Field emission and thermal breakdown are the main mechanisms limiting the accelerating gradient of niobium radiofrequency cavities. Diagnostic tools to study these mechanisms include quality measurements, thermometry and microscopy. Results presented here demonstrate that micron-size, conducting particles are the source of field emission. Thermal breakdown is caused by a variety of defects such as inclusions, pits and submillimeter-size particles. Techniques developed to minimize field emission and thermal breakdown include clean-room assembly, high-power processing (to avoid field emission) and the use of high-purity niobium (to avoid thermal breakdown). With these techniques, accelerating gradients of 20-30 MV/m can be achieved in niobium cavities.
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