Article

# Integration of superconducting magnets with cryogen-free dilution refrigerator systems

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## Abstract

There has been much recent research interest into “cryogen-free” dilution refrigerators. Cryogen-free systems have some advantages from a safety and convenience point of view as liquid cryogens are unnecessary. However, this also makes integrating the low-temperature system with a high magnetic field environment much more challenging. Here we shall describe recent successes of integrating superconducting magnets and dilution refrigerators into one system requiring a single pulse tube cooler. The resulting environment provides experimental temperatures between 7 mK and 30 K and magnetic fields up to 12 T. We shall describe the effects of AC loss heating in such systems on the pulse tube refrigerator when the field is ramped and also the effects of eddy current heating on the mixing chamber in sweeping fields.

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... The approach adopted here is the integration of cryogen-free dilution refrigerators and superconducting magnets [18], with the entire system running from a single pulse tube cooler, with a "bolt-on" nuclear refrigerator to extend the accessible temperature range to below 1 mK, thus maintaining compatibility with a wide range of experimental applications. Given the extreme precautions taken with nuclear adiabatic demagnetization cryostats to create ultra-low mechanical noise environments in order to minimize heat leaks due to vibration of the nuclear stage, the feasibility of realising this goal was not apparent; this has been addressed in the work reported here. ...
... The pulse tube coolers used on cryogen-free dilution refrigerators are known to be a source of mechanical noise, particularly on systems where the room temperature, first (∼ 50 K) and second stage (∼ 3 K) components of the pulse tube coldhead (the volume in which the gas expands) are not vibrationally decoupled from the dilution refrigerator [19]. Initial measurements of cryogen-free refrigerator and magnet systems [18] did not show a significant increase in the base temperature of the refrigerator when the installed magnet was persistent at its full field of 12 T. However in those tests the mixing chamber plate only experienced the fringing field of the magnet, not the full field to which any nuclear refrigerant would be exposed. To our knowledge it has not been demonstrated previously that this potential obstacle to the implementation of nuclear demagnetisation techniques in a cryogen-free environment could be overcome. ...
... In view of the concerns about the possible effects of vibrational heating, both in the pre-cool phase and the warm-up following demagnetization, we opted for PrNi 5 as our coolant. The dilution refrigerator used in this work, an Oxford Instruments Triton 200, has been described previously [18]. The typical performance parameters are a cooling power in excess of 200 µW at 100 mK, and a base temperature below 10 mK. ...
Article
Full-text available
We report experimental demonstration of the feasibility of reaching temperatures below 1 mK using cryogen-free technology. Our prototype system comprises an adiabatic nuclear demagnetisation stage, based on hyperfine-enhanced nuclear magnetic cooling, integrated with a commercial cryogen-free dilution refrigerator and 8 T superconducting magnet. Thermometry was provided by a current-sensing noise thermometer. The minimum temperature achieved at the experimental platform was 600 {\mu}K. The platform remained below 1 mK for over 24 hours, indicating a total residual heat-leak into the experimental stage of 5 nW. We discuss straightforward improvements to the design of the current prototype that are expected to lead to enhanced performance. This opens the way to widening the accessibility of temperatures in the microkelvin regime, of potential importance in the application of strongly correlated electron states in nanodevices to quantum computing.
... The cryogen free dilution refrigerator [1] has been an enabling technology for a range of experimental applications. The rapid uptake of these machines, particularly in the field of quantum information processing, has been driven by their ease of use, large experimental plates and the ability to integrate superconducting magnets [2]. The fact that these systems can be designed without an inner vacuum chamber means installing experimental services can be much more straightforward, and the desire to maximise the experimental payload has led to the design of ever larger and more powerful systems for these applications [3]. ...
... The top-level design concept for these systems is similar to that described previously [2], with a single vacuum space, however the size of the experimental plates has been reduced to keep the system compact. The cold environment is generated by a pulse tube cooler from Sumitomo [7] which has a remote rotary valve and a nominal cooling power at the second stage of 0.9 W at 4.2 K. ...
... It is possible to integrate an experimental superconducting magnet, cooled by the pulse tube via a high thermal-conductivity link [2]. ...
Article
Full-text available
We report the design and performance characteristics of a compact cryogen-free platform. The system is based around a continuous 1 K pot which operates using a small (10 m³ h⁻¹) room temperature circulation pump. The pot cools an experimental plate to ≈ 1.2 K, and has a cooling capacity of 100 mW at a temperature ≈ 1.9 K. Cooling the pot from room temperature to < 2 K takes around 12 hours. The temperature range of the platform can be lowered to < 50 mK with the addition of a small dilution refrigerator, using the 1 K pot as a pre-cooling stage for the circulating ³He. The dilution stage has a typical (continuous) cooling capacity of 30 µW at 100 mK (300 µW at 250 mK) and is designed to operate with just 3 litres of (NTP) ³He.
