Optimization of the Shielded pair geometry for surface resistance measurements of the LHC beam screen

For the measurement of beam induced losses in the RF and microwaverange arising from the surface resistance of the LHC liner a dedicatedtechnique has been developed. A sample of the liner with a typicallength of about one meter is converted into a shielded pair coaxialresonator. The shielded pair approach using two parallel innerconductors, is required as both the conductivity of these two innerconductors and the conductivity of the inner liner surface change asa function of a DC magnetic field and temperature in a cryostat.These two conductors are operated in the odd and even mode, returning two Qvalues for each resonance. There are several degrees of freedom forthe diameter and lateral spacing of these two conductors. Taking into account end effects and different sources of mechanical positioning errors, parameters for an optimum geometry are discussed and the corresponding measurement errors on the surface resistance areestimated.