Niobium in superconducting RF cavities

Niobium is the favorite metal for the fabrication of superconducting accelerating cavities. While the majority of these cavities are formed from niobium sheet material a large number of copper cavities have been made with a thin niobium film on the inner surface produced by sputter coating. The resonators are operated well below the transition temperature of niobium (9.2K). A high thermal conductivity in the cavity wall is needed to guide the dissipated radio frequency (RF) power to the liquid helium coolant. In the case of bulk niobium cavities this requires niobium of exceptional purity with a gaseous contamination below 10 ppm (by weight) and a tantalum contents of less than 500 ppm. The Nb material must be free of foreign inclusions or metallurgical defects down to a scale of 20 µm. Considerable care must be applied during handling or machining the Nb parts in order to avoid any additional contamination. Most bulk niobium cavities are fabricated by deep drawing of half-cells from sheet material and electron beam welding. Hydroforming the accelerating structure from a seamless pipe is a cost-effective alternative. This technology has yielded excellent results in single-cell cavities and should be available for multicell structures in the near future. Final cleaning of the finished cavity by chemical or electrochemical methods and rinsing with ultraclean high-pressure water are essential steps to achieve a defect-free inner Nb surface such as needed in RF cavities at high fields. The production of niobium for cavities is a challenge for the fabrication process: low tantalum contents of the raw material, stringent vacuum requirements during electron-beam melting of the ingots, clean and well controlled conditions during sheet rolling, cutting and recrystallisation heating. Some Nb producers have developed adequate procedures. Open communication between these companies and the user laboratories was very helpful to reach the required specification. Up to now a total amount of about 25 tons of high purity niobium has been purchased for the fabrication of superconducting cavities. During the last years typically 2 tons per year have been ordered. There is worldwide interest in new superconducting accelerators for elementary particle physics and fourth generation synchrotron light sources, in particular Free Electron Lasers in the ultraviolet and X-ray regime. Therefore a continuously growing demand for high purity Nb is expected. The proposed linear collider project TESLA requires 500 tons of high purity niobium at a fabrication schedule of three years. (AU)