Development of a Cold End and High-Efficiency Valves for a One-Watt 10 Kelvin Cryocooler

This work focuses on the development and principal analysis of the cold end for a prototype cryogenic machine, a cryocooler, capable of demonstrating the feasibility and implementation of a miniaturized, multistage Collins cycle. This machine represents a proof-of-concept test rig used to unite several pre-existing technologies into a single device. The machine was used to validate analytical thermodynamic results as well as to work through unanticipated challenges arising from integration of various independent technologies into a single system. The machine developed in part through this work is the precursor to a modular, three-stage cyrocooler capable of achieving 1 watt of cooling at 10 Kelvin. The successful three-stage machine can achieve two-fold increase in efficiency over existing cryocoolers capable of sustained cooling at 10 Kelvin.

The availability of a small machine capable of sustained cooling at 10 Kelvin enables many technologies reliant upon cheap cryogenic working fluid. Current applications include chilling military sensory equipment and enabling better resolution for satellite-based observation. However, foreseeable applications include cheap, low-temperature superconductors as well as desktop-sized supercomputers. Many classes of small cryocoolers are currently in use today. These systems include pulse tubes, stirling machines, and Gifford-McHahon cycles. However, each of these cryocooler types is either prohibitively wasteful with respect to energy usage or simply impractical for substantial cooling applications at 10 Kelvin.

The motivation for compacting an industrial-scale machine into a device a few cubic feet in volume is threefold. First, it is desirable to capture the positive attributes of the Collins cycle in a device small enough to be dedicated to a single electronics package or capable of being plugged into a standard wall outlet. Second, a miniaturized Collins cycle is competitive with existing small cyrocoolers used in military and space-based applications. Third, shrinking the system’s size allows incorporation of several technologies that would be impractical in a larger machine. These technologies include the electromechanical valves upon which this work focuses.

The Collins cycle is an industrial-scale helium liquefaction process usually carried out by large machines. As with any cryogenic machine, it is prudent to thermally isolate the working components of the cold end to mitigate thermal leaks, which detract from the cycle’s cooling power. In conventional helium liquefaction machines, thermal isolation is achieved by driving all mechanical components via cams connected to long, thin linkages. These long linkages retard heat transfer from room temperature to the cold end, but they necessitate a machine of large size.

The primary means of miniaturizing the cycle is removing the various cams and linkages operating the valves and piston expander in favor of other driving mechanisms. In fact, the mechanical complexities associated with conventional actuation mechanisms make them impossible to implement as the size of the system drops. To operate the scaled-down cryocooler, alternate mechanisms were developed in this project.

This project focused on the cold end and the expander, which contain all of the hardware to carry out the adiabatic expansion of the working fluid to drop its temperature. This process is the coldest part of the cycle, and all of the mechanisms had to operate through a huge temperature range from room temperature down to the coldest operating temperature of the apparatus.

Directing the flow of working fluid through the expander at the cold end is a pair of cold valves of novel electromagnetic design. These valves are actuated by a control current generating a magnetic field that attracts the valve disk away from the valve ports. This action allows the valves to be mechanically opened and controlled very precisely by a pulse of electrical current. When the control current is not on, the cold valves act as check valves. They are held closed by the gas pressure difference across the valve ports and by a permanent magnet acting as a valve spring.