SPS Experiments on Long-Range Beam-Beam Compensation

1
SPS Experiments on Long-Range Beam-Beam
Compensation
G. Burtin, J. Camas, G. de Rijk, G. Ferioli,
J.-J. Gras, S. Jackson, J.-P. Koutchouk, J.
Wenninger, F. Zimmermann, with help from lots
of groups!

• Summary
• Motivation
• LHC SPS Simulations
• Scaling

• MDs in 20022003
• BCT, PMT
• Future Devices
• Summary again

2
Summary

• SPS BBLR Set Up models Long-Range Beam-Beam
  Collisions
• in the LHC so far 33 MDs performed in 2002
  2003
• Tune shift orbit distortions well understood
  they allow precise
• determination of beam-wire distance
• Beam lifetime and loss measurements indicate LHC
  parameters
• at the edge (10 less crossing angle -gt beam
  lifetime of 4 h)
• Clear shrinkage of emittance due to BBLR its
  dependence on
• current and distance using calibration by
  scraper, this was
• converted into diffusive aperture (d.a.)
  Irwins scaling law
• confirmed (d.a.Sqrt(I)) d.a. might be only 2s
  in LHC?!
• Direct diffusion measurements started (limited
  by PMT
• signals, by scraper speed software), so far
  difficult
• Further MDs (upgraded scraper) analysis (e.g.,
  1000 turns)
• foreseen
• Two more devices in 2004 will demonstrate
  correction efficiency
• also allow comparison of various crossing
  schemes

3
Long-Range Beam-Beam Collisions

• perturb motion at large betatron amplitudes,
  where particles come close to opposing beam
• cause diffusive aperture (Irwin), high
  background, poor beam lifetime
• increasing problem for SPS, Tevatron, LHC,...
• that is for operation with larger of bunches

LR encounters
SPS 9
Tevatron Run-II 70
LHC 120
4
LHC 4 primary IPs
and
30 long-range collisions per IP
120 in total
partial mitigation by alternating planes of
crossing at IP1 5 etc.
5
result of weak-strong simulations for LHC
center of other beam
Y. Papaphilippou F.Z., LHC 99
diffusive aperture
6
Long-Range Beam-Beam Compensation for the LHC

• To correct all non-linear effects correction
  must be local.
• Layout 41 m upstream of D2, both sides of
  IP1/IP5

Phase difference between BBLRC average LR
collision is 2.6o
(Jean-Pierre Koutchouk)
7
Scaling from LHC to SPS
perturbation by wire
relative perturbation
 
