Bunch Compression for the International Linear Collider

1
Bunch Compression for the International Linear
Collider

• ILC-Americas Meeting at SLAC
• October, 2004

2
Bunch Compressor Requirements

• Compress bunch (duh!)
• Hourglass effect limits minimum ß gt sz
• Short bunch better for wakefields, energy spread
• Orient polarization
• Vertical in MDR, longitudinal at IP
• Generally need capability to set tune arbitrary
  orientation
• Geometry match
• Transfer beam from MDR to main linac
• Preserve beam quality
• Emittance, energy spread, 6-D jitter,
  polarization, MPS
• Collimate backgrounds generated in MDR

3
ILC MDR Extracted Beam

• As presently envisioned, beam parameters out of
  ILC MDR are challenging
• 6 mm RMS length, 0.13 RMS energy spread (e)
• 5 GeV/c momentum
• 0.3 mm RMS length desired at IP

4
TESLA TDR / USCold Design

• Single stage 20 x bunch compression
• Increase RMS energy spread to 2.8
• Solenoid based spin rotation
• 2 rotators, 2 solenoids each
• Emma reflector between solenoids to cancel
  coupling
• 8º arc between rotators
• H/V chicanes to transfer from MDR line to linac

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TDR Design (2)
Three requirements on the compression part of the
system
Prevent MDR synchronous phase variation from
becoming main linac phase variation
Achieve the desired RMS bunch length given the
MDR beam distribution
Use the RF curvature to compensate the T566 of
the compressor
Given a chosen frequency and initial energy, the
3 free parameters (V, f, R56) are determined by
these 3 relations.
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TDR Design (3)
TDR choice was to use main linac RF (so 1.3 GHz)
and MDR energy (5 GeV). This leads to parameters
V 890 MV, f -113º, R56 0.215 m
To achieve the desired R56 with acceptable SR
emittance growth, an 86 meter wiggler is used
rather than a chicane. Running the RF 23 degrees
from the zero crossing results in a reduction of
the beam energy by about 400 MeV (ie, the beam
enters the linac with 4.6 GeV, not 5.0 GeV).
7
TDR Design (4)
Spin rotation from vertical to horizontal
polarization is achieved by using a solenoid.
The solenoid strength needed is given by
At 5 GeV (16.68 T.m), a total BL of 26.2 T.m of
solenoid is needed per rotation.
Since the solenoid in question would rotate the
flat beam through 45º, the spin rotation is
performed by a pair of equal-strength solenoids
with an Emma rotator between them (I in x, -I in
y) each solenoid performs half the spin rotation
(ie, each is 13.2 T.m). Because of chromatic
issues in the Emma rotator, this should be done
at the lowest possible energy spread. With 2
rotators separated by an 8º bend (which rotates
horizontal to longitudinal polarization), any
polarization state is achievable at the IP.
8
Problems and Limitations in TDR / USCold Bunch
Compressor

• Required compression at the limit of what a
  single stage can do
• Very large espread into linac causes serious
  emittance growth
• No path to shorter bunches if someday required
• No collimation included in design
• Spin rotator solenoids enormous
• Probably need field comparable to LCD solenoid
• Geometry set for TESLA TDR configuration
• Almost certainly will need to be changed
• Tuning and operations studies nowhere near as
  mature as for main linac

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Options for a Dual Bunch Compressor Design in ILC

• NLC required a much larger compression factor
  than ILC
• About a factor of 50
• LCLS, SPPS, XFEL also require very aggressive
  compression
• Typical approach is 2 bunch compressors with some
  acceleration in-between
• Keep RMS energy spread low
• Can we do that for ILC?

10
More on Dual BCs

• Consider NLC design
• BC1 similar to TDR BC but scaled to NLC
  parameters
• Smaller compression factor (9)
• Linac to accelerate from 2 GeV to 8 GeV (2.9 GHz)
• BC2
• Factor of 5.5 compression
• Preserve BC1 longitudinal phase
• Arc ? X-band RF ? chicane (telescope in Ez plane)

11
Dual BCs (3)

• Can ILC use a pair of 90º BCs?
• Would convert MDR phase to linac phase
• OK if MDR phase transients jitter small
• Advantages
• Simplicty pair of fairly similar systems inline
  with one another
• No large arc required (lots of optics issues)
• No depolarization in arc from large energy spread
• Disadvantages
• Aforementioned phase sensitivity
• Arc allows steering feed-forward from MDR to linac

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Dual BCs (4)

• 1.3 GHz RF system for 2nd BC looks pretty bad
• Cant achieve T566 compensation V cos f so
  large beam is stopped dead!
• 3.9 GHz looks more promising
• Only 1 GeV needed (for beam _at_ 10 GeV)
• Only 10º off-crest needed
• Existing 3.9 GHz structures somewhat
  disappointing

13
Collimation

• TDR design did not include collimation
• Was foreseen to have collimation in principle
• In practice nobody ever put the deck together
• ILC should probably have some
• E,z, and at least one each x/x/y/y
• MPS collimators between BC and main linac?
• Issues well understood
• Should include in decks check impact on
  performance early (so we can stop worrying about
  it)

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Conclusions

• At least 3 options exist for ILC BC design
• All have advantages and disadvantages
• How shall we decide?
• Need input on phase stability out of MDR
• Need to understand emittance dilutions
• Will 2nd BC cause more problems in BC than it
  cures in linac?
• Other (geometry) considerations
• Benefits to long turnaround between MDR and
  linac?
• Real impact of large 3.9 GHz RF system
• Ie,
• Plus theres other stuff to do (like collimation!)

15
What SLAC Can Contribute

• SLAC has a lot of expertise in the design and
  performance study of challenging BCs
• SLC RTL BC seemed pretty challenging at the time
• SPPS BC even more exciting
• NLC BC1/BC2 system
• Lots of SLAC people obsessed with emittance
  preservation
• Existing mini-group of international emittance
  zealots (Laptops at Dawn group) interested in
  this problem