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E lectron cloud challenges for future circular colliders

This talk discusses the challenges and conditions required to suppress electron clouds in future circular colliders, including the SEY curves of surfaces, beam and machine parameters, surface coatings, and photoelectron production from synchrotron radiation.

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E lectron cloud challenges for future circular colliders

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  1. Work supported by the Swiss State Secretariat for Education, Research and Innovation SERI Electron cloud challenges for future circular colliders E. Belli, L. Mether, P. Dijkstal, G. Iadarola, G. Rumolo Acknowledgements: S. Arsenyev, I. Bellafont, R. Kersevan, D. Schulte ECLOUD’18Isolad’Elba 3-7 June, 2018

  2. The Future Circular Collider Study The future circular collider study covers: • FCC-hh: • A 100 km circumference p-p collider • 16 T magnets  100 TeVc.o.m energy • 1011 p/bunch with 25 ns • FCC-ee: • An e+-e- collider as a possible first stage • Z, WW, H (ZH) and ttbar production modes • HE-LHC: • LHC tunnel with FCC-hh magnets and technology • 27 TeVc.o.m energy, HL-LHC beam parameters • 2.2 × 1011 p/bunch with 25 ns • FCC-he: • p-e- collisions with e- from energy recovery linac HE-LHC FCC

  3. The Future Circular Collider Study The future circular collider study covers: • FCC-hh: • A 100 km circumference p-p collider • 16 T magnets  100 TeVc.o.m energy • 1011 p/bunch with 25 ns • FCC-ee (not covered here): • An e+-e- collider as a possible first stage • Z, WW, H (ZH) and ttbar production modes • HE-LHC: • LHC tunnel with FCC-hh magnets and technology • 27 TeVc.o.m energy, HL-LHC beam parameters • 2.2 × 1011 p/bunch with 25 ns • FCC-he (not covered here): • p-e- collisions with e- from energy recovery linac HE-LHC FCC

  4. Electron cloud challenges Electron clouds can cause transverse instabilities, tune shift and spread, emittance growth, losses, heat load and vacuum degradation • These effects can effectively be mitigated by preventing the build-up of e-clouds, and future machines should ideally be designed to operate without e-cloud • To this end we need to identify the required conditions to sufficiently suppress e-cloud build-up These conditions will depend, among others, on the following aspects, which will be reviewed in this talk: • SEY curves of the surfaces • Beam and machine parameters • Surface coatings • Photoelectron production from synchrotron radiation

  5. SEY model The predictions from e-cloud simulations rely on confidence in the SEY curves used • We use the Cimino-Collins SEY model, parameterising measured SEY curves of the LHC beam screens: with: with: R. Cimino et al., Phys. Rev. Lett. 93, 014801

  6. SEY model The predictions from e-cloud simulations rely on confidence in the SEY curves used • We use the Cimino-Collins SEY model, parameterising measured SEY curves of the LHC beam screens: • Dependence on incidence angle: with: with: R. Cimino et al., Phys. Rev. Lett. 93, 014801

  7. SEY model The predictions from e-cloud simulations rely on confidence in the SEY curves used • We use the Cimino-Collins SEY model, parameterising measured SEY curves of the LHC beam screens: • Dependence on incidence angle: with: with: • New SEY measurements on LHC beam screens at different stages of surface conditioning and comparison with SEY model in simulations are on-going. • See talks: • V. Petit, Tuesday morning • L. Sabato, Wednesday morning R. Cimino et al., Phys. Rev. Lett. 93, 014801

  8. Effect of beam configuration The experiments in colliders tend to prefer to divide the design beam current in lower intensity bunches with shorter spacing to reduce event pile-up • This generally makes e-cloud (and other two-stream effects) worse • For both the FCC-hh and the HE-LHC, beams with 12.5 ns and 5 ns bunch spacing are more prone to e-cloud build-up than 25 ns beams In the FCC-hh the 12.5 ns beam, with 5 × 1010 p/b, is most prone to build-up In HE-LHC, with 2 × the beam current of FCC-hh, the 5 ns beam is most prone to build-up HE-LHC Dipole at 1.3TeV e-/m FCC-hh Dipole at 50 TeVSEY = 1.3

  9. Multipacting thresholds The multipacting thresholds tell us when and where coating is necessary to suppress e-cloud build-up • Multipacting thresholds for build-up in FCC-hh • (Highest SEY without build-up) • Main observations above threshold: • Heat loads with 25 ns are moderate compared to synchrotron radiation ~28.4 W/m • Stability is critical: central densities are above the instability threshold in most cases if build-up occurs, in particular in the quadrupoles and for the smaller bunch spacings 25 ns, 50TeV 25 ns, 3.3TeV Scaled to device length in arc cells