... The main part of the dilution refrigerator is given by the TritonUHV, a cryogen free 3 He-4 He dilution refrigerator 13,14 for UHV applications, designed and constructed by Oxford Instruments (OI). [15][16][17][18][19] The Triton is subdivided into five different temperature levels which are thermally decoupled from each other as can be seen in Fig. 2. The main work against the outer room temperature thermal radiation is performed by the first stage of a pulsed tube refrigerator with remote motor (PTR, Cryomech PT-410 RM 20 ). It is connected to the first level plate (15) and cools the first level down to 45 K where a cooling capacity of 31.5 W is given. ...
... [15][16][17][18][19] The Triton is subdivided into five different temperature levels which are thermally decoupled from each other as can be seen in Fig. 2. The main work against the outer room temperature thermal radiation is performed by the first stage of a pulsed tube refrigerator with remote motor (PTR, Cryomech PT-410 RM 20 ). It is connected to the first level plate (15) and cools the first level down to 45 K where a cooling capacity of 31.5 W is given. Similar to the first level, the second level is cooled by the PTR second stage (14), achieving a temperature of 4.2 K at 0.9 W. Below the second level, the inner circuit of the 3 He-4 He dilution refrigerator is located, including the temperature levels three, four, and five. ...
... Since the PTR is only in thermal contact with the first two levels, a bypass connected by heat exchangers to each level is used for cooling the lower three levels down from room temperature. 15 Therefore, some of the warm 3 He-4 He mixture is used to cycle through the bypass. Under PTR cooling, the mixture is compressed with decreasing temperature until the whole mixture circulates through the bypass. ...
Article
A new ultra-low temperature experiment including a superconducting vector magnet has been developed for soft x-ray absorption spectroscopy experiments at third generation synchrotron light sources. The sample is cooled below 50 mK by a cryogen free (3)He-(4)He dilution refrigerator. At the same time, magnetic fields of up to ±7 T in the horizontal direction and ±0.5 T in the vertical direction can be applied by a superconducting vector magnet. The setup allows to study ex situ and in situ prepared samples, offered by an attached UHV preparation chamber with load lock. The transfer of the prepared samples between the preparation section and the dilution refrigerator is carried out under cryogenic temperatures. First commissioning studies have been carried out at the Variable Polarization XUV Beamline P04 at PETRA III and the influence of the incident photon beam to the sample temperature has been studied.
... The cryogen-free dilution refrigerator [1], often integrated with a superconducting magnet [2], has become the workhorse of many low-temperature laboratories around the world, and such systems are routinely used to attain temperatures below ∼ 10 mK. In some fields of research, such as quantum information processing, these cryogen-free machines are preferred to "wet" systems using liquid 4 He as the installation of experimental services is more straightforward, for a discussion see [3]. ...
... The overall layout and principle of operation of this new system (single vacuum space, precool circuit, Joule-Thomson condensing stage, automated operation etc.) is similar to smaller systems that have been described previously [2] and so here we focus on the enhancements this new platform offers. ...
... Pulse tube coolers are a source of mechanical vibration and are normally decoupled from the refrigerator plates using high thermal-conductivity, flexible copper-braids [2]. However, the conductivity of these braids is finite, so there is always some δT between the pulse tube stage and the plate. ...
Article
Full-text available
We report the introduction of a new cryogen-free dilution refrigerator experimental platform that provides significant performance enhancements, in several key areas, over the current generation of systems. In particular the ability to: install more experimental services; install higher-field experimental magnets; dissipate more power at the ∼ 4 K stage; and to attain higher cooling powers and lower base-temperatures (below 3.5 mK) at the mixing chamber plate.
... With the installation of high-frequency wiring these refrigerators have been developed into measurement systems for circuit quantum electrodynamics [10] and superconducting qubits [11]. The integration of superconducting magnets [12], with the entire system able to be run from a single pulse-tube cooler, has enabled a wider range of experiments (those requiring magnetic fields) to be performed using this cryogen-free technology [13]. ...
... With no boil-off considerations, cryogen-free systems have evolved to be much wider than their wet counterparts with experimental plates (to which services can be mounted) typically several hundred mm in diameter [12]. This has enabled more and / or more complex services to be installed on dilution refrigerator systems, in particular bulky signal conditioning elements such as cryogenic amplifiers, microwave components (bias-tees, circulators, switches etc.) and filtering (such as metal powder filters, for example [14] and the references therein). ...
... Below their superconducting transition temperature these cables provide very low attenuation and have a small thermal conductivity [15] so in many cases are the ideal solution to this problem. However, with cryogen free dilution refrigerators enabling experiments over extended temperature ranges [12] some care needs to be taken, as the electrical performance of these lines will change (attenuation will increase) dramatically above their transition temperature. ...
