Stirling pulse tube cryocoolers for space science missions
Keywords: space detectors
phasors
astronomy
cryocoolers
instrumentation
astrophysics
compressors
pulse tube
thermodynamics
Stirling
cryogenics

Cryocoolers are compact and effective devices that provide cooling to cryogenic temperature. They serve to reduce thermal noise on focal plane assemblies in space telescopes, cool down superconducting devices, and present many other applications for a wide range of technologies. A Stirling cryocooler operates by running the Stirling refrigeration cycle and reaches high efficiencies. They have an established spaceflight heritage, and in the case of the Oxford-style cryocooler, they employ flexure bearing springs that enhance their longevity and render them suitable for long lifetime space missions. In more recent times, pulse tube cryocoolers have garnered great interest specifically due to the benefit they pose of having no moving parts at the cold end. This is beneficial for astrophysics purposes where vibration requirements are especially stringent.
In this thesis, a hybrid Stirling pulse tube cryocooler (SPTC) was developed, which incorporates a warm end displacer as is used in Stirling systems, whilst hybridised with a pulse tube assembly at the cold end. This enabled the pulse tube to reach higher efficiencies and for the displacer to recover PV expansion power back into the system at the warm end. This was developed in a coaxial configuration using an active displacer with an additional motor. The subsequent aim of this research project was to reiterate this design for a coaxial SPTC that reaches comparable cooling capabilities, whilst running the displacer passively. The objective in doing so was to remove the second phase in the power electronics and reduce the complexity of the active displacer design. The active displacer SPTC was successful in experimentally cooling to a cold temperature of 80K with an input power of 79W, an optimum cooling power of 2.4W and a relative Carnot efficiency of 8.3%. Similarly, the designed, built, and tested passive displacer SPTC achieved cooling to 80K with an optimum input power of 97W, a cooling power of 3W, and a relative Carnot efficiency of 9.4%. Numerical simulations were completed using Sage software for both SPTCs and the models were validated with the experimental findings to show similar trends and an average error of approximately 20% for cooling power and efficiency.
As part of the design and physical development of the passive displacer SPTC, new design flexure bearing springs underwent stiffness, fatigue, and resonance testing in order to characterise their behaviour and performance. This enabled a suitable selection for the passive displacer. Moreover, the suitability of a strain gauge circuit in a full Wheatstone bridge configuration was investigated as a novel technique for in-situ cryocooler stroke measurements. The technology was found to be successful in serving this purpose, and further, was not subject to drift or phase lag, which makes it an advantageous technology compared to commonly used LVDTs which are subject to the latter. A good frequency response was also demonstrated by conducting a range of impact tests on the spring-strain gauge setup and assessing the strain gauge bridge response.
Finally, theoretical and computational studies were undertaken to better understand the inner workings of the pulse tube cryocooler. Phasor analyses were conducted to derive mathematical expressions for mass flow, pressure pulse, and the phase angle between the two. These in turn enable the cooling power and efficiency of a given system to be readily obtained. Fluid flow studies were undertaken to gain insight into the hydrodynamics within the coaxial regenerator and thermal simulations were completed using ESATAN-TMS to quantify conductive and convective losses in the SPTC assembly, where strong agreement with an error of under 7% was achieved between the thermal model and the experimentally validated Sage model.