  10. Single bunch stability threshold Threshold electron densities for beam stability can be estimated with beam dynamics simulations with PyECLOUD/PyHEADTAIL • Example from FCC-hh with 25 ns beam at 3.3 TeV in a dipole field, without any stabilizing mechanisms More details on simulation mechanism and capabilities in talk by A. Romano, Wednesday • Over 17 000 turns (~ 5 s ): instability threshold around 1-2.5 x 1011 m-3 • Compare to analytic estimate scaled to dipole length: 7.5 x 1010 m-3 •  Analytic estimate slightly pessimistic, by factor 2-3

  11. Evolution during a fill E-cloud effects don’t scale linearly with intensity  some effects may get worse with the intensity burn-off during luminosity production fills • A scan for HE-LHC decreasing intensity and emittance in uniform steps shows a significant effect for the 12.5 and 25 ns beams, in particular in the quadrupoles (also dipoles for 12.5 ns) Quadrupole 13.5TeV, 25 ns Quadrupole 13.5TeV, 25 ns For lower intensities, the central density approaches the instability threshold at low SEY The multipacting threshold decreases significantly with decreasing intensity

  12. Evolution during a fill E-cloud effects don’t scale linearly with intensity  some effects may get worse with the intensity burn-off during luminosity production fills • A scan for HE-LHC decreasing intensity and emittance in uniform steps shows a significant effect for the 12.5 and 25 ns beams, in particular in the quadrupoles (also dipoles for 12.5 ns) • This effect should be taken into account when considering the need for coating: • Multipacting thresholds from build-up simulation(defined as highest SEY without build-up)

  13. Evolution during a fill E-cloud effects don’t scale linearly with intensity  some effects may get worse with the intensity burn-off during luminosity production fills • A scan for HE-LHC decreasing intensity and emittance in uniform steps shows a significant effect for the 12.5 and 25 ns beams, in particular in the quadrupoles (also dipoles for 12.5 ns) • This effect should be taken into account when considering the need for coating: • Multipacting thresholds from build-up simulation(defined as highest SEY without build-up) In several cases it is clear that even a fully scrubbed Cu surface would not be sufficient to suppress e-cloud build-up

  14. Coating materials Considered options for surface coating in the FCC hadron machines: • Amorphous carbon (a-C) coatings, with maximum SEY around 1.0 – 1.05 • Laser treated surfaces (LASE), with maximum SEY below 1.0, Emax at high energy We cannot determine which coating is viable in any given case without detailed SEY curves for the coating materials to use in simulations • To illustrate the possible effects, let’s look at the effect of shifting only Emax in the SEY curve used in the simulations: Emax 25 ns beam Emax 25 ns beam FCC-hh Quadrupole 3.3 TeV FCC-hh Quadrupole 50 TeV

  15. Coating materials Considered options for surface coating in the FCC hadron machines: • Amorphous carbon (a-C) coatings, with maximum SEY around 1.0 – 1.05 • Laser treated surfaces (LASE), with maximum SEY below 1.0, Emax at high energy We cannot determine which coating is viable in any given case without detailed SEY curves for the coating materials to use in simulations • To illustrate the possible effects, let’s look at the effect of shifting only Emaxin the SEY curve used in the simulations: 12.5 ns beam Emax Emax 12.5 ns beam FCC-hh Dipole 3.3 TeV FCC-hh Quadrupole 50 TeV

  16. Coating materials Considered options for surface coating in the FCC hadron machines: • Amorphous carbon (a-C) coatings, with maximum SEY around 1.0 – 1.05 • Laser treated surfaces (LASE), with maximum SEY below 1.0, Emax at high energy We cannot determine which coating is viable in any given case without detailed SEY curves for the coating materials to use in simulations • To illustrate the possible effects, let’s look at the effect of shifting only Emaxin the SEY curve used in the simulations: Disclaimer: This should not be taken as proof of the effect of any given surface, but to illustrate how important it is to know the SEY curve of any potential materials! 12.5 ns beam Emax Emax 12.5 ns beam FCC-hh Dipole 3.3 TeV FCC-hh Quadrupole 50 TeV

  17. Cloud distribution The cloud distributions need to be identified in order to determine where beam screen coating is required • Distributions for FCC-hh in the main arc elements with 25 ns beam: • Drift • Multipactingon all sides, hot spots along axes, but threshold high enough that coating should not be necessary • Quadrupole • Coating is required at 45° to the horizontal plane • Dipole • The coating should cover the full top and bottom of the chamber

  18. Cloud distribution The cloud distributions need to be identified in order to determine where beam screen coating is required • Distributions for FCC-hh in the arc dipoles with different beams: • 5 ns • Bunchintensity2 × 1010p • 12.5 ns • Bunchintensity5 × 1010 p • 25 ns • Bunchintensity 1 × 1011 p

