R＆D of Niobium Copper Clad Seamless Superconducting RF Cavities

对于下一代高能物理，需要一个质心碰撞束能量为500-1000 GeV的电子/正电子线性对撞机。超导直线加速器：特斯拉是这种机器的候选者之一。现在DESY正在通过国际合作推动特斯拉的未来计划。对于特斯拉，主要问题是1)开发高梯度超导射频腔，其场梯度高于25 MV/m， 2)成本有效地制造这种优秀的腔。迄今为止，KEK已开发出40 MV/m的高梯度腔体，采用KEK原创技术：水平旋转连续电抛光，该技术是在TRISTAN sc项目中开发的。基于我们sc空腔研发的背景，我们提出了第二个研发课题的想法，即使用铌铜包层管水成型sc空腔。该方法将大大减少使用昂贵铌的材料，并消除腔体制造过程中复杂而昂贵的电子束焊接，从而实现经济有效的腔体生产。该研究项目的目的之一是通过意大利KEK和INFN-LNL、德国DESY和美国TJNAF的国际合作，研究铌(Nb)/铜（Cu）包覆腔的工作原理。在这项研究中，我们成功地制作了40MV/m的高梯度Nb/Cu包覆腔，并证明了我们的想法是优秀的。另一个目的是为未来的多腔体开发一种长包层管。我们提出用HIP键合法制备Cu/Nb/Cu夹芯管，然后用现有的管拉拔技术对其进行拉长。我们成功地做了一根长包层管。我们用该管制作了Nb/Cu包覆无缝腔。最后我们估算了Nb/Cu 9电池腔的制造成本。结果表明，与电子束焊接铌体腔相比，其价格比电子束焊接铌体腔便宜三倍。

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.