Working Group 2 Closing Summary

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Working Group 2Closing Summary

• T. Sen, W. Fischer,
• J.P. Koutchouk,

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2- Motivation for the LHC Upgrade

• The crossing angle shall be increased due to
• the reduction of ß
• the increased bunch current and number of bunches
• the possibly increased interaction length
  (long-range)
• The geometric luminosity loss becomes rapidly
  unacceptable

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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 t 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
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Quadrupole aperture with BBLR
Wire compensation has the potential to reduce the
aperture required significantly
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Dynamic aperture with wire compensation
DC wire compensation increases the DA of a
nominal bunch by 2sat most tunes. But it
decreases the DA of the extreme PACMAN bunch at
most tunes.
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The specification of the frequency (439kHZ) needs
more study
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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 lt 7 s
• Effect is very tune dependent
• How important are machine nonlinearities and
  other time dependent effects?
• Did they change with the beam-beam separation?

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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 or d2 measured 04/28/05, scan
  4

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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
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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)
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RHIC BBLR design drawing
pleasecomment

• Main features
• elliptic copper bar (a/b 59)
• air cooled heat sinks
• on vertically movable stand (60mm movement)

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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
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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.

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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

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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.
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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

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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,

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

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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.

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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.