High-field Second Generation Cable with Sub-Millimeter Filaments
In this Phase I SBIR Brookhaven Technology Group (BTG) proved feasibility and de-risked a novel technology for reel-to-reel manufacturing of mm-wide exfoliated YBCO filaments. Currently, second generation (2G) wire is the most promising conductor for high-field magnets such as accelerator dipoles and compact fusion devices and for military power distribution systems. The key element of the wire is a thin HTS layer deposited on a flexible substrate. The thick and expensive substrate, which becomes incorporated in the 2G conductor, increases cost and reduces electrical and mechanical performance of the wire, thus preventing the 2G wire technology from reaching larger markets. BTG exfoliation technology eliminates these shortcoming by exfoliating the YBa2Cu3O7 (YBCO) layer from the substrate. However, in order to reduce the magnetization loss of the cable, the filaments need to be sliced in mm-wide filaments. In Phase I, BTG explored the slicing methods that prevent the edge damage and maintain the critical current density at a level > 90% as the filament width is reduced from 10 mm to 1 mm. During the Phase I effort we manufactured and tested short a focal length head for a 10.9 μm CO2 laser and performed test cuts. Also, we explored filament slicing using recently developed CW and pulsed fiber lasers operating at 1.06 μm wavelengths. The experiments were performed at the SPI laser lab in San Jose, CA and IPG Photonics lab, Marlboro, MA. Condition of the superconducting layer was tested using magneto-optic imaging at the National High Magnetic Field Laboratory. Additionally, the filaments were extensively characterized at the BTG facility, Stony Brook, NY. Magneto-optic imaging and scanning electron microscopy were used to identify the slicing defects that are responsible for the critical current density reduction in mm-wide filaments. The defects develop as a result of the filament overheating by the laser beam. The excessive heat melts the solder attaching the YBCO layer to the metal foil, thus nucleating edge cracks in the YBCO layer. We show that pulsed fiber laser system deposits the minimum amount of thermal energy and does not produce detectable edge defects in the exfoliated filament. However, the laser beam needs to be focused on the tape surface within 100 µm in order to minimize the edge damage. The critical current measurements of the filaments sliced by the pulsed laser indicate the edge damage is below 50 µm, compared with 200 µm for the CW systems. We manufactured up to 10 meters of 1 mm wide exfoliated filament using a laser slicing system. The filaments were coated with a 10 µm layer of Sn-Pb solder in order to protect the silver coating and a provide high-conductance connection between filaments in a cable. The coating temperature and coating duration were optimized to eliminate the effect of the coating on the critical current. We modified the cabling machine for handling 1 mm wide filaments. We manufactured 8 approximately 1.5 meter lengths of 1 mm wide cable, comprised of 4 and 8 filament layers. The cable demonstrated critical current density of 120 A at 77 K for a 4 filament cable and 230 A for a 8 filament cable. The allowable twist was measured at 40 mm pitch.
- Research Organization:
- Brookhaven Technology Group Inc., Stony Brook, NY (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), High Energy Physics (HEP)
- DOE Contract Number:
- SC0017797
- OSTI ID:
- 1432717
- Type / Phase:
- SBIR (Phase I)
- Report Number(s):
- DOE-BTG-SC0017797
- Country of Publication:
- United States
- Language:
- English
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