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Electron cloud studies for the HL-LHC: where do we stand and the next steps

Electron cloud studies for the HL-LHC: where do we stand and the next steps. G. Iadarola , G. Rumolo. Many thanks to: H. Bartosik , R. De Maria, O. Dominguez, L. Mether , R . Tomas. 11th HiLumi WP2 Task 2.4 meeting - April 30, 2014. Outline. Studies for the arc main magnets

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Electron cloud studies for the HL-LHC: where do we stand and the next steps

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  1. Electron cloud studies for the HL-LHC: where do we stand and the next steps G. Iadarola, G. Rumolo Many thanks to: H. Bartosik, R. De Maria, O. Dominguez, L. Mether, R. Tomas 11th HiLumi WP2 Task 2.4 meeting - April 30, 2014

  2. Outline • Studies for the arc main magnets • Scaling with bunch intensity • 200 MHz option • 8b+4e scheme • “Doublet” scrubbing beam • Studies forthe inner triplets • New triplets in IP1/5 • Present triplets in IP1/8 • Studies for the matching sections – preliminary considerations

  3. Outline • Studies for the arc main magnets • Scaling with bunch intensity • 200 MHz option • 8b+4e scheme • “Doublet” scrubbing beam • Studies for the inner triplets • New triplets in IP1/5 • Present triplets in IP1/8 • Studies for the matching sections – preliminary considerations

  4. Bunch intensity dependence for the arc main magnets Dipole Quadrupole

  5. Bunch intensity dependence for the arc main magnets • Underlying mechanism: • When the SEY decreases the energy window for multipacting becomes narrower Dipole Quadrupole • For high bunch intensity the e- spectrum drifts to higher energies Quadrupole – SEY=1.3

  6. Bunch intensity dependence for the arc main magnets Dipole Quadrupole

  7. Bunch intensity dependence for the arc main magnets Dipole Quadrupole • In dipoles the multipacting region becomes wider for higher intensities

  8. Bunch intensity dependence for the arc main magnets Dipole Quadrupole • Warning: dependencies rely on assumptions on Emax and uniform SEY on the wall • Provided that we manage to access a low SEY regime, increased bunch intensity should be acceptable for heat load, effect on the beam still to be assessed

  9. 200 MHz option: effect on e-cloud in the dipoles Thanks to O. Dominguez • If option with 200 MHz main RF is adopted, longer bunches will have a positive impact also on electron cloud • Impact of bunch length on e-cloud to be studied experimentally in Run 2 (e.g. to mitigate degradation at low energy)

  10. 8b+4e fallback scenario Thanks to O. Dominguez 25 ns (72b. per PS batch) 8b + 4e (48b. per PS batch) 8b + 4e (56b. per PS batch) Measured in 2012 (Fill 3429) • Filling pattern with 25 ns spacing interleaving batches of 8 bunches with gaps of 4 slots, showed a significantly increased multipacting threshold compared the standard scheme • backup scheme in case safe operation with 25 ns beam is hampered by e-cloud (still 50% more bunches wrt 50 ns) • The effectiveness of this scheme will have to be confirmed during Run 2

  11. “Doublet” scrubbing beam • Operation with 25 ns will rely on SEY reduction by beam induced scrubbing • To operate with 25 ns beams (~2800b.) it is mandatory to achieve lower values in SEY compared to what was achieved in Run • Dedicated scrubbing beams with hybrid bunch spacing (5+20 ns) are presently under study (e-cloud enhancement shown experimentally at SPS) • If possible to be used for scrubbing already during Run 2 (SPS and LHC) LHC dipole Scrubbing beam (5+20) ns Standard beam 25 ns Electron density [a. u.] PyECLOUD simulations

  12. Outline • Studies for the arc main magnets • Scaling with bunch intensity • 200 MHz option • 8b+4e scheme • “Doublet” scrubbing beam • Studies for the inner triplets • New triplets in IP1/5 • Present triplets in IP1/8 • Studies for the matching sections – preliminary considerations

  13. Inner triplets for HL-LHC upgrade Present D1 Q1 Q3 Q2 (A/B) • IP1, 4 TeV, β* = 0.6 m • More details on electron cloud studies for the inner triplets can be found in: • For the present triplets: • http://indico.cern.ch/event/270441/ • For the HL LHC triplets (including IP2 and IP8): • https://indico.cern.ch/event/278323/ • https://indico.cern.ch/event/303742/ Q1 (A/B) Q3 (A/B) D1 HiLumi Q2 (A/B) • IP1, 7 TeV, β* = 0.15 m

  14. A look to the EC buildup – HL-LHC triplets • Few snapshots of the electron distribution  HL-LHC triplets develop thicker stripes along field lines farther from the center of the chamber • HL-LHC (2.20 x 1011 ppb) Present (1.15 x 1011 ppb)

