Crab system for the ILC

1
Crab system for the ILC
LC-ABD 24/10/2005
Graeme Burt - Lancaster University Philippe
Goudket - ASTeC Lili Ma - ASTeC Alexander
Kalinin - ASTeC Carl Beard - ASTeC Amos
Dexter - Lancaster University
2
What might a superconducting crab cavity system
look like?
4m-10m
to IP
Cryostat
Reference from beam

• Anticipated requirement for 20mrad crossing is 4
  ? 9 - cell cavities per linac
• Need space for cryostat, input/output couplers,
  tuning mechanisms

Reference Phase
Reference for crab cavities on other beam
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Key technical issues

• Position of crab cavities in the BDS
• Type of crab cavity
• Frequency of operation
• How phase jitter and field stability affects
  luminosity
• Effect and extraction of the lower order mode
• Effect of wakefields/higher order mode damping
• Performance of phase control systems

4
Technical issues relating to choice of crab
cavity system frequency

• In favour of 1.3 GHz
• At 1.3GHz an IOT could be used which is 10 times
  more phase stable than a klystron, however a
  solid state source can be used at either
  frequency.
• Wakefield voltages are reduced as they are
  proportional to frequency for a given energy,
  however attenuation of HOM energy is also
  reduced.
• In favour of 3.9 GHz
• Dimensions of the cavity are smaller as they are
  roughly inversely proportional to frequency.
• The physical phase tolerance is relaxed for
  higher frequencies, though more understanding of
  phase control issues is required.
• Transverse voltage required is three times less
  than it would be at 1.3GHz.

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Work in progress

• A study of quadrupole effects on crab rotation.
• A study of cavity to cavity phase tolerance and
  cavity to beam phase tolerance.
• Study of frequency for crab cavity system.
• Electromagnetic design studies.
• Alternative cavity designs.
• Evaluation of Fermilab 3.9GHz CKM cavity.
• Planning of phase control experiments, measuring
  cavity to cavity jitter on existing
  superconducting cavities and the performance of
  phase control systems.
• Study of higher order modes.

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Beam dynamics - Action of Quadrupoles
Dashed back of bunch Solid front of bunch
2nd Quad
Crab Location
1st Quad
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Location of the crab cavity (effect on space
required)
Active length (m)
1.3GHz
3.5
2.5
The full crab cavity system length will be equal
to the active length multiplied by a packing
factor (typically 2-4)
1.5
3.9GHz
0.5
CC distance from IP (m)
10
20
30
40
50
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e/e- beam-to-cavity phase error Transverse
deflecting dipole mode

• 15 degrees beam to cavity error allowed, as
  bunches arriving late get more kick hence bunches
  always line up as they approach the IP

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Technical issues on cavity to cavity phase jitter
electron bunch
?x
positron bunch
Interaction point

• Tolerance for a 20mrad crossing angle with 1.3
  GHz cavities
• 3 voltage stability ?
• 2 degrees cavity to beam phase error ?
• 0.022 degrees phase jitter unknown, thought to
  be possible

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Fermilab cavity, other cavities and technical
advancement

• Current deflecting cavity exists at Fermilab, 13
  cells per cavity, 6MV/m at 3.9GHz
• We are studying variations on current cavity
  design for 1.3, 2.6 and 3.9 GHz.
• Also undertaking more general studies.

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Order of modes in an dipole cavity
TM010 accelerating mode
Higher order modes
TM110v
TE111v
Need to extract the fundamental mode
TM011
frequency
Extraction of the lower order mode and the higher
order modes is essential to minimise disruption
of the beam. v-vertical h-horizontal
TE111h
TM110h crabbing mode
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Study of cavity shape
As the beampipe radius is increased the
transverse voltage decreases and the cell-to-cell
coupling increases.
Beampipe radius
13
Crab cavity aperture evaluation

• Work carried out by Sasha Drozhdhin (SLAC)
  evaluated the effect of photons from the bends on
  the crab cavity assuming different cavity
  apertures.
• According to the calculations, photons (mean
  energy ltEgt30keV) could hit elements inside the
  beampipe, with less than 1.5cm clearance from the
  axis, such as the LOM coupler.
• RD needs to be done to evaluate the effect of
  this photon flux and whether careful placement of
  masks can reduce the flux hitting the cavity.

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Planned work

• Numerical multipacting study for dipole cavities.
• Development of models of phase control systems.
• Development of a phase control system suitable
  for the crabbing system.
• Warm models of the Fermilab cavity to investigate
  trapped modes.
• Evaluate minimal cavity beam clearance
• Development of couplers and LOM/HOM damping
  systems.
• Study of cryostat requirements.
• Undertake Daresbury ERL Prototype phase control
  experiment.

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Likely configuration for phase control system
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Phase control system development

• Tests need to be carried out to understand the
  difficulties that need to be overcome in order to
  achieve the required phase stability.
• Can phase be effectively controlled over the
  distances involved?
• How well would current phase control systems cope
  with the crab cavity system requirements and
  where would improvements be needed?

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Planned experiment on performance of 1.3GHz phase
control system
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Wakefield study - Low power warm cavity model
Input coupler
HOM coupler
VNA

• Construction of a dipole mode cavity model to
  investigate higher and lower order modes.

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Conclusions

• Completed tasks
• Calculations of phase/amplitude parameters
• Comparison of cavity design options
• Current and future RD topics
• Phase control experiment and phase control system
  design
• Investigation of lower order mode damping
• Wakefield/trapped mode investigation
• Multipacting in dipole mode cavities
• Aperture limits
• RF system model
• Klystron/IOT performance