Oxford Pulse Tube Incorporating COaxial Regenerator (OPTICOR)

Pulse tube coolers are small low temperature refrigerators that can provide cooling for electronic devices such as infra-red detectors, superconducting devices and gamma ray detection in homeland security systems. Temperatures as low as 10 K can be obtained with a single stage helium-filled pulse tube but 30 K would be more typical, as lower temperatures would use a multi-stage approach.

Pulse tubes can be thought of as a Stirling cycle cooler that relies on a gas column to act as a displacer; http://en.wikipedia.org/wiki/Pulse_tube_refrigerator. A reciprocating piston (the compressor) drives a flow through a regenerator into a tube (the 'pulse tube'); heat is rejected at the interface between the compressor and the regenerator, and heat is absorbed at the other end of the regenerator. The performance of the pulse tube can be improved greatly by connecting the pulse tube to a buffer volume through an orifice or inertance tube. As this can very bulky and have limited possibilities for control, there are advantages in having a warm-end expander (in effect an oscillating piston). This can be designed to respond to the pressure variation generated by the compressor, with a phase lag and amplitude determined by the system dynamics.

The pulsating flow can be produced by a system of valves with a high pressure gas supply, but this approach can only operate at low frequencies (a few Hz) due to the operation of the valves. In most recent applications a reciprocating piston is used which can be either mechanically or electromagnetically driven, and these typically operate at between 30 and 100 Hz. As the gas needs to be free of oil and other contaminants (to avoid fouling of the regenerator and cold-end heat exchanger), then it is sensible to use an 'Oxford-style' compressor. The Oxford-style compressor was developed over 30 years ago for a Stirling cycle cooler and its key features are:

* a spring suspension system that is radially stiff to provide accurate linear motion of a piston in a cylinder,
* a small radial clearance between the piston and cylinder (of order 8 micron) so there is negligible leakage and no contact (so no wear).
* an electromagnetic drive (originally like the voice-coil of a loudspeaker).

More recently compressors with moving magnets have been developed at Oxford. A crucial step was the design of a compressor in EP/E036899/1 'Development of a Miniature Refrigeration System for Electronics Cooling' that used a moving magnet with a stationary drive coil. This leads to a cheaper, low moving mass system that can operate at high frequency - this not only increases the specific output, but also leads to lower seal leakage losses and resistive losses in the drive coils. The work to be undertaken in this project will use a design that has been developed from the refrigeration compressor.

The simplest pulse tubes have a linear configuration (compressor, hot-end heat exchanger, regenerator, cold-end heat exchanger, 'pulse tube' volume). But these have the disadvantage of the cold-end being in the middle so a 'U' tube arrangement is used. However, there are flow losses associated with the 'U' bend that can be eliminated by a novel radial flow and concentric tube arrangement. This will be combined with a warm-end expander, and a single dynamic balancer to provide perfect balance of the pulse tube.

Stirling cycle coolers, in general, have a better performance than pulse tubes, but pulse tubes are simpler and better suited to higher frequency operation. The latest moving magnet compressor motor is capable of high frequency operation. Furthermore, high frequency operation leads to lower compressor clearance seal leakage losses, since this power loss and the Ohmic power loss are essentially independent of frequency. Therefore a pulse tube operating at high frequency (say 90 Hz) will be more efficient than at lower frequencies and be more compact than a Stirling cycle cooler.

Who will benefit from this research?

Cryocoolers are widely used on equipment such as infra-red detectors, superconducting devices and gamma ray detection in homeland security devices. Medical applications are also increasing, notably the cryopreservation of tissue samples, stem cells, embryos and spermatozoa. For example, Cambridge-based Asymptote manufacture a Stirling cycle based low cost, portable liquid nitrogen-free cryocooler http://asymptote.co.uk/EF600.php that has an operating cost that is 1% of liquid nitrogen based systems http://www.grantinstruments.com/ef600m-controlled-rate-freezer/. Cryosurgery or cryotherapy is another important medical application of cryogenics, but the rate of cooling is better matched to liquid cryogens. This would require running the pulse tube to liquefy air and the need to remove other species such as carbon dioxide and water vapour.


How will they benefit from this research?

This project would showcase a new design of clearance seal compressor with a moving magnet motor. Since developed in Oxford around 1980 the clearance seal concept has been widely copied, but the 3rd generation of moving coil compressor (a collaboration between Engineering Science, Hymatic and NGAS dating from the 1990s) has led to a compressor design with high efficiency and the greatest specific output). However, the moving magnet compressor to be used here allows operation at higher frequencies, which is of particular benefit to pulse tubes, as it increases the specific output.

Pulse tubes have notable advantages over 'traditional' Stirling cooler cold heads:
1) manufacturing costs are significantly lower due to reduced part counts and the removal of tight tolerances between running components
2) Pulse tube design lends itself to long life maintenance free applications due to the lack of moving components.
3) On axis vibration is greatly reduced as a consequence of not having the moving components (moving mass) of a Stirling cold head. This means that the technology is pertinent to applications where exported vibration may be an issue such as gamma ray detection and Infra-Red detection applications.

These advantages present the opportunity for lower cost devices that are easier to manufacture especially when quantities become more significant. Future potential applications such as Square Kilometre Array (SKA) require antennas to be cooled to sub 50 K, and the solution has been traditionally pointed towards Gifford McMahon (GM) cooling. However, GM is a technology that uses mechanical valves, is inefficient and requires routine maintenance. SKA is an application where the many thousands of Antennas demands a technology that has low to zero maintenance and higher efficiencies such as Pulse Tube cooling.

This project will facilitate the introduction to the UK of Pulse Tube manufacture, by organisations such as RAL and companies such as Hymatic. Licensing of any IP would be by ISIS Innovation, who already license technology and know-how generated by the Oxford Cryogenics Engineering Group, notably to Honeywell Hymatic and NGAS.