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This paper presents experimental results and numerical evaluation of a Vuilleumier (VM) type pulse tube cryocooler. The cryocooler consists of three main subsystems: a thermal compressor, a low temperature pulse tube cryocooler, and a Stirling type precooler. The thermal compressor, similar to that in a Vuilleumier cryocooler, is used to drive the low temperature stage pulse tube cryocooler. The Stirling type precooler is used to establish a temperature difference for the thermal compressor to generate pressure wave. A lowest no-load temperature of 15.1 K is obtained with a pressure ratio of 1.18, a working frequency of 3 Hz and an average pressure of 2.45 MPa. Numerical simulations have been performed to help the understanding of the system performance. With given experimental conditions, the simulation predicts a lowest temperature in reasonable agreement with the experimental result. Analyses show that there is a large discrepancy in the pre-cooling power between experiments and calculation, which requires further investigation. [ABSTRACT FROM AUTHOR]

Micromachined Joule–Thomson (JT) coolers are attractive for cooling small electronic devices. However, microcoolers operated with pure gases, such as nitrogen gas require high pressures of about 9 MPa to achieve reasonable cooling powers. Such high pressures severely add complexity to the development of compressors. To overcome this disadvantage, we combined a JT microcooler with a thermoelectric (TE) pre-cooler to deliver an equivalent cooling power with a lower pressure or, alternatively, a higher cooling power when operating with the same pressure. This hybrid microcooler was operated with nitrogen gas as the working fluid at a low pressure of 0.6 MPa. The cooling power of the microcooler at 101 K operating with a fixed high pressure of 8.8 MPa increased from 21 to 60 mW when the precooling temperature was reduced by the thermoelectric cooler from 295 to 250 K. These tests were simulated using a dynamic numerical model and the accuracy of the model was verified through the comparison between experimental and simulation results. Based on the model, we found the high pressure of the microcooler can be reduced from 8.8 to 5.5 MPa by lowering the precooling temperature from 295 to 250 K. Moreover, the effect of TE cooler position on the performance of the hybrid microcooler was evaluated through simulation analysis. [ABSTRACT FROM AUTHOR]

This paper is concerned with the design, development and performance testing of a cryogenic system for use in high cooling power instruments for ground-based environmental testing. The system provides a powerful tool for a combined environmental test that consists of high pressure and cryogenic temperatures. Typical cryogenic conditions are liquid hydrogen (LH2) and liquid oxygen (LO2), which are used in many fields. The cooling energy of liquid nitrogen (LN2) and liquid helium (LHe) is transferred to the specimen by a closed loop of helium cycle. In order to minimize the consumption of the LHe, the optimal design of heat recovery exchangers has been used in the system. The behavior of the system is discussed based on experimental data of temperature and pressure. The results show that the temperature range from room temperature to LN2 temperature can be achieved by using LN2, the pressurization process is stable and the high test pressure is maintained. Lower temperatures, below 77 K, can also be obtained with LHe cooling, the typical cooling time is 40 min from 90 K to 22 K. Stable temperatures of 22 K at the inlet of the specimen have been observed, and the system in this work can deliver to the load a cooling power of several hundred watts at a pressure of 0.58 MPa. [ABSTRACT FROM AUTHOR]

The magnetocaloric effect is the thermal response of a material to an external magnetic field. This manuscript focuses on the physics and the properties of materials which are commonly used for magnetic refrigeration at cryogenic temperatures. After a brief overview of the magnetocaloric effect and associated thermodynamics, typical requirements on refrigerants are discussed from a standpoint of cooling power density optimization. Finally, a compilation of the most important properties of several common magnetocaloric materials is presented. [ABSTRACT FROM AUTHOR]

Temperature span, cooling power, work, and efficiency are key performance metrics describing active magnetic regenerators used as refrigerators. Numerical methods to calculate these parameters efficiently and accurately are needed. This paper proposes a set of semi-analytic relationships to determine cooling power and magnetic work as a function of temperature span, regenerator characteristics, operating conditions, and magnetic field waveform. Appropriate effective values for magnetocaloric effect and specific heat are defined. Semi-analytic predictions for cooling power and work are validated against experimental data for room-temperature tests as well as a more complex numerical model describing magnetic work. The semi-analytic approach is found to predict performance well over the range of operating conditions considered. [ABSTRACT FROM AUTHOR]

