Electron Cloud in the International Linear Collider ILC Mauro Pivi work performed while at SLAC and the ILC Damping Ring Working Group High Luminosity Workshop May, 2013

high L Electron Cloud in a nutshell In the vacuum chambers of an accelerator, electrons are generated by photons, ionization, etc. e - are accelerated by the passing bunches, hit the chamber and multiply due to surface Secondary Electron Yield (SEY) After few bunches pass, a cloud of electrons may form The cloud couples with the beam to cause beam instabilities and emittance increase, beam losses and lower luminosity The electron cloud has been observed in several machines as PEP-II, KEKB, LHC, Daphne, CesrTA, and others It is a Very High Priority Issue for the ILC, CLIC, SuperKEKB, SuperB with ultra-low emittance

In a positron or proton storage ring, electrons are generated by a variety of processes, and can be accelerated by the beam to hit the vacuum chamber with sufficient energy to generate multiple “secondary” electrons. Under the right conditions, the electron “cloud” density can reach high levels and can drive the beam unstable and increase the beam size decreasing the collider luminosity. The Luminosity Challenge: Electron Cloud Effect 25 ns Electron cloud in the LHC 25 ns

27-30 May, 2013 Electron cloud in the Linear Colliders high L 4 While at SLAC, coordinating the ILC electron cloud Working Group (WG) WG milestones: development of mitigations for the electron cloud that lead to reduction of Damping Rings circumference from 17km to 6km (2006) and then to 3km (2010) Latest years goal: Develop mitigations and give recommendation for the ILC

Recommendation of Electron Cloud Mitigations 5 Clearing Electrodes KEKB Grooves w/TiN coating Clearing Electrode C ESR TA Grooves on Cu Stable Structures Reliable Feedthroughs Manufacturing Techniques & Quality amorphous-Carbon CERN SLAC KEK INFN Frascati CesrTA

Electron Cloud Mitigations Evaluation Criteria high L 6 Efficacy Photoelectric yield (PEY) Secondary emission yield (SEY) Ability to keep the vertical emittance growth below 10% Cost Design and manufacturing of mitigation Maintenance of mitigation –Ex: Replacement of clearing electrode PS Operational –Ex: Time incurred for replacement of damaged clearing electrode PS Risk Mitigation manufacturing challenges: –Ex: ≤1mm or less in small aperture VC –Ex: Clearing electrode in limited space or in presence of BPM buttons Technical uncertainty –Incomplete evidence of efficacy –Incomplete experimental studies Reliability –Durability of mitigation –Ex: Damage of clearing electrode feed- through Impact on Machine Performance Impact on vacuum performance –Ex: NEG pumping can have a positive effect –Ex: Vacuum outgassing Impact on machine impedance –Ex: Impedance of grooves and electrodes Impact on optics –Ex: x-y coupling due to solenoids Operational –Ex: NEG re-activation after saturation May 2013 The Working Group (about 50) met at a dedicated Workshop to evaluate technologies and give recommendation on electron cloud mitigations

Structured Evaluation of EC Mitigations high L May 2013

27-30 May, 2013 Aggressive mitigation plan needed to obtain optimum performance for 3.2km positron damping ring and to pursue the high current option Summary of Electron Cloud Mitigation Plan for the ILC high L 8 Baseline Mitigation Recommendation - EC Workshop, Cornell University Baseline Mitigation Recommendation - EC Workshop, Cornell University M. Pivi, S. Guiducci, M. Palmer, J. Urakawa on behalf of the ILC DR Electron Cloud Working Group

Mitigations: Wiggler Chamber with Clearing Electrode Thermal spray tungsten electrode and Alumina insulator 0.2mm thick layers 20mm wide electrode in wiggler Antechamber full height is 20mm Joe Conway – Cornell U. ILC Wiggler chamber

Mitigations: Dipole Chamber with Grooves 20 grooves (19 tips) 0.079in (2mm) deep with 0.003in tip radius 0.035in tip to tip spacing Top and bottom of chamber Joe Conway – Cornell U. ILC Dipole chamber

27-30 May, 2013 Electron cloud assessment in the ILC Damping Ring for 2013 TDR report WG latest years goal: Estimate the electron cloud effect by simulations including full mitigation plan For the ILC Technical Design Report (TDR) 2013

