Beam Loss Issues of ERL Accelerators

1
Beam Loss Issues of ERL Accelerators

• CY Yao, L. Emery, M. borland, A. Xiao
• Advanced Photon Source

2
Acknowledgement

• Thanks Rod Gerig and Efim Gluskin for their many
  suggestions and support.

3
Introduction

• Basic parameters of proposed APS ERL upgrade
• Beam energy 7 GeV
• Max. beam current 100 mA
• Injector energy 10 MeV
• Spent beam energy 10 MeV
• Total number of SRF cavities 350
• RF frequency 1.3 Ghz
• Micro bunch length 2 ps
• Beam emittance 0.022 µm-rad
• Number of passes 2

Figure 1 Layout of Proposed APS-ERL upgrade.
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Table1 Operation modes
Numbers scaled from G Hoffstaetters talk at 06
ERL workshop
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Consequence of beam loss Radiation hazards

• The APS-ERL has 1 MW of beam power of injector
  beam.
• Although the linac is power limited, even without
  energy recovery it can still generate 2.5 MW of
  beam power.
• A small fraction of beam loss presents a high
  radiation hazards and must be controlled.
• Current APS safety envelope is 308 W, or beam
  loss rate of 44 nA. This is only allowed to last
  1 hour (most credible incident).
• Argonne/DOE requires below 500 mrem/year for
  controlled area and 100 mrem/year for
  uncontrolled area.
• Current APS beam loss during top-up operations
  21 pA. The radiation dose level is measured below
  100 mrem/year, which allows to Re-designate the
  experimental hall as uncontrolled area.
• We need to control beam loss in APS and TAA to
  similar level. It is a challenge.

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Consequences of beam loss equipment damage and
activation

• Radiation from beam loss damages undulators
• Direct beam hit can damage frond-end devices and
  vacuum chambers.
• Heat deposit on the SRF cavities can cause
  quenching and operation downtime.
• Radiation activates accelerator parts that may
  impact hands on maintenance. This probably is not
  a big problem for electron machine.

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Consequence of beam loss increase both equipment
and operations cost

• Increase the need for better shielding---high
  construction budget.
• Increase the cooling requirement and the cost of
  the cryogenic system of SRF cavities.
• A 10 W/cavity heat load requires 3.5 kW of
  additional cooling capacity.
• Additional cost 35M.
• Increase the operational power and cost of the
  cryogenic system.
• Total heat load of 3.5 kW at 2ºK requires
  estimated of 3.5 MW of wall power.
• Additional operation cost 1.75M/year.
• In order to maintain this level of heat load,
  beam loss at any single point of the SRF linac
  must be lt 10 nA.

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Beam loss mechanisms

• Beam halo formation.
• Gas scattering.
• Intra-beam scattering.
• Emittance growth
• CSR and ISR effect
• Wake field and other instability
• Beam instability
• Beam break up (BBU) in the linac.
• Full or partial beam loss due to incidents
• Radiation protection.
• Machine protection.
• Beam dump due to failure of equipment.

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Beam Halo

• Beam halo can be formed in many parts of the
  accelerator structure.
• Dark current of the electron gun.
• Stray laser light can produce dark current in a
  photo-cathode gun.
• Space charge effect .
• Non-linearity of lattice.
• Field emission at the SRF cavities.
• Mismatch of the beam transport.
• Scattered particles that are not lost but form
  beam halo.

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Beam halo related R D work

• Low dark current low emittance gun development.
• Field emission study in high gradient
  superconducting RF cavities.
• Computer simulation of halo formation in electron
  gun and periodic focusing beam transport system.
• Development of beam diagnostics for halo
  characterization.
• Study halo formation process with existing APS
  linac and guns.
• Development of collimation configurations to
  eliminate halo particles in the early stage, such
  as combination of beta function collimation.

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Gas Scattering

• Beam loss due to gas scattering is a concern for
  any accelerator.
• Beam can be scattered and lost both in energy
  aperture and transverse aperture.
• Beam energy, vacuum pressure, gas composition,
  longitudinal and transverse acceptance are the
  main factors.
• At the APS and TAA areas the conditions for gas
  scattering are similar to current APS.
• Gas scattering in the current APS ring is very
  small compared to intra-beam scattering.
• We don't think gas scattering is a serious
  problem for APS-ERL.
• R D work
• Optimize lattice design that maximizes both
  transverse and energy acceptance in APS and TAA
  area.
• Develop simulation tools to include the injector
  and linac.
• Assess the effect of gas scattering on the beam
  emittance and heat load on the SRF cavities of
  linac.

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Intra-beam Scattering

• lntra-beam scattering is the main cause of beam
  loss of APS storage ring during normal
  operations.
• Will this be worse for the ERL?
• Preliminary estimates with elegant simulation
  shows that the highest loss rate is 50 nA for
  the high flux mode.
• Further study is need
• Lattice optimization.
• Upgrade elegant to simulate with acceleration.
• Find detailed distribution of the beam loss
  around the facility.
• Assess the impact on beam emittance, the linac
  SRF cavities and radiation safety.

