Hydroforming of elliptical cavities

Abstract

Activities of the past several years in developing the technique of forming seamless (weldless) cavity cells by hydroforming are summarized. An overview of the technique developed at DESY for the fabrication of single cells and multicells of the TESLA cavity shape is given and the major rf results are presented. The forming is performed by expanding a seamless tube with internal water pressure while simultaneously swaging it axially. Prior to the expansion the tube is necked at the iris area and at the ends. Tube radii and axial displacements are computer controlled during the forming process in accordance with results of finite element method simulations for necking and expansion using the experimentally obtained strain-stress relationship of tube material. In cooperation with industry different methods of niobium seamless tube production have been explored. The most appropriate and successful method is a combination of spinning or deep drawing with flow forming. Several single-cell niobium cavities of the 1.3 GHz TESLA shape were produced by hydroforming. They reached accelerating gradients E acc up to 35 MV=m after buffered chemical polishing (BCP) and up to 42 MV=m after electropolishing (EP). More recent work concentrated on fabrication and testing of multicell and nine-cell cavities. Several seamless two-and three-cell units were explored. Accelerating gradients E acc of 30-35 MV=m were measured after BCP and E acc up to 40 MV=m were reached after EP. Nine-cell niobium cavities combining three three-cell units were completed at the company E. Zanon. These cavities reached accelerating gradients of E acc ¼ 30-35 MV=m. One cavity is successfully integrated in an XFEL cryomodule and is used in the operation of the FLASH linear accelerator at DESY. Additionally the fabrication of bimetallic singlecell and multicell NbCu cavities by hydroforming was successfully developed. Several NbCu clad singlecell and double-cell cavities of the TESLA shape have been fabricated. The clad seamless tubes were produced using hot bonding or explosive bonding and subsequent flow forming. The thicknesses of Nb and Cu layers in the tube wall are about 1 and 3 mm respectively. The rf performance of the best NbCu clad cavities is similar to that of bulk Nb cavities. The highest accelerating gradient achieved was 40 MV=m. The advantages and disadvantages of hydroformed cavities are discussed in this paper.

Metal forming technology for the fabrication of seamless Superconducting radiofrequency cavities for particle accelerators

“…ICNFT 2015 The hydroformed cavity RF results are recently reported in literature [13]. In summary the nine-cells produced by joining the three-cell units achieve at 2 K a maximum accelerating field of 35 MV/m, with a Q-value at the highest gradients exceeding 1E+10.…”

Niobium superconducting rf cavity fabrication by electrohydraulic forming

“…Spinning is currently used at CERN to produce half-cells of copper which undergo electron-beam welding at the equator in order to obtain a full cavity [3]. At DESY, hydroforming was implemented to produce full cells by starting from seamless niobium tubes [4][5]. All these traditional forming processes present several drawbacks; in particular, deep-drawing cannot produce complex shapes with high accuracy and according to literature [6] it requires high-tonnage hydraulic presses when shaping large half-cells.…”

“…Concerning hydroforming, its main advantage is the production of a complete cell without the critical equator welds; however it requires several forming and calibrations stages and a careful optimization of the forming parameters. Hydroforming produces surfaces which are rougher than the one obtained by deep-drawing and it still requires the removal of 100-200 μm by buffered chemical polishing [5]. Another drawback of hydroforming is that only the outside shape of the cell is formed with precision, while the accuracy of the rf surface highly depends on the thickness and tolerance of the starting tube [5].…”

“…Hydroforming produces surfaces which are rougher than the one obtained by deep-drawing and it still requires the removal of 100-200 μm by buffered chemical polishing [5]. Another drawback of hydroforming is that only the outside shape of the cell is formed with precision, while the accuracy of the rf surface highly depends on the thickness and tolerance of the starting tube [5]. This requires a high control of the dimensions of the initial tube and of the hydroforming parameters [5].…”

“…Another drawback of hydroforming is that only the outside shape of the cell is formed with precision, while the accuracy of the rf surface highly depends on the thickness and tolerance of the starting tube [5]. This requires a high control of the dimensions of the initial tube and of the hydroforming parameters [5].…”