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We report on the measurement of the surface impedance of thin sub-micron non-evaporable getter (NEG) coatings on a copper substrate, as used for distributed pumping in the vacuum system of the Swiss Light Source upgrade (SLS 2.0). Given the low electrical conductivity of NEG, a sub micron thickness with well known properties is required to avoid heat up and beam instabilities. Measurement frequencies around 100 GHz are required to obtain a good measurement sensitivity for these tiny coatings. A quasi-optical test stand based on a Fabry Perot principle uses one spherical mirror containing input and output couplers and a planar NEG coated probe mirror. 3D printing allowed to produce the spherical mirror together with the RF coupler and waveguide flanges in one integral part, which required only minimal additional processing. The setup is calibrated with the help of probe mirrors fabricated from bulk stainless steel and copper. For coated probes, we clearly saw the impedance changes caused by coatings with thicknesses in the 300 to 600 nanometer range and were able to prove their compatibility with the impedance requirements of the accelerator.
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