WG3b since Snowmass

1. WG3b since Snowmass S. Guiducci LNF-INF On behalf of ILC Working Group WG3b GDE meeting LNF 7-9 December 05

2. Baseline Configuration recommendation • Coordination of DR activity started at 1st ILC Worksop, KEK, November 2004 • Injector conveners: G. Dugan, M. Kuriki, S. Guiducci • Progress was reviewed at the 2nd ILC Workshop at Snowmass, in August 2005. We were not ready at that time to make any recommendations. • Final results were reported at the damping rings meeting at CERN, November 2005. • Damping Ring conveners: J. Gao, S. Guiducci, A. Wolski • Participants in the meeting agreed recommendations on the DR configuration.

3. Baseline Configuration recommendation • Nearly 50 participants Contributions from more than a dozen institutions in all the three regions. D. Alesini* (INFN) A. Babayan (YPI) I. Bailey (CI) K. Bane (SLAC) D. Barber* (DESY) Y. Cai* (SLAC) W. Decking (DESY) A. Dragt* (UM) G. Dugan (Cornell) E. Elsen* (DESY) L. Emery* (ANL) J. Gao* (IHEP) G. Gollin* (UIUC) S. Guiducci* (LNF) S. Heifets* (SLAC) J. Jones (ASTeC) E.-S. Kim* (POSTECH) H. S. Kim* (CHEP) K. Kubo (KEK) M. Kuriki (KEK) S. Kuroda (KEK) O. Malyshev* (ASTeC) L. Malysheva* (CI) F. Marcellini* (LNF) C. Mitchell* (UM) T. Naito (KEK) J. Nelson* (SLAC) K. Ohmi* (KEK) Y. Ohnishi* (KEK) K. Oide (KEK) T. Okugi* (KEK) M. Palmer* (Cornell) M. Pivi* (SLAC) P. Raimondi (LNF) T. Raubenheimer (SLAC) I. Reichel* (LBNL) M. Ross* (SLAC) D. Rubin* (Cornell) D. Schulte* (CERN) G. Stupakov* (SLAC) A. Tomonori* (KEK) J. Urakawa* (KEK) J. Urban* (Cornell) M. Venturini* (LBNL) L. Wang (SLAC) R. Wanzenberg* (SLAC) A. Wolski* (LBNL) M. Woodley (SLAC) G. Xia* (DESY) A. Xiao (ANL) F. Zimmermann (CERN) * 34 participants in the DR meeting at CERN, November 9-11, 2005

4. Status of the debate… Snowmass - WG3b Summary • The injection/extraction kickers should be strip-line (or similar) devices powered by fast pulsers. • “Conventional” kicker technology has developed so that 17 km or 6 km damping rings are feasible. 3 km rings may also be possible, but at present have higher technical risk. • It is still important to document thoroughly the work that has been done on alternative kicker technologies. • Further studies are needed to make a firm decision on the circumference. However, a very promising option appears to be a 6 km circumference ring, possibly using rings in pairs to provide adequate bunch spacing (for electron cloud, bunch number increasing…) • Other options need further information and debate. • We have an organized international effort to produce the necessary information. • We have a plan for presenting a well-documented recommendation to the GDE.

5. Seven representative lattices were assembled by end of April 2005 • The goal was to apply analysis tools and procedures systematically to each of the seven reference lattices. • An “arbitrary” naming scheme was chosen to promote objectivity. • We did not set out to choose one of the lattices. Our goal was to understand the issues based on the results of studies of these reference lattices, and use that understanding to make a recommendation for a configuration, not a design.

6. DR Configuration Study Task Forces were formed to co-ordinate activities • 1: Acceptance Issues • Y. Cai and Y. Ohnishi • 2: Vertical Emittance Tuning • J. Jones and K. Kubo • 3: Classical Instabilities • K. Bane, S. Heifets, G. Stupakov • 4: Space-Charge Effects • K. Oide and M. Venturini • 5: Electron-Cloud Effects • K. Ohmi and M. Pivi • 6: Fast-Ion Effects • E.-S. Kim, D. Schulte, F. Zimmermann • 7: Polarization • D. Barber • 8: Kicker Technology • M. Ross and T. Naito • 9: Cost Estimates • S. Guiducci, J. Urakawa and A. Wolski • 10: Availability • J. Nelson An enormous amount of work was completed in a little over six months. Results are being written up: far too many results to do justice in a short presentation. There were close to 50 contributors, with activities co-ordinated by the Task Force leaders. Most Task Forces carried out thorough studies of all (or nearly all) reference lattices. Results were continually cross-checked between two or more researchers.

