High purity niobium for superconducting accelerator cavities

Superconducting niobium microwave cavities are becoming increasingly important for particle accelerators in a variety of applications: electronpositron machines for high energy particle physics, continuous beam heavyion and electron accelerators for nuclear physics and for free electron lasers. Critical to the performance of r.f. superconducting accelerator devices is the quality of industrially produced sheet niobium, both in terms of low interstitial content as well as freedom from harmful inclusions. This review will discuss progress in producing large quantities of high purity niobium to fulfill this need and summarize the state of the art of r.f. superconductivity in the context of applications to accelerators under construction or under contemplation. Achievement of the full potential of r.f. superconductivity is closely tied to further advances in niobium quality. By realization of this potential, this technology may eventually play a parallel role for electron accelerators as that of superconducting magnets in today's proton accelerators.