1. Groove Mitigation and Plans
M. Pivi
and L. Wang (SLAC) Y. Suetsugu, H. Fukuma (KEK) M. Palmer (Cornell) G. Arduini, E. Chapochnikova (CERN)
AEC09
CERN
12-13 October 2009

2. Grooves as mitigation
Grooves strongly damp electron multiplication by trapping mechanism. Most efficient schemes are:
Rectangular shape in field-free sections
Triangular shape in magnets
Showing here:
Early SEY measurements on test samples, 2006
PEP-II groove chambers, 2007
KEKB groove in wiggler, 2008
CesrTA groove in chicane, 2009

3. Groove samples SEY measurements, SLAC
M.Pivi and G. Stupakov, SLAC
PEP-II chamber test samples
Tested a variety of grooved samples with several different geometries
1 mm
Special surface profile design, Cu OFHC. EDM wire cutting. Groove: 0.8mm depth, 0.35mm step, 0.05mm thickness.
Un-coated copper sample SEY < 0.8. Smaller SEY~0.6 with deeper grooves
Ref: A. Krasnov LHC-Proj-Rep-617

4. Design - Groove Extrusions - SLAC
FAN EVENTUALLY HITS “BOTTOM” OF SLOT FOR FULL SR STRIKE
LIGHT PASSES THRU SLOTS BETW FINS
BECAUSE FAN IS “THICKER” THAN FIN
FIN TIPS= I.D. OF CHAM
FAN HITS HERE FIRST
M. Pivi et al, SLAC
Linear Collider R&D: Built Groove chambers by Al extrusion, TiN coated and installed in straight section PEP-II LER for ILC tests.

5. Groove Chambers in PEP-II
4 chambers alternating groove and standard (flat or smooth) chambers in PEP-II beam line, straight section.
Grooved
chamber
Flat
chamber e+
Standard (flat) chambers installed as reference.
All chambers TiN coated
Electron detectors

6. Groove Chambers in PEP-II
Groove chambers in field-free section, a factor ~20 lower e- cloud current
All 4 chambers re-deployed to CesrTA

7. Grooves in dipole magnet
Grooves located on upper and lower side of chamber where e- cloud develops. Include diagnostics.
TiN-coated
Re-deployed to CesrTA
Grooves + TiN chamber best performances in ongoing chicane tests
M. Pivi and L. Wang design, SLAC

8. Dipole Mitigation Comparisons
Current scan in L3 Chicane, 1x45 e+, 14ns, 5GeV
Note: Al signal is divided by 20 to show on the same scale
Grooved chamber has 5mm deep 20° triangular grooves with TiN coating
Performance:
TiN+Grooves significantly better than TiN alone
Both TiN and grooves significantly outperform the bare Al surface, as expected
CesrTA Mitigation Studies: LCWA09
M. Palmer Cornell

9. 4ns Chicane B-field Scans 1x45x1 mA, 4ns, 5GeV, e+ Resonance structure
Plots show
Central collector (near beam axis)
Collectors near edge of vacuum chamber
17 collectors in each RFA
Grooves + TiN Al
TiN
M. Palmer Cornell 9 9

10. Grooves tests in KEKB Groove and smooth inserts coated with TiN
5mm groove insert (Left) TiN-coated, manufactured by SLAC
Smooth insert (Right) used as reference
Swapped at same location in KEKB wiggler
Located on one side of chamber opposite to collector
M. Pivi, L. Wang (SLAC) Y. Suetsugu and H. Fukuma (KEK)

11. Electron Currents Vr = 0 V 1585 bunches (Bs ~ 6 ns) ~1600 mA
Y. Suetsugu and H. Fukuma (KEK), M. Pivi, L. Wang (SLAC)

12. Smaller Grooves
New smaller ~2mm groove SLAC-KEK design. Ongoing tests at KEKB.
Sharpness of groove geometry can be a manufacturing challenge due to demanding e- cloud tolerances
Y. Suetsugu, H. Fukuma KEK, M. Pivi, L. Wang SLAC

13. Groove Tests in the SPS Planning for tests in SPS dedicated area:
Depth of groove limited to 1 mm due to vacuum chamber aperture requirements.
Stainless steel insertion
Tight tolerances on roundness of groove tips and valleys: < 100um.
Groove on one side of test chamber only
Insertion in existing liner or modified liner

14. Groove Tests in the SPS
SPS liner for e- cloud tests

15. Groove Tests in the SPS
Liner for SPS tests
Sep 2009

16. Groove Tests in the SPS St st. Groove insert bolt to liner
Variable incursion into chamber, actually 1.5mm

17. Extruded aluminum insertion
Aluminum triangular groove, SLAC. Depth 1.9mm, Opening angle 20o, radius top 95um, radius valley 144um
Tip
Tip
Valley
Lanfa Wang, SLAC
Back-up: 1.9mm aluminum + coating grooves (pictures above), manufactured at SLAC.
Extruded aluminum groove.
April 2008
Mauro Pivi - SLAC

18. Effect of triangular grooves on Impedance
L. Wang et al. SLAC
Impedance enhancement factor for the triangular grooved surface with round tips. Note that this is valid for frequencies ω such that c/ ω >> W; for example, for W~3mm this means n<6e11 Hz.
ref. [L. Wang et al. FRPMS079, Proceedings of PAC07.]

