2122 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011 Joint Design and Test for the SCH Hongyu Bai, Todd Adkins, Scott T. Bole, Iain R. Dixon, Emsley L. Marks, George E. Miller, Patrick D. Noyes, Thomas A. Painter, Robert F. Stanton, Hubertus W. Weijers, and Ting Xu Abstract—The NHMFL Series Connected Hybrid (SCH) magnet will provide an energy-efficient 36 T to the DC user facility by em- ploying a 20 kA superconducting outsert coil in series with a resis- tive insert. The magnet outsert consists of three concentric layer- wound sub-coils using three different grades of Cable-in- Conduit Conductors (CICC). The electrical joints in the supercon- ducting outsert require low DC resistance to minimize the refriger- ation requirement at the operational 4.5 K temperatures and low AC losses to ensure good stability against ramping operation re- quired by the users. There are four internal splice joints in the outsert, which are to joints with the same design configuration. There are another two terminal joints between the outsert and the two NbTi buslines, which connect the out- sert terminals to the two current leads. The two to NbTi terminal joints are of identical configurations. All of the joints will be praying-hands configuration with an operation current of 20 kA. The R&D for the joins has been carried out at the NHMFL. The joints design and test results are discussed in this article. Index Terms—Cable-in-conduit conductor, joint, series con- nected hybrid, superconducting magnet. I. INTRODUCTION T HE NHMFL Series Connected Hybrid (SCH) magnet will provide an energy-efficient 36 T to the DC user facility by employing a 20 kA superconducting outsert coil in series with a resistive insert [1]–[3]. The superconducting outsert of the Se- ries Connected Hybrid (SCH) magnet consists of three concen- tric layer-wound sub-coils using three different grades Cable-in Conduit Conductors (CICC) [2], [3], which are known as the High Field (HF), Middle Field (MF) and Low Field (LF) conductors. The four internal splice (layer to layer) joints in the outsert are joints with the same design con- figuration. There are another two terminal joints between the terminals of outsert and two Buslines, which are made of NbTi CICC and connect the outsert terminals to the two cur- rent leads. The two terminal joints are with the same configuration. All of the joints will be praying-hands con- figuration with an operational current of 20 kA. The CICC joint development has been carried out at the NHMFL and the activ- ities are to design the proper joint for the SCH, gain fabrication Manuscript received August 02, 2010; accepted October 29, 2010. Date of publication December 17, 2010; date of current version May 27, 2011. This work was supported by the US National Science Foundation Division of Ma- terials Research under Grant DMR-0603042, and by Helmholtz Center Berlin. The test was supported in part by the US National Science Foundation Cooper- ative Agreement DMR-0654118, by the State of Florida, and by the DOE. The authors are with the National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32301 USA (e-mail: bai@magnet.fsu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2010.2091387 experience, and characterize and qualify the performance of the joint. The electrical joints in the superconducting outsert require low DC resistance to minimize the refrigeration requirement at the operational 4.5 K temperatures and low AC losses to en- sure good stability during the ramping operation required by the magnet users. A solder-less prototype sample joint was the first manufactured and tested in 2008 [4]. It used a different but sim- ilar cable from the SCH. Two joints with the real cable of SCH: an HF-MF splice joint and an LF-Busline terminal joint, were later developed and tested after the first joint was tested. This article discusses the splice joints and terminal joints design and test results. II. JOINT DESIGN The design of the joints for the SCH should balance the joint DC resistance and AC loss and keep the mechanical integrity. The solder-less joint style has been chosen for the SCH based on the merits of low AC losses and no required handling for the cable after heat treatment since the joints will be heat treated with the outsert coil together in a furnace after they are fabricated. The design criteria are: joint DC resis- tance should be lower than 2 due to the cryogenic cooling requirements; AC loss of the joint should not be the weak link on the cryogenic stability margin; no degradation is allowed during thermal cycling and load cycling; mechanically robust and leak tight at a pressure of 30 bar and 4.5 K. The splices and terminals joints are designed to be placed outside the SCH outsert and aligned with the magnetic axial field. The maximum background field on the joints is about 2 T. This configuration gives a lower background field and preferred orientation that reduces AC losses in the joints. It also provides a low background field for the NbTi bus. The magnitude of the magnetic field at the ends of the coil is too high for NbTi cable, which excludes the choice to install the terminal joints at the ends of the coil. Fig. 1 shows the location of the joints. A. Splice Joint The four splice joints in the series connected hybrid are one HF-MF joint, one MF-LF joint and two LF-LF joints. The ar- rangement of the joints limits the length of each CICC to less than 600 meters. The splice joint consists of two ca- bles with a Cu saddle between them. The cables and Cu saddle are enclosed in a stainless steel joint box and are compressed to obtain a void fraction of 20% in the cable area. After com- pression, the joint box is sealed by welding. Fig. 2 shows the HF-MF joint made for the test. The MF-LF joint and the LF-LF joint have the similar design, but smaller cables. The configura- tion provides a long term vacuum tightness and also avoids the dissimilar metal bonding of copper and stainless steel, which 1051-8223/$26.00 © 2010 IEEE