Low Pressure Ratio Cascaded Joule-Thomson Cryogenic Coolers

This thesis presents the design, fabrication, and characterization of several coldstages intended to investigate the feasibility of a low pressure ratio three stage cascaded Joule-Thomson (JT) based cryogenic cooler. The coldstages investigated use single component refrigerants operating at pressure ratios of less than 0.5:0.1 MPa. Low pressure ratio single component refrigerants offer an order of magnitude higher specific refrigeration capacity but a narrower temperature range of cooling when compared to mixed refrigerants. Cryogenic temperatures can be reached by cascading several self-contained JT refrigeration cycles. A micro-fabricated polyimide based single stage coldstage is built using micro-fabrication techniques developed in previous works. Several processing challenges are identified and recommended solutions and best practices are implemented. Control of the JT restriction is difficult as the polyimide micro-channels expand under high internal pressure. The coldstage is run in a high pressure environment to press the JT restriction into the correct geometry and cools to 280 K. To better control the JT restriction, a glass capillary based coldstage design is identified. The coldstage uses a glass capillary to form the JT restriction. The coldstage cools to 267 K and 283 K when run in a vacuum environment and atmosphere respectively as calculated. The coldstage is integrated with a mini-compressor and intermittently cools to 288 K in atmosphere. The glass capillary design is further developed to make a two stage cascaded coldstage with a Kapton tube-in-tube inter-stage heat exchanger. The coldstage cools to 228 K with a net refrigeration power of 150 mW. A three stage coldstage is demonstrated using the same design. The coldstage cools to 193 K with net refrigeration power of 15 mW. The coldstages presented demonstrate the feasibility of low pressure ratio cascaded JT cryogenic coolers. The efficiencies of the coldstages are low because of large natural heat leak into the systems and inefficiencies of the inter-stage heat exchangers. Future work can improve efficiencies through design optimization of the inter-stage heat exchangers and implementation of more significant thermal insulation. An in-depth review of micro-fabrication techniques based on mechanical design, thermal design, and quality and ease of fabrication is needed for future scalability