Development of Autocascade Cryocooler

Beam line optics for x-ray facilities such as synchrotrons need to avoid thermal-gradient-driven local ball and crystalline lattice distortions even during intense heating with radiant fluxes in excess of 100 W/mm2. Silicon cooled near 125 K exhibits zero coefficient thermal expansion, and therefore will not exhibit these deformations even under intense heating. While several researchers have demonstrated this effect numerically and experimentally, the means to provide the cooling is largely accomplished with a liquid nitrogen stream generated by a high-cost gas-phase cryocooler (i.e., Stirling, Gifford McMahon, etc.). The DOE wishes to develop a low-cost cryocooler that can provide 250W of cooling at 125 K with a selling price of less than $50,000. Mainstream Engineering Corporation (Mainstream) proposes an autocascade vapor compression system to achieve the required cooling capacity and temperature. The autocascade system uses a refrigerant mixture and a series of heat exchanger stages. Each stage condenses a refrigerant constituent out of the mixture, expands it to a colder temperature, then uses that cooling to condense the next lower volatility refrigerant. By using this approach, conventional HVAC refrigerant components can be used (i.e. compressors, condensers, liquid-vapor phase separators, control valves, etc.). The use of readily available high-volume components drives down the cost relative to gas-cycle cryocooler that use high precision, low-volume components (close tolerance pistons and displacers, regenerators, etc.). During Phase I, high-fidelity models and simulations will be developed to demonstrate the system level operation. Optimization of refrigerant mixture composition as well as control valve behavior will be determined. It is anticipated that a four constituent mixture and four stage autocascade will be required to have 250 W of cooling in the temperature range of 99-125 K. Once the high-fidelity model is created, Mainstream will demonstrate the concept of the autocascade including the controls on a two stage autocascade. Phase I will conclude with the design of the four stage autocascade vapor compression system using the initial model and experimental results. The components for the autocascade vapor compression system are all COTS HVAC components. Mainstream has preliminarily sized this system and has a target selling price of $39,200 including 25% profit. While this cryocooler will primarily be used in synchrotron optics facilities, a secondary use would be to incorporate it as a topping cycle for other cryogenic coolers for high- and low-temperature superconductors, MRIs, and IR sensing devices.