beambeam wire compensation

1
beam-beam wire compensation crab cavities for
Super-LHC
Frank Zimmermann
KEK Accelerator Physics Seminar 27 October 2005
2
schematic of LHC upgrade paths (F. Ruggiero)
3
(1) need for beam-beam compensation

• nominal LHC parameters are challenging at the
  edge
• 20 geometric luminosity loss from crossing
  angle
• chaotic particle trajectories at 4-6s due to
  long-range
• beam-beam effects
• if we increase bunches or bunch charge, or
  reduce b
• long-range beam-beam effects require larger
  crossing angle
• but geometric luminosity loss would be
  inacceptable!

4
Piwinski angle
luminosity reduction factor
nominal LHC
5
Piwinski angles for lepton colliders LHC
6
Impact of crossing angle?
Lepton colliders Strong-strong beam-beam
simulations predict an increase in the KEKB
beam-beam tune shift limit by a factor 2-3 for
head-on collision compared with the present
crossing angle. This is the primary motivation
for installing crab cavities. The
simulations correctly predict the present
performance. K. Ohmi
Hadron Colliders
RHIC operates with crossing angles of /- 0.5
mrad due to limited BPM resolution and diurnal
orbit motion. Performance of proton stores is
very irreproducible and frequently occurring
lifetime problems could be related to the
crossing angle, but this is not definitely
proven. W. Fischer Tevatron controls crossing
angle to better than 10 mrad, and for angles of
10-20 mrad no lifetime degradation is seen.
V. Shiltsev

7
Experiment at SPS Collider K. Cornelis, W. Herr,
M. Meddahi, Proton Antiproton Collisions at a
Finite Crossing Angle in the SPS, PAC91 San
Francisco
Q0.45
qc500 mrad
Qgt0.7
qc600 mrad small emittance
8

• to boost LHC performance further various
  approaches
• have been proposed
• increase crossing angle AND reduce bunch length
• (higher-frequency rf reduced longitudinal
  emittance)
• J. Gareyte J. Tuckmantel, HHH-20004
• 2) reduce crossing angle apply wire
  compensation
• J.-P. Koutchouk
• 3) crab cavities ? large crossing angles w/o
  luminosity loss
• R. Palmer, 1988 K.Oide, K. Yokoya, 1989 KEKB
  2006
• 4) collide long intense bunches with large
  crossing angle
• F. Ruggiero, F. Zimmermann, 2002

9
baseline upgrade parameters invoke shorter or
longer bunches
F. Ruggiero, F. Zimmermann, HHH-2004
beam-beam compensation with wires or crab
cavities would change the optimum beam parameters
and could greatly affect the IR layout
10
minimum crossing angle from LR b-b
Irwin scaling coefficient from simulation
note there is a threshold - a few LR encounters
may have no effect! (2nd PRST-AB article with
Yannis Papaphilippou)
minimum crossing angle with wire compensator
need dynamic aperture of 5-6 s wire
compensation not efficient within 2 s from the
beam center
independent of beam current
11
(2) wire compensation BBLR

• SPS studies
• simulations
• LHC situation
• RHIC experiment
• US LARP
• pulsed wire

12
SPS experiment 1 wire models LHC long-range
interaction
extrapolation to LHC beam- beam distance, 9.5s,
would predict 6 minutes lifetime
13
SPS experiment two wires model beam-beam
compensation
Qx0.31
beam lifetime
no wire
2 wires
1 wire
vertical tune
lifetime is recovered over a large tune range,
except for Qylt0.285
14
New Simulation Tool BBTrack Purpose of the
code Weak-strong simulations of long-range and
head-on beam-beam interactions and wire
compensation. Author Ulrich Dorda,
CERN Programming language FORTRAN90 Homepage
http//ab-abp-bbtrack.web.cern.ch/ab-abp-bbtrac
k/
Other codes, used in the past WSDIFF (F.
Zimmermann, CERN) http//care-hhh.web.cern.ch/CAR
E-HHH/Simulation Codes/Beam-Beam/wsdiff.htm BBSIM
(T. Sen, FNAL) http//waldo.fnal.gov/tsen/BBCODE/
public
15
simulated stability region in x-y plane with1 2
SPS wires
19mm (8s)
y
unstable
x
stable
un- stable
stable
-19mm (8s)
19mm (8s)
-19mm (8s)
-19mm
19mm
two wires
one wire
Yellow stable with two wires unstable with
one
Green unstable with one wire stable with two
U. Dorda
16
simulation of wire compensation for the SPS
experiment
unstable
stable
different initial betatron phases
one wire
1 sigma
unstable
stable
two wires
U. Dorda
8 sigma
3 sigma
0
17
unstable particles jump between phase-space
ellipses when they approach the wire (or, in LHC,
the other beam)
x
x
U. Dorda
18

