Low Temperature Regenerator and Pulse Tube Losses

All-digital radio communication between the various forces of the U.S. military is required to improve communications to the level needed in the near future. Such communication requires analog-to-digital converters of speeds only possible with low temperature superconducting electronics. The replacement of existing rack-mounted rf electronic communication equipment with superconducting digital equipment requires the development of compact 4 K cryocoolers that can provide about 0.1 W of refrigeration at 4 K with much less than 1 kW of input power. Existing 4 K cryocoolers require at least 1 kW of input power and are too large and heavy for military operation. We have determined that major sources of entropy generation in such cryocoolers are those in the 4 K regenerative heat exchanger (regenerator) and the 4 K pulse tube component. The real-gas effect in the ⁴He working fluid is also a major source of loss in these 4 K cryocoolers. The objective of this program is to determine how to increase the efficiency of the 4 K stage by a factor of about four while still being able to operate at a relatively high frequency of 30 Hz so that the compressor can be made compact. 

Goals of the program are (a) to develop accurate equations for transport properties for ³He from 4 K to 300 K and to high pressures (at least 2.5 MPa), (b) incorporate these ³He equations along with equation of state for ³He in a new numerical computer code (REGEN3.3) that models the behavior of regenerators down to 4 K using either ³He or ⁴He, (c) use the model to determine optimum operating conditions and regenerator materials, (d) develop an apparatus that can separate and measure various losses in 4 K regenerators and pulse tubes, (e) measure losses in 4 K regenerators and pulse tubes and compare with model predictions, and (f) modify the model where appropriate.

Activities to date in the program are the development of the equations for the viscosity, thermal conductivity, and surface tension for ³He from 0.003 K to 1000 K and for pressures from 0 to 20 MPa. A Debye equation of state for similar temperatures and pressures was developed at Zhejiang University in China, and in collaboration with them both the equation of state and the transport equations were incorporated into the new NIST software known as REGEN3.3. This software follows from early NIST software for modeling the behavior of regenerators, but incorporates the ³He properties as well as those of ⁴He and includes improved numerical procedures to decrease the running time by a factor of ten or more (minutes instead of hours). The software was made publicly available in March 2008 on the NIST website. The new software was used to find the optimum operating conditions for both ³He and ⁴He working gases as well as the optimum regenerator materials. Figure 1 shows the 2nd law efficiency of the 4 K regenerator with various regenerator materials using ³He and ⁴He working gases. A factor of four to five efficiency improvement is indicated when using ³He and the optimum material combination. Figure 2 shows the apparatus constructed to measure losses in regenerators and pulse tubes at temperatures down to 4 K.