Article
Full-text available
Researchers attempting to study quantum effects in the solid-state have a need to characterise samples at very low-temperatures, and frequently in high magnetic fields. Often coupled with this extreme environment is the requirement for high-frequency signalling to the sample for electrical control or measurements. Cryogen-free dilution refrigerators allow the necessary wiring to be installed to the sample more easily than their wet counterparts, but the limited cooling power of the closed cycle coolers used in these systems means that the experimental turn-around time can be longer. Here we shall describe a sample loading arrangement that can be coupled with a cryogen-free refrigerator and that allows samples to be loaded from room temperature in a matter of minutes. The loaded sample is then cooled to temperatures ∼ 10 mK in ∼ 7 hours. This apparatus is compatible with systems incorporating superconducting magnets and allows multiple high-frequency lines to be connected to the cold sample.
... On the other hand, for the cooldown from room temperature to a temperature of ~ 10 K, exchange gas precooling is no longer possible. Instead, an additional cooling loop is necessary in the cryostat (figure 2) where a forced flow of helium gas cools all the components in the DR to the base temperature of the PTR [7]. A separate lab compressor may be used to circulate the helium in the loop, or a small fraction of the helium of the PTR is branched off into it (figure 2) [8]. ...
... CF-DRs can be combined with dry superconducting magnets (figures 2,3). At the time of this writing, the highest field commercially available with a dry DR is 15 T with a 57 mm bore of the magnet [7]. Top loading and bottom loading ports are available with these DRs, and even the exchange of cold samples (~ 4 K) is technically feasible. ...
Article
We review briefly our first cryogen-free dilution refrigerator (CF-DR) which was precooled by a GM cryocooler. We then show how today's dry DRs with pulse tube precooling have developed. A few examples of commercial DRs are explained and noteworthy features pointed out. Thereby we describe the general advantages of cryogen-free DRs, but also show where improvements are still desirable. At present, our dry DR has a base temperature of 10 mK and a cooling capacity of 700 μW at a mixing chamber temperature of 100 mK. In our cryostat, in most recent work, an additional refrigeration loop was added to the dilution circuit. This 4He circuit has a lowest temperature of about 1 K and a refrigeration capacity of up to 100 mW at temperatures slightly above 1 K; the dilution circuit and the 4He circuit can be run separately or together. The purpose of this additional loop is to increase the cooling capacity for experiments where the cooling power of the still of the DR is not sufficient to cool cold amplifiers and cables, e.g. in studies on superconducting quantum circuits or astrophysical applications.
... We have implemented this goal within the EU quantum Flagship project QMiCS together with Oxford Instruments Nanotechnology Ltd. (OINT). Our cryogenic link with a total length of 6.6 m reaches temperatures below 35 mK and connects a home-built dry dilution refrigerator with a commercial Triton500 dilution refrigerator from OINT [125], both dilution refrigerators reaching below 20 mK. During the design phase of the cryogenic link, several unique features were incorporated. ...
Preprint
Full-text available
The field of propagating quantum microwaves has started to receive considerable attention in the past few years. Motivated at first by the lack of an efficient microwave-to-optical platform that could solve the issue of secure communication between remote superconducting chips, current efforts are starting to reach other areas, from quantum communications to sensing. Here, we attempt at giving a state-of-the-art view of the two, pointing at some of the technical and theoretical challenges we need to address, and while providing some novel ideas and directions for future research. Hence, the goal of this paper is to provide a bigger picture, and -- we hope -- to inspire new ideas in quantum communications and sensing: from open-air microwave quantum key distribution to direct detection of dark matter, we expect that the recent efforts and results in quantum microwaves will soon attract a wider audience, not only in the academic community, but also in an industrial environment.
... In the dry DR which we introduced several years ago [1], and in most commercial cryostats there is only one cooling circuit, namely the He-3/He-4 dilution circuit [2]. An additional condensation stage (sometimes referred to as "pot") is not necessary as the condensation of the He-3 flow is taken over by the pulse tube refrigerator. ...
Article
He-3/He-4 dilution refrigerators (DR) are very common in sub-Kelvin temperature research. We describe a pulse tube precooled DR where a separate He-4 circuit condenses the He-3 of the dilution loop. Whereas in our previous work the dilution circuit and the He-4 circuit were separate, we show how the two circuits can be combined. Originally, the He-4 loop with a base temperature of ∼ 1 K was installed to make an additional cooling power of up to 100 mW available to cool cold amplifiers and electrical lines. In the new design, the dilution circuit is run through a heat exchanger in the vessel of the He-4 circuit so condensation of the He-3 stream of the DR is done by the He-4 stage. A much reduced condensation time (factor of 2) of the He-3/He-4 gas mixture at the beginning of an experiment is achieved. A compressor is no longer needed with the DR as the condensation pressure remains below atmospheric pressure at all times; thus the risk of losing expensive He-3 gas is small. The performance of the DR has been improved compared to previous work: The base temperature of the mixing chamber at a small He-3 flow rate is now 4.1 mK; at the highest He-3 flow rate of 1.2 mmol/s this temperature increases to 13 mK. Mixing chamber temperatures were measured with a cerium magnesium nitrate (CMN) thermometer which was calibrated with a superconducting fixed point device.