7. Configuration studies were concluded in early November • Two reports are now in preparation, containing the results of the configuration studies and the configuration recommendations: • ILC Damping Rings Configuration Recommendation Summary Report • http://www.desy.de/~awolski/ILCDR/DRConfigurationStudy_files/DRConfigRecommend.pdf • Completed. • ILC Damping Rings Configuration Studies Detailed Report • http://www.desy.de/~awolski/ILCDR/DRConfigurationStudy_files/DRConfigRecommendSummary.pdf • In progress (180 pages). • Nearly all contributions have been collected. • Expected completion in early 2006. • Configuration Recommendation has been presented to the GDE Executive Committee by Andy Wolski (SLAC, November 17th) • http://cbp.lbl.gov/people/wolski/Wolski-DRConfigRecommend-2.pdf

8. BC recommendation meeting • CERN, November 9-11, • Thanks to Gilbert Guignard for hosting the meeting at CERN. • 34 participants • 21 presentations on the results of the task forces were presented. and are available on the web: • http://www.desy.de/~awolski/ILCDR/CERNDampingRingProceedings.htm • One afternoon and the following morning were devoted to discussions. • For each configuration item, the participants agreed on: • the relevant issues and their significance; • the risks associated with each issue for each of the configuration options; • a recommendation for the baseline and alternative configurations.

9. Nominal Parameters and performance specifications

10. Circumference and Layout • The critical choice for the DR was the circumference and layout recommendation • I’ll describe the options, the issues and the process which led to the recommendations. • For each issue was attributed a significance for the circumference choice (A,B,C) and a risk parameter (from 1 to 4). • Final recommendations came from the discussion, not a mathematical formula.

11. Classification of “Significance” and “Risk”

12. Circumference optionsfrom TESLA dogbone 17 Km to 6 & 3 Km 3 or 6 km rings can be built in independent tunnels “dogbone” straight sections share linac tunnel 3 Km 6 Km Two or more rings can be stacked in a single tunnel

13. Issues for the circumference choice • Kickers • Injection/extraction kickers are more difficult in a shorter ring. • R&D programs are proceeding fast, it is expected a demonstration for a 6 km circumference. • Electron cloud effect • Shorter rings have a closer bunch spacing, which greatly enhances the build-up of • electron cloud. Electron cloud density is dominant in the wiggler and in the dipole. Electron cloud instability could limit the stored current or increase the vertical beam size in the positron ring. R&D programs on mitigation tecniques are in progress at different storage rings. • Acceptance • Given the high average injected beam power injection efficiency has to be ~100% for the nominal positron distribution. The dogbone damping rings have a small acceptance, while the nearly circular 6 km ring has the largest acceptance. • Ion effects • Fast ion instability could limit the current in the electron ring. Fill pattern and vacuum pressure are more significant than the circumference for the severity of the effect. Gaps in the fill and very low vacuum levels will be necessary to mitigate ion effects.

14. Issues for the circumference choice • Space charge • The incoherent space-charge tune shift is proportional to the ring circumference. The coupling bumps used to reduce this effect in the dogbone ring could be some risk for the vertical emittance. • Tunnel layout • Sharing the linac tunnel reduces the time available for commissioning and reduces the availability. • Stray fields in the linac tunnel could adversely affect the vertical emittance • of the extracted beam. • Cost • Smaller rings have lower cost. Dogbone shape allows tunnel cost saving.

15. Issues for the circumference choice • Availability (Significance: C) • The larger number of components in a larger ring is likely to have an adverse impact on reliability. • Classical collective effects (Significance: C) • Classical collective effects as: resistive-wall instability, HOM coupled-bunch instabilities, microwave instability, and intrabeam scattering are of potential concern. Issues such as bunch charge, bunch length, momentum compaction, beam-pipe diameter etc., are determinant rather than the circumference. These effects should be manageable in any of the proposed circumference options. • Low-emittance tuning (Significance: C) • Achieving the specified vertical beam emittance in the damping rings is important for producing luminosity. However, there is an additive emittance dilution in all the systems downstream of the damping rings. There is little evidence that the circumference of the damping ring in itself has an impact on the emittance sensitivity to misalignments and tuning errors. • Polarization (Significance: C) • Studies suggest that depolarization should not be a major issue in any of the configuration options under consideration.

