Low-frequency magnetic interference in a SQUID gradiometer caused by a Cryotiger gas-mixture cooler

A Cryotiger^® gas-mixture cooler was applied for cooling of three high-T_c SQUID magnetometers. These SQUID magnetometers were mounted on an alumina holder in an axial gradiometer configuration. From 20 Hz upward, the system noise was about 0.1 pT/√Hz. Below this frequency, the noise gradually increased to a level of 10 pT/√Hz at 1 Hz. This low-frequency excess noise appeared to be due to remnant magnetization of the Cryotiger cold head. Movement of magnetic cold-head parts with respect to the SQUIDs are induced by pressure fluctuations in the heat exchanger lines. By using one SQUID as a reference for the cooler noise, a first-order gradiometer can be formed in which the cooler noise is eliminated. To establish a proper second-order gradiometer either a fourth SQUID has to be added, or the spatial separation between cold head and SQUIDs has to be increased significantly.     

Construction and tests of a heart scanner based on superconducting sensors cooled by small stirling cryocoolers

At the University of Twente, a heart scanner has been designed and constructed that uses superconducting devices (superconducting quantum interference devices (SQUIDs)) to measure the magnetic field of the heart. A key feature is the elimination of liquid cryogens by incorporating cryocoolers. In the design, two coolers are operated in counter-phase to reduce the mechanical interference. In addition to the application of ferromagnetic shields around the compressors, the magnetic cooler interference is reduced by placing the SQUID magnetometers coplanar with respect to the coolers. In this way, the cooler noise was reduced to a level below the intrinsic sensor noise: 0.16 pT/√Hz. A temperature of 60 K was realised with a cool-down time of about 2 h. The corresponding heat load to the coolers is roughly 0.9 W. Magnetocardiograms were recorded inside a magnetically shielded room.     

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

In order to achieve turnkey operation, we plan to use cryocoolers to cool a SQUID magnetometer system. To minimize the magnetical and mechanical interference from the coolers, we intend to switch them off during the actual measurements. Consequently, a thermal storage unit (TSU) is required with sufficient capacity at an appropriate temperature (<77 K). In a feasibility study, we consider a load of 0.5 W from the SQUID sensor unit and an operating time of 10 h. 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 liquid and the corresponding well-suited temperature (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. Experiments show an overall capacity of the system around 85% of the expected theoretical value. Suggestions for improvements are given so as to arrive at a TSU capacity of 15 Wh.     

10W/80K高频同轴脉冲管制冷机的实验性能

介绍了一款大冷量高频单级同轴脉冲管制冷机的基本结构、数值模拟和实验性能。其线性压缩机采用Redlich动磁式直线电机驱动,压缩活塞对置布置,使用板弹簧支撑和间隙密封技术,80 K温区工作时的电机效率在83%以上。膨胀机的蓄冷器和脉冲管为同轴型布置,这种结构使冷头与器件之间的耦合非常方便。使用数值软件对制冷机整机和调相部件进行数值模拟,并对模拟结果进行实验验证。对制冷机的运行频率和制冷性能进行实验研究,制冷机在210.3 W输入电功时能获得10 W/80 K的制冷性能,比卡诺效率为12.66%,运行频率为62 Hz,整机重量小于5.5 kg。     

Pulse tube cooler for flight hyperspectral imaging

A new version of TRW's miniature pulse tube cooler system maintains the short wave infrared–focal plane array (SWIR–FPA) (with wavelength spectrum of 0.9–2.5 μm in the hyperspectral imaging spectrometer for the Hyperion Instrument) interface at a temperature of 110 K. The cooler provides the nominally required cooling load of 0.84W to the FPA via a cold thermal strap, at 72% stroke consuming 14.7 W of electrical power, when the heat reject temperature is at 300 K. This cooler can operate up to 90% stroke, having 1.5 W cooling load, thus having 79% performance margin for the Hyperion mission. Before the installation and operation of the cooler onto the instrument, both the mechanical and the electronics assemblies underwent the environmental tests of launch vibration, thermal vacuum cycling, and burn-in. The cooler performance in terms of mechanical efficiency, electronics efficiency, load lines, temperature stability, self-induced vibrational force reduction, ripple current reduction, and magnetic radiated emission was measured and are reported here.     