  19. Cloud distribution The cloud distributions need to be identified in order to determine where beam screen coating is required • Distributions for FCC-hh in the arc dipoles with different beams: • Effect on impedance • The effects of a surface coating have to be taken into account also on the impedance of the beam screen: • For the FCC-hh, an a-C coating has been estimated to increase the beam screen impedance with about 30%, whereas a LASE coating is expected to give a higher contribution (S. Arsenyev) • customized coating material or coating fraction to maximize e-cloud benefit, while minimizing impact on impedance? • 5 ns • Bunchintensity2 × 1010p • 12.5 ns • Bunchintensity5 × 1010 p • 25 ns • Bunchintensity 1 × 1011 p

  20. Photoelectrons Future colliders with higher energy beams typically also produce more synchrotron radiation than current machines • The radiation in itself can be a problem for the vacuum and cryogenics • May also significantly increase the amount of photoelectrons produced Studies show that photoelectrons could raise the central density above the instability threshold below the multipactingthreshold, depending on their production rate • In FCC-hhthis is especially the case for the 12.5 and 5 ns beams Detailed knowledge of the photoelectron yield as a function of photon energy and incidence angle would be necessary to predict the production rate, unless there is a sufficient margin so that even a very high yield cannot cause problems 12.5 ns, 50TeV Dipole

  21. FCC-hh beam screen In the FCC, the problem has been addressed with a beam screen design that directs the synchrotron radiation into an ante-chamber on the side of the main chamber • Planned to be used both in FCC-hh and HE-LHC The design has evolved during the project study: • 2015  2017 • Larger slit, saw-tooth surface at SR impact point, straight edges • Impact expected mainly in the drifts, due to the cloud distribution in magnetic fields • 2017  2018 • Smaller vertical aperture (by 2 mm) for manufacturing purposes • Could impact results also in dipoles and quadrupoles 2015 2017 2018 C. Garion, J. Fernandez Topham et al

  22. FCC-hh beam screen Effect of beam screen design on multipacting thresholds: 2015  2017 The threshold in the drift is pushed higher, no other significant effect 25 ns, 3.3 TeV 25 ns, 50 TeV Scaled to device length in arc cells

  23. FCC-hh beam screen Effect of beam screen design on multipacting thresholds: 2015  2017 The threshold in the drift is pushed higher, no other significant effect 2017  2018 The drift threshold is increased further, and the dipole threshold is slightly increased as well 25 ns, 3.3 TeV 25 ns, 50 TeV Scaled to device length in arc cells

  24. Photoelectrons with FCC-hh beam screen Photon absorption patterns have been studied with ray-tracing simulations (I. Bellafont) To model photoelectron production in this complex geometry in the e-cloud simulations, the possibility to assign a photoelectron yield to each segment (of arbitrary length) of the chamber walls was implemented I. Bellafont Photoelectron distributions based on ray-tracing studies used in the e-cloud simulations: • Conservative estimate (10% yield):photoelectron flux 5×1010 – 5×1011 e/cm2/s in main chamber • LASE estimate :photoelectron flux 109 – 1010 e/cm2/s in main chamber

  25. Photoelectrons with FCC-hh beam screen Photon absorption patterns have been studied with ray-tracing simulations (I. Bellafont) To model photoelectron production in this complex geometry in the e-cloud simulations, the possibility to assign a photoelectron yield to each segment (of arbitrary length) of the chamber walls was implemented Photoelectron distributions based on ray-tracing studies used in the e-cloud simulations: • Conservative estimate (10% yield):photoelectron flux 5×1010 – 5×1011 e/cm2/s in main chamber • LASE estimate :photoelectron flux 109 – 1010 e/cm2/s in main chamber FCC-hh12.5 ns, 50 TeV Simulations confirm that both cases are viable • Central electron densities remain below the instability threshold for all bunch spacingswhen the SEY is below the multipacting threshold

  26. Conclusions Future machines should be designed to suppress e-cloud build-up, by employing surface treatments where needed • For the FCC-hh and HE-LHC coating will be required to achieve this goal • Short bunch spacings (< 25 ns) are generally more challenging To reliably estimate when, where and which kind of coating is needed, detailed knowledge of the SEY curves of all considered materials is required • Several measurements on LHC Cu surface exist, less data available for potential surface coatings The impact of coatings on impedance (and cost) must be considered and surface potentially tailored to maximize benefit with minimum overhead Photoelectrons may enhance build-up and/or drive the beam unstable below the multipacting threshold  sufficient suppression of their production must be ensured • Achieved in FCC-hh and HE-LHC with adapted beam screen design

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