G.C.F. Venhorst

Fabrication of a micro cryogenic cold stage using MEMS-technology

This paper describes the design and production process of a variety of reliable micro cryogenic coolers. The different cold stages are based on an optimized design found during a study which was done to maximize the cold-stage effectiveness. Typical cold-stage dimensions are 30 × 2 × 0.5 mm with an expected net cooling power varying from 10 mW to 20 mW at a tip temperature of 96 K. A cold stage consists of a stack of three fusion bonded D263T glass wafers. The production process has 7 lithography steps and roughly 100 process steps. In order to determine the maximum bend, shear and bond stresses inside a 175 µm thick D263T glass wafer, several pressure tests were performed.

Long-life vibration-free 4.5 K sorption cooler for space applications.

A breadboard 4.5 K helium sorption cooler for use in vibration-sensitive space missions was developed and successfully tested. This type of cooler has no moving parts and is, therefore, essentially vibration-free. The absence of moving parts also simplifies scaling down of the cooler to small sizes, and it contributes to achieving a very long lifetime. In addition, the cooler operates with limited dcs so that hardly any electromagnetic interference is generated. This cooler is a favorite option for future missions such as ESAs Darwin mission, a space interferometer in which the sensitive optics and detectors can hardly accept any vibration. The system design consists of a hydrogen stage cooling from 80 to 14.5 K and a helium stage establishing 5 mW at 4.5 K. Both stages use microporous activated carbon as the adsorption material. The two cooler stages need about 3.5 W of total input power and are heat sunk at two passive radiators at temperatures of about 50 and 80 K-radiators which are constructed at the cold side of the spacecraft. We developed, built, and tested a demonstrator of the helium cooler. This demonstrator has four sorption compressor cells in two compressor stages. Test experiments on this cooler showed that it performs within all specifications imposed by ESA. The cooler delivered 4.5 mW at 4.5 K with a long-term temperature stability of 1 mK and an input power of 1.96 W. So far, the cooler has operated continuously for a period of 2.5 months and has not shown any sign of performance degradation.

Development of a 4K Sorption Cooler for ESA's Darwin Mission: System-Level Design Considerations

ESA’s Darwin mission is a future space interferometer that consists of six free-flying telescopes. To guarantee a proper mechanical stability of this system, hardly any vibration of the optical system with integrated cryocoolers can be tolerated. This paper presents the system design of a 4.5 K, 10 mW vibration-free sorption cooler chain, of which the helium stage is currently in development under an ESA-TRP contract. A sorption cooler is a favorite option because it has no moving parts and it is, therefore, essentially vibration-free. A two-stage helium/hydrogen cooler is proposed which needs 5 Watts of input power and which applies two passive radiators at 50 K and 80 K. The paper includes the following aspects: system modelling, radiator configurations, activated carbons, different multi-stage cooler options, and integration aspects of the compressor cells with the radiators.

Microcooling Developments at the University of Twente

At the University of Twente, research is continued on micro cooling in which cooler components are manufactured by means of micromachining (MEMS). Two projects are carried out in parallel: one on recuperative coolers and one on regenerative coolers. The micro recuperative coolers incorporate counter-flow heat exchangers and Joule-Thomson expansion stages that are machined in glass. A prototype device is currently under construction. The cold finger measures 28 mm × 2.2 mm × 0.8 mm (max dimensions). It is designed to generate a net cooling power of 10 mW at 96 K at a nitrogen flow of 1 mg/s at a high pressure of 80 bar and a low pressure of 6 bar. The gas flow can be supplied from a high-pressure reservoir or from a small sorption compressor. Research on micro regenerative coolers is primarily focused on the design and the realization of the regenerator. Test samples were manufactured with regenerator matrix dimensions of typically 25 µm. Design considerations, prototypes and experiments in both research projects are presented and discussed.