Time domain measurements of the sub-THz response of different coatings for beam pipe walls

Modern accelerators and light sources often require special treatment of the vacuum chamber surface in order to avoid undesirable effects and maximize machine performance. Coatings with Non Evaporable Getter compounds and amorphous Carbon have been extensively tested and used with very effective results since they allow to reduce the secondary electron emission from pipe walls and therefore the relevant beam instability. An electromagnetic characterization of such coatings is therefore fundamental to build a reliable impedance model. We present here a method based on time domain measurements of an electromagnetic wave passing through a tailored waveguide, where the material under test is deposited on a planar slab. This configuration allows an easy measure of samples having a homogeneous coating thickness and a reasonable area, with parameters chosen in order to have a good signal-to-noise ratio, avoiding at the same time problems due to peel-off and blistering during deposition. The study on the electromagnetic response is performed in the frequency range from 0.1 to 0.3 THz, corresponding to the first transverse electric (TE) mode propagation inside the designed waveguide.