... The paper published in Physical Review B [23] became the first publication where a powerful cryogen-free dilution refrigerator has been used in a neutron scattering experiment. Today dilution refrigerators of this type are commercially available from a number of companies such as Leiden Cryogenics [26], Air Liquide [27], Oxford Instruments [28], LTLab [29], Cryoconcept, Janis, BlueFors and Ice Oxford. In neutron scattering experiments, an ultra-low temperature sample environment is predominantly required for small samples. ...
Article
Full-text available
A global shortage of helium gas can seriously jeopardise the scientific programmes of neutron scattering laboratories due to the use of cryogenic sample environment in the majority of the neutron scattering experiments. Recently developed cryogen-free technology allows a significant reduction or even a complete elimination of liquid helium consumption. Here we review the impact of the cryogen-free revolution on cryogenic equipment used at large neutron facilities, such as cryostats, dilution refrigerators, superconducting magnets and other cryogenic systems. Particular attention is given to the newly developed superconducting magnets for neutron diffraction and spectroscopy experiments. Use of the cryogen-free approach, as well as cutting-edge superconducting magnet technology and advanced neutron optics allows researcher to achieve extraordinary performance in their experiments, opening up new opportunities in neutron scattering research.
... Currently Oxford Instruments, again in collaboration with ISIS, is developing a split pair superconducting magnet for neutron scattering experiments; the magnet is going to be attached to the 4K shield of the E-18 refrigerator. The design of the magnet is based on a prototype successfully tested by Oxford Instruments [8]. ...
Article
Full-text available
Most neutron facilities have a fleet of cryostats providing low temperature and high magnetic fields for sample environment. This large scale usage of cryogenic equipment requires significant resources and can create a number of problems including health and safety issues and the considerable cost of the required cryogens. The last problem has become more significant due to the increasing costs of liquid helium caused by global helium supply problems. The ISIS facility has an internal development programme intended to gradually substitute all conventional cryogenic systems with cryogen free systems preferably based on the pulse tube refrigerator. The programme includes a number of development projects which are aiming to deliver a range of cryogen free equipment including a top-loading 1.5 K cryostat, superconducting magnets in re-condensing cryostats and cryogen free dilution refrigerators. Here we are going to describe the design of these systems and discuss the results of prototype testing.
... The design and operation of this dilution refrigerator has been described elsewhere [1]. Although both a bottom and top loading version of the same concept have been developed by the authors, only the top loading version will be discussed here. ...
Article
Full-text available
We present results from system tests of top and bottom loading cryogen-free dilution refrigerators which enable cool-down times from room temperature to mK temperatures in 6 to 8 hours. The loading and unloading processes take only a few minutes to perform and the cooling procedure is fully automated. Sample temperatures of less than 10 mK have been achieved with up to 8 coaxial cables and 25 DC-wires connected to the sample holder. We also present cool-down tests of a beam line dilution refrigerator with a 35 kg mass installed on the mixing chamber. A heat pipe was developed to accelerate the cooling of large experimental payloads and the mass was cooled from room temperature to 30 mK in less than 28 hours.
... The returning 3 He/ 4 He gas flow of the dilution circuit and the 4 He stream of the 1K-stage are first cooled and purified in two charcoal filters which are linked to the 1 st stage of the PTC via copper braided ropes. Then both gas streams are cooled in identical hxs at the first stage, at Our precool circuit is very similar to the one described in detail in other work by Batey et al. 7 . A helium gas stream is cooled in hxs by the two stages of the PTC to the temperature of the second stage; this cold helium stream is thermally connected by hxs to the mounting plates of the 1K-stage, the still, the 50 mK plate of the DR and the mixing chamber. ...
Article
At the Walther-Mei{\ss}ner-Institut (WMI), a new cryogen-free 3He/4He dilution refrigerator (DR) has been completed; the cryostat will be employed to cool experiments on superconducting quantum circuits for quantum information technology and quantum simulations. All major components have been made at the WMI. The DR offers lots of space at the various stages of the apparatus for microwave components and cables. E. g., the usable space at the mixing chamber has a height of more than 60 cm and a diameter of 30 cm (mixing chamber mounting plate). To cool cables and cold amplifiers, the DR is equipped with a separate 4He-1K-loop which offers a cooling power of up to 100 mW near 1K. The refrigeration power of the still is 18 mW at 0.9 K; the diameter of its mounting plate is 35 cm. The cryostat rests in an aluminum trestle on air springs to attenuate building vibrations. It is precooled by a Cryomech PT410-RM pulse tube cryocooler (PTC) which is mechanically decoupled from the vacuum can of the cryostat by a bellows assembly. The two stages of the PTC are thermally connected to the DR via copper ropes. There are no nitrogen cooled traps with this DR to purify the gas streams of the 3He and 4He loops; instead, charcoal traps are mounted inside the DR at the first stage of the PTC. The dilution unit has three heat exchangers; its base temperature is 11 mK and its cooling power is 300 {\mu}W at 100 mK.
... Advances in cryogen-free technologies now make low millikelvin temperature platforms widely accessible 28 , with prospects of commercial solutions in the microkelvin range 29,30 . However, cooling electrons in semiconductor devices is challenging. ...