• sensitivity to 2nd wires
• transverse position
• SPS data
• BBSIM simulation
• T. Sen
• BBtrack simulation
• U. Dorda

19
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

current-carrying wires
Phase difference between BBLRC average LR
collision is 2.6o
(Jean-Pierre Koutchouk)
20
simulated LHC tune footprint with w/o wire
correction

• .16s
• .005s
• .016s

Beam separation at IP
MAD
(Jean-Pierre Koutchouk, LHC Project Note 223,
2000)
21
for future wire beam-beam compensators -
BBLRs -, 3-m long sections have been reserved
in LHC at 104.93 m (center position) on either
side of IP1 IP5
22
LR collisions at IP1 5 for nominal bunch
LR collisions at IP1 5 for extreme PACMAN bunch
long-range collisions only
without BBLR compensation
U. Dorda BBTrack
tune footprints for starting amplitudes up to 6s
in x and y
23
LR collisions BBLR at IP1 5
LR collisions at IP1 5
nominal bunch
compensated
long-range collisions only
with without compensation
U. Dorda BBTrack
tune footprints for starting amplitudes up to 6s
in x and y
24
LR collisions BBLR at IP1 5
LR collisions at IP1 5
extreme PACMAN bunch
overcompensated
long-range collisions only
with without compensation
U. Dorda BBTrack
tune footprints for starting amplitudes up to 6s
in x and y
25
head-on LR collisions in IP1 5
head-on, LR BBLR
nominal bunch
LR compensated
4,10
-1,1
long-range head-on collisions _at_ IP1 5
with without compensation
U. Dorda BBTrack
tune footprints for starting amplitudes up to 6s
in x and y
26
head-on, LR BBLR
PACMAN bunch
head-on LR collisions in IP1 5
LR over- compensated
long-range head-on collisions _at_ IP1 5
with without compensation
U. Dorda BBTrack
tune footprints for starting amplitudes up to 6s
in x and y
27
LHC tune scan for nominal bunch, 45 deg. in
x-y-plane
red unstable (strong diffusion), blue stable
0
U. Dorda, BBTrack
long-range head-on
10s
stability of nominal bunch improves for almost
all tunes
0
long-range head-on wire compensation
0.3
0.8
10s
Qy
28
LHC tune scan for PACMAN bunch, 45o in x-y-plane