27-30 May, 2013 Electron cloud assessment: 3-step Simulation plan Photon generation and distribution by Cornell U. in BENDs with grooves - LBNL In WIGGLERS with clearing electrodes - SLAC In DRIFT, QUAD, SEXT with TiN coating - Cornell U. Beam move freely interacting with cloud - SLAC 2. Evaluate electron cloud build-up 1. Map the photoelectron distribution 3. Evaluate beam Instability map

Photon rates, by magnet type and region dtc03 Used Synrad3d a 3D simulation code that includes the ring lattice at input and full chambers geometry (3D photon tracking, photon stops, antechambers, reflectivity, etc.) G. Dugan Cornell U. Photon azimuthal distributions in various chamber types

27-30 May, 2013 high L Evaluation results: Electron Cloud in Drift Regions, with Solenoid field (40 G) Solenoid fields in drift regions are very effective at eliminating the central cloud density J. Crittenden, Cornell U. Chamber-average cloud density Near-beam cloud density

27-30 May, 2013 high L Electron Cloud in Quadrupoles Trapping of electron in quadrupole field: the electron cloud density does not reach equilibrium after 8 bunch trains. J. Crittenden, Cornell U.

27-30 May, 2013 high L Electron Cloud in Quadrupoles Electron cloud density (e/m 3 ) Electron energies (eV) J. Crittenden, Cornell U.

27-30 May, 2013 high L Electron Cloud in arc Sextupoles Electron cloud density (e/m 3 ) Electron energies (eV) J. Crittenden, Cornell U.

27-30 May, 2013 high L Wiggler Magnets: Clearing Electrodes Modeling of clearing electrode: round chamber is used Clearing Field (left) & potential (right) L. Wang, SLAC

27-30 May, 2013 high L +600V 0V +600V+400V +100V L. Wang, SLAC Wiggler magnets: Effect of Clearing Electrodes on Electron Cloud Distribution

27-30 May, 2013 ElementCloud density [e/m 3 ]% occupancy Drifts0e1066 Bends4.e Quads in arcs0.16e129.8 Sextupoles in arcs0.135e Wigglers1.5e Quads in wiggler region1.2e Average3.5e10 Simulation with full lattice This expected cloud density is already promisingly low. Next step is to compute if this cloud density destabilizes the beam. Summary of Electron Cloud distribution along the ILC DR with mitigations implemented

27-30 May, 2013 high L Last step: evaluate beam instability Used C-MAD parallel code ( M. Pivi while at SLAC et al. ): electron cloud instability, Intra-Beam Scattering IBS. Allows uploading the full SPS lattice from MAD for increased realistic simulations. Simulations challenge: very flat beams in ILC DR

27-30 May, 2013 Calculation of emittance growth and beam instability There is a clear threshold to exponential growth between 3÷5 e11 e/m 3 cloud density. Though, below the instability threshold, there is a persistent linear emittance increase 1.Upload full MAD lattice 2.place electron clouds along ring with varying the cloud density 3.Track beam

27-30 May, 2013 Result: Vertical emittance growth with full lattice Expected average cloud density with mitigations is 3.5e10 e/m 3 The fractional emittance growth in 300 turns = Beam Store time in ILC DR =18550 turns Thus estimated emittance growth in turns ~ 10% 10% emittance growth is acceptable, Mitigations are effective in the ILC DR! Estimated Vertical emittance growth with full lattice 3.5e10 e/m 3

27-30 May, 2013 high L Summary of Electron cloud in the ILC During last years, developed novel mitigations in a multi-laboratory collaborative effort Recommended mitigations for each DR region Intensely developed simulation codes Methodically evaluated electron cloud effect With mitigations, the electron cloud density is well below the instability threshold A persistent slow emittance growth due to electron cloud is an acceptable 10% Mitigations are effective in DR

25 By=0.3 T;  =99% SEY=1.0 SEY=1.1 SEY=1.2 SEY=1.3 Snapshot of the electron (x,y) distribution “just before” the passage of the last bunch SuperB: Buildup in the arcs Dipoles HER Arc quadrupole vacuum chamber (CDR) Theo Demma ECLOUD12 Workshop

SuperB: Summary from talk at Electron Cloud Workshop 2012 Simulations indicate that a peak secondary electron yield of 1.1 and 99% antechamber protection result in a cloud density below the instability threshold. Planned use of coatings (TiN, ?) and solenoids in SuperB free field regions can help. Ongoing studies on mitigation techniques (grooves in the chamber walls, clearing electrodes) offers the opportunity to plan activity for SuperB. 26 Theo Demma ECLOUD12 Workshop Challenging!