Cryocoolers for Space Applications Research

A brief introduction to coolers used in space

Passive coolers are those which require no input power and there are two types in common use:

Passive coolers have been used for many years in space science applications due to their relatively high reliability and low vibration levels, however these are now joined by coolers requiring input power, so called `active' devices (also termed 'cryocoolers') Active coolers use closed thermodynamic cycles to transport heat up a temperature gradient to achieve lower cold-end temperatures at the cost of electrical input power. The first long-life active cooler system successfully operated in space was a cluster of four rhombic-drive, grease lubricated Stirling cycle machines launched in 1978 aboard DOD 1-78-1 developed by Phillips and cooling two gamma ray detectors. Although these showed significant performance degredation on-orbit they operated sufficiently well to keep the payload operating until it was destroyed during a successful test of an anti-satellite interceptor in 1985. Since this first generation the flexibility and reliability of active coolers have proved major contributors to the success of many missions. The major types of active cooler are:

1. Stirling cycle. These coolers are based on causing a working gas to undergo a Stirling cycle which consists of 2 constant volume processes and two isothermal processes. Devices consist of a compressor pump and a displacer unit with a regenerative heat exchanger, known as a `regenerator'. Stirling cycle coolers were the first active cooler to be used successfully in space and have proved to be reliable and efficient. Recent years have seen the development of two-stage devices which extend the lower temperature range from 60-80K to 15-30K.
2. Pulse tube. Pulse tube coolers are similar to the Stirling cycle coolers although the thermodynamic processes are quite different. They consist of a compressor and a fixed regenerator. Since there are no moving parts at the cold-end reliability is theoretically higher than Stirling cycle machines. Efficiencies approaching Stirling cycle coolers can be achieved and several recent missions have demonstrated their usefulness in space.
3. Joule-Thompson. These coolers work using the well known Joule-Thomson (Joule-Kelvin), effect. A gas is forced through a thermally isolated porous plug or throttle valve by a mechanical compressor unit leading to isenthalpic cooling. Although this is an irreversible process, with correspondingly low efficiency, J-T coolers are simple, reliable, and have low electrical and mechanical noise levels. A J-T stage driven by a valved linear compressor, similar to those used for Stirling cycle and Pulse Tube coolers, will be flown on the planned Planck telescope mission (expected to be launched in 2007).
4. Sorption. Sorption coolers are essentially J-T coolers which use a thermo-chemical process to provide gas compression with no moving parts. Powdered sorbent materials (e.g. metal hydrides), are electrically heated and cooled to pressurize, circulate, and adsorb a working fluid such as hydrogen. Efficiency is low but may be increased by the use of mixed working gases. Demonstration models have already been flown and they are expected to useful in long-life missions where very low vibration levels are required, such as the planned Darwin mission to image the atmospheres of extra-solar planets.
5. Reverse Brayton. Reverse/Turbo Brayton coolers have high efficiencies and are practically vibration free. Coolers consist of a rotary compressor, a rotary turbo-alternator (expander), and a counterflow heat exchanger (as opposed to the regenerator found in Stirling or Pulse Tube coolers). The compressor and expander use high-speed miniature turbines on gas bearings and small machines are thus very difficult to build. They are primarily useful for low temperature experiments (less than 10K), where a large machine is inevitable or for large capacity devices at higher temperatures (although these requirements are quite rare). A recent application of this class of cooler was the Creare device used to recover the NICMOS instrument on the Hubble Space Telescope.
6. Adiabatic Demagnetization. Adiabatic Demagnetization Refrigeration (ADR), has been used on the ground for many years to achieve milli-Kelvin temperatures after a first stage cooling process. The process utilizes the magneto-caloric effect with a paramagnetic salt. These coolers are currently under development for space use.
7. 3He coolers. In addition to its use as a stored cryogen the properties of can be used to achieve temperatures below 1K with closed cycle "Sorption coolers" (above 250mK), and dilution refrigerators (above 50mK). The former are scheduled for use in the SPIRE and PACS instruments aboard the Herchel satellite whereas the latter will be used on the Planck mission.
8. Optical cooling. In recent years the principle of optical cooling has been developed and demonstrated. The principle of anti-Stokes fluorescence in Ytterbium doped Zirconium Fluoride is used to provide vibration-free solid-state cooling. The principles of this technique are being developed and it will be many years before they are ready for space applications.
9. Peltier effect coolers. Solid-state Peltier coolers, or Thermo-Electric Converters (TECs), are routinely used in space to achieve temperatures above 170K (e.g. the freezers aboard the Interational Space Station). These devices work on the same principle as the Seebeck effect, but in reverse: the creation of a temperature difference between two dissimilar metals by application of a current.

Advantages/disadvantages of different types of cooler technology

Usually the coolers can be run at lower or higher temperatures with correspondingly lower or high heat lifts.

TABLE

Some examples of missions using the above coolers

TABLE

• Missions are listed as vehicle/instrument.
• Design lifetime has been quoted if the instrument is yet to be launched or failed due to another component.
• Excluding electronics.
• STS/BETSE was a technology demonstrator.
• UARS/ISAMS figures per cooler running at 83% stroke.

Stirling cycle coolers at the University of Oxford

In 1978 the Department of Engineering Science was awarded a contract to construct a small, long life Stirling cycle cryocooler for cooling an infrared instrument called ISAMS (Improved Stratospheric and Mesospheric Sounder), aboard the UARS (Upper Atmosphere Research) platform. These coolers were jointly developed by groups in the Physics and Engineering Departments at Oxford and at the Rutherford Appleton Laboratory. The two devices were designed to provide 1 W of total focal plane and detector cooling. The instrument was launched in September 1991 aboard the Space Shuttle and operated for 180 days before the instrument failed due to a chopper problem. During this time the cooler worked to specifications and was a powerful demonstration of the usefulness of long-life cryocoolers in space.

The ISAMS cooler was the result of extensive development over 10 years by the three groups involved and incorporated several important design features to prevent the failure and performance degradation commonly associated with other mechanical coolers and the first generation devices mentioned above.

• Split design - The cooler consisted of compressor and displacer units connected with a gas transfer pipe. This simplified the testing and integration of the units.
• Clearance seals - To maintain the compression pressure the compressor and displacer units must have good seals. Space-based coolers cannot use fluid bearings due to contamination issues and contact bearings have a limited lifetime. High tolerance clearance seals were therefore developed using low friction organic plastics.
• Spiral flexures - These are flat springs with high radial stiffness allowing maintenance of the close tolerances required for the clearance seals.
• Linear motor - Essentially a loud-speaker type motor, this allowed the cooler to avoid the wear and vibration problems associated with rotary drives by minimizing side loads.