Diagnostics for Superconducting r.f. Cavities

In the past a variety of diagnostic methods have been developed which allow to sense the "overall" behaviour of superconducting r.f. cavities. Besides these methods temperature and X-ray mapping techniques as well as visual inspection give a more detailed insight into energy loss and electron phenomena of a superconducting cavity. With these techniques regions with increased r.f.-losses or electron emitting areas can be localised. It is hoped that these techniques together with surface diagnostic methods will give a better insight in the nature and causes of weak spots and will provide means for avoiding or eliminating such regions by adequate surface treatments. Since the beginning of work on superconducting cavities the diagnostic methods allowing an information on the "inner life" of a superconducting cavity have been constantly developed and improved. In a first stage information were mainly gained by the r.f.-signals transmitted and reflected by the cavity. It is astonishing how many information a careful analysis of the amplitude, frequency and time behaviour of r.f. signals alone can already reveal like e.g. the overall losses, their nature, field and time dependence, the coupling factors, resonant and non-resonant electron loading, higher mode excitation, nature and behaviour of field limitations and breakdowns. Even some localisation of regions with increased r.f.-losses or of breakdown regions can be obtained in cavities where different modes can be excited.