Article
Full-text available
Two-dimensional electron gases (2DEGs) with high mobility, engineered in semiconductor heterostructures host a variety of ordered phases arising from strong correlations, which emerge at sufficiently low temperatures. The 2DEG can be further controlled by surface gates to create quasi-one dimensional systems, with potential spintronic applications. Here we address the long-standing challenge of cooling such electrons to below 1 mK, potentially important for identification of topological phases and spin correlated states. The 2DEG device was immersed in liquid ³He, cooled by the nuclear adiabatic demagnetization of copper. The temperature of the 2D electrons was inferred from the electronic noise in a gold wire, connected to the 2DEG by a metallic ohmic contact. With effective screening and filtering, we demonstrate a temperature of 0.9 ± 0.1 mK, with scope for significant further improvement. This platform is a key technological step, paving the way to observing new quantum phenomena, and developing new generations of nanoelectronic devices exploiting correlated electron states.
... Today dilution refrigerators of this type are commercially available from a number of companies such as Leiden Cryogenics [18], Air Liquide [19], Oxford Instruments [20], LTLab [21], Cryoconcept, Janis, BlueFors and Ice Oxford. ...
Chapter
Full-text available
Today almost a quarter of all neutron scattering experiments performed at neutron scattering facilities require sample temperatures below 1K. A global shortage of helium gas can seriously jeopardize the low temperature experimental programs of neutron scattering laboratories. However the progress in cryo-cooler technology offers a new generation of cryogenic systems with significantly reduced consumption and in some cases a complete elimination of cryogens. Here we discuss new cryogen free dilution refrigerators developed by the ISIS facility in collaboration with Oxford Instruments. We also discuss a new approach which makes standard dilution refrigerator inserts cryogen-free if they are used with cryogen-free cryostats such as the 1.5K top-loader or re-condensing cryostat with a variable temperature insert. The first scientific results obtained from the neutron scattering experiments with these refrigerators are also going to be discussed.
... Today dilution refrigerators of this type are commercially available from a number of companies such as Leiden Cryogenics [30], Air Liquide [31], Oxford Instruments [32], LTLab [33], Cryoconcept, Janis and BlueFors. ...
Chapter
Full-text available
A global shortage of helium gas can seriously jeopardise the scientific programmes of neutron scattering laboratories due to the use of cryogenic sample environment in the majority of the neutron scattering experiments. Recently developed cryogen-free technology allows a significant reduction or even a complete elimination of liquid helium consumption. Here we review the impact of the cryogen-free revolution on cryogenic equipment used at large neutron facilities, such as cryostats, dilution refrigerators, superconducting magnets and other cryogenic systems.
... Advances in cryogen-free technologies now make low millikelvin temperature platforms widely accessible [28], with prospects of commercial solutions in the microkelvin range [29,30]. However cooling electrons in semiconductor devices is challenging. ...
Preprint
Full-text available
Two-dimensional electron gases (2DEGs) with high mobility, engineered in semiconductor heterostructures host a variety of ordered phases arising from strong correlations, which emerge at sufficiently low temperatures. The 2DEG can be further controlled by surface gates to create quasi-one dimensional systems, with potential spintronic applications. Here we address the long-standing challenge of cooling such electrons to below 1$\,$mK, potentially important for identification of topological phases and spin correlated states. The 2DEG device was immersed in liquid $^3$He, cooled by the nuclear adiabatic demagnetization of copper. The temperature of the 2D electrons was inferred from the electronic noise in a gold wire, connected to the 2DEG by a metallic ohmic contact. With effective screening and filtering, we demonstrate a temperature of 0.9$\,\pm\,$0.1$\,$mK, with scope for significant further improvement. This platform is a key technological step, paving the way to observing new quantum phenomena, and developing new generations of nanoelectronic devices exploiting correlated electron states.
... Such a temperature may be routinely provided in the lab by 3 He / 4 He dilution. However, when there is a requirement for the receiving system to be mounted upon a telescope that will follow astronomical targets across the sky this can be problematical for traditional implementations of the dilution refrigerator (DR) because of their complex gas-handling system and limitations in tilt angle (although ± 30°operation is claimed by Batey et al. [1] and the Janis ASTRA dilution refrigerator is specified to operate over an asymmetric tilt range). A novel design was developed by Benoit and Pujol [2], that dispensed with the need for a still to draw 3 He out of the mixing chamber, instead employing a "vortex pump" effect. ...
Article
We present a 3He / 4He dilution refrigerator designed for cooling astronomical mm-wave telescope receivers to around 100 mK. Used in combination with a Gi?fford-McMahon closed-cycle refrigerator, 4He and 3He sorption-pumped refrigerators, our cryogen-free system is capable of achieving 2 microW cooling power at 87 mK. A receiver attached directly to the telescope optics is required to rotate with respect to the downward direction. This scenario, of variable tilt, has proved difficult for typical dilution refrigerators, but our design has a geometry chosen to allow tilt to 45 degrees and beyond.
