Second Generation Superconducting Cable with Exfoliated YBCO Filaments

High temperature superconducting wire (HTS) technology has a significant potential to become the wire of choice for helium‐free, ultra‐high field magnet systems. High‐field, helium‐free magnet systems are attracting more interest as the world supply of helium is depleted and new science facilities are demanding magnetic field levels much higher than accessible by niobium‐based superconductors. Currently, HTS wire is manufactured as a thin, 1 micron‐meter, superconducting layer deposited on a 1 cm wide 100 micron‐meter thick metal substrate. Due to this geometry, a magnet wound from the wire suffers from high losses. Additionally, the mechanically weak interface between the superconductor layer and the ceramic buffer is known to be the most common source of HTS magnet failure. Brookhaven Technology Group (BTG) will develop and demonstrate the feasibility of a new type of cable bundled from exfoliated HTS filaments. The approach is based on successful Phase I experiments on stress‐induced exfoliation of HTS filaments. The technology eliminates the HTS‐buffer interface thus enabling mechanically strong narrow filaments. In Phase I, the filaments were demonstrated to retain up to 90% of the original critical current and possessed tensile strength in excess of 500 MPa. In Phase II, we will develop technology for bundling and twisting the filaments into a narrow, 1‐2 mm diameter cable in a reel‐to‐reel mode. This geometry will deliver significantly improved quench stability of the cable and enhance the traditionally low tensile strength of the conductor. The cabling system will include an exfoliation unit, a laser slicing attachment and a bundling‐twisting machine. The cable will be tested both at 77 K and at 20 K, 8 Tesla field. The mechanical tests will be performed at National High Field Magnetic Laboratory. After the cable test, a solenoid coil will be wound using a bundling method that reduces AC loss of the coil by a factor of five as compared with a coil wound using 10 mm wide YBCO wire. The AC loss reduction will be confirmed by direct measurements of the loss by use of the rotating calorimeter facility at Wright Patterson Air Base Laboratory. Our cable technology will provide a pathway for efficient and economical high‐temperature superconducting magnets operating > 20 K. Such a magnet could be cooled by conduction, thus not requiring a supply of helium. High temperature operation ensures that the materials comprising the magnet have approximately 50 times more specific heat than at 4.2 K. This makes the high‐temperature magnet quench‐resistant. Quench‐resistant magnets can be ramped faster and they are more efficient and reliable. These features are very appealing for many applications outside of particle physics facilities. For example, there is a large market for MRI machines in countries with undeveloped helium re‐fill infrastructure. Keywords: YBCO wire, superconducting magnets, MRI machines.