For some IR sensor applications, may be necessary to meet two critical performance requirements with the same system design configuration: very fast cooldown time (seconds to reach Sensor operating temperature) and long system operational run times (enabling the system to operate for thousands of hours without maintenance or service). Also, in some cases, the ability to abort a mission and re-use the device at a later date can be a desirable feature and adds operational flexibility.
The requirements for achieving very quick cooldown time to operating temperature and maintaining long operational run times are challenging to realize for applications where weight, size and power are a premium. Applications such as a seeker on a missile or a surveillance sensor, are preferably small, lightweight, portable and adaptable. So there is generally a trade-off between quick cooldown time and operational run time because of size and weight constrains.
Cryocoolers designed for applications requiring very fast turn on times are generally based on the Joule Thomson (J-T) effect because of the very high rates of cooling achievable with this cooling cycle. As a result applications requiring rapid IR sensor activation typically use J-T cooling approaches because fast cooldown times are crucial to the program. However, some variations of J-T type coolers suffer from relatively short run times because of the size, weight and power penalty associated with running these coolers for long periods of time. J-T cryostats can be made very small, lightweight and compact but lack operational run time. Although longer operational times can be realized by supplying a J-T cooler with large reservoir volumes of very high pressure gasses or very large compressors to supply very high pressure gasses, such solutions add to the size, weight and power to the device.