Experimental study of a cascade pulse tube cryocooler with a displacer

Recovering the expansion power in pulse tube cryocooler is of great utility in improving cooling efficiency. Using a second-stage cooler after a primary cooler to produce extra cooling power is an effective way especially when the cooling temperature is not very low. In the configuration, the two coolers are connected by a displacer which is used as a phase shifter. In this paper, experimental investigations were conducted to study this system. Firstly, the performance of the overall system and separated cooler was respectively presented. To better understand the displacer, phase relation, mechanical resistance and displacement were then clarified. In addition, the power consumption distribution of the cascade cryocooler was discussed. Finally, both numerical and experimental comparisons were made on the displacer-type and tube-type cryocooler. The experimental results show that the displacer-type cryocooler has superior performance due to the better phase-modulation capability and less power loss. With the input electric power of 1.9 kW and cooling temperature of 130 K, the overall system achieved a cooling power of 371 W and a relative Carnot efficiency of 24.5%.     

有限元法在辐射制冷器热分析中的应用

辐射制冷器主要为红外探测器提供合适的低温工作条件,是星载红外遥感探测系统的重要组成部分。建立了W型辐射制冷器的有限元模型。利用AN SYS分析软件对辐射制冷器热变形进行了分析,提出了减小2个冷头相对位移的措施,为辐射制冷器的优化设计和红外相机的光学补偿提供了理论依据。     

Advances in a 300 Hz thermoacoustic cooler system working within liquid nitrogen temperature range

With the combined advantages of high reliability, compact size and low electromagnetic interference, a high frequency operating thermoacoustic cooler system, i.e. a pulse tube cooler driven by a thermoacoustic heat engine, is quite promising for space applications. This article introduced a high frequency standing-wave thermoacoustic heat engine-driven pulse tube cooler system working around 300 Hz with axial length being 1.2 m. To improve the thermal efficiency of such system, an optimization has been carried out, both analytically and experimentally, by observing the influence of the dimensions of the stack, the hot buffer length and the acoustic pressure amplifier tube length. So far, a no-load temperature of 68.3 K has been obtained with 4.0 MPa helium and 750 W heating power. With 500 W heating power, a no-load temperature of 76.9 K and 0.2 W cooling power at 80 K have been achieved. Compared with former reports, the performance has been improved.     

分离型二级脉管制冷机的实验研究第一部分20～40 K温区单级大功率脉管制冷机

为了满足液氦温区分离型二级脉管制冷机第二级预冷的需要,设计制作了1台20～40K温区单级大功率脉管制冷机.采用额定功率为6 kW的压缩机驱动该制冷机,最低制冷温度达13.8K,刷新了单级脉管制冷机最低制冷温度纪录.该制冷机在40 K可获得高达55.9 W的制冷量,基本可以满足15～40 K温区超导磁体等冷却的需要.着重分析了频率、充气压力和不同压缩机对系统制冷性能的影响,测试了长时间运行中系统性能的变化情况.     

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

ESA's 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 Darwin's far-away orbit L2 where earth-radiation is limited.In this paper, first Darwin's 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.     

Study of metastable superconducting detector response to irradiation using SQUID-readout

We describe the results of experiments on the³⁵S irradiation of a metastable superconducting detector between 2.0 and 3.4 K using a second-derivative rf SQUID readout operated in an integral mode. The detector consisted of 3×10⁶ tin grains, distributed in diameter from 10–25 micron. Comparison with computer simulation based on a hot border model consistent with the measurement protocols yields reasonable agreement despite the large timing uncertainties associated with the measuring systematics of the SQUID system.     

