LHC Impedance Coffee Meeting Minutes

November 13^th, 2009

LHC collimators impedance calculations

• Context:

As soon as the LHC starts, an MD might be organized in the weeks to come, to measure the imaginary part of the beam coupling impedance in the LHC (with one bunch at low intensity, the tune with respect to the intensity will be measured). Before that, Elias, Benoît and Nicolas would like to give an estimation of the tune shift that should be measured due to collimators, for different settings (energy, collimator jaws opening, etc.). Ralph would like to be sure that the settings used in the computation (and provided by Adriana) are realistic with respect to what is likely to be done during the MD.
 

• Summary of discussions:
   
  • Ralph said the main point is that the 19 collimators of IR7 will probably be moved simultaneously and set each time to the same half gap (in terms of s of the beam), from full opening (58 to 60 mm full gap) to minimum gap (which is 0.8mm - full gap - for the TCLA collimators, and 0.5mm - full gap - for the TCP and TCSG ones). The optics will be those of injection, the energy 450 GeV and (maybe) 1.18 TeV, and there will be one single bunch with 2.10⁹ to 4.10¹⁰ protons.
  Elias said this was the plan foreseen for the calculations, with a computation of the tune shift for each opening settings chosen. If the tune shift is not negligible when moving the 19 collimators together, one then could try to move less collimators at a time and observe the tune shift.
   
  • Elias and Ralph mentioned the possible issues that may occur during the first measurements (Qmeter available? bunch length measurements? Chromaticity (~2)? transverse feedback?)
     
  • Adriana will provide collimator settings ("colgap" file) for 2 (extreme) cases, one with injection settings everywhere except in IR7 where the collimators will be set at half gap=1s, and the other with full opening of the collimators everywhere except again in IR7 (1s).
     
  • Ralph mentioned that he was very interested in trying to see if the inductive bypass effect could be seen. Benoît said this will probably not be the case with a single-bunch beam.
     
  • Ralph said that probably there will not be any transverse feedback working before Christmas, therefore a relevant question is: up to what intensity can we go without feedback ? Elias said that before the end of the year Nicolas should be able to run MTRISM (Alex Koschik's code) which is the tool to investigate this, as for that problem we have to look at multibunch instabilities. In particular, the effect of octupoles is implemented in MTRISM. Then next year Nicolas should try to extend the single bunch code Headtail to the multibunch case.

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