Vladimir Shiltsev Considerations on Beam-Beam Compensation with Electron Lenses in LHC April 27, 2006.

Vladimir ShiltsevVladimir Shiltsev

Considerations on Considerations on Beam-Beam Compensation Beam-Beam Compensation with Electron Lenses in LHCwith Electron Lenses in LHC

April 27, 2006April 27, 2006

4/27/06, eBBC in LHC - Shiltsev 2

Content: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)…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 Head-on and Long-Range BBCompensationBBCompensation

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/J<0.1% e-pbar centering

e-beam shaping

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dc

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Yu.Alexahin

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E-lenses in RHIC and LHCE-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 beta’s Better have much wider e-beam than p-beam + ~uniform j(r) Needed dQ~0.006 ; max tuneshift dQ~0.009 achieved

LHC (and RHIC) can benefit from head-on beam-beam compensation Electrons compensate prtons – that’s good! DC beam = no HV pulsers! (even better) Need 2 lenses – one per beam; at equal beta’s 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_max~0.01 which is achievable see next slide

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basics of head-on beam-beam basics of head-on beam-beam compensationcompensation

• If beam sizes and shapes are matched (e=p)

• for LHC N_p=1.1e11, N_ip=4, for 10kV electrons (beta=0.2) one needs N_e=4.4e11 or J_e=1.2 A in L=3 m long e-beam

• approx Gaussian e-current distribution with rms = 0.3-0.5 mm

… or donut shape?

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e-Shape controle-Shape control

Gaussian

Optimized

(6-Gauss)

Just an example

(see footprint on previous slide)

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Challenges of Head-On Challenges of Head-On CompensationCompensation

Total current is not a challenge

Optimum beam profile (vs bunchlength/beta*, Qx,y) -?

Compression to 0.3-0.5 mm is doable, though not easy

Keeping beam straight within 0.03-0.05 mm

Beam-beam centering within 0.03-0.05 mm

Will bending sections do harm?

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eLens configurationseLens configurations

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TEL-2 B-field line straightnessTEL-2 B-field line straightnessS.KamerdzhievX.L.ZhangV.Shiltsev

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Proposed Action PathProposed 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$ M&S

Demonstrate head-on compensation 2009-2010~200k$ SWF

Build TELs for LHC 2011-2012 ~1100k$ M&S

Install TELs in LHC and commission 2012-2014 ~200k$ SWF

Total: 3,000k$

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Step 1: Theory and Simulations: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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slidesslidesbackup

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Proposed Action PlanProposed 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 E-lenses in RHIC and LHC: General Considerations Considerations

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a=1.7mmL=2mE=980GeVbeta_e=0.2

Scaling: need a_e = a_p and about same dQ~0.01 Two factors:

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For RHIC: beta_x = 20 m for Tev: beta_x=100 gamma=250 gamma=1000Thus (B_c/B_main)=4/5 x (B/B)_Tevatron easy!

For LHC: beta_x = 200 m for Tev: beta_x=100 gamma=7000 gamma=1000Thus (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) Tevatron Electron Lens #1 (TEL-1)

+ HV Modulator, HV+HC PSs, Cryo, QPs, Vacuum, Controls, Diagnostics, Cables

90 deg bend

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Tevatron Electron Lens #1 in the Tevatron Tunnel, sector F48Tevatron Electron Lens #1 in the Tevatron Tunnel, sector F48

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Tuneshift dQTuneshift dQhorhor=+0.009 by TEL=+0.009 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: ConfinedBeam-Beam at Low-beta Now: Confined

7th order resonances:

Q=4/7=0.571 -

HIGH LOSSES

12th order resonances:

Q=7/12=0.583 -

Bad lifetime

5th order resonances:

Q=3/5=0.600 –

EMITTANCE BLOWUP

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