LHC Impedance Coffee Meeting Minutes

January 12^th, 2010

NEG coating :

• Context:

Need to compute the impedance of NEG coated chambers for the CLIC BDS. What are the properties of NEG? Do they affect the impedance of CLIC and LHC?

• Summary of discussions:
   
  • Fritz mentioned the talk by David Seebacher on measurements of NEG coated glass rods at SPSU (March 24th, 2010). The conclusion was "Both the carbon and the NEG coating behave like bad conductors at the frequency range looked at (2 to 5 GHz). Therefore they have only a very small, negligible influence on the impedance." Looking at David's graphs, the carbon coating does not change the resonance frequencies of the glass rod but changes the amplitude (indicating the coating is resistive but almost not capacitive), while the NEG coating changes the resonance frequencies of the glass rod similarly to a brass rod (with slightly lower amplitudes, indicating a behaviour similar to a bad conducting metal).
     
  • However, for the very short bunch of the CLIC BDS, the frequency range of interest extends to the THz range. Very little is known or measured about the properties of the NEG coating at these frequencies, and the results obtained by extrapolation of the EM properties should be used with a lot of care.
     
  • Fritz said that classical RF measurement setups enable measurements up to 50 GHz. Some measurements could be tried at THz frequencies using optical methods, but the quantities are expected to be small, so that a way to modulate the measured signals should be found.
    
     

LHC collimators :

• Context:

Need to give advice on the impedance of LHC collimators for phase 2.

• Summary of discussions:
   
  • Fritz said that we should compare a collimator made of (a) graphite, (b) graphite with copper coating, (c) dielectric, (d) dielectric with copper coating, with respect to the following properties (1) real impedance, (2) imaginary impedance, (3) coupled bunch growth rates, (4) heating and (5) tune shift.
     
  • Elias said that he already did some computations using the formula of Sacherer (see for instance PAC09 paper and ABP/RLC meeting 2006), but that he would like Nicolas to look at the coupled bunch complex tune shifts with Sacherer for all the coupled bunch modes and with HEADTAIL, and Benoit to produce TMCI plots. This should enable to see what optimum trade-offs could be made.

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