Theoretical and experimental study of a gas-coupled two-stage pulse tube cooler with stepped warm displacer as the phase shifter

A compact and high efficiency cooler working at liquid hydrogen temperature has many important applications such as cooling superconductors and mid-infrared sensors. This paper presents a two-stage gas-coupled pulse tube cooler system with a completely co-axial configuration. A stepped warm displacer, working as the phase shifter for both stages, has been studied theoretically and experimentally in this paper. Comparisons with the traditional phase shifter (double inlet) are also made. Compared with the double inlet type, the stepped warm displacer has the advantages of recovering the expansion work from the pulse tube hot end (especially from the first stage) and easily realizing an appropriate phase relationship between the pressure wave and volume flow rate at the pulse tube hot end. Experiments are then carried out to investigate the performance. The pressure ratio at the compression space is maintained at 1.37, for the double inlet type, the system obtains 1.1 W cooling power at 20 K with 390 W acoustic power input and the relative Carnot efficiency is only 3.85%; while for the stepped warm displacer type, the system obtains 1.06 W cooling power at 20 K with only 224 W acoustic power input and the relative Carnot efficiency can reach 6.5%.     

Cascade pulse-tube cryocooler using a displacer for efficient work recovery

Expansion work is generally wasted as heat in a pulse-tube cryocooler and thus represents an obstacle to obtaining higher Carnot efficiency. Recovery of this dissipated power is crucial to improvement of these cooling systems, particularly when the cooling temperature is not very low. In this paper, an efficient cascade cryocooler that is capable of recovering acoustic power is introduced. The cryocooler is composed of two coolers and a displacer unit. The displacer, which fulfills both phase modulation and power transmission roles, is sandwiched in the structure by the two coolers. This means that the expansion work from the first stage cooler can then be used by the second stage cooler. The expansion work of the second stage cooler is much lower than the total input work and it is thus not necessary to recover it. Analyses and experiments were conducted to verify the proposed configuration. At an input power of 1249 W, the cascade cryocooler achieved its highest overall relative Carnot efficiency of 37.2% and a cooling power of 371 W at 130 K. When compared with the performance of a traditional pulse-tube cryocooler, the cooling efficiency was improved by 32%.     

50 mK cooling solution with an ADR precooled by a sorption cooler

CEA/SBT is currently developing a 2.5 K-50 mK cooling solution composed of a small demagnetization refrigerator (ADR) precooled by a sorption cooler, equivalent to the high temperature stage of a two-stage ADR system. Thanks to the use of this dual technology, a low weight cooler able to reach 50 mK with a heat sink up to 2.5 K can be designed. Because the sorption cooler is probably the lightest solution to produce sub-Kelvin temperatures, these developments allow us to propose a solution to face the drastic reduction in the mass budget of space missions like SPICA or IXO. The European Space Agency (ESA) is funding the development of an engineering model able to produce 1 μW net heat lift at 50 mK. It is sized so that the sorption cooler provides an additional 10 μW at 300 mK. The ESA main requirements are an autonomy of more than 24 h and a recycling time smaller than 8 h. We present the design of the system able to meet these requirements as well as the expected performances and preliminary measurements.     

Performances of the 50 mK ADR/sorption cooler

CEA/SBT is currently testing a 50 mK cooler developed in the framework of a European Space Agency Technological Research Program targeted for the Advanced Telescope for High Energy Astrophysics space mission. This cooler is composed of a small demagnetization refrigerator pre cooled by a sorption cooler stage. This Engineering Model is able to produce 1 μW of net heat lift at 50 mK and an additional 10 μW at 300 mK provided by the sorption cooler stage. The autonomy of the cooler is 24 h, and once the low temperature phase at 50 mK is over, it can be recycled in about 8 h with 10 μW and 100 μW available at respectively the 2.5 and 15 K heat sinks. These performances are in agreement with the European Space Agency requirements.In this paper, we present the detailed thermal performances of the cooler in nominal conditions as well as sensitivity measurements of the variation of the heat sink and the cold end temperatures.     

