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

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

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

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.

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

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

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

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

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

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)

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

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

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

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

Groove Tests in the SPS Liner for SPS tests Sep 2009

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

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

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.]

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.

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.

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.

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

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)

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.

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]

Report Document ILC Damping Ring 15 Layout and parameters 8 [Pivi / Guiducci] Photon and Photoelectron Production 10 [Dugan] Simulation build-up 20 [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

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.

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).