Microwave Performance of Niobium/Kapton Superconducting Flexible Cables

Flexible superconducting Nb microstrip transmission line cables, with lengths of ~5 cm, were fabricated using 2 mil (50.8 μm) thick E-series Kapton films. A 50 nm Ti layer was used to improve the adhesion of sputter-deposited 250-nm-thick Nb signal line and ground plane metallization. The signal traces of the microstrip were directly interfaced with edge-launch SMA connectors and measured up to 14 GHz in liquid He. For comparison, we also fabricated normal conductor (Ti/Cu) microstrip transmission lines with a similar design. The microwave performance of the Nb-based transmission lines indicates significantly lower intrinsic insertion loss (less than ~0.02 dB/cm) compared to the Cu-based microstrip measured at 4.2 K to have a loss of ~0.57 dB/cm.