Measurements on the conductivity of copper coating with resonant cavity

Purpose To reduce the beam coupling impedance of the vacuum chamber made of poorly conducting material, a layer of high-conductivity metal, such as copper, is often coated on its inner surface. As the natural bunch length of modern accelerators is about several millimeters, its beam spectrum can reach tens of GHz. In this case, the skin depth of copper is of the same order of magnitude as its surface roughness, and its electrical properties can be different from that in DC, which will influence the beam coupling impedance. Therefore, the electrical property of copper coating at high frequency needs to be investigated. Methods In this paper, the method of resonant cavity is adopted to measure the coating conductivity, which is based on the relation between the quality factor of the cavity and material conductivity. Results Three different resonant modes are tested in the measurement, among which the H011 mode shows the best performance. The results also indicate that surface roughness of copper can have an influence on its effective conductivity at high frequency. Conclusion The H011 mode is suitable for measuring high-conductivity materials. When the skin depth of copper is comparable to or larger than its surface roughness, its effective conductivity will be significantly reduced.