1051-8223 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TASC.2016.2640760, IEEE Transactions on Applied Superconductivity 4MOr2A-05 1 The Bundle-Barrier PIT Wire Developed for the HiLumi LHC Project B. Bordini, A. Ballarino, M. Macchini, D. Richter, B. Sailer, M. Thoener, K. Schlenga Abstract — For the HiLumi (HL) LHC project, CERN is developing dipole and quadrupole magnets based on state of the art high–J c Nb 3 Sn wires that are expected to operate at 1.9 K and at fields larger than 11 T. Two different types of Nb 3 Sn wires are considered for the project: the Powder In Tube (PIT) and the Restacked Rod Process (RRP) conductors manufactured respectively by Bruker-EAS and Oxford Superconducting Technology (OST). During the last 18 months, CERN and Bruker-EAS have being collaborating to develop a new variant of the PIT conductor in order to further improve its electromechanical properties. This collaboration led to the introduction of an additional Nb barrier around the whole bundle of filaments that allowed drastically reducing the effect of mechanical deformation and of the heat treatment cycle on the Residual Resistivity Ratio (RRR) of the stabilizing wire copper. Furthermore the new wire has already a slightly larger engineering critical current density with respect to the previous generation of PIT wire and it has the potential to further improve. In this paper the bundle-barrier PIT wire is presented together with the critical current, magnetization and RRR measurements carried out at CERN to: characterize its electro- mechanical properties; quantify the effect of the filament size on the critical current performance and; study the effect of the heat treatment cycle. Index Terms— HL-LHC, Nb 3 Sn, PIT, wire. I. INTRODUCTION N THE framework of the High Luminosity upgrade of the Large Hadron Collider (HL-LHC) [1]-[3], CERN is developing high field magnets (B>10 T) based on high-J c Nb 3 Sn wires. In order to create space for the installation of additional collimators in the dispersion suppressor regions of LHC, some Nb-Ti main dipoles will be replaced with shorter 11 T Nb 3 Sn magnets with the same integral magnetic field [2], [4], [5]. Furthermore the Nb-Ti quadrupoles of the interaction regions will be substituted with larger aperture Nb 3 Sn quadrupoles [6]-[9]. By doubling the quadrupoles’ aperture, CERN is expecting to quadruple the peak luminosity of the LHC [6], [7]. All these Nb 3 Sn magnets will be wound with 40- strand Rutherford cables; for the dipole magnets the wire diameter is 0.7 mm while for the quadrupoles is 0.85 mm. CERN procurement strategy is based on double sourcing: some of the magnets (both the dipoles and the quadrupoles) will use the Powder in Tube (PIT) wires produced by Bruker- Work supported by the High Luminosity LHC Project at CERN. B. Bordini, A. Ballarino, M. Macchini, D. Richter are with the European Organization for Nuclear Research, CH-1211 Geneva 23, Switzerland (email coresponding author: Bernardo.Bordini@cern.ch) . B. Sailer, M. Thoener, K. Schlenga are with Bruker EAS GmbH, Ehrichstraße 10 Hanau, Germany. EAS and some others the Rod Restack Process (RRP) wire by Oxford Superconducting Technology (OST). During the last six years CERN has procured several tens of kilometers of these types of wires in order to study their characteristics and to manufacture Rutherford cables used in model magnets. In particular during the last 18 months CERN and Bruker-EAS have being collaborating to develop a new variant of the PIT conductor in order to further improve its electromechanical properties. This collaboration led to the introduction of an additional Nb barrier that, surrounding the whole bundle of filaments, drastically reduced the effect of mechanical deformation and of the heat treatment cycle on the Residual Resistivity Ratio (RRR) of the stabilizing wire copper. The characteristics of this new wire are presented together with the measurements performed at CERN to quantify its performance. In particular, the second section summarizes: the properties of the non-barrier PIT wire received by CERN for the HL-LHC project and; the reasons for developing the new bundle-barrier PIT. The third section is dedicated to the bundle-barrier PIT wire: the electrical characteristics of the round wire, the RRR degradation due to mechanical deformations and, the effects of the filament size on the wire performance are presented. In the last section, before the conclusions, possible routes to further improve the PIT critical current are discussed. II. THE NON-BARRIER PIT WIRE - OLD LAYOUT This section summarizes the electro-mechanical performance of the non-barrier PIT wires (old layout, i.e. without the additional barrier) that were received at CERN for the HL-LHC project. The geometrical characteristics of these wires are collected in the first four columns of Table I while Fig. 1 a-c shows the wires’ cross sections. The results of the critical current I c , copper-to-non-copper ratio and RRR measurements performed by Bruker-EAS on round wires are reported below in the sub-section A. For each billet there are three different I c , copper-to-non-copper ratio and RRR samples: one taken from the initial part of the billet, one from the center and one from the final part. The I c measurements were carried out at 4.22 K on ITER like Ti-6Al- 4V barrel [10]. Each sample was tested at six different magnetic fields (from 12 T to 17 T), which allowed estimating quite accurately the Kramer upper critical field B c2 * at 4.22 K and at the given strain of the I c measurement. B c2 * was calculated after having corrected the I c data in order to account for the self-field of the sample [11]. The RRR measurement consists in measuring the electrical resistance of a 80 mm-long wire sample at 293 K and 20 K; the RRR is the ratio between these two values. I