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The control of cryogenic temperatures is usually accomplished by a passive stage, exploiting the combined effect of a thermal mass connected to a thermal resistance; by an active control, often of a PID type, based on the combination of a dedicated sensor, a heater and a controller; or by a combination of the two. Such a system typically uses a controlled stage that is isolated from the source of the fluctuations by a thermal isolator. Controlled insertion of heat into this stage counters the temperature fluctuations reaching the stage. Inherent to this type of system is the insertion of heat into the controlled stage that eventually reaches the cold end of the cooler, reducing the net heat lift available. The larger the thermal isolation, the smaller the reduction of the net heat lift, but with the attendant increase in the interface temperature. Any scheme that can reduce the penalty associated with the loss of heat lift or the temperature offset would be attractive in terms of cooler performance. If the cooler system has a recuperative heat exchanger between the coldest heat sink and a higher temperature precooler, a different approach can be used. In this paper we describe a novel control approach capable of passively damping low frequency fluctuations, requiring minimal reduction of cooler heat lift and minimal temperature increase of the cold end interface. This alternative scheme is based on the idea of controlling the temperature of a section of the recuperative heat exchanger between the coldest precooler and the cold end of the cooler and it has been tested on a 20 K hydrogen sorption JT cooler. |
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