“Dry” dilution refrigerator with pulse-tube precooling

We present construction details and performance measurements of a “dry” dilution refrigerator which is precooled by a pulse-tube refrigerator; no cryo-liquids are required to operate this cryostat. The dilution refrigerator is equipped with a double mixing chamber and reaches a lowest temperature of 4.3 mK. The ³He throughput of the refrigerator was between 100 and 300 μmol/s. The total heat leak into the mixing chamber was determined to be 75 nW. For thermometry we had a ³He melting-curve thermometer at our disposal. Ease of operation and reliability are the characteristics of this type of cryostat.     

Development of a thermodynamic model for a cold cycle 3He-4He dilution refrigerator

A thermodynamic model of a ³He-⁴He cold cycle dilution refrigerator with no actively-driven mechanical components is developed and investigated. The refrigerator employs a reversible superfluid magnetic pump, passive check valves, a phase separation chamber, and a series of recuperative heat exchangers to continuously circulate ³He-⁴He and maintain a ³He concentration gradient across the mixing chamber. The model predicts cooling power and mixing chamber temperature for a range of design and operating parameters, allowing an evaluation of feasibility for potential ³He-⁴He cold cycle dilution refrigerator prototype designs. Model simulations for a prototype refrigerator design are presented.     

Concepts for a low-vibration and cryogen-free tabletop dilution refrigerator

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.     

He/He dilution refrigerator precooled by Gifford-McMahon cooler II. Measurements of the vibrational heat leak

In this paper we describe improvements and changes to a ³He/⁴He dilution refrigerator (DR) which is precooled by a Gifford-McMahon (GM) refrigerator instead of a helium bath cryostat. The general layout of this “dry” DR has been described in detail before. First we report about the installation of two more step heat exchangers into the dilution unit to reduce its final temperature, which was thus lowered from 15 to 8.1 mK. To measure these temperatures we inserted a ³He melting-curve thermometer which is described briefly. The heat leak into the mixing chamber, predominantly caused by vibrations of the GM cooler, was measured with a second dilution unit which was equipped with a double mixing chamber (DMC); the method of measuring the heat leak with a DMC is explained. The vibrational heat leak was found to be 0.14 (±0.02) μW.     

Proposed designs for a “dry” dilution refrigerator with a 1 K condenser

Recent development of “dry” dilution refrigerators has used mechanical cryocoolers and Joule-Thomson expansion stages to cool and liquefy the circulating ³He. While this approach has been highly successful, we propose three alternative designs that use independently-cooled condensers. In the first, the circulating helium is precooled by a mechanical cooler, and liquified by self-contained ⁴He sorption coolers. In the second, the helium is liquefied by a closed-cycle, continuous flow ⁴He refrigerator operating from a room temperature pump. Finally, the third scheme uses a separate ⁴He Joule-Thomson stage to cool the ³He condenser. The condensers in all these schemes are analogous to the “1-K pot” in a conventional dilution refrigerator. Such an approach would be advantageous in certain applications, such as instrumentation for astronomy and particle physics experiment, where a thermal stage at approximately 1 K would allow an alternative heat sink to the still for electronics and radiation shielding, or quantum computer research where a large number of coaxial cables must be heat sunk in the cryostat. Furthermore, the behaviour of such a refrigerator is simplified due to the separation of the condenser stage from the dilution circuit, removing the complex interaction between the 4-K, Joule-Thomson, still and mixing chamber stages found in current dry DR designs.     

Operation of superconducting magnet with dilution refrigerator insert in zero boil-off regime

The combination of high magnetic field and ultra-low temperatures has proved to be indispensable for a broad range of condensed matter physics experiments. However problems with the global helium supply have raised significant concern about affordability of conventional cryogenic equipment. The latest developments in cryo-cooler technology offer a new generation of cryogenic systems in which the cryogen consumption can be significantly reduced and in some cases completely eliminated. We have demonstrated a new high magnetic field - ultra-low temperature neutron scattering sample environment system based on re-condensing technology. In our tests we have shown that the 9 T superconducting magnet, built for the ISIS facility, can be run with a dilution refrigerator insert in continuous zero boil-off regime without any additional cooling.     

Integration of superconducting magnets with cryogen-free dilution refrigerator systems

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.     

