Damping Rings

1
Damping Rings

• James Jones
• ASTeC, Daresbury Laboratory

2
Personnel

• ASTeC
• Oleg Malyshev Vacuum Studies
• James Jones Low emittance tuning studies
• Engineering Support
• Liverpool University
• Andy Wolski Low emittance tuning, Vacuum Design
• 1 RA currently employed looking partially at
  impedance effects
• RA 7.1.4 Vacuum Design (LC-ABD 2)
• RA 7.2.1 Vacuum Design (LC-ABD 2)
• Larisa Malysheva Polarisation issues (As part
  of a wider collaboration)

3
Damping Rings must provide very high quality,
very stable beams
ILC DR PEP II LER HERA e SPring-8
Energy 5 GeV 3.5 GeV 27.5 GeV 8 GeV
Circumference 6694 m 2199 m 6360 m 1435 m
Average current 400 mA 2450 mA 58 mA 100 mA
Number of bunches 2767 5782 1588 156 2016
Particles per bunch 21010 11010 71010 3.71010 0.151010
Extracted bunch length (rms) 6 mm 12 mm 9 mm 6 mm
Horizontal normalised emittance 8 µm 200 µm 1000 µm 90 µm
Vertical normalised emittance 0.02 µm 10 µm 180 µm 0.09 0.27 µm
Damping ring parameters are very demanding in
terms of beam stabilityNo operating machine
meets all the parameters simultaneously.
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Previous Work

• ASTeC was, and is, committed to damping ring work
  as part of the EUROTeV framework
• Both O. Malyshev and J. Jones have small but
  significant work packages within this framework
• EUROTeV WP 3 Task 2 deals with electron cloud
  issues within the positron damping ring, and
  mitigation schemes. O. Malyshev was a major
  contributor to vacuum simulations for this task.
• EUROTeV WP 3 Task 3 deals with low emittance
  tuning simulations for the damping rings. The
  work is coordinated by J. Jones.

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Vacuum Simulations

• Complete simulations of damping ring vacuum
  systems, including both the dipole induced photo
  desorption, along with analysis of pump location
  and speeds
• Conventional vacuum technology does not allow to
  reach required vacuum after 100 Ah conditioning

• NEG coated chamber provides cheapest and simplest
  vacuum solution for dumping ring
• less number of pumps
• less pumping speed required
• lower bakeout/activation temperature (180C in
  stead of 250-300C)
• low SEY to suppress e-cloud effect

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Low Emittance Tuning

• Have a large scale emittance tuning simulation
  environment for the RDR damping rings.
• Includes full orbit, dispersion and coupling
  correction.
• Models the effects of ATL-like ground motion on
  the time evolution of the output emittances.

Initial Correction Only
Full
CO
None
Full Correction every 6 Days
7
Major issues for beam stability

• Electron cloud effects in the positron damping
  rings
• One of the top priorities for damping rings RD
  already receiving major attention from groups
  around the world.
• Ion effects in the electron damping rings
• Still some uncertainty in likely impact on
  damping rings performance. Can probably be
  mitigated with feedback systems and a
  well-designed vacuum system.
• Impedance-driven beam instabilities
• Wide experience from operating facilities we
  expect the damping rings to operate in a
  challenging regime.
• Long-range wake fields can drive multibunch
  instabilities, and couple jitter from
  freshly-injected bunches to damped bunches
  awaiting extraction.
• Short-range wake fields can drive single-bunch
  instabilities, which can appear as emittance
  increase, or a bursting type of instability.
• These effects require careful study, with beam
  dynamics models closely connected to the
  technical design of the vacuum system.

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Task 2.1 Goal 1

• Evaluate the effects of beam loading,
  injection/extraction transients and long-range
  wake fields in the damping rings under a range of
  operational conditions.

Using a time-domain simulation code, we studied
the coupling of injection jitter to damped
bunches in the NLC damping rings.
t 0 ms
t 10 ms
Similar (or stronger) effects are expected in
ILC. Studies must include a detailed impedance
model (resistive wall and HOMs), lattice model,
radiation damping and feedback system. Our
present code does include these effects
alternatives may be available (e.g. MULTI-TRISIM).
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Task 2.1 Goal 2
Evaluate impedance-driven instability thresholds
and growth rates.

• Single-bunch instabilities are diverse and
  complicated. There is a lot of operational
  experience of these effects, but a good
  understanding for any given machine generally
  requires a lot of hard, detailed work.
• Single-bunch instabilities were a major problem
  for the SLC damping rings eventually, the vacuum
  chamber had to be rebuilt.

Single-bunch instability in the SLC damping
rings. Left Experimental observation(B.
Podobedov, BNL). Right Simulation(K. Oide, KEK).
We shall collaborate with LBNL and SLAC in the
construction of impedance models (using technical
designs of the vacuum chamber, to be performed in
Task 2.2) and the evaluation of the resulting
instabilities.
10
Task 2.1 Goal 3
Develop techniques for low-emittance tuning.

• Lowest achieved vertical emittance (after
  significant effort) is 4.5 pm in KEK-ATF. The
  ILC specification is for 2 pm.

Several techniques (orbit/dispersion/coupling
correction orbit response matrix analysis) work
well in simulation, but practical implementation
with the necessary accuracy and precision is
still extremely challenging. We need to
demonstrate a technique that can be routinely
applied to a (6 km) ring to achieve vertical
emittance of 2 pm on a regular basis.
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Task 2.1 Goal 3
Develop techniques for low-emittance tuning.