Article
In this technical report a cryogen-free 1 K cryostat is described where the pot of the 4He refrigeration unit is precooled by the 2nd stage of a pulse tube cryocooler (PTC) from room temperature to T ∼ 3 K via a shunt made from sintered alumina (SA); the total mass of the 1 K stage is 3.5 kg. SA has high thermal conductivity at high temperatures; but below ∼50 K the thermal conductivity drops rapidly, almost following a T3-law. This makes SA an interesting candidate for the construction of a thermal shunt, especially as the heat capacity of metals drops by several orders of magnitude in the temperature range from 300 K to 3 K. At the base temperature of the PTC, the heat conduction of the shunt is so small that the heat leak into the 1 K stage is negligible.
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Helium-3,4 dilution refrigeration is indispensable for low temperature science and engineering as it is the only method which provides temperatures between 0.3 K and 0.005 K for unlimited working periods. Cryogen-free dilution refrigerators are about to replace traditional cryostats with liquid helium precooling. The dilution circuit is always precooled by a two-stage pulse tube cryocooler; therefore, refrigeration capacities are available to the experimentalist at the temperatures of the two stages of the pulse tube cooler, and furthermore at three temperatures of the dilution circuit (~ 0.7 K - still, 0.1 K - heat exchanger, ~ 0.01 K - mixing chamber). However, there are quite a few applications (e.g. quantum information processing or astro-physics) where the cooling power of the still near ~ 1K is not sufficient to cool amplifiers and electric lines. In our work we present a dilution refrigerator where a He-4 cooling circuit has been added in the cryostat to the dilution circuit. This He-4 circuit provides up to 60 mW of refrigeration capacity in addition to the cooling capacity of ~ 30 mW of the still. The dilution circuit and the 1Kcircuit can be operated together or separately.
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The magnetic-field fluctuation thermometer (MFFT) is a noise thermometer that has been developed for thermometry in sub-Kelvin refrigerators. Meaningful MFFT operation requires a prior measurement at one known temperature in the temperature range of operation. From this reference measurement a set of calibration parameters is determined that characterizes the thermometer. Aiming at improving the measurement quality of the MFFT and extending its acceptance, we have developed a setup and procedure for reference measurements at temperatures directly traceable to the PLTS-2000. The reference measurements allow the assessment of methods to determine the uncertainty of a MFFT measurement. Temperature measurements with relative uncertainties of <1% can be obtained within measurement times of ≈10 s.
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The purpose of this article is to describe several concepts of how to cool a modern tabletop dilution refrigerator (DR) with a cryogen-free pulse tube cryocooler (PTC). Tabletop DRs have come more and more into the focus of scientists, recently, because they offer easy access to the mixing chamber mounting plate from all directions and because of their very short cooldown times. However, these milli-Kelvin coolers are precooled with LHe which makes their handling inconvenient and often expensive. In the paper it is explained how a cryocooler can be directly coupled to a DR unit making the use of LHe superfluous. Furthermore, concepts are discussed where a tabletop DR is cooled by a remote PTC; PTC and DR are mounted in separate vacuum containers which are connected by a stainless steel bellows tube. This kind of apparatus would offer an extremely low level of vibration at the mixing chamber mounting plate.
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Thermometry over broader temperature ranges below 4 K usually requires the application of different thermometers to have sufficient precision and accuracy. We have developed dc SQUID-based noise thermometers which cover the temperature range from 4 K to below 10 mK. The working principle is based on the detection of thermal magnetic flux noise generated by noise currents in a metallic temperature sensor. The noise thermometers exhibit a linear characteristic via the Nyquist relation, are fast, compact, and easy to use. The current sensing noise thermometer and the magnetic field fluctuation thermometer we present can be designed to meet different requirements concerning speed and accuracy of the temperature measurement. In both variants the temperature sensor and the SQUID are located at the same temperature level. Experimental results will be given for the characteristics of the thermometers as well as for comparison measurements with the PLTS-2000. A commercial noise thermometer version now is available through the company MAGNICON.
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Regenerators with finite losses are capable of absorbing a limited amount of heat at intermediate temperatures along their length. This paper discusses a simple analytical model and a rigorous numerical model of regenerator behavior under the influence of heat input or heat removal at intermediate temperatures as well as the influence of a steady mass flow superimposed on the oscillating mass flow within the regenerator. The finite time-averaged enthalpy transport through the regenerator undergoes a discontinuity at the location of the heat input to satisfy the First Law of Thermodynamics. The discontinuous enthalpy flow leads to a discontinuous temperature gradient in the axial direction and to an increase in the regenerator loss that must be absorbed at the cold end. However, the increased loss is less than the heat input at the intermediate temperature, which allows the regenerator to provide a certain amount of cooling without the need for a separate expansion stage. This phenomenon is particularly useful for shield cooling and for precooling a gas continuously or at discrete regenerator locations prior to liquefaction at the cold end. For continuous precooling the total heat load can be reduced by as much as 23%. A comparison is made of the system performance with and without intermediate heat input under various conditions. The paper presents design guidelines to determine the amount of heat a regenerator is capable of absorbing at various temperatures. Methods for optimizing the location of discrete heat inputs are presented. The analytical and numerical models are in very good agreement with each other and are consistent with very limited experimental data.
