CERN, Geneva
MPI-K, Heidelberg
Cockcroft Institute, Warrington, Cheshire
The surface resistance RS of superconducting cavities can be obtained by measuring the unloaded quality factor Q0. RS can vary strongly over the cavity surface but the value obtained is only an average over the whole surface. Furthermore surface analysis tools are difficult to apply inside a cavity. A more convenient way consists of investigating small samples. They can be cheaply manufactured and easily duplicated. RF cavities excited in the TE011 mode with a sample attached as the cover plate are often used for material characterization. However, there is the drawback of relatively large size at the frequencies of interest concerning accelerator applications. At CERN a compact Quadrupole Resonator has been developed for the RF characterization of superconducting materials at 400 MHz. In addition the resonator can also be excited at multiple integers of this frequency. Besides RS it enables determination of the critical RF magnetic field, the thermal conductivity and the penetration depth of the attached samples, at different temperatures. The features of the resonator will be compared with those of similar RF devices and first results will be presented.
CERN, Geneva
Individual niobium superconducting RF (SRF) cavities for accelerator application are nowadays performing up till the believed limitations (surface magnetic field B of ~ 200 mT and a low field surface resistance Rs of a few nΩ). It is also observed that Rs may increase with B (dubbed Q-slope/Q-drop). A theoretical expression for this increase was derived from the two-fluid model, depending on typical parameters such as the electrical conductivity, penetration depth, temperature T, frequency f, A least square fit of measured data against the theoretical expression for Rs allowed to determine these parameters and to compare them with generally accepted values. The measured data consisted of about 1400 quadruples (Rs, B, f, T) collected from cavity tests of a very broad provenience in shape, cell number, frequency, surface treatment, niobium quality With this approach it is hoped that stochastic factors cancel out and that the fundamental parameters of the niobium metal prevail. A quantitative explanation for the Q-slope/Q-drop is proposed which is based on the number of normal electrons located at the uppermost niobium surface layer that increases with T and B.
