Instabilities & Coherent Effects group meeting CERN, 02/16/2011 Experience with Beam-Beam Effects at the Tevatron Incoherent injection/acceleration/squeeze Incoherent collisions - emittance initiate collisions - lifetime in colliding beams Beam-beam compensation Observations of coherent beam-beam modes Y. Alexahin (Fermilab APC) Tevatron - Y. Alexahin ICE meeting 02/16/2011 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 Circumferencekm6.28 Beam energyGeV980 ** cm28 Proton bunch lengthcm~ 50 Tunes, hor / ver / Bunches / beam 36 Protons / bunch 2.7 Pbars / bunch 0.9 Normalized emittance proton / pbar mm mrad 18 / 9 Total beam-beam tuneshift, proton / pbar / D0 CDF (B0) B17H C49H D11H D48H A49H C17V B11H C49V A49V A17V B11V D11V Run II Initial Separator injection/acceleration/squeeze only 2 separators B17H and C17V were collisions separators formed closed 3-bumps between IPs in each step 13 of the squeeze (out of 17 initially) the transition from injection to collision helix took place Tevatron - Y. Alexahin ICE meeting 02/16/2011 3 Typical Early Run II Comfort Plot Tevatron - Y. Alexahin ICE meeting 02/16/2011 4 Early Run II Debacle Impact of beam-beam effect on peak luminosity (V. Shiltsev) (design total proton intensity = = 10000e9) Record Run I average initial lumi = 27e30 Tevatron - Y. Alexahin ICE meeting 02/16/2011 5 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 150) Remove unused C0 Lambertsons (the tightest aperture restriction) and cover F0 Lambertsons with foil to reduce impedance Tevatron - Y. Alexahin ICE meeting 02/16/2011 6 Figure of Merit for Beam Separation 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 Separation in respective (betatron) sigmas Tevatron - Y. Alexahin ICE meeting 02/16/2011 7 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 Tevatron - Y. Alexahin ICE meeting 02/16/2011 8 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 10 12, compared to 6 in store Tevatron - Y. Alexahin ICE meeting 02/16/2011 9 150 GeV vs Chromaticity (data mining by V. Shiltsev) With introduction of octupoles in Jan the chromaticity for protons and pbars was lowered to 3/0. Tevatron - Y. Alexahin ICE meeting 02/16/ Effect of Octupoles on Pbar 150 GeV Tevatron - Y. Alexahin ICE meeting 02/16/ Pbar 150 GeV Tevatron - Y. Alexahin ICE meeting 02/16/ Effect of Octupoles on Proton Pbar Injection Tevatron - Y. Alexahin ICE meeting 02/16/ Particle Losses at Different Stages Tevatron - Y. Alexahin ICE meeting 02/16/ More Comforting Comfort Plot Tevatron - Y. Alexahin ICE meeting 02/16/ Incoherent BB 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 5 th and 12 th order resonances Large BB-induced split in chromaticity: C h pbar - C h proton ~ 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 Tevatron - Y. Alexahin ICE meeting 02/16/ Nearest Parasitic IPs upstream PIPs not seen by 1 st proton and last pbar bunches in trains downstream PIPs not seen by last proton and 1 st pbar bunches in trains (d)(u) IPdx [mm] (pbar-prot) dy [mm] (pbar-prot) x [m] y [m] A B C D Beam separation and optics nearest parasitics (ideal optics): Tevatron - Y. Alexahin ICE meeting 02/16/ Beam-Beam Pbar Tuneshifts 1.7 GHz Schottky 07/27/2004 Analytics QhQv PACMAN bunches have much lower tunes do not see 5 th order Tunes go down with time due to proton emittance growth (mostly by IBS) Calculations satisfactorily reproduce measurements Tevatron - Y. Alexahin ICE meeting 02/16/ 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 d x,y Tevatron - Y. Alexahin ICE meeting 02/16/ Beam-Beam Chromaticity: Measurements vs. Calculations Antiproton chromaticity measured with 1.7GHz Schottky monitor (store 3678, 07/28/04). Bare lattice chromaticity: C x =10.5, C y =11.5 on the