red unstable (strong diffusion), blue stable
0
U. Dorda, BBTrack
long-range head-on
10s
stability of extreme PACMAN bunch decreases for
almost all tunes
0
long-range head-on wire compensation
0.3
0.8
10s
Qy
29
tune scan for nominal bunch
wire compensation
LHC
without wire
wire increases dynamic aperture by 2s
U. Dorda, BBTrack
30
tune scan for PACMAN bunch
without wire
LHC
wire over- compensation
dc wire reduces dynamic aperture by 2s
U. Dorda, BBTrack
31
6-D effects? - nominal LHC optics
IP5
IP5
up to 1 m vertical disper- sion in the triplet
position of BBLR
position of BBLR
position of BBLR
position of BBLR
IP1
IP1
position of BBLR
position of BBLR
32
chromaticity from LRBB wires
B. Erdelyi T. Sen, 2002
d beam-beam or beam-wire distance in s D
dispersion h dispersion in s nLR number of LR
encounters
e.g., d9.5, nLR30, D0.6 m, b3000 m ? Q0.25
chromaticity from long-range collisions or wire
is a small effect
33
Long-Range BB Experiment in RHIC, 28 April 2005,
Wolfram Fischer, et al., 1 Bunch per Ring
Beam lifetime vs transverse separation -
Initial test to evaluate the effect in RHIC.
collision at main IP
10 min. lifetime
collision at s10.65m
(1) Experiment shows a measurable effect.(2) The
beam loss is very sensitive to working point.
34
Long-Range BB Experiment in RHIC, 28 April 2005,
Wolfram Fischer et al., 1 Bunch per Ring
more data sets
collision at s10.65m
Some time stamps have to be adjusted (used time
of orbit measurement, not orbit change)
parameters other than the orbit were changed -
not shown. Scan 4 is the most relevant one.
collision at s10.65m
35
BBTrack simulations for RHIC
increased particle loss for separation below
6s consistent with experiment
effect sensitive to the precise tune value, also
consistent with experiment
particles surviving over 300k turns vs. shift in
y tune for a constant beam- beam distance of 3s
initially 10000 particles are distributed on a
grid extending from -8 to 8s in x and y
unperturbed tunes are Qx0.27, Qy0.28
particles surviving over 300k turns vs.
beam-beam distance initially 10000 particles are
distributed on a grid extending from -8 to 8s in
x and y unperturbed tunes are Qx0.27, Qy0.28
36
US LHC Accelerator Research Program Task
Sheet Task Name Wire compensation of beam-beam
interactions Date 23 May
2005 Responsible person (overall lead, lead at
other labs) Tanaji Sen (FNAL, lead), Wolfram
Fischer (BNL) Statement of work for FY06
Statement of work for FY07 CERN
Contacts J.P. Koutchouk, F. Zimmermann

• Design and construct a wire compensator (either
  at BNL or FNAL)
• Install wire compensator on a movable stand in
  one of the RHIC rings
• Theoretical studies (analysis and simulations)
  to test the compensation and robustness
• Beam studies in RHIC with 1 bunch / beam at flat
  top 1 parasitic interaction.
• Observations of lifetimes, losses, emittances,
  tunes, orbits for each b-b separation.
• Beam studies to test tolerances on beam-wire
  separation w.r.t. b-b separation,
• wire current accuracy, current ripple

• Beam studies with elliptical beams at the
  parasitic interaction, aspect ratio close
• to that of the beams in the LHC IR quadrupoles
• Compensation of multiple bunches in RHIC with
  pulsed wire current.
• Requires additional voltage modulator

37
not to degrade lifetime for the PACMAN bunches,
the wire should be pulsed train by train
LHC bunch filling pattern
example excitation patterns (zoom)
38
specifications for pulsed wire compensator
88.9 ms/-0.0002 ms
23.5 ms/-0.02 ms
(variation with beam energy is indicated)
high repetition rate turn-to-turn stability
tolerance
39

• approaches towards solution
• earlier design for pulsed LHC orbit correction
• by Corlett Lambertson (LBNL)
• was expensive 10 years ago
• fast kicker developments for ILC (KEK, UK)
• fast switching devices for induction rf (KEK)
• contacts with industry
• collaboration with US LARP
• advice by Fritz Caspers and other CERN
• colleagues
• start paper study Ulrich Dorda
• if promising solution is found, possibly lab
  test
• test in RHIC (2007?)

40

• merits of wire compensation
• long-range compensation was demonstrated
• in SPS using 2 wires (lifetime recovery)
• simulations predict 1-2s gain in dynamic
• aperture for nominal LHC
• allows keeping the same or smaller
• crossing angle for higher beam current
• ?no geometric luminosity loss
• challenges plans
• further SPS experiments (3rd wire in 2007)
• demonstrate effectiveness of compensation
• with real colliding beams (at RHIC)
• study options for a pulsed wire

41
(3) Crab Cavities
s.c. crab cavity production at KEKB
42
Super-KEKB crab cavity scheme
2 crab cavities / beam / IP
Palmer for LC, 1988 Oide Yokoya for storage
rings, 1989
first crab cavities will be installed at KEKB in
early 2006
43
history of s.c. crab cavity developments

