# Presentation: 408865

## Vladimir Shiltsev

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

## Content:

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

## 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 “

## Head-on and Long-Range BBCompensation

- 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

- Yu.Alexahin

## 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_ 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

##          basics of head-on beam-beam compensation

-   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?

##          e-Shape control

- Gaussian

- Optimized
- (6-Gauss)

- Just an example
- (see footprint on previous slide)

## Challenges of Head-On Compensation

- 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?

##          eLens configurations

## TEL-2 B-field line straightness

- S.Kamerdzhiev
- X.L.Zhang
- V.Shiltsev

## 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$   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$_

## 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

## slides

**backup**

## 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)

## E-lenses in RHIC and LHC: General Considerations 

- a=1.7mm
- L=2m
- E=980GeV
- beta_e=0.2

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

- For RHIC: beta_x = 20 m	for Tev: beta_x=100
-                  gamma=250			 gamma=1000
- Thus	    (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=1000
- Thus	    (B_c/B_main)=1/3.5 x (B/B)_Tevatron  not easy, 						       but doable

## Tevatron Electron Lens #1 (TEL-1)  

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

- 90 deg bend

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

## Tuneshift dQhor=+0.009 by TEL

- _Three_ p-bunches in the Tevatron, the TEL acts on _one_ of them

## Beam-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

- p

- pbar