Performance Test of 20-Stacked Two-Tapes-Bundled REBCO Pancake Coils for Upgrading of 25-T Cryogen-Free Superconducting Magnet to 30 T

For next generation of high field cryogen-free superconducting user magnets, we adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration, a fluorine-coated polyimide tape as the inter-bundle insulation and thin FRP plates glued on the coil edges. To validate our coil design, we investigated electromagnetic and current-voltage properties of 20-stacked REBCO pancake coils wound with Fujikura EuBCO tapes with the robust coil concept at 4.2 K under a background field of 14 T. The inner and outer diameters and total height of the coils are 68 mm, 266 mm, and 101 mm, respectively. The average of measured hoop and axial strain on the outermost winding at the mid plane of the 20-stacked coils were 0.23% and −0.15% respectively at a hoop stress of 370 MPa estimated by the
BJR
relation. The
I
-
V
properties of the coils showed no anomalous behavior up to 25 T totally generated by the coils and background field, suggesting that our coil structure has an advantage against large electromagnetic stress.