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Low temperature nuclear orientation thermometry, in particular gamma -ray anisotropy thermometry, is discussed both from a theoretical and practical point of view. Detailed information is given on the most often used gamma -ray anisotropy thermometers, along with a comprehensive description of the gamma -ray anisotropy technique. The **6**0Co in (hcp) cobalt single crystal gamma -ray anisotropy thermometer is discussed in considerable detail since it is used more frequently in comparison experiments with other primary thermometers. Recent experimental results using gamma -ray anisotropy thermometers are also reviewed.
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The Planck sorption cooler cold end utilizes Joule–Thomson expansion of hydrogen to achieve temperatures below 18 K. Porous stainless steel sintered elements function as the J–T restriction. We report the flow characteristics of these porous flow restrictors, as a function of temperature over the range 19–300 K at constant pressure, and as a function of pressure at 1.4–5.5 MPa (200–800 psi) at constant temperatures of 19 and 300 K. The J–T restrictors were fabricated in three separate batches, using the same process and materials. Some comments are made regarding the uniformity of results from various fabrication batches.
Article
Helium liquefaction with a two-stage 4 K pulse tube cryocooler is introduced in this paper. The helium liquefier has a feature of precooling helium gas to be liquefied by using inefficiency of the second stage regenerator in the pulse tube cryocooler. This process reduces enthalpy of the incoming helium gas when entering the condenser and significantly increases the condensation rate. Numerical analysis predicts the precooling heat load on the second stage regenerator, decreases the PTC second stage cooling capacity by only 11% of the heat actually absorbed into the regenerator. A prototype pulse tube helium liquefier was built, which has two precooling heat exchangers on the first stage cold head and the second stage regenerator. It continuously liquefies helium with a rate of 4.8 l/day under normal pressure while consumes 4.6 kW power input.
Article
The magnetic field dependence of the resistance of commercial 1000Ω thick film chip resistors between 0.03 and 0.6K in fields from zero to 18T for five resistors and up to 32T for one of those five shows both a positive and a negative magnetoresistance that is temperature dependent. In the field range between 4 and 32T, and over the temperature measurement range, the resistance is proportional to B1/2, and once an individual thermometer is calibrated, it can easily be used to determine the temperature at any field within the range. Measurements of the effect of repeated thermal cycling of these resistors from 300 to 77 and 4.2K show that the resistance continually changes up to 120 cycles to 77K or below then becomes stable.
Article
This report describes the development of low-vibration cooling systems with pulse-tube (PT) cryocoolers. Generally, PT cryocoolers have the advantage of lower vibrations in comparison to those of GM cryocoolers. However, cooling systems for the cryogenic laser interferometer observatory (CLIO), which is a gravitational wave detector, require an operational vibration that is sufficiently lower than that of a commercial PT cryocooler. The required specification for the vibration amplitude in cold stages is less than ±1 μm. Therefore, during the development of low-vibration cooling systems for the CLIO, we introduced advanced countermeasures for commercial PT cryocoolers. The cooling performance and the vibration amplitude were evaluated. The results revealed that 4 K and 80 K PT cooling systems with a vibration amplitude of less than ±1 μm and cooling performance of 4.5 K and 70 K at heat loads of 0.5 W and 50 W, respectively, were developed successfully.
Article
We present thermal conductance measurements of different types of bolted joint at sub-Kelvin temperatures. Joints containing sapphire surfaces provided good thermal isolation; the mechanism appears to be the existence of a very small area over which the surfaces are actually in contact. Various configurations were measured at temperatures between 100 mK and 4 K. The best joint contained sapphire discs separated by diamond powder and had a conductance of 0.26 µWK −1 (T/1 K) 2.9 , where T is temperature. A mechanical support structure constructed from similar joints, but using alumina powder, had a measured heat leak of 2.57 µW between 80 mK and 1.1 K and was capable of supporting a mass of over 10 kg. Joints between metal surfaces provided good thermal conduction; a bolted joint between copper and a beryllium-copper alloy (C17510 TF00) had a measured conductance of 46 mW K −1 at 100 mK, increasing linearly with temperature. Measurements were also made on a copper-copper compression joint using differential thermal contraction to provide the clamping force. The performance was approximately an order of magnitude worse than for the bolted joint. These measurements were all made as part of the development programme for the SCUBA-2 astronomical instrument; the design requirements were achieved for both insulating and conducting joints. Cryogenics 47(5-6):333-342, 2007 http://dx.doi.
