R＆D of Niobium Copper Clad Seamless Superconducting RF Cavities

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For high-energy physics in the next generation, an electron/positron linear collider with a colliding beam energy 500-1000 GeV at center of mass is required. Superconducting linear accelerator : TESLA is one candidate as such a machine. Now DESY is promoting the TESLA future plan by international collaboration. With TESLA, main issues are 1) to develop high gradient superconducting RF cavities with a higher field gradient than 25 MV/m and 2) to fabricate such excellent cavities cost effectively. To date KEK has developed high gradient cavities with 40 MV/m by KEK original technology : horizontally rotating continuous electropolishing, which was developed in TRISTAN sc project.By such a background of our sc cavity R&D, we propose an idea for the second R＆D issue, in which one uses niobium copper clad tubes to hydro-form sc cavities. This method will lead to a considerable material reduction with expensive niobium and eliminating the complicated and expensive electron beam welding in cavity fabrication process, thus results in cost effective cavity production. One aim of this research program is to investigate how works the niobium (Nb)/copper (Cu) clad cavity by international collaborations between KEK and INFN-LNL in Italy, DESY in Germany and TJNAF in USA. In this study, we have successfully fabricated a high gradient Nb/Cu clad cavity with 40MV/m and demonstrated our idea is excellent.Another purpose is to develop a long clad tube for future multi-cell cavities. We proposed to make a Cu/Nb/Cu sandwiched tube by HIP bonding, then to elongate it using established tube drawing technique. We have successfully made a long clad tube. We have made Nb/Cu clad seamless cavities from the tube. Finally we estimated the fabrication cost for a Nb/Cu 9-cell cavity. The result says the price will be cheaper by a factor three comparing with electron beam welding niobium bulk cavity.