16. Kickers • The length of the TESLA DR and the idea of the dogbone shape (to save tunnel length) were originated by the anavailability of ultra fast kickers. 17Km were needed to accommodate ~3000 bunches with 20 ns bunch distance. • Three different type of fast pulsers have been tested on a strip line kicker at ATF(KEK). All of them have very short rise/fall time (~3ns) and fulfil nearly all of the requirements for the damping ring injection. R&D programs are in progress in various laboratories both on the pulser and on the electromagnetic design of the electrode. With the ATF kickers’ strength, nearly 10 stripline electrodes are needed to reach the required injection/extraction angle. R&D programs are rapidly proceeding and the task force participants are confident that: • - kickers for a 6 Km (i.e. 6 ns bunch spacing) are a “low risk” issue • - kickers for the 3 Km ring are considered at present a high risk.

17. Injection/extraction kickers beam test at ATF kick angle [rad] kick angle [rad] kick timing [ns] kick timing [ns] • Rise/fall time < 3ns • a tail of a few percent extends for ~7ns: • R&D on the pulser • Cancellation with two kickers at p J. Urakawa, for ATF collaboration

18. General considerations: kicker length and pulse length Tf-2L/c=4B/c Generator pulse shape ILC VIN VT L=kicker length Tr=rise time length Tf=flat top length B=bunch length TB=bunch spacing Tf 2TB t t Tr Tr 2L/c+Tr 2L/c+Tr Kicker impulse response (ideal case) VT Injected bunch Stored bunches 2L/c DANE Injection upgrade VT t assuming Tr=300ps 2TB t VT=2.5 MV

19. Design completed

20. Y. Cai

21. Single bunch instability threshold and simulated electron cloud build-up density M. Pivi, K. Ohmi, F. Zimmermann, R. Wanzenberg, L. Wang, T. Raubenheimer, C. Vaccarezza, X. Dong

22. ILC DR Task Force 6 Recommendation Summary • The instability limit is more likely to be exceeded in smaller rings. • Larger bunch spacing Damping Rings with a larger synchrotron tune and/or momentum compaction are preferable. • In order of preference: MCH, DAS, TESLA, BRUx2, OCSx2, BRU, OCS. • It’s a technical challenge to stably reduce the SEY below 1.1-1.2 • Redflag: KEKB Annual Report 2005 “The electron cloud effect still remains the major obstacle to a shorter bunch spacing, even with the solenoid windings” [1]. • If the SEY can be reduced in magnets, the 6 km BRU and OCS can be feasible. • Promising cures as microgrooves and clearing electrodes need further R&D and full demonstration in accelerator. • Larger wiggler apertures may be helpful to reducing the cloud density below threshold in 6km rings • In the short bunch spacing 3 km DR, multipactoring arises even at low SEY~1, developing the highest cloud densities (see Snowmass 05 talks) therefore should be discarded as possible candidates. M.Pivi, K. Ohmi, R. Wanzenberg, Zimmermann, SLAC, Nov 2005

23. Suppressing e- cloud in magnetic field regions • Microgrooves. Groove spacing comparable with e- Larmor radius. R&D status: laboratory tests at SLAC very successful in magnetic free regions, measured reduction to SEY < 0.7. Building chamber for installation in dipole region in PEP-II. • Clearing electrodes: simulations show that likely electrodes can suppress electron cloud in magnetic field regions, but need further R&D and studies (Impedance, support …). R&D at KEKb. • Photon absorbers to reduce reflectivity *

24. smooth surface Rectangular groove surface By=0.19T Rectangular grooves in BEND: SEY Parameters rectangular groove: period = 250 um depth = 250 um width = 25 um Simulated secondary yield of a rectangular grooved surface in a dipole field compared with a smooth surface (field free reference). Groove dimensions in wiggler ~10-100 um. 1cm wide stripe with grooves. • Possible solution: need laboratory and accelerator tests in dipole field

26. ACCEPTANCE: Dynamic Aperture with Multipole Errors and Single-Mode Wigglers MCH OCS 16 km sz=9mm 6 km sz=6mm TESLA/S-Shape DAS/PI 17 km sz=6mm 17 km sz=6mm Y. Cai, Y. Ohnishi, I. Reichel, J. Urban, A. Wolski

27. Comparison of different wiggler models and tracking codes One-mode is an ideal, infinite pole width, wiggler DA for TESLA DR with CESRc or one-mode wiggler model M. Venturini, ILC DR Meeting - CERN 10 Nov 05

