High-current density CORC® wires for use in accelerator magnets
Next generation accelerator magnets capable of generating magnetic fields in excess of 16 T require high-temperature superconductors (HTS) such as Conductor on Round Core (CORC®) cables and wires, wound from RE-Ba 2 Ca 3 O 7-δ (ReBCO, Re = rare earth) coated conductor tapes.1 The severe operating requirements of an engineering current density (J e ) of at least 400 Amm -2 at 4.2 K and 20 T, high operating currents of 5-20 kA, tolerance to Lorentz stresses in excess of 150 MPa, and in many cases cable bending diameters as small as 30 mm, make accelerator magnets one of the most challenging applications for any HTS material. Advanced Conductor Technologies (ACT) has been developing CORC® cables and wires wound from ReBCO tapes containing substrates of 30-50 µm thickness to meet these requirements. Although these requirements could each be met separately with current CORC® technology, meeting the requirements at the same time requires thinner, more powerful CORC® wires wound from tapes with even thinner substrates. The purpose of this Phase I SBIR program was to determine the feasibility to develop ReBCO tapes containing 25 µm thick substrates on an industrial scale and wind thinner CORC® wires from these tapes that could eventually meet the stringent performance specifications for accelerator magnets. The results of the Phase I program clearly demonstrated the feasibility of developing thinner, more powerful CORC® wires using ReBCO tapes with 25 µm substrate that would likely meet the stringent requirements for use in accelerator magnet by the end of the potential Phase II program. SuperPower Inc., as the subcontractor on the Phase I program, was successful in developing tapes with 25 µm substrate at minimum piece lengths of 30-40 m with a clear path towards industrial production. A total length exceeding 350 meters of 2 mm wide ReBCO tape with critical current (I c ) of between 45 and 65 A at 77 K was delivered to ACT during the Phase I program. The thinner tapes allowed us to reduce the CORC® core (former) from 2.5 mm to 2.0 mm, resulting in a higher current density conductor with enhanced flexibility. Several CORC® wires were manufactured to reach the Phase I goal of a Je of at least 300-400 Amm -2 at 20 T at a minimum bending diameter to less than 50 mm. Several CORC® wires were wound and analyzed after bending by identifying the location of peak stresses and causes of tape damage. Data was compared to 3D finite element models that included parametric studies to identify key parameters leading to the observed tape failure modes. The models suggest that flexibility is heavily dependent on the friction coefficient between tapes in a CORC® wire and the width of the tapes used. These studies provide a clear and straightforward path to improve conductor flexibility within the proposed Phase II program to reach our goal of minimum bending diameters below 30 mm with minimal I c reduction. We were successful in developing a flexible, 3.3 mm thick, high-J e CORC® wire containing 30 ReBCO tapes that carried > 80 % of its original I c after bending to diameters down to 44 mm. The CORC® wires showed no cyclic degradation due to in-field testing up to 57 stress cycles at 12 T. Another CORC® wire of 3.4 mm thickness had a J e as high as 451 Amm -2 when extrapolated to 20 T at a bending diameter of 63 mm, which is a new high-field performance record. We were also able to design a CORC® wire with increased tensile strength of 240 MPa at 76 K. The results of the Phase I program allow us to propose a very strong Phase II program with a high likelihood of reaching the ultimate goal of CORC® accelerator magnets wires with I c exceeding 8-10 kA and J e exceeding 600 Amm -2 at 20 T while bent to a 30 mm diameter.