Super-LHC Crab Ambitions H. Padamsee, LARP Meeting October 3, 4, 2005

1
Super-LHC Crab AmbitionsH. Padamsee, LARP
MeetingOctober 3, 4, 2005
2
Intro Remarks

• Derived from experts
• Ohmi, Zimmerman, and Ruggiero

3
Several Possible Paths to Super-LHC, All
Challenging

• Goals increase luminosity to 10 X nominal
  (1x1034)
• In several stages, with different approaches
• Beam-beam compensation with wires
• Reduce beta
• Increase bunch charge
• Reduce emittance
• Increase number of bunches
• Reduce bunch length
• Large angle crossing with crab compensation to
  allow head-on collisions
• Upgrade time scale 2012 - 2015
• Several different paths arranged into phases 0,
  1, 2..

4
Attractive Features of Crab Crossing Option with
Large crossing angle (0.3 - 8 mrad)

• Avoids parasitic collisions
• Avoids geometric loss of luminosity for crossing
  angles larger than beam aspect ratio
• Aspect ratio of LHC beam is 0.17mm/7.5 cm 0.23
  x10-3.
• Two beams can pass through separate magnetic
  channels, without sharing a quadrupole aperture.
• The crab cavities lead to a simplified IR design.
• Strong-strong beam-beam simulations predict an
  increase in the KEK-B beam-beam tune shift limit
  by a factor 2-3 for head-on collision compared
  with the present crossing angle.

5
(No Transcript)
6
(No Transcript)
7
Kick Voltage
8
(No Transcript)
9
The bunch is synchronized to pass through the
center of the cavity at the zero crossing of the
magnetic field. The head and tail of the bunch
are deflected in opposite directions. The bunch
oscillates in the magnetic field of the insertion
quads and reach their peak amplitude at the
interaction point.
10
Crab Voltage
To realize head-on collision effectively, the
beam deflection angle f should be equal to the
half crossing angle ?c. Voltage V required for a
deflection angle is
LHC R12 30 m

• Super-LHC needs
• 1/2 Crossing angle max 0.15 mrad - 4 mrad,
• Frequency 400 to 1200 MHz,
• E.g. Vc (400 MHz) 28 MV/mrad gt 112 MV for max
  angle

11
Crab Crossing Requirements

• Rotate the bunch without deflecting the bunch
• Jitter of RF phase of crab cavity induces a
  random transverse offset at the collision point,
  which causes diffusion due to beam-beam
  interaction.
• Jitters of RF phase of main accelerating cavity
  causes a deviation of timing of beam arrival at
  the crab cavity.
• Since there is no damping mechanism due to the
  absence of synchrotron radiation, a slow
  diffusion can be serious for hadron colliders.
• Emittance can grow

12
Noise Issues

• amplitude noise introduces small crossing angle
• tolerance 0.1 jitter from emittance growth
• phase noise causes beam-beam offset
• tolerance on LHC IP offset random variation
• Dxmax10 nm
• ? tight tolerance on left-right crab phase and
• on crab-main-rf phase differences

Df lt0.012o (Dtlt0.08 ps) at qc1 mrad 400 MHz Df
lt0.04o (Dtlt0.28 ps) at qc0.3 mrad 400 MHz
13
Comparison of timing tolerance with other
applications
KEKB Super-KEKB ILC Super-LHC
sx 100 mm 70 mm 0.24 mm 11 mm
qc /- 11 mrad /-15 mrad /-5 mrad /- 0.5 mrad
Dt 6 ps 3 ps 0.03 ps 0.08 ps
IP offset of 0.001 sx
IP offset of 0.2 sx
? not more difficult than ILC crab cavity
Zimmerman
14
SC Crab/Deflecting Cavities Efforts Worldwide

• CERN/Karlsruhe SC deflecting cavity for
  separating the kaon beam 1970s, 2.86 GHz
• Cornell 1500 MHz crab cavity 1/3 scale models
  1991for very high average current 1 A
• KEK 500 MHz crab cavity with extreme polarization
  1993 - Present, 1-2 A beam current, - Hosoyama
  WG3
• KEK 2003 new crab cavity design for Super-KEKB,
• 10 A beam current, 3 mm bunch length,
• more heavily damped (coaxial waveguide)
• Fermilab CKM deflecting cavity - 2000 - present
• Daresbury is studying crab cavities for ILC, 2005
• Crab cavities for ultra-fast light sources (LBNL,
  Argonne)
• Crab cavities or super-LHC (upgrades)

15
Around the World

• LHC
• 1/2 Crossing angle max 0.15 mrad - 4 mrad,
• frequency 400 to 1200 MHz,
• E.g. Vc (400 MHz) 28 MV/mrad gt 112 MV max
• CESR-B studies,
• 12 mrad crossing angle,
• kick 2 MV,
• one 500 MHz cavity, Es lt 25 MV/m
• KEK-B crab crossing
• Most advanced development
• 500 MHz, Vc 1.44 MV
• ILC crab crossing
• Crossing angle 20 mrad, w/o crab lum loss is
  80
• kick voltage 6 MV at 3.9 GHz or 18 MV at 1.3
  GHz
• 6.5 MV/m
• Most likely candidate Fermilab 3.9 GHz structure

