TURBO BRAYTON
The recent years saw, in Europe, the developments of coolers to meet Earth Observation mission requirements that are capable to provide significant cooling power at an operational temperature around 50K (for IR detection). Those single stage Stirling or Pulse Tube coolers are mechanisms that involve moving parts at low frequency (in the compressor and in the Cold Finger for the Stirling) which induce microvibrations.
Even if active microvibration cancellation and careful screening and manufacturing of the coolers parts can lead to low exported microvibrations (< 100 mN in all directions at all harmonics), more and more stringent system level induced microvibration requirements can lead to very complex solutions to overcome those vibrations (e.g. suspended coolers and radiators, flexible thermal link assemblies that degrade the overall thermal performances).
In order to minimize significantly the exported vibration level generated by conventional cryocooler, vibration-free technologies need to be developed for next generation of instruments.
To achieve this performance, either Sorption based cryocoolers or turbo-brayton can be proposed. The Sorption cooler offers the advantage to be free of mechanism source of vibration while the Turbo-brayton rejects the exported vibration far above the critical bandwidth. The Turbo-Brayton cycle cryocooler uses miniature, high-speed turbomachines and high-effectiveness recuperators to provide efficient cooling with low vibration and high reliability. Gas bearings are used in the miniature machines to support the rotors, which operate at speeds of 100,000 to 600,000 rpm. The low-mass rotors are the only moving parts in the systems, and because they are precision balanced, the systems are inherently vibration-free. No supplemental vibration canceling electronics or hardware is required. The gas bearings also provide non-contact operation, so performance degradation resulting from wear or the accumulation of debris is absent. These systems are generally capable of maintenance free operating lives of 5 to 20 years.
The Turbo-Brayton cryocooler developed is arranged in a conventional configuration including a 2 stages compressor, a recuperative heat exchanger, the expansion turbine and the thermal heat exchanger interfaced with the customer load (focal plan, detector, thermal shield…). These components may be integrated into a compact package or distributed over fairly large areas, interconnected by lengths of tubing. Refrigeration can be delivered to multiple loads at either a single temperature or several different temperatures.
The second type of delivery can be accomplished either by multi-staging an integral cooler or by combining several cryocoolers at the appropriate interfaces. Cooling loads and thermal interfaces may be separated by large distances without significant effects on overall system efficiency. Thus, the turbo-Brayton cryocooler can be implemented in a variety of ways in space applications.
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