Impedance RF Measurement Coffee Meeting Minutes

November 10^th, 2008

Electron Cloud mitigation carbon coating impedance:

• Context:

                We do not know the electrical characteristics of the thin carbon coating (< 1 micron) on the metal substrate, especially the permittivity.
                As a consequence, we can not estimate the beam impedance. However can we measure the effect on an RF bench?
 

• Summary of discussions:
   
  • Sergio mentioned that the amorphous carbon coating DC conductivity could differ from the bulk graphite DC conductivity by two orders of magnitude.
     
  • The expected effect of the coating is very small. Therefore, we need to use a very sensitive technique to be able to measure the impedance. Fritz was suggesting the resonator method described in http://doc.cern.ch/archive/electronic/other/generic/public/cer-000204033/scanned.pdf
     
  • Indeed, surface impedance calculations were performed by matching TEM fields at the boundaries (by Sergio see first results), and beam impedance resistive wall calculations were performed by using a multi layer beam pipe (by Benoit see first results). These are preliminary results and their consistency remains to be checked. However, both calculations show that the effect of a submicron layer of carbon coating is transparent in the frequency range of interest. Some more calculations should be performed with higher permittivity values (both real and imaginary).
     
  • At the same time Fritz also mentioned that some beam measurement showed that the effect of NEG coating on the imaginary part of the beam impedance was not negligible (see paper1, paper2 and paper3).  From these papers, the transverse impedance is reported to increase significantly when NEG coated vessels are included in the ring.
     
  • Therefore a clarification is needed.
     
  • After the ECloud workshop next week, we will decide whether a better assessment of the impedance is needed.
    
     

AOB:

• The institute for applied physics at the university of Frankfurt (Prof. Dr. Ulrich Ratzinger) agreed to help at performing RF measurements on dielectrics.

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