Vladimir Shiltsev

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Considerations on Beam-Beam Compensation with
Electron Lenses in LHCApril 27, 2006

• Vladimir Shiltsev

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Content

• The idea, how it works, Tev experience
• eBBC in LHC and RHIC
• Possible plan to proceed

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Original Idea Was (1997)
to compensate (in average) space charge forces
of positevely charged protons acting on
antiprotons in the Tevatron by interaction with
a negative charge of a low energy high-current
electron beam
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Head-on and Long-Range BBCompensation
Yu.Alexahin

• Tev Run II 36x36 bunches in 3 trains
• compensate beam-beam tune shifts
• a) Run II Goal
• b) one TEL
• c) two TELs
• d) 2 nonlinear TELs
• requires
• 1-3A electron current
• stability dJ/Jlt0.1
• e-pbar centering
• e-beam shaping

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E-lenses in RHIC and LHC

• In Tevatron TELs to compensate bunch-by-bunch
  tune shift due to parasitic beam-beam
  interactions ?
• Requires pulsed e-beam (400-800ns)expensive HV
  modulator
• Need 2 lenses one vert and one horizontal at
  unequal and large betas
• Better have much wider e-beam than p-beam
  uniform j(r)
• Needed dQ0.006 max tuneshift dQ0.009 achieved
  
• LHC (and RHIC) can benefit from head-on beam-beam
  compensation ?
• Electrons compensate prtons thats good!
• DC beam no HV pulsers! (even better)
• Need 2 lenses one per beam at equal betas
• Large beta is OK (at least better than in
  Tevatron as sigma_z/beta is not as big as in Tev
  where it is 2)
• E-beam j(r) should be Gaussian to match protons
  at IPs easy!
• Need dQ_max0.01 which is achievable ? see next
  slide

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basics of head-on beam-beam compensation

• If beam sizes and shapes are matched (ep)

• for LHC N_p1.1e11, N_ip4, for 10kV electrons
  (beta0.2) one needs N_e4.4e11 or J_e1.2 A in
  L3 m long e-beam

• approx Gaussian e-current distribution with rms
  0.15-0.2 mm
• or donut shape?

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e-Shape control
Optimized (6-Gauss)
Gaussian
Just an example (see footprint on previous slide)
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Challenges of Head-On Compensation

• Total current is not a challenge
• Optimum beam profile (vs bunchlength/beta, Qx,y)
  -?
• Compression to 0.15-0.2 mm is doable, though not
  easy
• Keeping beam straight within 0.02-0.03 mm
• Beam-beam centering within 0.02-0.03 mm
• Will bending sections do harm?

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eLens configurations
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TEL-2 B-field line straightness
S.Kamerdzhiev X.L.Zhang V.Shiltsev
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Proposed Action Path

• Form an LHC eCompensation Task Force with a
  charge to perform feasibility study in 1 year
• Goal is to explore parameter space and
  effectiveness of head-on BBC in LHC and RHIC
• Cost 1.5FTE (200k) in SWF 50 for LARP
• In case of positive outcome, next steps may
  include
• Design of the TEL for RHIC 2007
  200k SWF
• Construction of TEL for RHIC
  2008-2009 1300k MS
• Demonstrate head-on compensation
  2009-2010 200k SWF
• Build TELs for LHC 2011-2012 1100k MS
• Install TELs in LHC and commission
  2012-2014 200k SWF
• Total 3,000k

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Step 1 Theory and Simulations

• Will Gaussian or truncated Gaussian e-current
  density distribution work (improve lifetime and
  reduce diffusion rates)?
• Straightforward tracking with a weak-strong code
• Is partial distribution helpful?
• Is there a better distribution?
• from first principles, theory, analytical
  consideration
• Effects are beta_TEL/beta/sigma_z or dP/P
• check in numerical tracking
• Importance of e-pbar(p) interaction in bending
  sections
• Which of three configurations is better?
• Is the choice tune dependent?
• Lifetime deterioration due to e-p misalignment
• e-beam straightness tolerances
• relative e-p displacement, angle
• Effect of low-frequency variations dJ, dX on beam
  lifetime
• Ion cleaning efficiency tolerances
• Cross-interaction with wires in LHC if there is
  any
• e-beam effect on coherent stability or
  strong-strong beam-beam effects

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slides

• backup

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Proposed Action Plan

• Calibrate TEL-2 BPMs with stretch wire (Vic,
  Seva, Brian)
• Measure SEFT profile (Seva)
• Assemble TEL-2 vacuum system and bake (Gennady
  John F)
• Perform EoS and SoS studies with TEL-1 (ZXL, VS,
  Seva)
• Get the current, study transmission vs
  B_gun/B_main (Seva, XL)
• Study e-beam orbit vs current (same)
• Tracking studies with LIFETRAC (Sasha Valishev)
• Calibrate TEL-2 BPMs with short e-pulses (Vic and
  Seva)
• Measure TEL-2 e-beam profile (Seva, Vic)
• Commission MARX generator pulser (Howie we)
• Move install TEL-2 FY06 during shtdn (Gennady,
  MSupp, et al)
• Commission TEL-2 with 980 GeV p-beam, studies
  with 2 TELs (all)
• Build parts for TEL-1 modification (Gennady,
  Seva, VS)
• Modify TEL-1 in FY07 shutdown (as in FY06)

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E-lenses in RHIC and LHC General Considerations
a1.7mm L2m E980GeV beta_e0.2
Scaling need a_e a_p and about same dQ0.01
Two factors
For RHIC beta_x 20 m for Tev beta_x100
gamma250 gamma1000 Thus
(B_c/B_main)4/5 x (B/B)_Tevatron ? easy!
For LHC beta_x 200 m for Tev beta_x100
gamma7000 gamma1000 Thus
(B_c/B_main)1/3.5 x (B/B)_Tevatron ? not easy,
but doable
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Tevatron Electron Lens 1 (TEL-1)
90 deg bend
HV Modulator, HVHC PSs, Cryo, QPs, Vacuum,
Controls, Diagnostics, Cables
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Tevatron Electron Lens 1 in the Tevatron Tunnel,
sector F48
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Tuneshift dQhor0.009 by TEL

• Three p-bunches in the Tevatron, the TEL acts on
  one of them

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Beam-Beam at Low-beta Now Confined
5th order resonances Q3/50.600 EMITTANCE
BLOWUP
pbar
12th order resonances Q7/120.583 - Bad
lifetime
p
7th order resonances Q4/70.571 - HIGH LOSSES