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Energy Aperture Optimization

• Intra-beam/Touschek scattering beam loss rate
  depends strongly on machine energy aperture.
• A Method was developed to directly minimize beam
  loss while varying sextupole settings. Energy
  aperture for the APS can be increased to 5 with
  this method, which can reduce beam loss
  substantially.
• Need to consider the impact on other parts of the
  machine High energy aperture at APS and TAA may
  increases beam loss in the linac.
• Need more realistic simulation to include such
  factors as orbit errors in the sextupoles,
  acceleration, etc.

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Collimation strategies

• Collimation has been applied successfully to many
  high energy accelerators.
• It removes halo particles and protects downstream
  equipment.
• Locations of the collimators are determined by
• location of beam halo source.
• equipment that needs protection.
• Lattice function (betatron collimation, energy
  collimation ).
• For APS ERL the main halo source is the injector,
  at the merger and the end of energy recovery.
• The areas that need protection are APS, TAA
  beamline areas and SRF cavities of the linac.

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A possible collimation scheme

• Preliminary simulation with EGS4 indicates that a
  10 cm lead collimator located at the entry of
  linac can reduce the energy deposit on the
  downstream linac structure to 9.
• RD work
• More realistic Monte Carlo simulation, possibly
  coupled with a tracking program such as elegant.

Figure 2 a possible collimation
scheme. courtesy of L. Emery
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Beam Abort System

• A beam abort system is needed to protect the APS
  and TAA areas and the linac SRF system.
• Loss of stored beam
• At APS storage ring 100 mA beam is safely dumped
  by simply shutting off RF power.
• The APS-ERL has the same stored energy per length
  of accelerator as APS now.
• Not a problem for radiation safety.
• For the SRF cavity the estimated heat deposit 6
  J/cavity.
• Injector beam
• Beam power up to 2.5 MW.
• Additional heat deposit 5 J/cavity, assuming a 1
  ms abort system reaction time.
• The beam loss can further reduced by combination
  of kickers and beam dumps.

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Beam Loss Monitoring

• For radiation protection, the commercially
  available Gamma and Neutron monitors are
  adequate.
• For the protection of SRF cavities of the linac
• 500 µs detection time.
• Sensitivity lt 10 nA of beam loss.
• Ion chambers, PMT based detectors or Cerekov
  detectors.
• Preliminary simulation indicates installing a
  monitor every 5 meters along the linac is
  sufficient.
• Another possibility is to directly measure beam
  current. But can it meet the required sensitivity
  and reliability?

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A proposed abort system

• One kicker is located at the linac entrance to
  deflect the injector beam to a beam dump.
• The second kicker is located before TAA area,
  which directs the accelerating beam to the spent
  beam dump.
• The third kicker is located at the entrance of
  APS area.
• Kicker requirement
• strength 1 mRad
• rise time 100 to 200 ns.

Figure 3A proposed abort/dump plan. Courte
sy of L. Emery
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Shielding Consideration

• In the APS and TAA beamline area
• The current shielding with some modification is
  adequate for incidental beam loss.
• Option of shielding improvement should be
  considered if the continuous beam loss can not be
  brought down to satisfactory level with lattice
  design and collimation.
• Linac and injector tunnel
• Enhanced shielding is required to handle the high
  beam power.
• Extra shielding is required for some local areas
  such as beam dumps and collimators.
• R D work
• Monte Carlo simulation with EGS4, MARS or other
  programs.

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Conclusion

• Beam loss of the proposed APS-ERL upgrade
  presents a challenge for accelerator design.
• Radiation dose in the APS/TAA area and the energy
  deposit on the linac SRF cavities due to
  continuous beam loss are the two main concerns.
• RD work should be carried out in these areas
• Research and understanding the various beam loss
  mechanisms in an ERL environment.
• Optimize lattice design for both high performance
  and low beam loss rate.
• Development effective collimation configuration.
• Development of a fast beam abort system.

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References

• 1 M. Borland, Optimization of ERL Energy and
  Undulator Parameters, OAG-TN-2007-021,
  http//www.aps4.anl.gov/operations/ops_www/APSOnly
  /oagTechnicalReports.shtml.
• 2 G. Hoffstaetter, Status of the Cornell ERL
  Project, FLS 2006 Workshop, working group2,
  http//adweb.desy.de/mpy/FLS2006/proceedings/HTML/
  SESSION.HTM.
• 3 Advanced Photon Source Safety Assessment
  Document, APS-3.1.2.1.0, June 1996.
• 4 A. Nassiri, ERL cost update, APS upgrade
  presentations, http//www.aps4.anl.gov/operations/
  ops_www/APSOnly/APS_Upgrade.html.
• 5 C. Chen, Halo Formation in Intense Linacs,
  Proc. Of LINAC1998, P. 729-733, 1998.
• 6 Y. Shimosaki, K. Takayama, Nonlinear-resonanc
  e Analysis of Halo-Formation Excited By Beam-Core
  Oscillation, Proc. of EPAC 2000, Vienna,
  Austria, P. 1330-1332, 2000.
• 7 A. Xiao, Estimate of Beam Loss Rate from
  Touschek Effect for APS-ERL Lattice,
  OAG-TN-2006-048, http//www.aps4.anl.gov/operation
  s/ops_www/APSOnly/oagTechnicalReports.shtml.
• 8 L. Emery, Beam Simulation and Radiation Dose
  Calculation at the Advanced Photon Source with
  shower, an Interface Program to the EGS4 Code
  System, Proc. of PAC 1995, P. 2309-2311.