No abstract

A thermal heat switch has been developed intended for cryogenic space applications operating around 100 K. The switch was designed to separate two pulse tube cold heads that cool a common focal plane array. Two cold heads are used for redundancy reasons, while the switch is used to reduce the thermal heat loss of the stand-by cold head, thus limiting the required input power, weight and dimensions of the cooler assembly. After initial evaluation of possible switching technologies, a construction based on the difference in the linear thermal expansion coefficients (CTE) of different materials was chosen. A simple design is proposed based on thermoplastics which have one of the highest CTE known permitting a relative large gap width in the open state. Furthermore, the switch requires no power neither during normal operation nor for switching. This enhances reliability and allows for a simple mechanical design. After a single switch was successfully built, a second double-switch configuration was designed and tested. The long term performance of the chosen thermoplastic (ultra-high molecular weight polyethylene) under cryogenic load is also analysed.

A two-stage Stirling-type U-shape pulse tube cryocooler driven by a 10 kW-class linear compressor was designed, built and tested. A special feature of the cold head is the absence of a heat exchanger at the cold end of the first stage, since the intended application requires no cooling power at this intermediate temperature.Simulations where done using Sage-software to find optimum operating conditions and cold head geometry. Flow-impedance matching was required to connect the compressor designed for 60 Hz operation to the 40 Hz cold head. A cooling power of 12.9 W at 25 K with an electrical input power of 4.6 kW has been achieved up to now. The lowest temperature reached is 13.7 K.