Present Status
The electron cloud instability is certainly a potentially serious limitation on damping ring performance, and is one of the most difficult effects to predict. Nonetheless, significant progress has been made recently, both in terms of simulations of the cloud build-up and the effects on the beam, and in development of low SEY coatings to suppress the cloud build-up.
The codes for simulation of the cloud build-up have been developed by a sustained effort of several people over many years. Available codes include detailed models of the chamber surface and magnetic field environment (including wiggler fields), and are starting to show good agreement with experimental data, where these are available.
Significant work has already been done in applying these codes to the damping rings; we believe that we have a good idea of the operating regime in the dog-bone lattice, for example, and the SEY limit below which the electron cloud will be suppressed [1].
A joint program over the past year between SLAC and LBNL on low SEY coatings has added to data previously obtained (e.g. at CERN), and shown very promising results from
TiN and NEG coatings (appropriately conditioned) [2], as well as from a novel technique of cutting grooves in the chamber surface. The focus has been directly on developing techniques applicable to linear collider damping rings. While producing the coatings is reasonably (though not completely) straightforward, characterizing the surface properties requires highly specialized apparatus (such as that at SLAC) and considerable care. The most promising surface preparations will be tested in the PEP-II B factory at SLAC beginning later this year.
The fast ion instability is another potentially serious limitation in the damping rings, but is difficult to study experimentally because it is only significant in the high-intensity, low emittance regime that the damping rings will need to operate in. Qualitative observations, and some measurements, have been made in a number of storage rings, but the most useful data by far have come very recently from the KEK-ATF. This is the only storage ring that combines the capability of routine operation with a vertical emittance below 5 pm, together with a laser-wire diagnostic capable of precision beam-size measurements in this regime, and the willingness to allow vacuum degradation for studies of ion effects. Comparison of data from the KEK-ATF with results of a simulation have shown excellent quantitative agreement [3].
Suggestions for Further Work

Continued application of existing codes to damping ring designs.

Continued development of low SEY surface preparations.
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Data collection and analysis from tests in PEP-II.
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Continued studies of fast-ion instability in KEK-ATF.

Further development of simulation codes for fast-ion instability.

Resources

Existing codes for simulation of the build-up of the electron cloud and the effects on the beam have been developed after considerable effort, and should continue to be used.

Apparatus for the production and measurement of low SEY surface preparations exists, and should continue to be used.
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Experiments in PEP-II, scheduled to begin in late 2004, will yield valuable data.
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KEK-ATF provides a unique opportunity for quantitative studies of fast-ion instability; however, it is unclear if this will continue operation.

Simulation codes for fast-ion instability exist, but may need further development.
Present Status
The specified extracted vertical emittance in a 5 GeV damping ring for a superconducting linear collider is 2 pm. The equilibrium vertical emittance will need to be below this
(depending on injected emittance and damping rate); the lowest vertical emittance achieved in any existing storage ring is presently a little below 5 pm in the KEK-ATF [4].
For a long time, simulation studies indicated a lower emittance should be possible than was actually achieved [5], although recently, there has been much better agreement between simulation and experiment [6]. An emittance of 5 pm was only achieved after considerable effort, including a complete upgrade of the BPM system, and detailed beambased alignment study [7].
A number of codes exist that can be used to simulate low-emittance tuning, including a wide variety of effects. There are also a variety of tuning algorithms that have been developed, including those used at the KEK-ATF, and the orbit-response matrix analysis technique used at the LBNL-ALS to achieve low emittance [8]. However, there is not good evidence that simulation results, without experimental validation, are reliable in the very low emittance regime of the damping rings. The experience from the KEK-ATF is that a very wide variety of effects, including dependence of BPM readings on temperature and beam current, can be significant, and that without an accurate model of the machine errors, it is difficult to achieve good agreement between simulation and experiment.
Suggestions for Further Work
Simulation codes exist that may be readily applied to studies of low-emittance tuning of the damping rings. However, confidence that the specified emittance for the damping rings can be achieved will only come from experience of operating an existing storage ring in a comparable parameter regime.
Resources

Simulation codes.