for constant normalized emittance the effect in
units of sigma is independent of energy and beta
function!
8
Experimental Set-Up in the SPS
IwireNb e c LR/lwire
Tech. Coord. J. Camas G. Burtin/BDI Help from
many groups
wire length
wire current
two 60-cm long wires with 267 A
current equivalent to 60 LHC LR collisions
(e.g., IP1 5)
9
nominal distance 19 mm (in the shadow of the arc
aperture)
water cooling
10
LHC long-range collisions SPS wire cause
similar fast losses at large amplitudes in
simulations
SPS wire
diffusive aperture
LHC beam
1 mm/s
1 mm/s
diffusive aperture
effect of the 1-m long wire at 9.5s from the
beam center, carrying 267 A current, resembles
the total number of long-range collisions in the
LHC
11
MDs in 2002 and 2003
20/08/2002, 26 GeV/c IC and PMT reading vs.
position (threshold!), tune shift _at_120 A
DQx,y-/0.0060 10/09/2002, 26 GeV/c
commissioned new inductive coil, DT gt18o at 275
A, blew up ey by damper chirps (WH), DQ Dy vs.
bump _at_120 A 267 A 23/09/2002, 55 GeV/c blew up
ey by damper, vertical aperture problem emittance
shrinks for beam-wire distances lt12 mm, beam
losses lifetime vs. separation 27/06/2003, 26
GeV/c new dipole corrector, emittance shrinkage
if BBLR is excited 04/07/2003, 26 GeV/c blew up
emittance by mismatch, emittance shrinkage vs.
wire excitation 21/08/2003, 26 GeV/c kick pencil
beam, emittance shrinkage vs. separation,
calibration by scraping, diffusion measurement
12
linear optics perturbations
orbit change Dd and tune shift DQ due to wire
(or due to other beam)
precise control of beam-wire separation!
13
MD-2002-3
comparison with MAD prediction (J. Wenninger)
orbit kick
Qy
Qx
14
MD-2002-23 beam-wire distance derived from
tune shift and from orbit change versus
prediction
prediction
15
evidence for diffusion vs. beam-wire distance in
SPS MD-2002-3
9 s
9 s
background up
lifetime drops
below 9s!
logarithmic scales!
LHC at the edge of lifetime drop!
8 s
compare with LHC simulation
9.5 s
16
MD-2003-1
final emittance reduced, if wire
is excited for large initial emittances it may
approach a constant value
dependence on initial emittance
17
dependence on wire current
experiment of July 4, 2003, confirms Irwins
scaling law beam shrinks since particles at
large amplitude are lost
18
Final emittance vs bump for 3 different BBLR
excitations
Reduction in emittance of the left outermost
point without excitation is consistent with
scraping at an amplitude of (19-13.4)mm 5.6 mm,
corresponding to 2.4 times the rms beam size of
2.3 mm. This is also the dynamic aperture for
267 A w/o bump.
dependence on beam-wire separation
19
Scraper at 3.05( 0.88)mm gives eOUT1.7 mm, no
BBLR
IN Scan at 100 ms
OUT Scan at 3200 ms
Note orbit bump and LRBB excitation from 1500 ms
Bump 11.6 mm for BBLR _at_67 A gives eOUT2.2 mm
Bump 9.4 mm for BBLR _at_267 A gives eOUT1.15 mm
20
Abel transformation of wire-scan data gives
change in (norm.) amplitude distribution
(Krempl, Chanel, Carli)
BBLR 267 A, bump 4.5 mm eout2.06 mm
scraping at 3.06 mm, eout1.71 mm
scraping at 2.06 mm, eout1.31 mm
BBLR 267A Bump 9.4 mm, eout1.15 mm
21
Calibration curve of measured final emittance vs
scraper position allows us to estimate effective
aperture due to BBLR excitation
Calibration measured final emittance vs. scraper
position
22
scaling to the LHC d.a. only 2-3 s?
23
Example of BCT PMT data
BBLR at 12725 ms, scraping at 13225 ms
only BBLR (at 12725 ms), w/o scraping
PMT
PMT
BCT
BCT
can we fit a diffusion constant?
on the right, scraper position is about 1s at
larger amplitudes the diffusion seems much faster
than the speed of the scraper
24
Extension 1 Compensation

• Goal demonstrate the correctability of the LR-BB
  effect for LHC conditions
• Betatron phase shift (2 degrees)
• Wire out of beam aperture (12 ? instead of 9.5 ?)
• Excitation error say 10
• Set-up a second identical device in LLS5 about 2
  m away vertical motion same quality (vacuum
  compatibility) larger safety vs SPS aperture.

25
Extension 2 Long-Distance Compensation
Crossing Schemes

• compensation over long distance with variable
  phase advance and independent xy orbit bumps at
  two wires y wire
• this will confirm the practicability of the
  compensation scheme for the LHC with reduced
  systematic cancellations explore tolerances
• compare effects of alternating x-y crossings at
  two locations (LHC baseline) with 2 times
  stronger collision at 45 degrees - inclined
  hybrid crossing - and with pure vertical or pure
  horizontal crossing other 2 wires this will
  probe the sensitivity to the LHC crossing scheme
  and assess present choice

26
schematic of 2nd and 3rd device
clearance required to stay in the arc shadow 19
mm (y), 51.5 mm (x), 26.5 mm (45 deg)
27
2nd 3rd devices will be mobile with wires in
different planes (G. Burtin)
28
Summary

• SPS BBLR Set Up models Long-Range Beam-Beam
  Collisions
• in the LHC so far 33 MDs performed in 2002
  2003
• Tune shift orbit distortions well understood
  they allow precise
• determination of beam-wire distance
• Beam lifetime and loss measurements indicate LHC
  parameters
• at the edge (10 less crossing angle -gt beam
  lifetime of 4 h)
• Clear shrinkage of emittance due to BBLR its
  dependence on
• current and distance using calibration by
  scraper, this was
• converted into diffusive aperture (d.a.)
  Irwins scaling law
• confirmed (d.a.Sqrt(I)) d.a. might be only 2s
  in LHC?!
• Direct diffusion measurements started (limited
  by PMT
• signals, by scraper speed software), so far
  difficult
• Further MDs (upgraded scraper) analysis (e.g.,
  1000 turns)
• foreseen
• Two more devices in 2004 will demonstrate
  correction efficiency
• also allow comparison of various crossing
  schemes