Seamless Nb Tubes for SRF Cavities

The problems with current pure niobium (Nb) superconducting radio frequency (SRF) cavities are that manufacture is expensive; cavity performance is often poor; and manufacturing results are inconsistent. These problems stem primarily from three factors: (i) the high cost of pure Nb, (ii) a poor and inconsistent microstructure in the starting niobium material, and (iii) seam welding to manufacture cavity strings. Severe plastic deformation (SPD) processing will be used to produce Nb and bi-metallic Cu-Nb tubes having fine-grains and a uniform/consistent microstructure for hydroforming into SRF cavity strings. Careful selection of Cu-Nb tube architecture, the plastic strain route, and heat treatment conditions during tube fabrication steps will lead to microstructures with favorable texture giving improved hydroformability and overall lower SRF cavity-string manufacturing costs. Phase I work included (i) SPD processing bulk RRR Nb, (ii) transforming the microstrructure in high purity (RRR grade) Nb tube for better hydroformability, (3) microstructure examination and mechanical testing of processed tube to assess quality for hydroforming into SRF cavities, (iv) trial extrusions of bi-mertallic Cu-Nb tube, and (v) design of procedures and scale-up equipment for Phase II. The most remarkable result was successful fabrication of Nb tube with a fine-grain uniform and repeatable microstructure with mechanical properties suitable for hydroforming. Phase II work will include construction of scale-up equipment for fabricating pure Nb and CuNb bimetallic tubes, SPD processing prototype scale thin wall Nb and CuNb tubes, hydroforming the processd tubes into prototype SFR cavities, and performance testing of the prototype SRF cavities. Cavity manufacturing and performance experiments will enable an evaluation of tube quality and commercialization potential. A cost/benefit analysis will assess economic viability of the new technology.Commercial Applications and Other Benefits: The commercial application is to apply this technology to the production of Nb and CuNb tubes for lower cost superconducting radio frequency cavities. Cabot, H.C. Starck, and ATI Wah Chang may be interested in using this technology for Nb tubular products. The processing approach could be applicable to the fabrication of other tube materials including tantalum, titanium and zirconium.