R＆D of Niobium Copper Clad Seamless Superconducting RF Cavities

对于下一代高能物理，需要一台对撞束能量在质心500-1000GeV的电子/正电子线性对撞机。超导直线加速器：特斯拉是这类机器的候选者之一。现在，DESY正在通过国际合作推动特斯拉的未来计划。有了特斯拉，主要的问题是：1)开发场梯度高于25 mV/m的高梯度超导射频腔；2)以成本效益的方式制造这样的高性能超导腔。到目前为止，KEK已经通过KEK的独创技术--水平旋转连续电抛光--研制出了40 mV/m的高梯度腔。基于这样的背景，我们提出了第二个研发课题的想法，即利用Nb铜管对Sc腔进行液压成形。这种方法将大大减少使用昂贵的Nb的材料，并消除腔体制造过程中复杂而昂贵的电子束焊接，从而产生具有成本效益的腔体生产。该研究项目的一个目的是通过意大利的KEK和INFN-LNL，德国的DESY和美国的TJNAF的国际合作，调查Nb(Nb)/铜(Cu)包层腔的工作原理。在这项研究中，我们成功地制作了40 MV/m的高梯度Nb/Cu包层腔，并证明了我们的想法是非常好的。另一个目的是开发一种用于未来多单元腔体的长包层管。我们提出了用热等静压键合的方法制作铜/Nb/铜夹层管，然后用现有的拉拔工艺对其进行拉长。我们已经成功地制作了一根长包覆管。我们用该管制作了Nb/Cu复合无缝腔。最后，我们估算了Nb/Cu9单元腔体的制作成本。结果表明，与电子束焊接Nb体腔相比，价格将便宜三倍。

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.