The advent of second generation (2G) YBa2Cu3O7 (YBCO) wire technology has spawned impressive technological progress since the first meter of a 2G wire was manufactured in 1995. Further developments in the field have been driven by existing and emerging applications, such as fault current limiters, transformers, and wind turbines . The 2G wires have the record high upper critical field and critical temperature, potentially enabling design of high-temperature superconducting magnets, which could be cooled with inexpensive single-stage cryo-coolers.
The core of the 2G wire technology can be described as a thin, <2 micron, YBCO layer deposited on a 50-100 micron thick metal substrate. In order to protect the substrate from oxidation a 30-40 nm, oxide stack is deposited on the substrate. The following figure shows the sequence of epitaxial layers in a RABiTS-based 2G wire (AMSC product) and the right panel shows the actual cross-section of an AMSC 344 conductor.
There are several problems with this architecture as far as magnet applications are concerned:
- High aspect ratio, > 1:1000, is the source of high magnetization (AC) losses, which can be as high as 10’s of J/m of wire.
- The superconducting layer is insufficiently stabilized because only the top stabilizer layer is in good electrical contact with the superconductor.
- Highly anisotropic mechanical properties. 2G wires are exceptionally strong in the direction along the tape, the tensile axial yield strength is approximately 500-600 MPa, however the c-axis pull (transverse) strength is > 10 times lower and the cleavage strength is almost negligible, < 1 MPa.
- The HTS tape allows only for pancake-type winding, which involves labor-intensive diagonal splicing.
- The geometry does not allow for a simple splicing of a multi-strand cable.