Researchers from Italy and France have proposed a new design for a superconducting refrigerator that uses a series of steps to more effectively cool objects down to temperatures near absolute zero.
Conventional superconducting refrigerators rely a array of superconductors (S), normal metals (N) and tunnel barriers (I) that are arranged in a symmetric configuration (SINIS or S2IS1IS2) to super-cool devices at both the microscopic and macroscopic scale. Cooling is achieved through the application of a voltage bias to the superconductors, which causes hot quasiparticles in the normal metal to move through the tunnel barriers to the superconductors, effectively cooling the normal metal. A similar phenomenon occurs in the case of a S2IS1IS2 configuration, albeit the hot quasiparticles jump from S1 to S2.
In comparison, the design for a multistage superconducting refrigerator, or “cascade cooler,” from M. Camarasa-G ́omez et al. published in the journal Applied Physics Letters adds an additional tunnel junction on each side (S2IS1INIS1IS2) of the SINIS design. In this case, when a voltage bias is applied to the S2, the hot quasiparticles tunnel from the normal metal to the S1 superconductors and then to the S2 superconductors, removing additional heat compared to the SINIS design. This multistage operation allows the refrigerator to easily cool a normal metal from a bath temperature of 0.5 K to 100 mK.
“[This is] a novel kind of electronic cooler based on hybrid superconducting tunnel junctions. A cascade geometry allows to cool a first superconducting stage, which is used as a local thermal bath in a second stage,” the researchers explain in their paper. “The correct operation of the device strongly depends on the matching between the resistances of the two kinds of tunnel junctions.” Furthermore, performance could be improved through the “decoupling of local phonon population from the thermal bath in a suspended metal geometry.”
The cascade cooling method could be easily implemented in a practical device, the researchers added, using a set of two additional junctions.
As electronic device temperatures continue to soar, designers are experimenting with more advanced cooling methods such as cascade cooling. Earlier this year, scientists at GE developed a refrigeration system that uses magnets arranged in a series of steps to cool a water-based fluid.
While progress was initially slow—it took the team five years to achieve cooling of just 2 degrees Fahrenheit—the research was expedited by the GE team’s creation of a new type of nickel-manganese alloy for magnets that could function at room temperatures.
The company hopes this cascading magnet technique could replace the traditional compressor and chemical refrigerant used in commercial refrigerators by the end of the decade.
