As cryogenic cooling is essential for achieving optimal performance in infrared (IR) detectors, it plays a critical role in enabling high sensitivity, low noise, and fast response times. Joule-Thomson (JT) cryocoolers, particularly those developed by CryoR, offer unique advantages for these demanding applications combining compact cold head size, and ultra-low vibration to meet the stringent requirements of modern tactical systems.
Many advanced IR detectors, particularly those operating in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) ranges, require cooling to cryogenic temperatures, often below 100 K, sometimes to higher temperature (around 150 K), and in some cases even down to a few Kelvins. Materials like Mercury Cadmium Telluride (MCT/HgCdTe) and Indium Antimonide (InSb) are prime examples of detector types that benefit immensely from cryogenic cooling.
Challenges in Cooling IR detectors
Obtaining the required cooling temperature and temperature stability, normally of a few tenths of Kelvins.
High heat flux dissipation is required, to absorb heat from miniature devices.
high efficiency allows low power consumption, which is crucial especially for mobile applications.
Tactical coolers must operate at various challenging conditions, where the most challenging is the ambient temperature, normally -40 ¸ 71 °C.
Avoiding the emission of vibrations (acoustic noise) and heat at the cold head.
Miniaturization is always beneficial, and sometimes mandatory.
Tactical cryocooler are located at the most precious part on the system and a failure has dramatic consequences.
Always important.
A comparison of the four common cooling technologies for IR detectors
| Cooling Technology | Stirling | Pulse Tube | Open cycle Joule-Thomson | Closed cycle Joule-Thomson |
|---|---|---|---|---|
| Cold Temperature | 20/150 K | 3/150 K | 4, 20, 27, 77, 87, 90, 111, 120, 170, 184 K, … | 67/200 K |
| Cooling power | < 105 W | < 104 W | Unlimited | < 106 W |
| Efficiency | High | Moderate | Low | Moderate |
| Vibration emission | Cold head: yes Compressor: yes | Cold head: no Compressor: yes | Cold head: no Ref. reservoir: no | Cold head: no Compressor: yes |
| Low SWaP | Low | Moderate | Low | Moderate |
| Reliability and availability | Low | Moderate | High | High |
| Cost | High | High | Low | Moderate |
Several cooling technologies are employed for IR detectors, each with its own set of advantages and limitations
These mechanical cryocoolers operate by a continuous flow of a pressurized refrigerant that enters the cold head, expends inside the cold head and leaves it at a low pressure. The cold head does not have moving parts and does not emit heat, therefore, has minimal negative interference on the IR detector. Moreover, its simple structure provides the smallest cold head among all other technologies. The thermodynamic cycle is inherently irreversible, meaning, consumes more power than the ideal option. Joule-Thomson coolers are divided into two main groups:
these coolers operate with high pressure refrigerants, usually pure gases (nitrogen, argon, and others) at a few hundreds of bars (normally 200 – 700 bar). Usually, it is inefficient to recompress the low-pressure refrigerant that leaves the cold head, on board, and these coolers are fed from a pressure vessel that provides pressurized refrigerant for a limited time. These coolers allow exceptional fast cooldown of a few seconds only, where all other technologies show cooldown of a few minutes.
these JT coolers use mixed-refrigerants (MR) that provide the same cooling power at much lower pressures, of a few tens of bars (normally 15 – 40 bar). Consequently, the MR is recompressed by a compressor to provide continuous operation. The advantages of the JT cold head are maintained, and the compressor, which indeed contains moving parts, may be located far from the cold head. This allows high reliability and availability and minimal negative interference on the IR detector.
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Stirling:
These mechanical coolers are widely used for cryocooling and can achieve very low temperatures. The thermodynamic cycle is ideally reversible (highest possible efficiency) and the cooler consists of a pressure wave generator, PWG, (usually called “compressor”) and a cold head, which must be close to each other. Both PWG and cold head have moving parts and therefore acoustic noise and MTBF are the main drawbacks of the technology.
Pulse Tube (PT):
Pulse tubes are Stirling type coolers with the absence of moving parts at the cold head, at the expense of efficiency and size. This mature technology is already implemented in many aerospace applications, replacing Stirling coolers, due to their improved reliability.
Where to use each technology?
fast cooldown, miniature cold head, and limited duration mission
miniature cold head, high reliability and availability, low cost
miniature efficient mobile applications
miniature reliable mobile applications
State of the art Joule-Thomson cryocooler, suitable for IR detector applications where vibration, size, reliability, and availability are paramount.
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