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LHCb VELO aperture verification with MADX

LHCb VELO aperture verification with MADX. M. Giovannozzi Acknowledgements: R. Appleby, R. De Maria, S. Fartoukh , M. Ferro Luzzi , B. Holzer. Forewords. Rob worked also on this subject with a Summer Student and the results are summarised in the final report.

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LHCb VELO aperture verification with MADX

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  1. LHCb VELO aperture verification with MADX M. Giovannozzi Acknowledgements: R. Appleby, R. De Maria, S. Fartoukh, M. Ferro Luzzi, B. Holzer MG - LEB meeting

  2. Forewords • Rob worked also on this subject with a Summer Student and the results are summarised in the final report. • The presentation by Massihas served as a basis for this study. • The presentation by Bernhard has served as input for this study. MG - LEB meeting

  3. Introduction - I • Nominal running conditions for LHCb (i.e., from LHC Design Report) • Spectrometer ramped with energy. It generates a horizontal crossing angle (so-called internal). • An additional horizontal crossing angle is superimposed (so-called external). • Considerations • A spectrometer polarity change generates a very large variation in the total crossing angle. • If assumption 1. is dropped, then there will be severe aperture issues for one of the spectrometer polarities for 25 ns beams. Half beam-beam separation MG - LEB meeting

  4. D2 separation dipole Introduction - II triplets triplets Highlights of key issues with LHCb spectrometer (from LHC-Project-Note-419) D1 separation dipole MG - LEB meeting

  5. Introduction - III • A possible solution is the use of a vertical external crossing angle. • It has been proposed already in the past (see W. Herr et al. LHC-Project-Note-419, LHC-PROJECT-Report-1009). • To note that the vertical external crossing angle has become the operational mode in 2012 (B. Holzer et al.). • Injection: standard horizontal external crossing angle • Top energy: transition to the vertical external crossing angle • It is mandatory to find a solution with a vertical external crossing angle from injection to top energy. • Comments: • Beam screen orientation is compatible with the nominal crossing scheme (i.e., crossing in the horizontal and separation in the vertical planes). • The potential aperture issues at injection with vertical crossing angle will be assessed by a dedicated aperture measurement during MD block 3. MG - LEB meeting

  6. Assumptions - I • The vertical external crossing angle is the preferred solution by LHCb. • It restores the symmetry between the configurations with different spectrometer polarities. • This is the scenario considered as the reference for running after LS2 and with HL-LHC. • The situation should become clearer after the tests during MD block 3. • Luminosity levelling could be performed either by • Beam-beam separation (routine operation) • Beta* variation (tested in MDs) • Ratio between levelled and virtual luminosity: 5 (this is the general assumption for HL-LHC). • VELO parameters: • Radius: 3.4 mm; Mechanical tolerance: 100 mm; Length: ±800mm around IP. • Aperture parameters: • Beta-beating: 20%; Closed orbit: 100 mm (IP stability during fill is around 50-60 mm). MG - LEB meeting

  7. Assumptions - II • Beam parameters (see also Bernhard presentation) • Post LS2 parameters depend on the performance reach of the injectors. Still to be discussed and unofficial. • Given the uncertainties, the approach has been to analyse worst case scenarios. MG - LEB meeting

  8. Assumptions - III • Reference beta*-> 3m • Beam-beam separation • l ≈ 100 mm (assuming 25 ns nominal emittance). This value has been added to the closed orbit budget in the aperture estimate (i.e., total closed orbit budget of 200 mm ). • Beta* levelling • b* in the interval 3 m, 15 m • Vertical external crossing angle (total) • Based on saturation of triplet aperture: about 478 mrad(assuming 25 ns nominal emittance). This provides 37 s of beam-beam separation. MG - LEB meeting

  9. Results • Beam-beam separation • VELO aperture (beginning of fill): 63 s(assuming 25 ns nominal emittance). • Beta* levelling • Vertical external crossing angle kept constant while b* is varied. • VELO aperture (beginning of fill): ≈31 s(assuming 25 ns nominal emittance). MG - LEB meeting

  10. Preliminary conclusions • Assuming that a vertical crossing angle will be used to separate the beams, then • VELO aperture (beginning of fill): 63 s(assuming 25 ns nominal emittance). • VELO aperture (beginning of fill): ≈ 31 s(assuming 25 ns nominal emittance). • The values have been obtained assuming worst case scenarios. • The crucial point is to assess whether the vertical external crossing angle can be used at injection with full spectrometer strength. • A MD session is planned on Saturday 10/10 from 0:00 to 2:00. • It is proposed that any decision is postponed until the results have been evaluated. MG - LEB meeting

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