Article
The thin-skinned structure in the crust of the northern Tianshan piedmont is explored by an 86-km-long, NS-trending deep seismic reflection profile through the Ürümqi depression of the north margin of the Tianshan mountains. On the CDP stacking section, the first- and second-row parallel to anticlines in the north margin of the Tianshan mountains are shown on the segment to the south of the Shihezi. The detachments, corresponding to the reflection events at TWT 2.5–3.0 s and 5.5–6.0 s respectively, join the crustal deep structure to the reverse fault-fold zone. The Manas thrust extends downwards in listric shape, merges into the detachment at about TWT 2.5 s and joins to the Qingshuihe thrust. The reflection events on 5.5–6.0 s are corresponding to the main detachment, which joins the lower Manas anticline, and finally converge to the Junggar Southern Marginal Fault. A 12–14 km-thick sedimentary basin exists on the region in the north to the Shihezi. The depth of the Moho discontinuity beneath the Junggar basin is about 45 km, and increases southwards to 50 km. The crustal structure inferred from the deep seismic sounding profile and the Bouguer anomaly in the same region is consistent with the image from the deep seismic reflection profile. The seismogenic model of the 1906 Manas earthquake is related to the fault system, which consists of the Qingshuihe thrust, the detachments and the shallow Manas ramp.
Article
In our article, experiments with a pulse tube (PTR) pre-cooled dilution refrigerator (DR) are presented, where an upgraded 3He condensation stage has been tested. The DR had a 3He flow rate of up to 1.1 mmol/s. The 3He gas entering the refrigerator was first pre-cooled to a temperature of ∼50 K at the first stage of the PTR. In the next cooling step, the 3He was run through a recently installed heat exchanger, which was attached to the regenerator of the second stage of the pulse tube cryocooler; at the outlet of this heat exchanger the temperature of the 3He was as low as ∼4 K. Due to the non-ideality of the helium gas, the second regenerator of a two stage PTR has excess cooling power which can be made use of without affecting the base temperature of this stage, and it is this effect which was put to work, here. Finally, the 3He was further cooled in a heat exchanger, mounted at the second stage of the PTR, before it entered the dilution unit of the cryostat.The installation of a heat exchanger at the regenerator of the second stage of the PTR is especially important for the construction of DRs with high refrigeration capacities; in addition, it allows for a plain design of the subsequent Joule–Thomson (JT) stage, and herewith facilitates considerably the construction of “dry” DRs. The condensation rate of the 3,4He mash prior to an experiment was increased. The pressure during condensation could be kept near 1 bar, and thus a compressor was no longer necessary with the modified apparatus.
Article
We present data and constructional details of a 3He/4He dilution refrigerator where a commercial two-stage Gifford-McMahon closed cycle refrigerator (Leybold RGD 510) precools the dilution unit, and thus this refrigerator does not require any external supply of liquid nitrogen or helium. The lowest temperature reached in the mixing chamber was 42 mK at a 3He flow rate of 1 × 10−4 mol s−1.
Article
A dilution refrigerator is described which is precooled by a two-stage pulse-tube refrigerator and thus no cryoliquids are needed to operate this millikelvin cooler. The cool-down process from room temperature to a precooling temperature below 3 K is automatic, and operating the dilution refrigerator is convenient. It takes about two hours to condense the mash; with just a standard rotary pump to circulate the in the dilution circuit and a very simple dilution unit we reached a final temperature of 15 mK.
Article
The vibrations of Gifford-McMahon (GM) and pulse-tube (PT) cryocoolers were measured and analyzed. The vibrations of the cold-stage and cold-head were measured separately to investigate their vibration mechanisms. The measurements were performed while maintaining the thermal conditions of the cryocoolers at a steady state. We found that the vibration of the cold-head for the 4 K PT cryocooler was two orders of magnitude smaller than that of the 4 K GM cryocooler. On the other hand, the vibration of the cold-stages for both cryocoolers was of the same order of magnitude. From a spectral analysis of the vibrations and a simulation, we concluded that the vibration of the cold-stage is caused by an elastic deformation of the pulse tubes (or cylinders) due to the pressure oscillation of the working gas.
Article
We have investigated the usability of a Coulomb blockade thermometer (CBT) at low temperatures around 50 mK and in high magnetic fields up to 27 Tesla. The experiments performed extend previous investigations both to lower temperatures and higher magnetic fields. We show that CBTs provide an easy way of magnetic field independent thermometry in an up to now problematic temperature range below the applicability of vapor pressure thermometry and above the working range of nuclear orientation thermometry, which are established methods of thermometry in magnetic fields.
Article
We describe the concept and experimental demonstration of primary thermometry based on a four-probe measurement of a single tunnel junction embedded within four arrays of junctions. We show that in this configuration random sample specific and environment-related errors can be avoided. This method relates temperature directly to Boltzmann constant, which will form the basis of the definition of temperature and realization of official temperature scales in the future.
A low temperature heat pump
• Gifford We Mcmahon
• Ho
Gifford WE, McMahon HO. A low temperature heat pump. In: Proceedings of the 10th international congress of refrigeration; 1959. p. 1.
A helium-3 dilution refrigerator
• Hall