ter Hjm Marcel Brake

Low-power cryocooler survey

A cryocooler survey has been performed on data of 235 cryocoolers, with cooling powers below some tens of watts and operating between 4 and 120 K. The state-of-the-art is discussed and trends are investigated on cooling performance, mass and size, cost, lifetime and reliability. The data were compared with earlier surveys to explore historical trends. Improvements in cooling performance were mainly in 80 K cryocoolers, an increase in efficiency by about a factor of 5. Coolers did not reduce in size significantly. Main reduction was in 80 K coolers because of the higher efficiency. In the survey, cost is related to input power, cooling power and operating temperature. Cost development is considered and related to learning curves. Since the mid-90s, the lifetime of Stirling-type cryocoolers has increased by one order of magnitude from typically 0.5 to 5 years or more. The confusion that exists on the term “reliability” is discussed.

Pressure drop of laminar gas flows in a microchannel containing various pillar matrices

The pressure drop of gas flows in a microchannel filled with a dense pillar matrix was investigated with specific attention to a pillar shape. Pillars of height 250 µm and aspect ratio of about 10 were etched in silicon using an optimized Bosch deep reactive ion etching process. The pressure drop head-loss coefficient due to compression and expansion of gas at the inlet and outlet of the pillar matrix was estimated to be about 1.4 for an opening ratio of 10. A comparison of friction factor correlations for circular pillar cross-sections agreed rather well with the correlations proposed for the macroscale. Experimentally determined friction factor correlations for several pillar cross-sections for Reynolds numbers in the range of 50–500 are presented. Among the various pillar cross-sections considered, sine-shaped pillars have the lowest friction factor. These pillar structures with low pressure drop but a rather large wetted area can be used quite effectively as regenerative materials enabling the development of microcryocoolers.

Vibration-free 5 K sorption cooler for ESA's Darwin mission

ESAs Darwin mission is an Infrared Space Interferometer that will search for terrestrial planets in orbit around other stars. It uses six free-flying telescopes that are stabilized with respect to each other to less than 10 nm by utilizing micro-Newton ion thrusters. As a consequence, hardly any vibration of the optical system with integrated cryocoolers can be tolerated. A sorption cooler is a favorite cooler option because it has no moving parts and it is, therefore, essentially vibration-free. An efficient two-stage helium/hydrogen sorption cooler is proposed with a cooling power of 10 mW at 5 K. It needs only 3 W of input power and applies two passive radiators at 50 and 70 K. Application of such low-temperature radiators is made possible by Darwins far-away orbit L2 where earth-radiation is limited. In this paper, first Darwins cooler requirements are discussed and different cryocooler options are compared. Next, sorption cooler operation is explained, after which six different sorption cooler configurations are described and compared.

Fetal magnetocardiography: clinical relevance and feasibility

We investigated the feasibility of a high-Tc SQUID system for fetal magnetocardiography (fetal MCG) aiming at a system without a magnetically shielded room and cooled by a cryocooler. The targeted SQUID resolution was 50 fT/√Hz (1–100 Hz). The research was performed along three lines: environmental noise suppression, cooling and low-Tc experiments. Environmental noise can be suppressed by forming second-order gradiometers from individual magnetometers. Concerning cooling, we investigated the applicability of commercially available coolers. In the low-Tc experiments, the medical relevance of fetal MCG was clearly shown. However, they also indicated that, in order to fully exploit the medical potential, the targeted resolution has to be 10 fT/√Hz. This increased resolution, in combination with the required high reliability of the sensors, will be hard to realize in high-Tc technology. This paper describes the results of the project and discusses the feasibility of a clinical system.

Insight into clogging of micromachined cryogenic coolers

Cryogenic microcoolers can be used to cool small electronic devices to improve their performance. The authors present a micro-cold-stage of only 0.05cm3 that cools to 96K, applying Joule-Thomson expansion in a 300nm high flow restriction. Critical in such a microcooler is the deposition of water molecules that migrate to the restriction and block the flow. Because the microcooler is made of glass the authors had the unique opportunity to monitor this phenomenon and combine this visualization with experimental data. This provides significant insight in the way this clogging develops and opens possibilities to realize stable operation.

Miniature 10-150 mW Linde-Hampson cooler with glass-tube heat exchanger operating with nitrogen

Two demonstrator versions of a small Linde-Hampson cooler with glass tube heat exchanger are presented. The fabrication of these coolers are described and experimental results are presented and discussed. One cooler with a length of 270 mm and an outer diameter of 0.67 mm has a cooling power of 60 mW at 88 K with a mass flow of 7.3×10−6 kg/s (0.35 ln/min1) nitrogen gas. The lowest temperature measured is 82 K at a mass flow of 4.2×10−6 kg/s. The other shorter cooler has a length of 105 mm and the same outer diameter. It could also reach 82 K with a flow of about 2×10−6 kg/s. In both cases nitrogen gas was supplied at a pressure of 10 MPa.

Cryogenic design of a high-Tc SQUID-based heart scanner cooled by small Stirling cryocoolers

A heart scanner based on high-Tc SQUIDs is currently under development at the University of Twente. It is intended to be used in standard clinical environments without a magnetically shielded room. In order to make the application simple to use, the SQUIDs will be cooled by small cryocoolers, thus realizing a turnkey apparatus. The aimed field resolution is 50 fTRMS Hz?1/2 in a measuring band of 0.1?100 Hz. The mechanical cooler interference is reduced by incorporating two coolers and operating them in counter phase. The magnetic cooler interference is reduced by positioning the coolers and the SQUIDs in a coplanar arrangement, and by separating the SQUIDs from the cold tips with a solid conducting thermal interface. A design is presented in which a temperature of 55 K is expected with a cool-down time of less than 1 h.

Bonding of a niobium wire to a niobium thin film

A method for bonding a niobium wire to a niobium thin film is described. The bonds are to be used as superconducting connections between wire-wound gradiometers and thin-film coupling coils on DC SQUIDS. The method is characterized by two steps. Firstly, the hardness of the niobium wire is reduced by a heat treatment. Secondly, the niobium film is covered with a thin layer of palladium to prevent it from oxidizing. Superconducting bonds were realized using an ultrasonic bonding technique. We tested the bonds and measured superconductivity (to a sensitivity level of 6 × 10-15 Ohm) with currents up to 80 mkA, which is equivalent to 10 times the dynamic range of the DC SQUID systems. Even at 80 mkA, limited by the measuring set-op, the critical current of the bonds is not reached

Fast Gas-Gap Heat Switch for a Microcooler

A sorption compressor requires heat switches to thermally isolate the cells during heating, and to connect them to a heat sink during cooling. The requirements for these heat switches are discussed and related to important compressor parameters. It is shown that under certain conditions a sorption compressor can be operated without heat switches at all. Furthermore, the static heat transfer behaviour of a gas gap is modelled in detail and compared with experiments on a 300 μm gas gap. Finally, the dynamics that limit the switching speed are discussed.

High pressure check valve for application in a miniature cryogenic sorption cooler

This paper presents a check valve with integrated filter that can stand gas pressures of more than 100 bar in the closed direction and which has a very low pressure drop at low absolute gas pressures in the forward direction. The check valve is designed as a part of a check valve unit for application In a miniature cooler for cryogenic temperatures (<120 K). This cooling system, which utilizes several micromachined components, will in this paper be introduced to the MEMS field.