High gradient superconducting niobium cavities - A review of the present status

Superconducting niobium cavities used in particle accelerators are limited in their rf performance by two phenomena: quench field levels below the theoretical limit of the material caused by defects, and field emission loading resulting from artificial contamination of the superconducting surfaces during surface treatment and assembly procedures. In recent years the community involved in SRF technology developments has successfully improved cavity performances by applying advanced surface treatment methods such as chemical polishing, electropolishing, tumbling, high temperature heat treatment, high pressure rinsing, "in situ" high peak power processing and clean room assembly procedures. In addition, improvements in the material properties such as thermal conductivity by "solid state gettering" and very strict QA methods, both in material inspection and during cavity fabrication, have resulted in cavity performance levels of E-acc up to 40 MV/m in monocells and gradients in the vicinity of 30 MV/m in multicell structures at Q-values of approximate to 10(10) at a temperature of 2K, More recently the fabrication of "seamless" cavities by spinning is being pursued with encouraging results. This process eliminates electron beam welds, which sometimes are the causes of performance degradations.