Impedance RF Measurement Coffee Meeting Minutes

June 9^th, 2008

• Transverse impedance of Ceramic collimator : Comparison of HFSS versus GdfidL (Alexej Grudiev - slides)
   
  • Alexej used a trick in GdfidLto let reflections take place at each side in the material of the longitudinally finite length beam pipe (see slide 2).
     
  • For a matched structure of finite length, increasing length leads to decrease the quality factor Q.
     
  • slide 4 shows that a small mesh size is very important to obtaining accurate results. However, the computation time strongly limits decreasing the mesh size. Typical GdfidL calculations can take up to a week to complete.
     
  • In the HFSS set up (slide 5), Fritz mentioned that the line equivalent cell should include both a DUT impedance in series and in parallel. Alexej agreed but pointed out that this model was the one used by everyone (both Tom's simulations and the measurements) for the moment, and that we should keep it for the time being to be able to compare with previous results. However, Fritz and Alexej agreed that this model is not probably accurate enough and should be revisited.
     
  • GdfidL seems to lose accuracy in certain types of materials compared to HFSS.
     
  • Elias will generate the theoretical plots for the same cases to compare HFSS, GdfidL and  the theory.

   

• First measurements of the transverse impedance of a full collimator assembly (Federico Roncarolo and Benoit Salvant - slides)

• Thanks to colleagues from ATB (Rocio, Nicolas, Sébastien, Oliver, and others), it has been possible to get a full collimator assembly for measurement for a couple of days. It was even possible to tilt the collimator back to horizontal in order to minimize the effect of coil sagitta on alignment.
   
• The measurements were performed in the main hall of building 252, i.e. there was no temperature regulation available. Therefore, measurements were performed very quickly (30 seconds) on both jaws and collimators to minimize the temperature impact.
   
• Alignment of the coil in the collimator is of course a big issue, especially when the measurement has to be set and performed quickly. However the wine thermometer technique helped us there:

 

• TBC

 

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