The objective of the project is to demonstrate that 3 mW of net refrigeration can be achieved at 77 K using less than 500 mW of input power. A schematic of the overall microcryocooler and the high temperature superconducting terahertz detector is shown in Figure 1. In scaling to very small sizes, scaling laws indicate thermal radiation and heat conduction can become serious problems. In order to show that proper scaling could be achieved, the major goal of the first 18 months (phase I) of the program was to demonstrate that sufficient thermal isolation at 77 K could be achieved. An experiment was devised using the same geometry of the heat exchanger and cold tip that showed we could achieve a thermal isolation at 77 K of greater than 40 000 K/W. The result showed that the cooler could be designed for a background heat leak from 300 K to 77 K of less than 6 mW. The use of an optimized mixed refrigerant with five components was calculated to provide sufficient refrigeration using the Joule-Thomson process with a pressure ratio of 16 to 1 and with a flow rate about ten times less than that using pure nitrogen gas. The low flow rate leads to a ten times smaller compressor and ten times less input power compared with a similar process using pure nitrogen. The lower flow also leads to a smaller heat exchanger and a significant reduction in radiation and conduction to the cold tip.
During phase II of the program, we have demonstrated that a temperature of 77 K could be achieved, but temperature oscillations occur below about 140 K, probably due to freezing of water vapor contaminants in the gas mixture in the 700 nm gap for the flow impedance in the cold tip. Figure 2 shows a photograph of the heat exchanger and cold tip. The heat exchanger consists of six 75 µm inside diameter glass capillaries enclosed by a larger glass capillary about 0.6 mm in diameter.
End Date:October 2010
Lead Organizational Unit:mml
University of Colorado
NIST/Boulder Clean Room, University of Colorado Clean Room, Cryogenic Technologies Vacuum and Cryogenic Facilities.
Ray Radebaugh (Thermophysical Properties Division)