pbar helix C x =12.5, C y =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 bunch # chromaticty bunch # LR chromaticty Tevatron - Y. Alexahin ICE meeting 02/16/ Scallops in Pbar Emittance Pbar emittance growth over first 10 of HEP (store /29/04) - clearly the work of the 5 th order resonances vN ( mm mrad) hN ( mm mrad) Tevatron - Y. Alexahin ICE meeting 02/16/ Diffusion due to Beam-Beam Resonances: Theory Tevatron - Y. Alexahin ICE meeting 02/16/2011 bunch # 11 bunch # 12 ax (sigmas) ay (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) 22 Diffusion due to 12th Order Resonances: a Puzzle Tevatron - Y. Alexahin ICE meeting 02/16/2011 axax 4 x +8 y 12 y 6 x +6 y 2 x +10 y ayay 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 Amplitude beat on 12th order resonances and their synchrotron satellites at Qx=.585, Qy=.575 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. 23 Pbar & Luminosity Losses vs. Separation Pbar NL losses at the beginning of 35 stores (MarchApril 2005) at indicated values of the helix size. Tevatron - Y. Alexahin ICE meeting 02/16/ 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 collisions abandoned New working point (close to 2/3 or 1/2) abandoned Tevatron - Y. Alexahin ICE meeting 02/16/ D0 CDF (B0) B17H C49H D11H D48H A49H C17V B11H C49V A49V A17V B11V D11V Run II Initial Separator Arrangement Tevatron - Y. Alexahin ICE meeting 02/16/ D0 CDF (B0) B17H C49H D11H D48H A17H C17V B11H C49V D11V A49V A17V B11V B48V D17V A49H Run II Final Separator Arrangement Tevatron - Y. Alexahin ICE meeting 02/16/ Collision Helix Upgrade *=28cm optics (fit): 13 separators 15 separators (A17H & B48V +) - more than 10% increase in separation at the nearest parasitics *=35cm optics (design): 12 separators 13 separators (D17V+) Tevatron - Y. Alexahin ICE meeting 02/16/ 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 2 nd order chromaticty (and therefore the total tunespread). Tevatron - Y. Alexahin ICE meeting 02/16/ Effect of 2 nd Order Chromaticity Compensation One more measure significantly improved proton lifetime: equalization of pbar bunch-by-bunch intensity. Presently beam-beam effects in collision impose ~5% tax on integrated luminosity Tevatron - Y. Alexahin ICE meeting 02/16/ BBC with TELs TEL2 operated on all p-bunches (not only the last bunches in trains, #12*k) raising the vertical tune. Both TEL1 and TEL2 improve proton lifetime significantly better than just lattice tune change nonlinear resonance compensation? - I dont 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. TEL1 increases the horizontal tune for p-bunch #13 which is normally too low close to 12 th order. Tevatron - Y. Alexahin ICE meeting 02/16/ Coherent Oscillations: Run Ib Observations 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. From V. Bharadwaj et al. Fermilab-TM-1970 (1996) - Nobody of the Tevatron people could recollect working with negative chromaticities nor were able to find any evidence in logbooks. Tevatron - Y. Alexahin ICE meeting 02/16/ 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 Tevatron - Y. Alexahin ICE meeting 02/16/ Dedicated Experiment on Colliding Beams Stability End-of-store study (N p =2 10 11, N a =2 ) 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 Tevatron - Y. Alexahin ICE meeting 02/16/ Bunch-by-Bunch Tune Monitor A. Valishev et al., EPAC08 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. Tevatron - Y. Alexahin ICE meeting 02/16/ 3x3 Beam-Beam Modes with TEL2 off/on Tevatron - Y. Alexahin ICE meeting 02/16/ Lessons Learned Head-on tuneshifts as high as 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 (12 th 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) Tevatron - Y. Alexahin ICE meeting 02/16/