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Structure Considerations

Structure Considerations. D. Schulte. Luminosity and Parameter Drivers. Luminosity spectrum. Beam Quality (+bunch length). Beam current. Approximate Parameter Derivation. Not how we chose parameters but how parameters are driven by physics.

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Structure Considerations

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  1. Structure Considerations D. Schulte

  2. Luminosity and Parameter Drivers Luminosity spectrum Beam Quality (+bunch length) Beam current

  3. Approximate Parameter Derivation Not how we chose parameters but how parameters are driven by physics Beam-beam effects define minimum charge to have full luminosity efficiency Luminosity efficiency requires structure aperture consistent with minimum charge Need to maximise RF to beam efficiency and all parameters follow Damping ring and BDS define minimum horizontal beam size at IP

  4. Long-range Wakefields • Once bunch charge is fixed main efficiency/current limitation comes from multi-bunch wakefields • Need to ensure stability of jittering beam • Different criteria • Coherent jitter of the full train • Expressed by FC • very likely • Independent jitter of each bunch • Expressed by FRMS • Possible • Worst initial pattern • Expressed by Fworst • Hopefully not likely

  5. Theoretical Model F gives the increase in luminosity due to long-range wakefields compared to the case with no long-range wakefield The final position of the bunches can be expressed as a function of the initial offset Each complex component of the vectors represents one bunch Aij depend on the wakefields With no wakefields A=1 Luminosity loss ΔL is proportional to

  6. Example: CLIC_G FRMS for kick only on the second bunch

  7. Remark: Static Errors Static errors of the main linac structures (scatters) are not very important

  8. Additional Trapped Mode The importance of a trapped mode depends very much on the frequency

  9. Example: Undamped Mode A mode with 88V/pCm-2 can yield an increase of FRMS by 20%

  10. Steps • Need wakefield in time domain or with a limited number of modes • Can perform theoretical analysis rapidly • Full simulation will take longer • Are expected to show better stability due to de-phasing in individual bunches • Do not want to rely on this

  11. First Result • Wakefields at different bunches from Alessandro • Point-like calculation done • FC=O(1012), FRMS=O(1010), Fworst=O(1013) • Will need to improve the structure • Long long-range wakefield by O(0.1) • But only envelope given • Change sign of every other kick and things look much better • Need a real wakefield • Need uncertainty estimates

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