A combined 3He-4He dilution refrigerator

A refrigerator capable of operating both in evaporation and dilution modes is described. The refrigerator provides preliminary cooling of investigated samples down to a starting temperature of about 0.3 K and permits the location of samples directly in the mixing chamber as well as their replacement at any stage of operation.The replacement of samples lasts about 20 min. The cooling time for a sample from room temperature down to 15 mK is about 3 h. The minimal temperature is 10 mK, and the ³He circulation rate is about 5 × 10^?4mols^?1.     

A 115 K thermoacoustically driven pulse tube refrigerator with low onset temperature

After the modifications of jacket type water coolers and stacks, and the optimizations of the openings of orifice and double inlet valves, a refrigeration temperature as low as 115.4 K has been achieved by a thermoacoustically driven pulse tube refrigerator. By operating the double inlet valve of the pulse tube refrigerator, the onset temperature of the thermoacoustic system decreases from 550 to 340 °C. It provides the possibility of utilizing the low-grade heat energy.     

On cycle-averaged pressure in a G-M type pulse tube refrigerator

An experimental investigation has been carried out on dynamical pressures of the viscous compressible flow oscillating at different locations in a Gifford-McMahon (G-M) type pulse tube refrigerator operating at cycle-steady states. Measurements show that the oscillating amplitude of the pressure was largest at the hot end of the regenerator while the cycle-averaged pressure was the largest in the reservoir. The latter characteristics can be explained based on a cycle-averaged and cross-sectional averaged of the governing equations for a compressible viscous oscillating flow. The reason why the cycle-averaged pressure of the compressible flow oscillating at low frequencies in a tube increases from the wave generator toward the reservoir is analyzed. In addition, the effect of the cycle-averaged pressure on the refrigeration performance is discussed, which can be used to explain why the system with proper asymmetric charging and discharging periods has a better performance than a symmetric one in a G-M type pulse tube refrigerator.     

Influence of resonance tube length on performance of thermoacoustically driven pulse tube refrigerator

A resonance tube is an important component of a thermoacoustic engine, which has great influence on the performance of the thermoacoustically driven pulse tube refrigerator. A standing wave thermoacoustic engine is simulated with linear thermoacoustics. Computed results show that an appropriate accretion of the resonance tube length may lead to a decrease of the working frequency and an increase of the pressure amplitude, which will improve the match between the thermoacoustic engine and the pulse tube refrigerator. The theoretical prediction is verified by experiments. A refrigeration temperature as low as 88.6 K has been achieved with an optimized length of the resonance tube, helium as working gas, and 2200 W of heating power.     

Effect of pulse tube volume on dynamics of linear compressor and cooling performance in Stirling-type pulse tube refrigerator

In a Stirling-type pulse tube refrigerator (PTR), the pulse tube volume affects the dynamic behavior of a linear compressor as well as the cooling performance of PTR. In this study, PTRs which have different pulse tube volume are tested and simulated. The simulation code is verified with the experimental measurement of piston displacement, pressure wave, input power and cooling capacity. And then, the power transfer from the electric power input to the cooling capacity is explained with the simulation results. The smaller pulse tube increases the resonant frequency of a linear compressor and suppresses the piston motion because it imposes larger gas spring effect and also larger gas damping effect to the piston. The smaller one allows larger power transfer from electric power to expansion PV work despite the smaller piston displacement, but shows less cooling capacity due to larger thermal losses.     

Design of the Cryogen-Free Cryogenic System for the CUORE Experiment

We present here the final design of the cryogenic system where the CUORE detector will be installed in 2010. It is a large cryogen-free cryostat cooled by pulse tubes and by a high-power dilution refrigerator. To avoid radioactive background, about 15000 kg of lead will be cooled to below 1 K and only few construction materials are acceptable. The detector assembly will have a total mass of about 1500 kg and must be cooled to less than 10 mK in a vibration-free environment. We discuss the adopted technical solutions, the results of the preliminary thermal analysis of the system, and its expected performance.      

Numerical method of inertance tube pulse tube refrigerator

A nodal analysis method for simulating inertance tube pulse tube refrigerators is introduced. The energy equation, continuity equation, momentum equation of gas, energy equation of solid are included in this model. Boundary condition can be easily changed to enable the numerical program calculate thermal acoustic engines, inertance tube pulse tube refrigerators, double inlet pulse tube refrigerators, and others. Implicit control volume method is used to solve these equations. In order to increase the calculation speed, the continuity equation is changed to pressure equation with ideal gas assumption, and merged with momentum equation. Then the algebraic equation group from continuity and momentum equation becomes one group. With this numerical method, an example calculation of a large scale inertance tube pulse tube refrigerator is shown.     