28. Topics of Acceptance Study • Dynamic aperture • Items to be considered: • Ideal lattice, Multipole errors, Nonlinear wigglers, (Machine errors) • Output: • g2Jy0-g2Jx0 plot, g2Jx0-d0 plot, Tune scan • Physical aperture • Aperture of wiggler section • Frequency map analysis (already reported at Snowmass) • Resonance structures • Injection efficiency • Input: Positron distribution • Output: Survived particle distribution

29. Risks Associated the Acceptance Studies • Lack of margin in acceptance especially for the off-momentum particles • Uncertainty of dynamic aperture in tracking compare to measurement, at best 20% agreement at SPS • Wiggler model is the best could be achieved. This inexplicitly assumes that we need large aperture and super conducting wigglers • Magnetic errors are also at the best can be achieved no room for any mistake • No misalignments and linear optical errors in the simulations yet. • Margin of acceptance is necessary for an adequate efficient collimation system in the damping ring. • Uncertainty in the actual distribution for the positron source Y. Cai

30. Circumference recommendation: Ion effects Mini-gap can reduces the growth rate of FII and tune-shift up to a factor of 10~20 Ion-density reduction factor (IRF) depends on fill-pattern, optics and the time during the damping. IRF=10 is guaranteed. The growth time with mini-gaps will be longer than 1 turn. Detail study is under the way to get a maximum IRF. L. Wang, T. Raubenheimer, Y. Cai, E.-S. Kim

31. Conclusions - Space charge • The winner is the OCS lattice [medium-size circumference (6.1Km), good symmetry properties] • The lattices shorter than 6km have not been analyzed in detail but they should be as good or better • The dogbone lattices are more vulnerable to space charge (as expected) but they still seem to offer patches of usable tunespace • Choice of working point may get in conflict with other requirements • Risk is higher • Augmented symmetry helps (‘S-shaped’ TESLA DR is better than the ‘C-shaped’ version) • Coupling bumps come at a cost as they excite new resonances and restrict region of usable tunespace • Effectiveness of coupling bumps seems dependent on lattice design • In general, they do not necessarily offer a decisive advantage • Still, installation may be recommended to add flexibility M. Venturini

32. Circumference recommendation: Space-charge effects Vertical (left) and horizontal (right) emittance growth from tracking MCH (16 km lattice) using Marylie-Impact. Top:Particles/bunch = 0 Middle: Particles/bunch = 2×1010 Coupling bumps OFF Bottom:Particles/bunch = 2×1010 Coupling bumps ON Errors will further reduceusable area of tune space K. Oide and M. Venturini

33. Preliminary Cost estimates A 3 km ring would have rather a lower cost than 6 km or 17 km rings. The additional tunnel in the 6 km rings makes the costs comparable to the 17 km rings. Two 6 km rings in a single tunnel is a higher cost than a 17 km ring.

34. An example from the Summary Report: the Circumference (4) The significance of each issue and the risk associated with each option are based on results from the configuration studies, which will be presented in the Detailed Report.

35. Recommendation for the circumference (baseline configuration) • Positrons: two rings of ~ 6 km circumference in a single tunnel. • Two rings are needed to reduce e-cloud effects unless significant progress can be made with mitigation techniques. • Preferred to 17 km due to: • Space-charge effects • Acceptance • Tunnel layout (commissioning time, stray fields) • Electrons: one 6 km ring. • Preferred to 3 km due to: • Larger gaps between minitrains for clearing ions. • Injection and extraction kickers ‘low risk’ • Estimated cost for 3x6 km rings is lower than 2x17 km.

36. Recommendations for the circumference (alternative configurations) • 1. If techniques are found that are sufficiently effective at suppressing the electron cloud, a single 6 km, or possibly smaller, ring can be used for the positron damping ring. This will save costs. • 2. If electron cloud mitigation techniques are not found that are sufficient for the baseline positron ring, then a 17 km ring is a possible alternative; this would require addressing space-charge, acceptance and stray fields issues. This will increase costs.