  15. Distribution of heat load – HL-LHC triplets • Heat load distribution along HL-LHC triplets + D1 • Build up more or less efficient at different locations mainly due to the different hybrid bunch spacings • The least efficient build up, i.e. lower heat load, at the locations of the long-range encounters (vertical dashed lines) • Values in D1 are comparable or higher than values in the quads

  16. Total heat load per element – HL-LHC triplets • Total heat load per element in HL-LHC triplets + D1 • Similar thresholds for quads and D1 • Values in D1 higher than values in the quads for high SEY values

  17. Total heat load on the triplet beam screen • Effect of larger bunch population and chamber size. For the same SEY: • - Similar energy of multipacting electrons • - Larger number of impacting electrons • Total heat load about x3 larger • e-cloud suppression can be obtained using low SEY coatings and/or clearing electrodes • Present triplets • (1.15 x 1011 ppb) • HiLumi triplets • (2.20 x 1011 ppb) Full suppression (SEY≈1 or clearing electrodes) Cu beam scr. SEY like 2012 Cu beam scr. SEY like 2012 SPS like a-C coating

  18. Scaling with bunch population - IP2 and IP8 • Electron cloud in present inner triplets, scaling with bunch population for one cut: • Wider multipacting region for high bunch intensity 1.1e11 ppb 2.1e11 ppb

  19. Scaling with bunch population - IP2 and IP8 • Electron cloud in present inner triplets, scaling with bunch population for one cut: • Wider multipacting region for high bunch intensity • Doubling bunch population leads to about x3 larger heat load • e-cloud suppression strategies needed also for these magnets • Detailed study for precise heat load estimation still needs to be performed Section far from long range encounter

  20. Outline • Studies for the arc main magnets • Scaling with bunch intensity • 200 MHz option • 8b+4e scheme • “Doublet” scrubbing beam • Studies for the inner triplets • New triplets in IP1/5 • Present triplets in IP1/8 • Studies for the matching sections – preliminary considerations

  21. Matching section IP5 - Present LHC Q4 BS2 Q5 BS1 Q6 BS1 Q7 BSArc b a D2 - BSD2 BSArc a=22, b=17.15 BS1 a=22.5, b=17.6 BS2 a=28.9, b=24 Beam off center BSD2 a=31.3, b=26.4 Beam screens can be rotated

  22. Matching section IP5 – HL-LHC Q4 BSHL Q5 BS2 Q6 BS1 Q7 BSArc b a D2 - BSHLD2 BSArc a=22, b=17.15 BS1 a=22.5, b=17.6 BS2 a=28.9, b=24 BSHL a=37, b=32 BSHLD2 a=41, b=36

  23. Matching section IP2 – Present LHC Q4 BS2 Q5 BS1 Q6 BS1 Q7 BSArc Q4 BS2 Q5 BS2 Q6 BS1 Q7 BSArc b a D2 - BSD2 D2 - BSD2 Inj. beam BSArc a=22, b=17.15 BS1 a=22.5, b=17.6 BS2 a=28.9, b=24 BSD2 a=31.3, b=26.4 Beam screens can be rotated

  24. Matching sections • In total 6 beam screen shapes (all rect-ellipse): BSArc a=22, b=17.15 BS1 a=22.5, b=17.6 BS2 a=28.9, b=24 BSD2 a=31.3, b=26.4 b a BSHL a=37, b=32 BSHLD2 a=41, b=36 • Many, many, many configurations in terms of B field/gradient, beam size and beam position considering energy ramp, squeeze, separation, β*-leveling etc… • Need for parametric scans to asses which of these dependencies strongly impact the EC buildup

  25. Matching sections • In total 6 beam screen shapes (all rect-ellipse): BSArc a=22, b=17.15 BS1 a=22.5, b=17.6 BS2 a=28.9, b=24 BSD2 a=31.3, b=26.4 b a • Some preliminary indication: effect of beam size and B field variation during the energy ramp (nominal intensity) BSHL a=37, b=32 BSHLD2 a=41, b=36 • Arc dipole • Arc quadrupole • Many manymany configurations in terms of B field/gradient, beam size and beam position considering energy ramp, squeeze, separation, β*-leveling etc… • Preliminary • Need for parametric scans to asses which of these dependencies strongly impact the EC buildup • Uniform train of 600b. Thanks to L. Mether

  26. Matching sections • In total 6 beam screen shapes (all rect-ellipse): BSArc a=22, b=17.15 BS1 a=22.5, b=17.6 BS2 a=28.9, b=24 BSD2 a=31.3, b=26.4 b a BSHL a=37, b=32 BSHLD2 a=41, b=36 • Many manymany configurations in terms of B field/gradient, beam size and beam position considering energy ramp, squeeze, separation, β*-leveling etc… • Next steps: • Effects of beam size • Effect of B gradient • Effect of beam position • Arc quadrupole (practically also matching quad with BS1) • Q4 IP2 (where the effect is stronger)

  27. Thanks for your attention!

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