19. Linear Collider R&D - Electron Cloud Working Group Charges
To evaluate electron cloud mitigation techniques, simulations and code benchmarking for the AD&I option. In particular, evaluate the differences between mitigations as grooves clearing electrodes, coating (TiN, TiZrV NEG and amorphous Carbon) regarding their feasibility, effectiveness, impact on the vacuum system, on the beam impedance and on costs, for different regions of the ILC DR as drifts, arc magnets and wigglers.

20. Working Group Charges
Mitigation techniques might be different for different regions of the damping ring.
To recommend a baseline solution along with alternate solutions for the electron cloud mitigations in the 6.4km Damping Ring.

21. Working Group Charges Furthermore, evaluate:
The proposal and options to reduce the DR circumference to 3.2 km.
The ‘upgrade’ potential from 6ns to 3ns bunch spacing in a 3km DR, immediately identified as bottleneck.
The current limits due to e-cloud for the 3.2 km DR.

22. Deliverable
Recommendation for the baseline and alternate solutions for the electron cloud mitigation in various regions of the ILC Positron Damping Ring.

23. Recommendation for mitigation table
DR element
% ring
Antechamber need
Coating
Additional Mitigation
Remarks
DRIFT in STRAIGHT 33 No
NEG
Solenoid
Grooves
DRIFT in ARC 56
Downstream of BEND only
BEND 7
Yes
TiN
WIGG 3
Electrodes
QUAD 1
Downstream BEND / WIGG
Preliminary table to be completed as input for Technical Design Phase. Goal is to turn all Red colors to Green as input for the recommendation.
Other mitigations under development (carbon coating CERN/CesrTA)

24. Report Document
A document produced by the working group by FY10 including the research work, a clear set of criteria for the recommendation and the recommendation itself.

25. Report Document
Recommendation on electron cloud mitigations for the ILC DR
Introduction 5
Request from AAP 5
Working Group Charges and Deliverables 5
Introduction Electron Cloud effect 10 [Zimmermann / Furman]
Mitigations description
Solenoid 10 [Seeman]
TiN coating10 [Suetsugu]
Carbon coating 10 [Calatroni / Palmer]
NEG coating 10 [Malishev]
Grooves and Impedance 10 [Pivi]
Clearing Electrodes and Impedance 10 [Suetsugu]
Feedback system 10 [J. Fox]
Conditioning 10 [Pivi / Suetsugu]
Other mitigations (more or less mature) 8
Overall Experience at CesrTA 20 [Palmer]
Overall Experience at PEP-II 20 [Pivi]
Overall Experience at KEK-B 20 [Oide]
Overall Experience at SPS 20 [Rumolo / Arduini]

26. Report Document ILC Damping Ring 15
Layout and parameters 8 [Pivi / Guiducci]
Photon and Photoelectron Production 10 [Dugan]
Simulation build-up [Demma / Furman]
Simulation single-bunch instability 20 [Pivi]
Impedance of mitigations 15 [Suetsugu / Bane]
Integrate CesrTA results to DR 20 [Palmer]
Process for mitigations recommendation 20
Set of criteria for a decision 20
External Reviews and Meetings 20
Mitigations cost 5 [Palmer]
Recommendation for Mitigations in the ILC DR 5
Summary 5

27. Summary
Overall grooves had excellent performances with e- cloud reduction in field free, dipole and wiggler sections of PEP-II, KEK-B and CesrTA.
Manufacturing groove insert for SPS tests. mm size grooves manufacturing challenge. Implementation in existing machines is challenging.
Looking at reducing groove depth to ~mm in long structures for Linear Colliders Damping Ring.
Recommendation for electron cloud mitigations in ILC DR.

28. Wakefield simulations PEP-II design
MAFIA simulations (A. Novokhatski) indicate that wake fields are not excited during the beam passage in PEP-II design. Very small losses at transition estimated 1.5E-04 V/pC. Quoting A. Novokhatski: actually, grooves may help damping the propagation of Higher Order Modes down the beam pipe (need of simulations)
Power losses due to image charge are contained (ex. PEP-II, dP/ds=1W/cm).