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METIS is the 'Mid-infrared ELT Imager and Spectrograph' for the European Extremely Large Telescope. This E-ELT instrument will cover the thermal/mid-infrared wavelength range from 3 to 14 μm and will require cryogenic cooling of detectors and optics. We present a vibration-free cooling technology for this instrument based on sorption coolers developed at the University of Twente in collaboration with Dutch Space. In the baseline design, the instrument has four temperature levels: N-band: detector at 8 K and optics at 25 K; L/M-band: detector at 40K and optics at 77 K. The latter temperature is established by a liquid nitrogen supply with adequate cooling power. The cooling powers required at the lower three levels are 0.4 W, 1.1 W, and 1.4 W, respectively. The cryogenic cooling technology that we propose uses a compressor based on the cyclic adsorption and desorption of a working gas on a sorber material such as activated carbon. Under desorption, a high pressure can be established. When expanding the high-pressure fluid over a flow restriction, cooling is obtained. The big advantage of this cooling technology is that, apart from passive valves, it contains no moving parts and, therefore, generates no vibrations. This, obviously, is highly attractive in sensitive, high-performance optical systems. A further advantage is the high temperature stability down to the mK level. In a Dutch national research program we aim to develop a cooler demonstrator for METIS. In the paper we will describe our cooler technology and discuss the developments towards the METIS cooler demonstrator.

This paper elaborates on the incorporation of in-plane electrical interconnects to the reflow bonding. After joining the tubes to silicon substrates, the electrically conductive lines will allow probing into the volume confined by the tube. Therefore methods of fabrication of electrical interconnects, which would survive the bonding and not alter the quality of the bond interface, are investigated.

Borofloat® (borosilicate 8330) glass is an important type of inorganic glass, both scientifically and commercially. During prolonged heat treatment of this glass above its glass transition temperature of 525°C, heterogeneous nucleation, and growth of cristobalite crystals occur. The kinetics and morphology of this crystal growth have been studied for heat treatment temperatures at and above 660°C. The activation energy for crystal growth is estimated to be 185±10 kJ/mol. This is attributed to the diffusion of boron, rather than sodium, being the rate-limiting step within the borosilicate framework. Contact with the atmosphere is shown to initiate the nucleation of cristobalite crystals, while deposition of a thin silicon nitride surface coating on the glass helps to prevent this nucleation.

Capillary bonding of glass tubes to silicon wafers has been elaborated. After proper preparation, tubes are placed on silicon substrates and specimens are annealed at sintering temperatures of glass. The resulting interconnect is capable of working above 70 bars of filling pressure and the bond is completely hermetic. However, due to high temperature annealing in air, surface devitrification has been observed on glass, which can be avoided by vacuum annealing

A micro Joule-Thomson cold stage was designed, built and tested, as part of the micro cooling research project at the University of Twente. The cold stage consists of a stack of three glass wafers. In the top wafer, the high-pressure line is etched as a rectangular channel with supporting pillars. The high-pressure line ends in a flow restriction and an evaporator volume that crosses the center wafer into the bottom wafer. The bottom wafer contains the low-pressure line, again etched as a rectangular channel containing supporting pillars, thus forming a counter-flow heat exchanger. A design aimed at a net cooling power of 10 mW at 96 K and operating with nitrogen as the working fluid was optimized based on the minimization of entropy production. The optimum cold finger measures 28 mm x 2.2 mm x 0.8 mm (max. dimensions). It should be able 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. A batch of 14 prototype coolers were made for 8 different designs, including a design of the theoretical optimum. Liquid nitrogen is collected in the evaporator, and since the low pressure is 6 bar, the temperature should be 96 K. However, due to some thermal resistance in the thermocouple attached to the cold tip, a temperature of 105 K was achieved. A net cooling power of 5 mW was measured. In the paper, the design of the coolers will be discussed along with experimental results.

At the University of Twente, research on the development of a sorption-based micro cooler is in progress. Because of the absence of moving parts, such a cooler is virtually vibration free and highly durable, which potentially results in a long lifetime. A miniature cryogenic cooler with these properties would be appealing in a wide variety of applications including the cooling of vibration-sensitive detectors in space missions, low-noise amplifiers and semi- and superconducting circuitry. The objective of the present project is to scale down a Joule-Thomson (JT) cold stage to a total volume of a few hundredths of a cm3. This size reduction introduces many problems. The proposed cold stage volume results in a restriction cross-sectional area of about a thousandth of a mm2 which may cause clogging problems. Flow channels with a cross-sectional area of a few hundredths of a mm2 will produce high pressure drops influencing the JT cycle. Furthermore, the micro channels must be capable of withstanding high pressures and maintaining a large temperature gradient over a relatively short length. The project aim is to develop a reliable micro JT cold stage that is fabricated out of one material with a relatively simple and reproducible fabrication method. The length of the cold stage is calculated at about 20 mm with a width of 1.7 mm and height of about 0.3 mm. The mass flow is in the order of one mg per second to create a net cooling power of 10 mW at 96 K. The final objective of the project is to integrate the cold stage, vacuum chamber and device into one compact design. This paper discusses possible solutions to the problems mentioned and presents a concept design of such a miniature JT cold stage.

An experimental study of low Reynolds number gas flow across pillars in a microchannel is presented. Various geometries of pillars are considered for systematic comparison. This is for the first time such an investigation is done. The pillars 25 um in diameter and height of 230 um are dry etched in silicon. a 4-point pressure drop measurement setup is used to avoid measuring losses in fittings. Among various geometries considered, shifted sine pattern and rhombus shaped structures with an aspect ratio of 1/3 have the lowest friction factor and circle staggered arrangement has the highest friction factor.

This paper presents the fabrication of a microcooler consisting of small stainless steel sorption compressor cells, micromachined silicon check valves, and a micromachined cold stage that incorporates glass-tube heat exchangers. The design, fabrication and experiments on the different elements are described. Two compressor cells were thermally cycled to investigate the dynamic behaviour. The cold stage could reach a stable temperature of 169 K with a cooling power of about 200 mW.

A heart scanner that can be equipped with up to 25 high-Tc SQUID magnetometers was designed at the University of Twente. In this design the mechanical cooler interference is reduced by operating two coolers in counterphase. The magnetic cooler interference diminished by positioning the coolers and the SQUIDs in a coplanar arrangement, and by separating the SQUIDs from the cold tips of the cryocoolers with a solid conducting thermal interface. In the present paper the construction of this cryogen-free high-Tc SQUID system is described and test results are presented. A SQUID plate temperature of 60 K was realized in about 2 hours. The corresponding heat load to the coolers is roughly 0.9 W. With a preliminary magnetic shield around the compressors a noise level of 0.6 pT/√Hz was measured in the frequency band 10 to 100 Hz and magnetocardiograms were recorded inside a magnetically shielded room. A further reduction of the noise level is expected after optimizing the mumetal shielding of the compressors.