Stirling-type pulse tube refrigerator (PTR) with cold compression: Cold compressor,colder expander

This research paper focuses on the performance prediction and its validation via experimental investigation of a Stirling-type pulse tube refrigerator (PTR) equipped with a cold linear compressor. When the working gas is compressed at cryogenic temperature, the acoustic power (PV power) can be directly transmitted through the regenerator to the pulsating tube without experiencing unnecessary precooling process. The required PV power generated by the linear compressor, furthermore, can be significantly diminished due to the relatively small specific volume of the working gas at low temperature. The PTR can reach lower temperature efficiently with higher heat lift at the corresponding temperature than other typical single-stage Stirling-type PTRs. Utilizing a cryogenic reservoir as a warm end and regulating the entire operating temperature range of the PTR will enable a PTR to operate efficiently under space environment.In this research, the experimental validation as a proof of concept was carried out to demonstrate the capability of PTR operating between 80 K and 40 K. The linear compressor was submerged in a liquid nitrogen bath and the lowest temperature was measured as 38.5 K. The test results were analyzed to identify loss mechanisms with the simple numerical computation (linear model) which considers the dynamic characteristics of the cold linear compressor with thermo-hydraulic governing equations for each of sub components of the PTR. All the mass flows and pressure waves were assumed to be sinusoidal.     

The experimental investigation of a pulse tube refrigerator with a ‘L’ type pulse tube and two orifice valves

In order to simplify the structure of the cold end of the pulse tube refrigerator (PTR) and have a better utilization of the cold energy of the system, a single stage four-valve pulse tube refrigerator (FVPTR) with a ‘L’ type pulse tube structure and two orifice valves at the hot end of pulse tube has been constructed. Verification experiments show that a two-orifice valve structure gives different adjustments to the gas flow rate of the hot end of the pulse tube than that of the one-orifice valve structure, a lowest temperature of 72 K was obtained at a frequency of 2.5 Hz under a system average pressure of 1.6 MPa with 200 mesh bronze screens as regenerator material, 20 mesh copper screens as stuffing material of heat exchanger. Due to the difficulty in manufacturing the thin ‘L’ type pulse tube, the wall thickness of the pulse tube in the experiment is relatively bigger than that of the ordinary pulse tube, which resulted in relatively big system loss and affected the minimum temperature of the system to a certain degree.     

Comparison of two models of a double inlet miniature pulse tube refrigerator: Part A thermodynamics

The cooling of electronic components is of great interest to improve their capabilities, especially for CMOS components or infrared sensors. The purpose of this paper is to present the design and the optimization of a miniature double inlet pulse tube refrigerator (DIPTR) dedicated to such applications. Special precautions have to be considered in modeling the global functioning of small scale DIPTR systems and also in estimating the net cooling power. In fact, thermal gradients are greater than those observed in normal scale systems, and moreover, because of the small dimensions of ducts (diameter), the pulse tube cannot be assumed to be adiabatic. Hence thermal heat conduction phenomena must be considered. Besides dead volumes introduced by junctions and capillaries cannot be neglected any more in front of the volume of the gas tube itself. The hydrodynamic and thermal behaviors of the cooler are predicted by means of two different approaches: a classical thermodynamic model and a model based on an electrical analogy. The results of these analysis are tested and criticized by comparing them with experimental data obtained on a small commercial pulse tube refrigerator.     

Low temperature heat exchanger for an He3He4 dilution refrigerator

The design and techniques for constructing a step-heat exchanger using sintered copper powder are reported. The results of applying it to a He³He⁴ combined dilution refrigerator are presented. The performance of the mixing chamber in a single-cycle mode is considered.     

Compact dilution refrigerator with a cryogenic circulation cycle of He3

A He³-He⁴ dilution refrigerator has been constructed which circulates He³ by using two alternately operating adsorption pumps which are in the same cryostat as the dilution refrigerator. The scheme described here organised the low temperature circulation cycle such that the construction and cryostat communications were essentially simplified, the dimensions of the arrangement were decreased which resulted in a circulation velocity of 10^?4 mole s^?1, which was sufficient to obtain a temperature in the order of 10 mK.     

A tiltable single-shot miniature dilution refrigerator for astrophysical applications

We present a ³He/⁴He dilution refrigerator designed for cooling astronomical mm-wave telescope receivers to around 100 mK. Used in combination with a Gifford–McMahon closed-cycle refrigerator, ⁴He and ³He sorption-pumped refrigerators, our cryogen-free system is capable of achieving 2 μW 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° and beyond.