STAR® Round Superconducting Wires
STAR® – Symmetric Tape Round – superconductor wires offer unparalleled electromechanical performance for many high magnetic field and high power applications. STAR® wires are the only superconductor wires available today with a diameter of just 1 – 2 mm, bend radius capability of 15 mm, and high critical currents over a wide range of temperatures from 4.2 K to 77 K. Other round superconductor wires cannot be bent to such small diameters or cannot be used at temperatures much higher than 4.2 K.
The enabling technology of STAR® wires is a novel symmetric superconductor tape technology developed by AMPeers and its partner, University of Houston. In a symmetric tape architecture, the RE-Ba-Cu-O (REBCO, RE = rare earth) superconductor film is positioned near the neutral plane by an optimum combination of substrate and copper stabilizer thickness. Additionally, the substrate – typically a high-strength alloy such as Hastelloy – is only 18 – 20 µm thick which provides excellent flexibility. Symmetric REBCO tapes can be bent to 0.8 mm in diameter with greater than 95% critical current retention, whereas standard REBCO tapes degrade even below 6 mm bend diameter.
STAR® wires exhibit excellent performance in high magnetic fields even when bent to a radius of 15 mm. STAR® wires of 1.3 to 2 mm in diameter have been fabricated by winding multiple symmetric REBCO tapes helically on 0.51-1.02 mm diameter copper former. An 1.3 mm diameter STAR® wire exhibited an engineering current density (Je) of 586 A/mm2 at 20 T. Another STAR® wire of 2.04 mm diameter made with 12 strands of symmetric tape exhibited a critical current (Ic) of 1396 A at 4.2 K, 24 T. Based on its alpha value of 0.77, a Ic of 1615 A is projected at 4.2K at 20 T.
AMPeers has demonstrated STAR wires with record-high Je of ~ 600 A/mm2 at 4.2 K, 30 T corresponding to Je of > 1,000 A/mm2 at 4.2 K, 20 T using high current REBCO tape strands made using Advanced Metal Organic Chemical Vapor Deposition (MOCVD). Advanced MOCVD tapes exhibit more than 3x the critical current of commercial-grade REBCO tapes.
AMPeers has scaled up symmetric REBCO tapes with 18 – 20 µm thick substrate to 100 meters and STAR® wires to over 60 meters. An 1.84 mm diameter, 61.5 meter STAR® wire has been demonstrated with a Ic of 370A at 77 K. A 23 m long, 1.95 mm diameter STAR® wire has been produced with a Ic of 482 A at 77 K.
STAR® wires are now being tested for use in ultra-high field accelerator magnets for High Energy Physics and other applications that require magnetic fields beyond 20 Tesla or temperatures above 4.2 K. Such high magnetic fields or higher temperatures are outside the realm of capability of present superconductor wire technology based on Nb-Ti and Nb3Sn. Because of their isotropic mechanical properties and capability to be bent to 15 mm radius, STAR® wires can enable complex magnet geometries such as canted cosine theta (CCT). A CCT magnet made with STAR® wires with a bend radius of 15 mm at the pole region showed no wire degradation after winding, confirming the robustness of STAR® wires. The magnet reached 78% of the short-sample prediction at 4.2 K at a peak current of 8.9 kA.Since STAR® wires are comprised of transposed tapes, they are also beneficial for applications requiring low AC losses.
Multi-strand, compact transposed STAR® cables have been demonstrated with 6 STAR® wires wound around a copper core. AMPeers and Lawrence Berkeley National Laboratory (LBNL) successfully demonstrated a 6-around-1 cable made of six STAR® wires and one Cu core with a twist pitch of 52 mm. The nominal cable diameter is 5.8 mm. STAR cables have been bent to a radius of 45 mm with less than 5% change in critical current. STAR® cables have been wound on to CCT dipole magnets with no degradation in critical current (1,462 A @ 77 K, self-field) and n value after winding.