Speaker
Description
This talk introduces SRF cavity fabrication research at KEK over the past five years. The common objective is to reduce the cost of SRF cavities. The first topic is a Nb/Cu cavity in which the cavity body is made of copper and the inside is coated with niobium to induce superconductivity. The inner surface of the accelerating cavity must be smooth, and a seamless cavity is ideal as a base for the coating. We developed the idea of manufacturing a seamless cavity from a single copper tube and successfully prototyped it by hydroforming. At CERN, we applied a niobium coating with a film thickness of about 5 µm by magnetron sputtering. RF performance tests were conducted at KEK, and the accelerating gradient reached 12 MV/m at 4 K and 16 MV/m at 1.85 K. We are currently working on further improvements to improve performance. After hydroforming, the inner surface of the cell becomes rough. We report our efforts to improve the roughness by polishing. The second topic is the manufacture of niobium SRF cavities by cold spray. Cold spray is a lower-temperature solid-state thermal spray process that deposits metal powder using a heated inert gas through a supersonic nozzle. When the material hits at supersonic speed and reaches the critical speed, the particles themselves are plastically deformed to form a film. We proposed a novel manufacturing method to form a thick niobium film on the surface of a mandrel by cold spray using niobium powder, then remove the mandrel to finish a hollow shape. We confirmed the feasibility of the proposed method for a 3 GHz one-cell cavity. We also carried out an RRR measurement of the cold-sprayed niobium specimen and obtained a value of 11. We report on these results. The third topic is the manufacture of a niobium foil cavity. We proposed a new approach to fabricating the cavity body by spinning a 1.5-mm-thick niobium sheet, and we prototyped a 3 GHz one-cell cavity. To improve the cavity's structural strength after spinning, we deposited a copper layer on the outer surface using cold spray. We report on the details of the manufacturing process.