Understanding And Overcoming Limitation Mechanisms In Nb3Sn Superconducting Rf Cavities

Nb3 Sn has the potential to significantly improve cryogenic efficiency and maximum fields in superconducting RF cavities, structures that impart energy to charged particle beams in large accelerators. Previous experiments demonstrated excellent cryogenic efficiency at small accelerating fields, producing cavities with surface resistance R s on the order to 10 nΩ, but it consistently increased strongly as the peak surface magnetic field exceeded the first critical field [MICRO SIGN]0 Hc1 [ALMOST EQUAL TO] 30 mT. This dissertation describes results from a new research program to investigate whether this behavior is fundamental and to determine what mechanisms ultimately limit RF superconductivity in this material. A chamber was designed and built for coating niobium substrates with a thin layer of Nb3 Sn via high temperature vapor deposition. After commissioning with samples, many coatings of single cell 1.3 GHz cavities were carried out. Several RF tests showed that small R s could be maintained up to fields significantly higher than Hc1 , showing that it is not a fundamental limitation. The field limitation encountered in these experiments was primarily quench, likely due to surface defects, based on results that include temperature mapping and high power pulsed measurements. Measurements of the temperature dependence of R s and microscopic investigations of the surface indicate that low tin content regions cause R s degradation, especially after material removal. A theoretical investigation showed that thick films have only slightly lower maximum fields than alternating layers of thin film superconductor and insulator on a bulk superconductor, and only for a small parameter range. The highest fields reached by a Nb3 Sn cavity in these experiments corresponds to an accelerating gradient of 17 MV/m, with a quality factor of 8 x 109 at 4.2 K. Cavities with this performance would significantly reduce costs in many applications, including large high duty factor linear accelerators and small-scale industrial accelerators. Additional development aided by increased understanding is expected to push performance even further