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Working Group 2 Closing Summary

Working Group 2 Closing Summary. T. Sen, W. Fischer, J.P. Koutchouk,. 2- Motivation for the LHC Upgrade. The crossing angle shall be increased due to the reduction of β * the increased bunch current and number of bunches the possibly increased interaction length (long-range)

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Working Group 2 Closing Summary

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  1. Working Group 2Closing Summary T. Sen, W. Fischer, J.P. Koutchouk,

  2. 2- Motivation for the LHC Upgrade • The crossing angle shall be increased due to • the reduction of β* • the increased bunch current and number of bunches • the possibly increased interaction length (long-range) • The geometric luminosity loss becomes rapidly unacceptable:

  3. Lessons from the SPS experiments No wires activated • Compensating 1 wire with another wire at nearly the same phase “works” • Compensation is tune dependent • Current sensitivity • Alignment sensitivity • Equivalent crossings in the same plane led to better lifetimes than alternating planes • Beam lifetime τ ~ d5 d is the beam-wire distance Higher power law expected given the proximity of high order resonances Nearly perfect compensation Both wires on 1 wire on

  4. Quadrupole aperture with BBLR Wire compensation has the potential to reduce the aperture required significantly

  5. Dynamic aperture with wire compensation DC wire compensation increases the DA of a nominal bunch by ~2σat most tunes. But it decreases the DA of the extreme PACMAN bunch at most tunes.

  6. The specification of the frequency (439kHZ) needs more study

  7. Lessons from RHIC experiment • Study at injection energy with 1 bunch and 1 parasitic interaction per beam • There is an effect to compensate, even with 1 parasitic • Drop in lifetime seen for beam separations < 7 σ • Effect is very tune dependent • How important are machine nonlinearities and other time dependent effects? • Did they change with the beam-beam separation?

  8. Lifetime versus separation SPS : t 5ms (d/s)5 [measured 11/09/04] Tevatron: t ~ d3[reported in F. Zimmermann, LTC 11/24/04] RHIC : t~ d4 ord2 [measured 04/28/05, scan 4]

  9. RHIC Simulation – Ji Qiang, LBNL Scan 2 – rms emittance vs. time 4.7s separation 5.54s 7.1s 5 sec real time Blue Difference between beams visible for scan 2 parametersLittle effect seen for scan 4 parameters Yellow

  10. RHIC BBLR design – locations RHIC Sector 5 (IR6) [picture mirrored] long-ranginteraction(vertical) long-rangcompensation(up) Dfx,y = 6 deg (b* = 1m) long-rangcompensation(down)

  11. RHIC BBLR design – drawing pleasecomment • Main features: • elliptic copper bar (a/b = 59%) • air cooled heat sinks • on vertically movable stand (60mm movement)

  12. RHIC BBLR design – parameters ~10x single bunch pleasecomment For now mechanical design for 125A-m But power up to a max of 30Am.Eases cooling

  13. Proposal - 1 FY06 Plan • Design and construct a wire compensator (BNL) • Beam-beam studies at top energy: beam separation and tune scan. No wire. • Theoretical studies (analysis and simulations) to test the compensation and robustness • Install wire compensator on a movable stand in one of the RHIC rings in 2006 shutdown FY07 Plan • Beam studies in RHIC with 1 proton bunch in at flat top and 1 parasitic interaction. • Test tolerances on: beam-wire separation, wire current accuracy, current ripple, phase advance to the wire. • Simulations to match experiments • Construct and install 2nd wire compensator and current modulator in 2007 shutdown.

  14. RHIC experimental program proposal • (d,Qy) scan at 100 GeV • Single and multiple long-range interactions Run-6 (2006) w/o BBLR (ask for 2x3hrs) Run-7 (2007) with 1 or 2 dc BBLR Run-8 (2008) with ac BBLR

  15. Challenges - 1 Sensitivity to alignment errors SPS experiments showed that the tolerance on the wire separation was ~3 sigma. Wire motion can be controlled to ~ 25 microns Sensitivity to current jitter We could introduce white noise on the wire to induce emittance growth. Tolerance on noise levels and benchmark simulations. Sensitivity to optics errors Impact of local coupling and spurious dispersion on compensation should be looked at.

  16. Challenges - 2 • Sensitivity to phase advance errors between the parasitics and the wire The phase advance can be changed over a wide range by moving the location of the parasitic. • Tune dependence of the compensation - RHIC tunes are close to the LHC tunes Tune scans of the compensation could be done. • Sensitivity to tune spread of the bunch. Do the different rates of emittance growth in RHIC and LHC matter? Perhaps not since the compensation is local • How important is it to use pulsed wires for compensating the PACMAN bunches, i.e. is it known that average compensation is not good enough for these bunches? Not known yet - will be studied further with simulations • If pulsed wires are required, what is the right frequency? Does every PACMAN bunch need a different current? Same as above

  17. Simulations What can we expect? • Reproduce the results of the beam-beam experiment at injection energy Important physics e.g. nonlinear fields including snakes, space charge, IBS, tune modulation,…? • Simulate 1 parasitic interaction at top energy. Is there a significant impact on the beam? Variation with separation of: dynamic aperture, emittance change, lifetime,… • Simulate 1 parasitic interaction and wire. Is compensation effective? Tolerances on: alignment, current strength and jitter, phase advance errors, non-roundness of “strong” beam, …

  18. LHC simulations & wire compensation Emittance growth J. Shi

  19. LHC simulations & wire compensation(2) Predicts that multipole compensation might also work for long-range but at high beam-beam tune shifts J. Shi

  20. Benchmarking simulations • Experimental evidence so far • SPS expt: variation of losses with wire currents, tunes, separations • RHIC experiment: variation of losses with beam-beam separation, tune variation • What is the common observable in experiments and simulations? • Hard to simulate lifetimes with good statistical accuracy, emittances often used • Experiments: hard to measure emittance changes over the small time scale of simulations

  21. Use of the Electron Lens • Footprint due to head-on collisions can be efficiently compressed with the electron lens • Requires a location where the beta functions are equal • Beam-beam interactions are a dominant source of emittance growth in RHIC. An electron lens in RHIC could help to improve performance. • Emittance growth is determined by the strength of nonlinearity • Beam tests in Tevatron (without parasitics) could be a useful first step.

  22. Summary • For the LHC upgrade, wire compensation has the promise of allowing smaller crossing angles (better use of aperture and higher luminosity) and higher intensities. “More luminosity earlier” • SPS experiments showed that the compensation principle works for 1 wire compensated by another. • RHIC experiment showed that there is an effect due to parasitic at 24 GeV. Needs to be repeated at 100GeV. • Propose installing a wire compensator in RHIC in 2006. Tests of the compensation principle in FY07 and beyond. • Simulation efforts need to be significantly ramped up in FY06. • Possibilities of using the electron lens for compensating headon beam-beam interactions in RHIC and perhaps LHC.

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