Measurements of ohmic losses of metallic reflectors at 140 GHz using a 3-mirror resonator technique

The reflectivity of metallic mirrors in the millimeter wave region does not only depend on the material, but also on the structure and roughness of th e surface. We have performed measurements of the reflectivity of various pl ane and grooved metallic and graphite samples at 140 GHz. The technique is based on the comparison of the quality factor of a 2-mirror reference reson ator with the quality factor of a 3-mirror resonator which has identical di mensions and includes the mirror to be tested. After a brief presentation o f the theory, the set-up is described and the reflection loss for various a luminium and copper mirrors as well as vacuum compatible materials for appl ications in thermonuclear fusion experiments are presented and discussed.