Electron Cloud in the International Linear Collider ILC

Electron Cloud in the International Linear Collider ILC Mauro Pivi work performed while at SLAC and the ILC Damping Ring Working Group Tau-Charm @ High Luminosity Workshop 27 -30 May, 2013

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, Cesr. TA, and others • It is a Very High Priority Issue for the ILC, CLIC, Super. KEKB, Super. B with ultra-low emittance 27 -30 May, 2013 Tau-Charm @ high L

The Luminosity Challenge: Electron Cloud Effect 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. 25 ns Electron cloud in the LHC 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.

Electron cloud in the Linear Colliders • 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 17 km to 6 km (2006) and then to 3 km (2010) Latest years goal: • Develop mitigations and give recommendation for the ILC 27 -30 May, 2013 Tau-Charm @ high L 4

Recommendation of Electron Cloud Mitigations amorphous-Carbon Grooves on Cu Manufacturing Techniques & Quality CERN Clearing Electrode CESRTA SLAC Grooves w/Ti. N coating KEK Reliable Feedthroughs Cesr. TA Stable Structures INFN Frascati Clearing Electrodes KEKB 5

Electron Cloud Mitigations Evaluation Criteria • The Working Group (about 50) met at a dedicated Workshop to evaluate technologies and give recommendation on electron cloud mitigations Efficacy Cost • Photoelectric yield (PEY) • Secondary emission yield (SEY) • Ability to keep the vertical emittance growth below 10% Mitigation manufacturing challenges: – Ex: ≤ 1 mm 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 feedthrough 27 -30 May 2013 – Ex: Replacement of clearing electrode PS • Operational – Ex: Time incurred for replacement of damaged clearing electrode PS Risk • • Design and manufacturing of mitigation • Maintenance of mitigation 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 Tau-Charm @ high L 6

Structured Evaluation of EC Mitigations Criteria for the evaluation of mitigations: Working Group rating Efficacy of Mitigation Costs Risks Impact on Machine Rating 10 1 4 4 Normalized Weighting 0. 53 0. 05 0. 21 27 -30 May 2013 Tau-Charm @ high L 7

Summary of Electron Cloud Mitigation Plan for the ILC Baseline Mitigation Recommendation - EC Workshop, Cornell University ILC Working Group Baseline Mitigation Recommendation Drift* Dipole Wiggler Quadrupole* Baseline Mitigation I Ti. N Coating Grooves with Ti. N coating Clearing Electrodes Ti. N Coating Baseline Mitigation II Solenoid Windings Antechamber Alternate Mitigation Amorphous Carbon or NEG coatings Ti. N Coating Grooves with Ti. N or Amorphous Carbon coating Clearing Electrodes or Grooves • Aggressive mitigation plan needed to obtain optimum performance for 3. 2 km positron damping ring and to pursue the high current option M. Pivi, S. Guiducci, M. Palmer, J. Urakawa on behalf of the ILC DR Electron Cloud Working Group 27 -30 May, 2013 Tau-Charm @ high L 8

Mitigations: Wiggler Chamber with Clearing Electrode • • Thermal spray tungsten electrode and Alumina insulator 0. 2 mm thick layers ILC Wiggler chamber 20 mm wide electrode in wiggler Antechamber full height is 20 mm Joe Conway – Cornell U.

Mitigations: Dipole Chamber with Grooves • • 20 grooves (19 tips) 0. 079 in (2 mm) deep with 0. 003 in tip radius 0. 035 in tip to tip spacing Top and bottom of chamber Joe Conway – Cornell U. ILC Dipole chamber

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

Photon rates, by magnet type and region dtc 03 Photon azimuthal distributions in various chamber types G. Dugan Cornell U. Used Synrad 3 d a 3 D simulation code that includes the ring lattice at input and full chambers geometry (3 D photon tracking, photon stops, antechambers, reflectivity, etc. )

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 Chamber-average cloud density Near-beam cloud density 27 -30 May, 2013 Tau-Charm @ high L J. Crittenden, Cornell U.

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 Tau-Charm @ high L

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

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

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

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

Summary of Electron Cloud distribution along Simulation with full lattice the ILC DR with mitigations implemented Element Cloud density [e/m 3] % occupancy Drifts 0 e 10 66 Bends 4. e 10 15. 14 Quads in arcs 0. 16 e 12 9. 8 Sextupoles in arcs 0. 135 e 12 5. 56 Wigglers 1. 5 e 10 2. 96 Quads in wiggler region 1. 2 e 12 0. 49 Average 3. 5 e 10 • This expected cloud density is already promisingly low. • Next step is to compute if this cloud density destabilizes the beam. 27 -30 May, 2013

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 Tau-Charm @ high L

Calculation of emittance growth and beam instability 1. Upload full MAD lattice 2. place electron clouds along ring with varying the cloud density 3. Track beam There is a clear threshold to exponential growth between 3÷ 5 e 11 e/m 3 cloud density. 27 -30 May, 2013 Though, below the instability threshold, there is a persistent linear emittance increase

Estimated Vertical. Result: emittance growth with full lattice Vertical emittance full lattice Expected average cloud density with mitigations is 3. 5 e 10 e/m 3 The fractional emittance growth in 300 turns = 0. 0016 Beam Store time in ILC DR =18550 turns Thus estimated emittance growth in 18550 turns ~ 10% emittance growth is acceptable, Mitigations are effective in the ILC DR! 27 -30 May, 2013

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 27 -30 May, 2013 Tau-Charm @ high L

25 Super. B: Buildup in the arcs Dipoles HER Arc quadrupole vacuum chamber (CDR) 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 Theo Demma ECLOUD 12 Workshop

26 Super. B: Summary from talk at Electron Cloud Workshop 2012 Challenging! • 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 (Ti. N, ? ) and solenoids in Super. B free field regions can help. • Ongoing studies on mitigation techniques (grooves in the chamber walls, clearing electrodes) offers the opportunity to plan activity for Super. B. Theo Demma ECLOUD 12 Workshop