Herman J. Holland

Characterization of micromachined cryogenic coolers

Micro cryogenic coolers can be used to cool small circuitry and improve their performance. The authors present a variety of micro coolers which are fabricated using MEMS technology production processes only. The typical dimension of a micro cold stage is 30 × 2.2 × 0.5 mm. It cools down to 96 K, applying Joule–Thomson expansion in a 300 nm high flow restriction and has a cooling power ranging from 10 mW to 25 mW. This paper discusses the operation of the micro cold stage and the characterization measurements done.

Thermodynamic considerations on a microminiature sorption cooler

The sorption/Joule-Thomson cycle is a promising cycle for microscale cooling of low-temperature electronic devices because the cycle lacks moving parts. This facilitates scaling down to small sizes, eliminates interferences, and contributes to achieving a long life time. A thermodynamic analysis is presented in which the behaviour of compressor and cold stage are analysed separately, leading to a better understanding of sorption coolers. Some fundamental possibilities to improve the thermodynamic efficiency are discussed, and as a part of this a novel two stage compressor concept is proposed.

A miniature Joule-Thomson cooler for optical detectors in space

The utilization of single-stage micromachined Joule-Thomson (JT) coolers for cooling small optical detectors is investigated. A design of a micromachined JT cold stage-detector system is made that focuses on the interface between a JT cold stage and detector, and on the wiring of the detector. Among various techniques, adhesive bonding is selected as most suitable technique for integrating the detector with the JT cold stage. Also, the optimum wiring of the detector is discussed. In this respect, it is important to minimize the heat conduction through the wiring. Therefore, each wire should be optimized in terms of acceptable impedance and thermal heat load. It is shown that, given a certain impedance, the conductive heat load of electrically bad conducting materials is about twice as high as that of electrically good conducting materials. A micromachined JT cold stage is designed and integrated with a dummy detector. The JT cold stage is operated at 100 K with nitrogen as the working fluid and at 140 K with methane. Net cooling powers of 143 mW and 117 mW are measured, respectively. Taking into account a radiative heat load of 40 mW, these measured values make the JT cold stage suitable for cooling a photon detector with a power dissipation up to 50 mW, allowing for another 27 to 53 mW heat load arising from the electrical leads.

Micromachined cryogenic cooler for cooling electronic devices down to 30 K

Cryogenic temperatures are required for improving the performance of electronic devices and for operating superconducting sensors and circuits. The broad implementation of cooling these devices has long been constrained by the availability of reliable and low cost cryocoolers. After the successful development of single-stage micromachined coolers able to cool to 100 K, we now present a micromachined two-stage microcooler that cools down to 30 K from an ambient temperature of 295 K. The first stage of the microcooler operates at about 94 K with nitrogen gas and pre-cools the second stage operating with hydrogen gas. The microcooler is made from just three glass wafers and operates with modest high-pressure gases and without moving parts facilitating high yield fabrication of these microcoolers. We have successfully cooled a YBCO film through its superconducting transition state to demonstrate a load on the microcooler at cryogenic temperatures. This work could expedite the application of superconducting and electronic sensors and detectors among others in medical and space applications

Clogging in micromachined Joule-Thomson coolers: Mechanism and preventive measures

Micromachined Joule-Thomson coolers can be used for cooling small electronic devices. However, a critical issue for long-term operation of these microcoolers is the clogging caused by the deposition of water that is present as impurity in the working fluid. We present a model that describes the deposition process considering diffusion and kinetics of water molecules. In addition, the deposition and sublimation process was imaged, and the experimental observation fits well to the modeling predictions. By changing the temperature profile along the microcooler, the operating time of the microcooler under test at 105u2009K extends from 11 to 52u2009h.