16
Cornell SC Crab Cavity

• Developed in 1991 for B-factories
• Single Cell Model Tested in 1992
• Advances at KEK based on Cornell
  concept/development

17
Issues Addressed at Cornell

• Kick voltage, peak surface fields
• Input power coupling requirements
• Polarization of crab mode
• Removal of Lower Order Modes
• TM010, TE110 (2 pol), and TM110 wrong pol.
• Use coaxial coupler with choke filter to reject
  desired crab mode
• Removal of HOM
• Large beam pipe/ferrite dampers
• 1/3 scale Nb cavity made and tested
• Kick field and peak fields achieved

18
TM110 Mode Properties
19
500 MHz TM110 Polarized Cavity Mode separation14
MHz
20
Nb Cavities Made - First
After overcoming one low field multipacting
barrier we were able to reach Epk 30 MV/m _at_ Q
1.2x109 in one polarization, and Epk 27.5 MV/m
_at_ Q 2.3x109 in the other polarization of the
TM110 mode. Both maximum fields were limited by
field emission. Needed 25 MV/m
21
Second Nb Cavity
Reached 22 MV/m Q gt 109
QHOM lt100 QLOM lt 100
22
Input Coupler
One requirementthere may be others For a 1 mm
displacement and a transverse voltage maximum of
2 MV, the effective accelerating voltage is 2
x104 Volts. A 1 amp beam current extracts or
gives back about 20 (x) kWatts for x mm
displacement. The required coupling of Qext
6.4x106/x (mm) can be accomplished by a coaxial
coupler similar to input couplers presently used
23
KEK Squashed Cavity
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
Fermilab Deflecting Cavity, 3.9 GHz, Candidate
for ILC Crab
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
Crab Cavity for Ultrafast Light Source
In a downstream radiating magnet or insertion
device, the time-correlated transverse kick
results in an angular or spatial distribution of
the radiation emitted by electrons along the
length of the bunch. The resulting extended
x-ray pulse with correlated distribution may be
imaged by asymmetrically-cut crystals or
aberration-corrected x-ray optics, to result in
an x-ray pulse of approximately picosecond
duration. After the bunch radiates, the kick
introduced is cancelled by a downstream
deflecting cavity. High repetition rates, in
principle as high as 500 MHz (ALS bunch rate),
may be achievable using this technique, providing
an ultrafast x-ray source with characteristics
suitable for a variety of time-resolved
photoemission and magnetism experiments.
35
LHC Crab System Issues

• What are the crossing angle choices early stage
  0.3 mrad, final stage 8 mrad? What are the
  required kick voltages? (next slide)
• What are the crab development/implementation time
  scales?
• Choice of rf frequency, 400 MHz, 800 MHz or 1200
  MHz
• How much free space is there near the interaction
  region for installing crab cavities?
• Estimate total length of cavity system, assuming
  filling factor 0.5?
• Single cells or multi-cells (number of cells?)
• How much polarization split needed for the TM110
  deflecting mode?
• Factors influencing choice of aperture (reduced
  by presence of coupler for TM010 mode)
• Optimum geometry for crab cells, choice of safe
  max surface fields
• Input power coupling requirements, Qext, power
  handling.
• To avoid deflecting the bunch, so as to keep
  beams in collision, the phase of the cavities
  must be highly stable how much is tolerable?
• How much phase jitter is tolerable to avoid
  beam-beam performance degradation?

36
Super-LHC Crab cavity voltage for different qcs
rf frequencies
crossing angle 0.3 mrad 1 mrad 8 mrad
800 MHz 2.1 MV 7.0 MV 56 MV
400 MHz 4.2 MV 13.9 MV 111 MV
200 MHz 8.4 MV 27.9 MV 223 MV
37
Higher Frequency Choice

• Less Total kick voltage required a 1/f
• But Wavelength must be gtgt than bunch length (7.5
  cm)
• Need fewer cavities, less free space
• Transverse dimensions smaller
• Aperture smaller, HOMs excitation stronger
• Can use IOT at 1200 MHz, better phase stability
• Higher frequency more tolerant of phase jitter

38
LHC Crab System Issues/2

• How to control phase jitter in crab cavity?
• What is the tolerance on crab kick amplitude?
• How much is the phase jitter due to the
  accelerating cavity, how much is tolerable?
• What bunch length, bunch charge, average current,
  bunch spacing to use for design of HOM damping?
  (Given several different luminosity upgrade
  scenarios for LHC)
• Estimate damping needed to avoid multi-bunch
  instability, given bunch spacing (e.g. 25 ns?)
• Estimate growth rate of coupled bunch
  instabilities caused by HOMs
• Compare to acceptable instability growth rate
  from accelerating LHC cavities.
• Tuning range for tuner
• Microphonics tolerance due to LOM coupler