• CERN/Karlsruhe sc deflecting cavity for
  separating
• the kaon beam, 1970s, 2.86 GHz
• Cornell 1.5 GHz crab cavity 1/3 scale models
  1991
• KEK 500 MHz crab cavity with extreme
  polarization,
• 1993-present, for 1-2 A current, 5-7 mm bunch
• length
• FNAL CKM deflecting cavity, 2000-present
• KEK 2003 new crab cavity design for Super-KEKB,
• 10 A beam current, 3 mm bunch length,
• more heavily damped (coaxial waveguide)
• Daresbury is studying crab cavities for ILC,
  2005
• Cornell and LBNL are interested in developing
• crab cavities for Super-LHC

H. Padamsee, Daresbury Crab Cavity Meeting,
April 2004
44
bunch shortening rf voltage
unfavorable scaling as 4th power of crossing
angle and inverse 4th power of IP beam size can
be decreased by reducing the longitudinal
emittance inversely proportional to rf frequency
crab cavity rf voltage
proportional to crossing angle independent of
IP beam size scales with 1/R12 also inversely
proportional to rf frequency
45
R12 R22(R11) from MAD
nominal LHC optics
R12,3430-45 m
R22,441
(from crab cavity to IP)
46
(No Transcript)
47
voltage required for Super-LHC
48
crab cavity voltage for different qcs rf
frequencies
800 MHz would be too high for nominal LHC bunch
length
49
tolerance on R22
z-dependent additional crossing angle
corresponding Piwinski angle should be small
not a problem
for qc1 mrad, sx12 mm, R1230 m, sz7.55 cm
50
geometric effect of R22 from crab cavity to IP
sx11.8 mm sz7.55 cm R1230 m
51
KEKB crab cavity
K. Ohmi, HHH-2004
1.5 MV_at_500 MHz

• Squashed cell operating in TM2-1-0 (x-y-z)
• Coaxial coupler is used as a beam pipe
• Designed for B-factories (1?2A)

1.5 m
Courtesy K. Akai
52
longitudinal space crab frequency
longitudinal space required for crab cavities
scales roughly linearly with crab voltage
desired crab voltage depends on rf frequency)
achievable peak field also depends on rf
frequency 2 MV 1.5 m, 20 MV 15 m frequency
must be compatible with bunch spacing wavelength
must be large compared with bunch length 400
MHz reasonable?
53
geometric effect of crab cavity frequency
54
crab cavity amplitude noise

• amplitude change introduces z-dependent crossing
  angle

insert R1230 m, nIP2, b0.25 m, Qc1 mrad,
sz7.55 cm ?1/e (De/dt) 1 per hour for
DVc/Vc0.1 random jitter
55
crab cavity phase noise
phase change causes beam-beam offset tolerance
on LHC IP offset random variation is Dxmax8 nm,
for an emittance growth of 10 per hour ? tight
tolerance on left-right crab phase and on
crab-main-rf phase differences
Df lt0.008o (Dtlt0.05 ps) at qc1 mrad 400 MHz Df
lt0.026o (Dtlt0.18 ps) at qc0.3 mrad 400 MHz
56
offset tolerance was estimated from the following
formula
p emittance growth due to random offsets
emittance growth from turn-by-turn random
collision offsets Dx
assuming fast filamentation and
considering beam-beam kick only
SuperLHC bx,y0.25 m, nIP2, xHO0.005, g7500,
ge3.75 mm
requiring less than 10/hr emittance growth
Dxrmslt8 nm 10-3s
57
comparison of timing tolerance with others
IP offset of 0.001 sx 8 nm
IP offset of 0.2 sx
IP offset of 0.02 sx
? not more difficult than ILC crab cavity
note XFEL requires 0.02 ps phase stability
58
diffusion rate due to crab cavity noise
BBSS simulation by K. Ohmi
factor 50 difference remains this difference
was a factor 1/1500 in Ohmi sans talk at
HHH-2004
with correlation time 100 turns, for the nominal
LHC
59
in addition to beam-beam offset, also the direct
dipole kicks from random crab cavity phase
jitter induce emittance growth (J.
Tuckmantel)
my estimate
example
(6x10-4 o)
?
0.004 ps!
10 times tighter tolerance!
this effect likely requires transverse feedback,
head-tail damping, or other scheme to suppress
the dipole motion, or it can eliminate the idea
altogether
60
emittance growth from noise including
decoherence feedback (Y. Alexahin, 1997)
g0.2 feedback gain, x0.01 total beam-beam tune
parameter, s00.645 related to the fact that only
a small fraction of the energy received from a
kick is imparted on the continuum eigenmode
spectrum
1 emittance growth per hour ?Dx1.5 nm with
feedback ?Dx0.6 nm w/o feedback
61
an important source of jitter, at lower
frequencies, are vibrations of the low-beta
quadrupoles
62
Christoph Montag
Nanobeam02
RHIC beam jitter magnet vibrations
IR triplet vibrations have been identified as
source of beam motion around 10 Hz vibration of
cold masses within the cryostat at 10 mm level
rather than vibration of triplet as a whole
modulated beam-beam interaction
causes emittance growth at start of each store
63
Vladimir Shiltsev
Nanobeam02

