Actively Cooled Current Leads for Superconducting Electronics Using Mixed-Gas Joule-Thomson Refrigeration

Abstract

Electrical leads used for the supply of current to superconducting magnets and electronics must span the temperature range from room temperature to cryogenic temperatures. Because the conventional materials used for such purposes (e.g., copper and aluminum) have both a finite electrical resistance and a significant thermal conductivity, operation of the leads results in both thermal generation and conductance. The resulting thermal loads must be removed from the cryogenic environment. This paper describes a method for integrating cryogenic refrigeration technology with current leads in an efficient and practical manner. The key to this concept is the use of a mixed-gas cooling cycle that absorbs the distributed refrigeration load continuously over the temperature range that it is generated, as opposed to allowing it to pass down to the cold end of the lead where the same energy flow constitutes a much higher entropy load on the cryocooler. Additional benefits of this technology include a more isothermal electronic package, as well as improvements in reliability, and reduction in size and mass. Mixed-gas working fluids can be used within Joule-Thomson devices to achieve a greater refrigeration effect for the same pressure span than is possible with a pure substance. This paper describes a computational tool that allows the composition of gas mixtures to be optimized for the case where the refrigeration load is not completely concentrated at the cold end, as is typically the case, but rather the refrigeration load is distributed over the entire temperature range. A genetic optimization algorithm was found to be the most robust and reliable technique for identifying optimum gas mixture composition. The thermodynamic advantage associated with accepting the refrigeration load at the temperature of its origin, rather than at the cold end, is quantified.