High frequency pressure oscillator for microcryocoolers

Microminiature pulse tube cryocoolers should operate at a frequency of an order higher than the conventional macro ones because the pulse tube cryocooler operating frequency scales inversely with the square of the pulse tube diameter. In this paper, the design and experiments of a high frequency pressure oscillator is presented with the aim to power a micropulse tube cryocooler operating between 300 and 80 K, delivering a cooling power of 10 mW. Piezoelectric actuators operate efficiently at high frequencies and have high power density making them good candidates as drivers for high frequency pressure oscillator. The pressure oscillator described in this work consists of a membrane driven by a piezoelectric actuator. A pressure ratio of about 1.11 was achieved with a filling pressure of 2.5 MPa and compression volume of about 22.6 mm(3) when operating the actuator with a peak-to-peak sinusoidal voltage of 100 V at a frequency of 1 kHz. The electrical power input was 2.73 W. The high pressure ratio and low electrical input power at high frequencies would herald development of microminiature cryocoolers.

The Application of Cryocoolers for Cooling a High-Tc SQUID Magnetometer

A multichannel high-Tc dc-SQUID based heart-magnetometer is currently under development in our laboratory. Since this system has to be simple to use, the cooling of the device should be established by means of a turn-key apparatus incorporating a cryocooler. Because of its magnetic interference, the cooler has to be separated from the SQUID unit. Therefore, an interface between the cooler and the SQUIDs is needed. Possibilities are a gas flow system or a conductive strip. A prototype closed-cycle gas flow system has been constructed and tested, in which helium gas is cooled by a Leybold Heraeus RG 210 Gifford-McMahon cryocooler. Then it is transported through a gas line of 2.5 meter length, and after that through a glass-epoxy heat exchanger on which the SQUIDs can be installed. With this system a temperature of 30 K can be established in about 2 hours (depending on the gas flow rate). Based on the results obtained with this configuration, a smaller system was designed incorporating two Signaal Usfa UP 7058 Stirling cryocoolers. Compared to the prototype the dimensions were reduced by roughly a factor 5.

Cooling a low noise amplifier with a micromachined cryogenic cooler

The sensitivity of antenna systems increases with increasing active area, but decreases at higher noise figure of the low-noise amplifier (LNA). Cooling the LNA locally results in significant improvement in the gain and in lowering the noise figure of the LNA. Micromachined Joule-Thomson (JT) coolers can provide a cryogenic environment to the LNA. They are attractive because they have no cold moving parts and can be scaled down to match the size and the power consumption of LNAs. The performance of a LNA mounted on a JT microcooler with dimensions of 60.0 × 9.5 × 0.72 mm(3) is reported in this paper. The microcooler is operated with nitrogen gas and the cold-end temperature is controlled at 115 K. The measured net cooling power of the microcooler is about 43 mW when the LNA is not operating. The power dissipation of the LNA is 26 mW, with a supply voltage of 2 V. At room temperature the noise figure of the LNA is 0.83 dB and the gain lies between 17.9 and 13.1 dB, in the frequency range of 0.65 and 1.05 GHz. Upon cooling to 115 K, the noise figure drops to 0.50 dB and the increase in gain varies in the range of 0.6-1.5 dB.

Micromachined Joule-Thomson coolers

A MEMS-based Joule-Thomson cold stage was designed and prototypes were realized and tested. The cold stage consists of a stack of three glass wafers. In the top wafer, a high-pressure channel is etched that ends in a flow restriction with a height of typically 300 nm. An evaporator volume crosses the center wafer into the bottom wafer. This bottom wafer contains the low-pressure channel thus forming a counter-flow heat exchanger. A design aiming 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. A batch of prototype coolers ranging from 20 to 40 mm was made for a flow of typically 1mgCs-1 at a high pressure of 80 bar and a low pressure of 6 bar. The design and fabrication of the coolers will be discussed along with experimental results. A specific issue that will be addressed is the clogging of the restriction due to the deposition of ice crystals. Furthermore, introductory experiments with multistage microcoolers will be discussed.

Sorption-based vibration-free cooler for the METIS instrument on E-ELT

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.