37. Recommendations summarized

38. Energy recommendation • Options: 3.7 GeV, 5 GeV, 6.8 GeV • Issues: Baseline recommendation 5 Gev Lower energy increases risk for collective effects, higher energy makes more difficult to tune for low emittance

39. Wigglers for ILC DR • Parameters • Bpeak 1.6 T • lw 0.4 m • Total length 165 m • Radiated energy 9.3 MeV • A high quality field is needed to achieve the dynamic aperture necessary for good injection efficiency: • Physical aperture A large gap is needed to achieve the necessary acceptance for the large injected positron beam: • a full aperture of at least 32 mm is highly desirable for injection efficiency • a full aperture of at least 46 mm is highly desirable to mitigate e-cloud effects

40. Technology Options • Field requirements have led to 3 suggested options: • Hybrid Permanent Magnet Wiggler • Superferric Wiggler • Normal Conducting Wiggler • Design Status • Hybrid PM based on modified TESLA design • Basic modified TESLA design (Tischer, etal, TESLA 2000-20) • 6 cm wide poles • Tracking simulations in hand • Next generation design (see note from Babayan, etal) • New shimming design • Improved field quality – field maps available at end of last week • Field fitting now underway, but no tracking studies yet • Superferric design based on CESR-c wiggler (Rice, etal, PAC03, TOAB007) • Tracking simulations in hand • No active design for normal conducting option • Will scale from TESLA (TESLA TDR) and NLC (Corlett, etal, LCC-0031) proposed designs Mark Palmer, ILCDR Meeting - CERN - 11 Nov 05

41. Field Quality • Significance: A • Primary Issue is Dynamic Aperture • 3 pole designs in hand: • Superferric with • DB/B ~ 7.7 x 10-5 @ Dx = 10 mm (CESR-c) • Shows acceptable dynamic aperture! • However, most designs approaching DA limit for Dp/p=1%! • Modified TESLA design (60 mm pole width) • DB/B ~ 5.9 x 10-3 @ Dx = 10 mm (TESLA A) • Dynamic aperture unacceptable! • Note that normal conducting designs (as is) are in this ballpark • Shimmed TESLA design (60 mm pole width) • DB/B ~ 5.5 x 10-4 @ Dx = 10 mm (TESLA B) • Detailed field map has just become available • Field fits and tracking studies not yet available • Concerned about potential impact on DA near Dp/p = 1% Mark Palmer, ILCDR Meeting - CERN - 11 Nov 05

42. ILC DR Wiggler Technology • Baseline • The CESR-c wigglers have demonstrated the basic requirements for the ILC damping ring wigglers. Designs for a superconducting wiggler for the damping rings need to be optimized. • Alternatives • Designs with acceptable costs for normal-conducting (including power consumption) and hybrid wigglers need to be developed, that meet specifications for aperture and field quality.

43. List of R&D • International Linear Collider Damping Ring Research and Development Projects web site (Thanks to G. Gollin) • http://www.hep.uiuc.edu/LCRD/ILCDR.html • Start discussion to create a global R&D plan for the Damping Ring • "Strawperson" list of design and engineering tasks • Table of comments, interests and planned activities • Categories: • Fast Kickers (HV pulsers, stripline kickers) • Feedback systems • Wiggler (design, beam dynamics) • High resolution BPMs • Fast Ion instability • Electron cloud (grooved metal surface, clearing electrodes) • Beam dynamics issues • Beam size monitors (X-SR, ODR, Laser wire) • Beam Based Alignment R&D • Feedforward system for the stabilisation of the extracted • superconducting RF cavity

44. List of R&D • International Linear Collider Damping Ring Research and Development Projects web site (Thanks to G. Gollin) • http://www.hep.uiuc.edu/LCRD/ILCDR.html • Laboratories, universities and industries • Diversified Technologies, Inc. (USA), University of Illinois (USA), KEK-ATF (Japan), LBNL (USA), CERN (EU), Cornell/CESR (USA), SLAC (USA), ANL (USA), DESY (EU), INFN-LNF (EU) • Infrastructures • ATF (KEK), SPS (CERN), DAFNE (LNF), ALS (BNL), PEP-II (SLAC), KEKb, APS (ANL), CESR (Cornell)

45. Final Remarks • The configuration recommendations presented here represent a consensus amongst the participants at the CERN damping rings meeting. • The damping rings community has demonstrated the ability for highly collaborative and well co-ordinated effort. • Next step is to continue to work in close collaboration to coordinate the required R&D activity and to prepare the RDR.

46. END

47. Risk associated with electron cloud simulations (..disclaimer) • ‘Build-up’ simulation codes give satisfactory agreement with experimental observation in existing accelerators. Codes benchmarking: agreement. • ‘Single-bunch’ threshold simulation codes agree qualitatively with some observations (chromaticity,..). Single-bunch simulation codes under development. • One should take a margin factor when comparing build-up and threshold. • For comparative ILC DR studies, train gaps not introduced (yet). Gaps likely reduce cloud density by certain extent. • Cloud space charge 2D: limit for wiggler simulations.

48. ILC DR Parameters