• The main facilities used so far for experimental
  studies of low-emittance tuning have been the
  KEK-ATF and the LBNL-ALS.
• The ATF will continue to be available.
• The main limitation so far has been the
  availability of personnel.
• Producing a high-quality beam from the storage
  ring will be essential for ATF2.
• Beam time at the ALS is generally available at
  monthly intervals.
• The main limitation tends to be the availability
  of staff to run the experimental studies.
• There are presently two serious proposals for
  future damping rings test facilities
• CESR-tf could start operations for damping rings
  studies as early as June 2008.
• HERA-DR could start operations in late 2009.
• In both proposals, low-emittance tuning would be
  an important part of the programme.
• Further opportunities are provided by other
  machines.
• Light sources, e.g. DIAMOND.
• KEK-B (proposed damping rings study programme
  starting in 2009, to include low-emittance
  tuning).

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Task 2.2 Goal 1

• Calculate the average pressure and pressure
  profile in the damping rings and, in the context
  of the results of these calculations, evaluate
  the technology options for the damping rings.

Calculation of the pressure in a section of the
ILC damping rings in two different scenarios for
the vacuum system, as a function of the spacing
between the pumps. Left Stainless steel
tube. Right NEG-coated tube. (O. Malyshev, ASTeC)
Initial evaluations have been performed, as part
of the EUROTeV programme, and have indicated the
benefits of NEG-coated vacuum chamber. Detailed
studies are now needed to evaluate the benefits
of NEG-coating, and to produce technical
specification for the vacuum system (apertures
antechambers material and coating pumping
locations pumping speeds etc.) Vacuum studies
must be well-integrated into studies of electron
cloud and ion effects.
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Task 2.2 Goal 2
Determine conditioning rates for NEG coatings
under various conditions.

• We know that
• the initial pressure in a NEG coated vacuum
  chamber activated at 180?C is better by two
  orders of magnitude than that in a stainless
  steel vacuum chamber baked in-situ to 300?C
• NEG outgassing rates reduce with accumulated
  photon dose. Data from the ESRF show that this
  reduction could be up to 2 orders of magnitude
• NEG coating simplifies and reduces the cost of
  the pumping system, and works to mitigate
  multipacting.
• In other words, we know that NEG coating is worth
  using.
• For the design of the vacuum system, we need to
  know the photon and electron-stimulated
  desorption yields
• as functions of photon or electron dose, up to
  very large doses
• as functions of photon or electron energy
• as functions of NEG activation temperature (from
  room temperature up to 250?C)
• after air vent to different pressures (from 10-6
  mbar to atmosphere), to determine whether
  recovery after an accident requires reactivation.
• As a result of the experimental studies
• we will be able to produce (for the TDR) a vacuum
  system design optimised for performance and cost,
  including spacing and required pumping speeds of
  the lumped pumps
• we will gain invaluable experience in the use of
  NEG coatings under a wide range of conditions.

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Task 2.2 Goal 3
Produce technical designs for components in the
vacuum chamber in the arcs and straights, and use
these designs for developing an impedance model.
Technical designs of components in the vacuum
chamber are essential for constructing an
impedance model. Need to include bellows,
flanges, tapers, pumping ports, BPMs,
antechambers, kickers and septa Close
collaboration with other technical groups (e.g.
instrumentation) is essential. Producing a
complete, detailed model is a significant amount
of work, but is essential for a reliable
evaluation of the impact of collective effects.
Calculation of trapped modes in PEP II bellows.
Higher-order mode heating is a significant
problem for PEP II, and a potential problem for
the ILC damping rings. (Cho Ng, SLAC).
We will collaborate with LBNL on the technical
design, and with SLAC on the impedance modelling.
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Task 2.2 Goal 3
Produce technical designs for components in the
vacuum chamber in the arcs and straights, and use
these designs for developing an impedance model.

• The goal of producing a detailed impedance model
  for the TDR, based on technical designs of the
  important components, is ambitious.
• The Damping Rings Workshop at Cornell, 26-28
  September, outlined a staged plan, with specified
  milestones towards the goal of a complete
  evaluation of the impedance-driven collective
  effects.
• Begin with constructing an impedance model based
  on scaling components from existing facilities,
  in parallel with the technical design of the
  damping rings vacuum.
• Proceed iteratively to improve the model, using
  the results of the scaled impedance model to
  guide the design work, so as to achieve a
  specified impedance budget.
• Our proposed work on the vacuum system fits
  extremely well with the timescales and
  methodologies.
• If the hoped-for contributions from other labs
  (LBNL and SLAC) are not provided, we still make
  an essential contribution towards a reliable
  impedance model.

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Final Words

• The damping ring work proposed addresses two
  critical and related issues for the ILC damping
  rings
• Dynamical effects that potentially limit beam
  quality and stability.
• Vacuum system specification and design.
• We will make a leading contribution to the ILC in
  these areas.
• The work we are proposing will produce results
  needed for the TDR on an appropriate timescale.
• We will collaborate with identified international
  partners to maximise the benefit of the resources
  that are available.
• Vacuum studies have the potential for industrial
  involvement, and a major contribution (gt 13 km of
  vacuum system) during construction.
• The tasks are closely connected to other work
  packages within Cockcroft, for example
• Effects of linac wakefields depend on beam
  stability from damping rings
• Instrumentation and feedback essential for
  maintaining stability