An Operation Circuit of a Micro-SQUID Magnetometer below 1?K

We have been developing a micro superconducting quantum interference device (??-SQUID) magnetometer for the study of quantum effects in ??m- or nm-sized magnetic materials. In the ??-SQUID magnetometer, the sample is placed on the SQUID loop directly and the flux is detected by the loop itself. The close proximity between the sample and the SQUID loop is of great advantage for highly sensitive detection of magnetization from the tiny magnetic materials. However, the heat release during the operation of the SQUID could cause a great problem in experiments below 1?K because of the close proximity of the SQUID and sample. Here we report a digital circuit for the ??-SQUID magnetometer which can reduce the heat release in the low temperature operation and whose parts are commercially available. The circuit consists of a FPGA-digital board with analog-to-digital and digital-to-analog converters, and an independent pre-amplifier. By using this circuit we succeeded in measuring quantum tunneling of magnetization in the single molecule magnet, Mn12Ph at 0.7?K.     

液氢温区单级高频多路旁通型脉冲管制冷机

为了既能降到液氢温区又能确保制冷机的温度稳定性,开展了仅采用长颈管,不使用双向进气进行调相的单级高频多路旁通型脉冲管制冷机的实验研究。首先用数值计算的方法获得了多路旁通开度是否最佳的判据。研制出的制冷机在充气压力1.73MPa,输入电功220W时,无负荷最低制冷温度能够降到23.6K,为目前所报道的在没有双向进气时单级高频脉冲管制冷机获得的最低温度。在达到稳定状态后,制冷机性能稳定,温度波动幅值小于0.1K。在220W输入电功下,能够在29.2K获得0.516W,34.3K获得1.0W的制冷量。     

Performance of the one-stage Joule–Thomson cryocooler fed with a nitrogen–hydrocarbon mixture and built from mass-produced components made for the refrigeration industry

This paper describes the theoretical performance and working parameters of a Joule–Thomson (J-T) cryocooler that is supplied with a nitrogen–hydrocarbon mixture and works in a closed cycle. Nowadays, they are the subject of intensive research in different laboratories around the world, especially in Asia and the USA. The industrial application of this type of cooler is significantly limited by the high values of working pressure for pure nitrogen. Supplying the system with a mixture of nitrogen and hydrocarbons makes it possible to reduce the level of the working pressure down to that which is achieved by commercially available compressors produced for the refrigeration industry. A theoretical analysis of the performance of the cooler is presented, along with the experimental results for different mixtures. The described cooler is characterized by high reliability, simple construction in the low-temperature section, and relatively low manufacturing costs. The system produces about 10 W of cooling power at an approximate temperature of 90 K. The cooling power can be used to cool down high-temperature superconductor magnets, in nanotechnology, for cryomedical applications, and to liquefy small amounts of nitrogen, argon, oxygen, or methane.     

Baseline design of a sorption-based Joule-Thomson cooler chain for the METIS instrument in the E-ELT

METIS, the Mid-Infrared E-ELT Imager and Spectrograph, is one of the proposed instruments in E-ELT (European Extremely Large Telescope). Its infrared detectors require multiple operating temperatures below 77 K. Therefore, active coolers have to be deployed to provide sub-liquid-nitrogen (sub-LN2) temperature cooling. However, the sensitive imaging optical detecting system also demands very low levels of vibration. Thus, the University of Twente proposed a vibration-free cooling technique based on physical sorption. In this paper, we describe the baseline design of such a sorption-based Joule-Thomson cooler chain for the METIS instrument, that is able to deliver cooling powers of 0.4 W at 8 K, 1.1 W at 25 K and 1.4 W at 40 K from a 70-K heat sinking. This design is based on working fluid selection, cascading cooler stages and operating parameter optimization. Also, the performance of the resulting cooler design is analyzed.     

Dynamic micromagnetic field measurement by stroboscopic electronbeam tomography

A stroboscopic electron beam tomography system for measuring the dynamic micromagnetic field of recording heads is presented. A pulsed electron beam, which is synchronized with the recording head driver, is scanned along the recording head surface from all directions. Integration of the magnetic field intensity along the beam path is calculated from the electron beam deflection angle. Intensity distributions of the dynamic magnetic field are calculated using a tomographic reconstruction algorithm. To obtain enough current even in pulsed electron beam operation, a high-brightness Ti/W thermal field emitter is used. This system was successfully applied in measuring the field distributions of a thin-film recording head, with 0.1 μm spatial resolution and 1 ns time resolution at an operation frequency of 30 MHz