SRF Requirements and Challenges for ERL-Based Light Sources

1
SRF Requirements and Challenges for ERL-Based
Light Sources

• Ali Nassiri
• Advanced Photon Source
• Argonne National Laboratory

2nd Argonne Fermilab Collaboration MeetingMay
18, 2007
2
Acknowledgements

• APS
• M. Borland, J. Carwardine, Y. Chae, G. Decker,
  L. Emery, R. Gerig, E. Gluskin,
• K. Harkay, R. Kustom, V. Sajaev, N. Sereno, C.
  Yao, Y. Wang, M. White
• JLAB
• G. Krafft, L. Merminga, R. Rimmer,

3
Outline

• Introduction
• SRF Requirement and Challenges
• Summary

4
Introduction

• Energy Recovery Linac (ERL) is a potential viable
  revolutionary option for future light sources.
• Argonne Advanced Photon Source is considering
  ERL for its upgrade
• Promise of very high brightness and transverse
  coherence
• Extremely low emittance, equal in both planes
• Very low energy spread
• Picosecond pulses
• Option for less current with high charge,
  femtosecond pulses.

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A Design Parameters Comparison
ILC1
Light Source ERL2
Beam Energy 500 COM 5 8 GeV
Average beam Current 9.0 100 mA
Bunch train repetition rate 5 1.3?109 Hz
RF duty factor 7.5?10-3 - 1?10-2 CW
Average accelerating gradient 31.5 20 MV/m
Cavity Quality factor 1?1010 gt 5?1010 (1?1011)
Beam pulse length 9.5?10-4 2?10-12 sec
Total AC power consumption 230 50 MW
1 Barry Barish, GDE/ACFA Closing Beijing
7/02/07 2 Ali Nassiri, APS MAC, Nov. 15-16,2006
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SRF requirements

• 7 GeV single pass cw linac
• 400 multi-cell SRF cavities for main linac
• Roughly 400 meter of rf linac
• 10 MeV, 100 mA Injector linac ( 1 MW RF power)
• Roughly 45 kW total losses ( dynamic and static
  losses) at 20K
• Large complex
• Extremely heavy cryogenic load
• Robust and reliable power couplers (FPC) and HOM
  dampers
• Complex low-level rf control for amplitude, phase
  stability and microphonics
• Acceptable RF systems reliability and
  availability for beam up time

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Cavity Main Parameters
Parameter Unit Value
Frequency MHz 1300/1408/704
Accelerating mode TM010 ? mode
Gradient MV/m 18/20
Quality factor Q0 2?1010 /1?1011
Number of cells 9/7/5 ( HOM problem)
R/Q ? 900/1200
Qext for input coupler 1?107
Cavity bandwidth at Qext Hz 400
Fill time ?s 500

• Multi-cell cavities with a larger number of
• cells would also improve linac packing
  factor,
• i.e., ratio of active length to total length
• This will reduce the cost of the ERL linac, BUT
• Strong HOM damping is essential with higher
• beam current which favors smaller number
• of cells

(per cavity for two beams)
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Superconducting modules for ERLs

• Superconducting modules for high average current
  ERL operation have not been yet been
  demonstrated.
• Issues ( among others) that must be addressed
  are
• CW operation resulting in fairly high dynamic and
  static heat loads.
• High-current operation and the resultant large
  HOM power that must be extracted to limit the
  cryogenic load and to ensure stable beam
  conditions (100s of watts)1.
• Small bandwidth operation ( almost negligible net
  beam loading), which makes the cavity operation
  particularly susceptible to microphonic detuning
• More rf power
• More complex LLRF system and controls

1 Ali Nassiri, APS MAC, Nov. 15-16,2006
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Cavity Designs for ERLs

• Effect of residual resistance on AC power
  consumption ( non-BCS surface resistance)

With state-of-the-art 7 n? residual resistance
With ideal 1 n? residual resistance
Multi parameters cost optimization is extremely
important.
Temperature dependent of Carnot efficiency of
the cryoplant is included.
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Quality factor

• To reduce refrigeration power, cavity quality
  factor should be improved
• ERLs need higher Q0 at moderate gradients
• Gradients of 15 to 20 MV/m is reasonable. It
  avoids field emission.

• To reduce refrigeration power, cavity quality
  factor should be improved
• ERLs need higher Q0 at moderate gradients
• Gradients of 15 to 20 MV/m is reasonable. It
  avoids field emission.

?
?CEBAF spec.
?CEBAF 12 GeV project spec.
?
? ERL design goal
?
Single-cell 1.3 GHz cavity tested at 1.6K at
Saclay
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Summary

• SCRF technology for CW machines is advancing at a
  fast pace.
• The fundamental principles of ERLs have been
  established.
• Technical challenges are
• Cryogenic design for ERL needs a new approach to
  improve refrigeration efficiency to reduce plant
  construction and operation costs.
• Design a high current CW-specific cryomodule to
  meet ERL design parameters requirement.
• Develop a robust HOM damping system for high
  average beam current operation
• Better understanding of field emission for high
  gradient in CW mode
• Improve cavity quality factor ( 1?1011)
• For CW operation highest fields are not
  important. Highest possible Q values at about 20
  MV/m are very critical.
• We are carefully considering the challenges
  presented by the ERL upgrade
• CW-SRF technology RD program for ERL will
  benefit from ANL-FNAL active collaboration
• We are ready to start

12
Acknowledgements

• M. Borland, J. Carwardine, G. Decker, L. Emery,
  R. Gerig, K. Harky, V. Sajaev, N. Sereno, M. White