Progress in Micro Joule-Thomson Cooling at Twente University

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

A nitrogen triple-point thermal storage unit for cooling a SQUID magnetometer

In order to achieve turnkey operation, the use is planned of cryocoolers to cool a SQUID magnetometer system. To minimize the magnetical and mech. interference from the coolers, they are switched off during the actual measurements. Consequently, a thermal storage unit (TSU) is required with sufficient capacity at an appropriate temp. (<77 K). In a feasibility study, a load of 0.5 W from the SQUID sensor unit and an operating time of 10 h are considered. To account for an increased load caused by the TSU itself, an overall capacity of 15 Wh is aimed at. The nitrogen triple-point is chosen because of the large latent heat involved in the transition from solid to liq. and the corresponding well-suited temp. (63 K). Furthermore, any safety risks involved with the use of nitrogen are small compared to alternatives. To contain the nitrogen, highly porous alumina is used. A structure was made in which layers of copper and porous material alternate, thus establishing a good thermal contact between the nitrogen and the casing of the TSU. Expts. show an overall capacity of the system around 85% of the expected theor. value. Suggestions for improvements are given so as to arrive at a TSU capacity of 15 Wh. [on SciFinder (R)

Long-life vibration-free 4.5 Kelvin sorption cooler for space applications

A breadboard 4.5K 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 dc’s so that hardly any electromagnetic interference is generated. This cooler is a favorite option for future missions such as ESA’s 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 80to14.5K and a helium stage establishing 5mW at 4.5K. Both stages use microporous activated carbon as the adsorption material. The two cooler stages need about 3.5W of total input power and are heat sunk at two passive radiators at temperatures of about 50 and 80K—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 4.5mW at 4.5K with a long-term temperature stability of 1mK and an input power of 1.96W. So far, the cooler has operated continuously for a period of 2.5months and has not shown any sign of performance degradation

Development of a stainless steel check valve for cryogenic applications

This paper describes the development of a check valve for use in a sorption compressor that will drive a 10 mW 4.5 K Joule–Thomson cryocooler. For the check valve extremely low backflow rates are tolerable at an operating temperature of the valve of 50 K. To fulfill these requirements, the sealing mechanism of the valve is based on a full metal to metal contact. In order to obtain sufficiently low leak rates, both parts were machined to a surface flatness in the order of 100 nm. In addition, the closing plate (boss) of the valve deforms (bends) slightly under pressure, forming itself to the opposite valve seat and thus reducing the gap between these parts. The measured leak flow at 50 K was 1.6 μg/s helium @ 16 bar pressure difference, which is well below the aim of 3 μg/s.\ud \ud The valve was subjected to an accelerated lifetime test of 300,000 cycles. It was observed that the leak flow through the valve during this test steadily decreased to a level of 0.15 μg/s after 100,000 cycles

Fabrication of a micro cryogenic cooler 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

Fabrication of a micro cryogenic cold stage using MEMS-technology \ud

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.\u