• Tevatron fast beam motion
• low-b quad vibration frequencies
• CHL compressors
• stand resonances
• beam/quad amplitude gt10

64
impedance of crab cavities
transverse impedance is an issue due to large
beta function rise time due to 1 crab cavity
rise time from 10 normal rf cavities with the
same voltage
K. Akai
65
Impedance of Super-KEKB Crab Cavity Design
K. Akai
horizontal
longitudinal
66
transverse space requirement
KEK-B Cryomodule Size
from H. Padamsee, summary talk at LHC IR Upgrade
meeting, FNAL 10/05
1.5 m
LHC 8 mrad crossing angle, after 30 m, gives
beam-beam separation 0.24 m
67
use lowest cavity mode -TM010- for 2 beams?
Forward beam
RF Cavity TM010
Dampers
Dampers
Return Beam
from H. Padamsee, summary talk at LHC IR Upgrade
meeting, FNAL 10/05
68

• merits of crab cavities
• practical demonstration at KEKB in early 2006
• avoids geometric luminosity loss, allowing
• for large crossing angles (no long-range
• beam-beam effect)
• potential of boosting the beam-beam tune
• shift (factor 2-3 predicted for KEKB)
• challenges proposed plans
• design prototype of Super-LHC crab cavity
• (Cornell and LBNL are interested)
• demonstration that noise-induced emittance
• growth is acceptable for hadron colliders
• (installation experiment at RHIC?)

69
Many thanks to Kazunori Akai, Gerard Burtin,
Jackie Camas, Fritz Caspers, Ulrich Dorda,
Wolfram Fischer, Jean-Pierre Koutchouk,
Kazuhito Ohmi, Katsunobu Oide, Yannis
Papaphilippou, Francesco Ruggiero,Tanaji Sen,
Vladimir Shiltsev, Jorg Wenninger,
70
appendix
71
diffusion rate from strong-strong simulation with
BBSS for nominal LHC
K. Ohmi, HHH-2004

• sx2sx02Dt t turn
• D1.4x10-15 Dxmm2

dz 0 0.005 0.01
72
tolerance from Ohmi sans strong-strong
simulation for nominal LHC
K. Ohmi, HHH-2004

• For Dx1.6 mm (df5 degree) and t100,
• D1.4x10-15 Dxmm2, where sx2sx02Dt, t
  turn.
• Tolerance is Dx0.016 mm, Df 0.05 degree for
  t100, and Dx0.0016 mm, 0.005 degree for t1,
  for luminosity life time 1 day

slightly worse than my simple estimate!?
for 300 mrad crossing angle and 400 MHz
73
analytic theory of beam-beam diffusion (T. Sen
et al., PRL77, 1051 (1996)M.P.Zorzano et al.,
EPAC2000)

• Diffusion rate due to offset noise. (round beam)

K. Ohmi, HHH-2004
74
comparison with the simulation
K. Ohmi, HHH-2004

• D(a1)ltDJ2gt1.5x10-25 m2/turn
• D(sim)(s-s02)2/b2 10-28 m2/turn
• need to check

3 orders of magnitude discrepancy!
analytical diffusion rate from
Sen-Ellison-Zorzano much larger than simple
estimate and strong-strong simulation!?!
has this discrepancy been resolved?