It is possible that the KEK-ATF may be tuned to achieve a vertical emittance below 2 pm. However, the very different structure of the lattice makes the relevance to the damping rings questionable.
Present Status
Given the high average injected beam power into the damping rings and the potential for radiation damage of critical components, the acceptance of the lattice is an important issue. Limitation of the acceptance from systematic nonlinearities in the wiggler field is a concern.
Techniques have recently been developed for efficient and accurate modeling of beam dynamics in a wiggler field. Some results were reported in [9]. Since then, the development has been completed of tools that will allow construction of an arbitraryorder power-series map representing the dynamics (including energy deviation) in the wiggler. In particular, the map is appropriate for fast symplectic tracking, and is based on an analytical fit to the wiggler field (produced by a magnet modeling code) with residuals that can be less than 0.05% [10]. For any given wiggler field map, the tools are fast and straightforward to apply. Combined with frequency map analysis for describing the nonlinear dynamics in the full lattice, an accurate picture of the acceptance in the damping rings can be constructed. Applied to the BL11 wiggler that had a severe impact on the beam dynamics in SPEAR2 [11], the technique has shown very good agreement with the limited experimental data.
Suggestions for Further Work

Application of existing tools for characterizing the nonlinear dynamics in the wiggler-dominated damping rings.

Further benchmarking of analysis tools, by collection of more detailed data in a wiggler-dominated storage ring, e.g. CESR-c.

Optimization of acceptance by modification of wiggler and lattice design.
Resources

Existing tools for analysis of the wiggler field, and characterization of the nonlinear dynamics in the damping rings.

Experimental data from an existing facility, e.g. CESR-c.

[1] M. Pivi, “Overview of Electron Cloud Studies for the Future Linear Colliders”,
Presented at ECLOUD04, Napa CA, USA, 2004. http://icfa-ecloud04.web.cern.ch/icfaecloud04/talks/Tuesday/SessionD/SessionDTalk3.pdf
[2] F. Le Pimpec et al, “The Continuing Story of Secondary Electron Yield
Measurements from TiN Coating and TiZrV Getter Film,” SLAC-TN-04-046,
August 2004. http://www.slac.stanford.edu/pubs/slactns/slac-tn-04-046.html
[3] T. Raubenheimer, “Fast Beam-Ion Instability in LC Damping Rings,” Presented at
ALCPG Workshop, Victoria BC, Canada, 2004. http://www.linearcollider.ca:8080/lc/vic04/abstracts/accelerator/dampingring/tor_ra ubenheimer.pdf
[4] Y. Honda et al, “Achievement of Ultralow Emittance Beam in the Accelerator Test
Facility Damping Ring,” Phys. Rev. Lett.
92 , 054802 (2004).
[5] K. Kubo et al, “Beam Tuning for Low Emittance in KEK-ATF Damping Ring,”
Proceedings EPAC 2000.
[6] K. Kubo, “Simulation Study of Low Emittance Tuning of the Accelerator Test
Facility Damping Ring at KEK,” Phys. Rev. ST Accel. Beams
6 , 092801 (2003).
[7] M.D. Woodley et al, “Beam Based Alignment at the KEK-ATF Damping Ring,”
Proceedings EPAC 2004.
[8] C. Steier et al, “Coupling Correction and Beam Dynamics at Ultralow Vertical
Emittance in the ALS,” Proceedings PAC 2003.
[9] A. Wolski et al, “Dynamical Aperture Study for the NLC Main Damping Rings,”
Proceedings EPAC 2004.
[10] A. Wolski and M. Venturini, “Frequency Map Analysis of Nonlinear Dynamics in the NLC Main Damping Rings,” In preparation.
[11] J. Safranek et al, “Nonlinear Dynamics in SPEAR Wigglers,” Proceedings EPAC
2000.