Superconducting Cavities from Niobium-Copper Material

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Abstract
For large-scale applications, such as superconducting LEP or TRISTAN, cavities made from Nb-Cu composite material offer several advantages over 100% Nb cavities -material cost reduction, defect stabilization, and cryostat simplification through pipe cooling application. Thermal model calculations presented show that if thicknesses less than 0.5 mm can be achieved for the Nb layer, surface defects can tolerate higher power levels. This improves the reliability of achieving target field levels, usually limited by thermal instabilities at defects. Calculations that model pipe cooling copper-backed Nb cavities show that even if coils are spaced as loosely as one every 10 cm, satisfactory cooling of defects can be accomplished. Nb-Cu composite material of good bond quality as well as NbCu cavities have been made. In one S-band cavity, Nblayer thickness 0.3 mm, Q0 > 2 × 109 and Eacc > 6.5 MV/m were achieved. In this cavity 0.5 mm diameter defects operated at 4.2 MV/m whereas in 100% Nb wall cavities they break down at 3.2 MV/m.

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