Hiroyuki Kamibayashi

The Performance of a Practical Size Epoxy Impregnated Pancake Coil Wound With a Polyimide Electro-Deposited (PIED) YBCO-Coated Conductor

A practical size epoxy impregnated single pancake coil, 30 mm in inner diameter and 73 mm in outer diameter, was fabricated using a polyimide electro-deposited (PIED) YBCO-coated conductor. The voltage-current characteristics achieved at 77 and 4.2 K reached coil critical currents without any degradation of coil performance. We reported similar results for a miniature test double pancake coil operated at 77 K. Because the thickness of the polyimide coating was as thin as 10-20 μm, the fraction of the YBCO-coated conductor in the coil winding was greater than 70%, and therefore high overall current density of greater than 500 A/mm2 was achieved at 4.2 K. The 3-D digital micrograph of the top end surface of the PIED YBCO-coated conductor single pancake coil winding infers that debonding of the epoxy from the polyimide coating reduces cleavage stress on the YBCO-coated conductor and results in no degradation in coil performance. PIED YBCO-coated conductors are reliable, easy to handle, and extensible to longer conductors; they are applicable to dry winding and soft polymer impregnated winding, in addition to epoxy impregnated winding as is widely used in Formvar-coated NbTi conductors used for nuclear magnetic resonance and magnetic resonance imaging, etc.

Use of a Thermal Grid to Increase Thermal Runaway Current for REBCO Pancake Coils Operated at 77 K

During the excitation of REBCO coils, premature thermal runaways occur. In the worst case, the conductor melts down. Premature thermal runaway is caused by partial conductor degradation in the coil winding, which sometimes occurs in coils fabricated from REBCO-coated conductors. In this paper, we have examined the use of a “thermal grid” method using a polyimide electro-deposited conductor and highly thermal conductive metal sheets as a countermeasure against premature thermal runaway. This thermal grid method increases the thermal runaway current for coils with partial conductor degradation. In addition, in the case of an actual thermal runaway, the use of a thermal grid suppresses the peak temperature reached by the coil.