The electromagnetic characterisation of different materials for the inner wall coating of beam pipes is a long-standing problem in accelerator physics, regardless the purpose they are used for, since their presence may affect in an unpredictable way the beam coupling impedance and therefore the machine performance. Moreover, in particle accelerators and storage rings of new generation very short bunches might be required, extending far in frequency the exploration of the beam spectrum and rendering therefore more and more important to assess the coating material response up to hundreds of GHz. This paper describes a time domain method based on THz waveguide spectroscopy to infer the coating properties at very high frequencies. The technique has been tested on Non Evaporable Getter thick films deposited by DC magnetron sputtering on copper plates.
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