Accelerator Physics

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Accelerator Physics

• Petr Ostroumov (ANL)
• March 15 , 2005

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Outline

• Basic concepts for the Linac design
• Choice of lattice parameters
• High-intensity beam physics
• Detailed design and simulations
• RD issues
• Conclusion

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8-GeV Linac conceptual design

• GOAL Design 8-GeV high-power linac in a
  cost-effective manner
• Apply ILC-style cavities above 1.2 GeV
• Feed multiple cavities from one klystron
• Develop and use S-ILC cavities (beta0.81) in the
  energy range 400 MeV-1.2 GeV
• Select sub-harmonic frequency for the front end
  1/4. Motivation multi-spoke cavities are
  developed at 345-350 MHz. Requires 30 less
  number of cavities compared to 433 MHz option.
  Klystrons are available from JHF developments.
• No need in SRF cavities below 15 MeV use 21
  RT-TSR with very high shunt impedance. Power
  distribution is similar to those for SRF cavities

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Linac conceptual design (contd)

• 325 MHz SSR, DSR and TSR from 15 MeV to 400 MeV
• Apply SC solenoid focusing to obtain compact
  lattice in the front end including MEBT
• JHF-SNS-style RFQ delivers axial-symmetric 3 MeV
  beam
• MEBT consists of 2 re-bunchers and a chopper.
  Smooth axial-symmetric focusing mitigates beam
  halo formation
• Beam matching between the cryostats adjust
  parameters of outermost elements (solenoid
  fields, rf phase)
• Frequency transition 14 at 400 MeV, matching in
  
• (?, ?W)-plane is provided by 90? bunch
  rotation
• Avoid beam losses due to H-minus stripping

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Linac Structure
Major Linac Sections Front end Squeezed
ILC-style ILS-style
325 MHz 1300 MHz 1300 MHz
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MEBT, Linac entrance, 28 mA
Chopper
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RT-TSR, f325 MHz
Shunt Impedance 160 M?/m - 80 M?/mIH
structure application for heavy ions GSI, LMU
(Munich), CERN-ISOLDA, TRIUMF-ISAC, several Labs
in Japan
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Energy gain per resonator vs distance
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Stability Diagram (transverse motion)
Unstable due to parametric resonance
Linac operating tunes (black dots) Stable for
all particles inside the separatrix
Defocusing Factor
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Defocusing Factor vs Energy
Defocusing factor
S-ILC 6 cavities/period ILC 16 cavities/period
S-ILC 4 cavities/period ILC 8
cavities/period Upper Limit for Defocusing
Factor
Beam Energy (MeV)
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Focusing lattice
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High intensity beam physics

• Current independent lattice (Developed at LANL
  for ATP, SNS )
• Smooth kT0 and kL0 avoid jumps in the
  transition, very important in the SC linac
• Space charge and higher order internal resonances
• Beam matching in the transitions
• Parameters should be close to equipartitioning
• Avoid obvious resonance ?T0 90?
• Watch for higher order resonances (Hoffmans
  Chart)
• Final verification by multi-particle simulations
• Front End
• The length of focusing period should be
  minimized SC solenoids provide shortest focusing
  period
• Analyze HOM and avoid excessive power losses on
  cavity walls

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RMS analysis wavenumebrs of T- and L-oscillations
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RMS analysis Phase advance
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Detailed design and simulation

• RFQ design
• Linac design on the base of rms equations (TRACE
  3D, TRACK-fitting)
• Simulations with the codes PARMILA, IMPACT (in
  progress) and TRACK
• End-to-end simulations TRACK
• All fields are 3-D, resonators fields - MWS,
  solenoid fields - EMS
• Most time-consuming work is beam matching in the
  transitions between different sections and
  cryostats

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End-to-end BD simulations (20K particles)
Beam current 30 mA-RFQ, 28 mA - Linac
aperture
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End-to-end BD simulations, Log scale
Beam current 30 mA-RFQ, 28 mA - Linac
aperture
RFQ MEBT RT-TSR SSR,DSR TSR
S-ILC, ILC
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Phase width (deg) along the linac
Beam current 30 mA-RFQ, 28 mA - Linac
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The same tune, 10 mA, 0 mA
10 mA-RFQ entrance Zero current
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Emittance growth
Emittance growth is almost negligible as soon as
beam is matched carefully.
30 mA (28 mA)
10 mA (9.3 mA)
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Phase space plots of 8-GeV beam
Beam current 9.3 mA
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RD issues

• Develop optimized design of the linac
• Error studies
• Study of different options of the linac to
  provide the most cost-effective design.
• Example frequency transition energy (110 MeV vs
  400 MeV)
• Detailed studies of HOM in all ILC-style cavities
  and in TSRs
• Iterate beam dynamics studies with hardware
  development
• Some code development work is required
• fitting in realistic fields
• support many different types of lattice structure
• include feedback loops of rf field for error
  simulations
• Apply different codes for BD simulations
• High-statistics simulations

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CONCLUSIONS

• Beam physics in the driver is predictable. Most
  problems are similar to the SNS and can be solved
  during RD phase
• New approach in hadron Linacs - Pulsed SC Front
  End-provides high-quality beams
• Extremely high accelerating gradients require
  shorter focusing periods. It is recommended to
  keep peak surface fields in the S-ILC section
  below 50 MV/m
• The concept of current-independent tune similar
  to those developed for the SNS works well for the
  Proton Driver the same 30-mA tune is good for
  all beam currents in the range from 0 to 30 mA
• RD work is required to eliminate uncertainties
  and provide cost-effective design and
  construction of the Linac