneshift decrease during a store with feeddowns • Damping of LB quad motion to reduce orbit response • Orbit feedback (essential!) • Lower chromaticity (possible due to reduced impedance) • Landau damping octupoles @ collisions – abandoned • New working point (close to 2/3 or 1/2) – abandoned Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  26. 26 Run II Initial Separator Arrangement CDF (B0) B17H B11V B11H C17V A49V A49H C49H A17V C49V D0 D11H D11V D48H Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  27. 27 Run II Final Separator Arrangement CDF (B0) B17H B11V B48V B11H C17V A49V A49H C49H A17V C49V A17H D0 D11H D11V D17V D48H Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  28. 28 Collision Helix Upgrade *=35cm optics (design): 12 separators 13 separators (D17V+) *=28cm optics (fit): 13 separators 15 separators (A17H & B48V +) - more than 10% increase in separation at the nearest parasitics Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  29. 29 Evolution of BB Tuneshifts After the commissioning of e-cooling in RR the problem with the proton lifetime became dominant. The cures implemented: - pbar “jacking” to increase emittance, - correction of chromatic beta-beat (A.Valishev) – beneficial by itself, but also reducing 2nd order chromaticty (and therefore the total tunespread). Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  30. 30 Effect of 2nd Order Chromaticity Compensation Presently beam-beam effects in collision impose ~5% tax on integrated luminosity Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  31. 31 BBC with TELs TEL2 operated on all p-bunches (not only the last bunches in trains, #12*k) raising the vertical tune. TEL1 increases the horizontal tune for p-bunch #13 which is normally too low  close to 12th order. • Both TEL1 and TEL2 improve proton lifetime significantly better than just lattice tune change – nonlinear resonance compensation? • I don’t think so, there is much simpler and effective mechanism: beta-beat. It happened so that TELs reduce -functions at IPs making the ration of proton/pbar sizes smaller. • No study of the beta-beat due to TELs performed. Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  32. 32 Coherent Oscillations: Run Ib Observations From V. Bharadwaj et al. Fermilab-TM-1970 (1996) “At collisions, the chromaticities are quite small and may even be slightly negative. (They are probably between about -5 and 5 units in both planes.) This seems to be a requirement for good particle and luminosity lifetimes. It has also been observed that when the beams are colliding they can tolerate chromaticities that would make a single beam unstable”. - Nobody of the Tevatron people could recollect working with negative chromaticities nor were able to find any evidence in logbooks. Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  33. 33 36x36 Spectra in Collision At initiate collisions nice quasi- and  mode excited (there is large difference in lattice tunes and intensities: Na=2232e9, Np=7855e9) but are quickly damped at nominal chromaticity Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  34. 34 Dedicated Experiment on Colliding Beams Stability • End-of-store study (Np=21011, Na=21010) on 04/21/05: • vertical chromaticity lowered from 10.5 to 2.5 units • chromaticity lowered from 7 to 1.5 units -> beams went unstable • Tev quenched due to pbar losses 1.7 GHz Schottky spectra in the proton (left) and antiproton (right) beams before the onset of instability (blue) and just before the quench (red) (data provided by A. Jansson) • Beam-beam tunespread failed to provide Landau damping • Pbars had a factor of 4 larger amplitude, very much in line with the rigid-bunch model predictions Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  35. 35 Bunch-by-Bunch Tune Monitor A. Valishev et al., EPAC’08 Clearly all bunches participate in coherent motion (but with different amplitude). Some “tickling” was applied to protons. Strong pbar line at ~0.563 is a puzzle – too low to be related to main bunches which experience head-on collisions, may be a mini-bunch is caught somewhere in between. Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  36. 36 3x3 Beam-Beam Modes with TEL2 off/on Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

  37. 37 Lessons Learned  Head-on tuneshifts as high as 0.025 can be tolerated in hadron colliders  Separation > 6 sigma, for a short time as low as 3 sigma  Low chromaticity (4) is essential for good lifetime  Multiple crossing of even weak high-order resonances (12th in the Tevatron case) can affect particles in the beam core (the mechanism first discussed by P. Sturrock in 1958 while at CERN)  There can be a non-trivial interplay between beam-beam and lattice nonlinearities (as testified by detrimental effect of octupoles in collision)  Beam-beam tunespread may be insufficient for colliding beam stability contrary to Run I indications and simplified theoretical predictions  Beneficial effect of electron lense exceeded expectations (but the reason is not completely understood) Beam-beam @ Tevatron - Y. Alexahin ICE meeting 02/16/2011

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