A Fully Functional Closed Cycle Cryosystem That Uses Less Than One Watt of Refrigeration at 4.5 K, for a 2.5 GHz Per Channel, 128 X 128 Channel Superconducting Switch

Abstract

Tektronix has designed, built and characterized an extremely low loss cryosystem, incorporating a closed cycle cryocooler, that is capable of providing the thermal and magnetic environment required to operate a niobium Josephson junction based superconducting digital switch with 128 x 128 Channels. The system is designed for seriai data rates of 2.5 Gbps on a total of 1000 individual lines, necessitating a distance of only 15 cm from 300K to 4.5 K. Thermal radiation shields and independent magnetic shields, interleaved with optimized blankets of MLI, in combination with the conductive and dissipative load from the data I/O cables, deliver a 1.6 W heat load to the first stage of the cryocooler at 800 K. The 4.5 K cold fmger incorporâtes a unique mounting stage that supports a ceramic multichip module and performs heat sinking for eight multi-channel I/O cables. The cable feedthroughs are demountable, and are assembled from inside the cryosystem.

The design of the cryosystem is optimized to the performance envelope of a single, closed cycle cryocooler, a Boreas B100. The design of the system is so precise, both thermally and mechanically, that primary data on thermal conductivity, yield strength and ultimate strength for certain critical materials were required to complete the design. State of the art CAD design and analysis tools were used to an extent that the system worked on the first assembly.

Characterization of the system using applied heat loads to mimic the presence of both I/O cables and a functioning MCM shows that the system meets the design goals without reaching the limits of the cryocooler.