High-temperature superconductivity power-grid technologies require a highly reliable and efficient cryocooler with cooling power of 100W to kilowatt level at liquid-nitrogen temperatures to produce cryogenic environments. This paper describes the design of a coaxial Stirling-type pulse tube cryocooler to meet this need. In the designed cryocooler, the regenerator and pulse tube are lengthened to avoid possible temperature inhomogeneity. In an experiment, the azimuthal temperature difference at the middle of the regenerator was less than 30K. With 7.6kW electric power input, the cryocooler offers more than 520W cooling power at 80K corresponding to a relative Carnot efficiency of 18.2%. When the cooling power was less than 370W, the efficiency is higher than 20%. [ABSTRACT FROM AUTHOR]

Highlights: [•] We have studied heat transfer between an atomized diphasic helium flow and walls. [•] This study is relevant to the cooling of the LHC superconducting final focus magnets. [•] The droplet mist and the deposited film were measured by optical and capacitive means. [•] The experimental non-linear global heat transfer is explained by spray cooling. [•] Superfluidity has a limited influence on the mist cooling power. [ABSTRACT FROM AUTHOR]

Abstract: Governing equations for the thermodynamics of an active magnetic regenerator (AMR) are used to analyze the performance of simplified cycles operating with idealized magnetocaloric material. An idealized material with a magnetocaloric effect that varies linearly with temperature is assumed. Results are presented for cases where balance and symmetry are equal to one () and the refrigerant specific heat is constant. Under these conditions, the temperature distribution, cooling power, work rate, and efficiency are determined for various values of entrained fluid thermal mass (R). A set of sample results are presented for each case as a function of utilization, temperature span, and conductance. Results from the simplified thermodynamic theory are briefly compared to experimental results from the literature. [Copyright &y& Elsevier]

Abstract: Cycle-averaged relationships for heat transfer, magnetic work, and temperature distribution are derived for an active magnetic regenerator cycle. A step-wise cycle is defined and a single equation describing the temperature as a function of time and position is derived. The main assumption is that the convective interaction between fluid and solid is large so that thermal equilibrium between fluid and solid exists during a fluid flow phase (regeneration). Relations for the temperatures at each step in the cycle are developed assuming small regenerative perturbations and used to derive the net cooling power and magnetic work at any location in the AMR. The overall energy balance expression is presented with transformations needed to relate the boundary conditions to effective operating temperatures. An expression is derived in terms of operating parameters and material properties when each location is regeneratively balanced; this relation indicates needed conditions so the local energy balance will satisfy the assumed cycle. By solving the energy balance expression to determine temperature distribution one can calculate work, heat transfer, and COP. [Copyright &y& Elsevier]

Abstract: Vibration-free miniature Joule–Thomson (JT) coolers are of interest for cooling a wide variety of devices, including low-noise amplifiers, semiconducting and superconducting electronics, and small optical detectors used in space applications. For cooling such devices, coolers are needed which have operating temperatures within a wide temperature range of 2–250K. In this paper, the optimization of the working fluid in JT cold stages is described that operate at different temperatures within that range. For each temperature, the most suitable working fluid is selected on the basis of the coefficient of performance of the cold stage, which is defined as the ratio of the gross cooling power to the change in Gibbs free energy of the fluid during compression. In addition, a figure of merit of the heat exchange in the counter-flow heat exchanger is evaluated that depends only on the properties of the working fluid. [Copyright &y& Elsevier]

Abstract: In contrast to the ideal case of unlimited size recuperator, any real Linde–Hampson machine of finite size recuperator can be optimized to reach the extreme rates of performance. The group of cryocoolers sharing the same size recuperator is optimized in a closed form by determining the corresponding flow rate which maximizes its rate of cold production. For a similar group of liquefiers an optimal flow rate is derived to maximize the rate of production of liquid cryogen. The group of cryocoolers sharing a constant and given flow rate is optimized by shortening the recuperator for reaching a maximum compactness measured by the cooling power per unit size of the recuperator. The optimum conditions are developed for nitrogen and argon. The relevance of this analysis is discussed in the context of practice of fast cooldown Joule–Thomson cryocooling. [Copyright &y& Elsevier]

Abstract: The thermal conductivity of liquid oxygen below 80K and pressures up to 1MPa has been measured using a horizontal, guarded, flat-plate calorimeter. The working equation of the calorimeter is based on the one-dimensional Fourier’s law. The gap between the calorimeter plates was measured in situ from a capacitance measurement. The cooling power to the calorimeter is provided by a two-stage Gifford–McMahan cryocooler. The absolute temperatures are measured using platinum resistance thermometers. The results are compared to existing data and analytical models. [Copyright &y& Elsevier]

Constellation-X is a multi-satellite X-ray astronomy mission presently being planned for launch in the 2010 time frame. Each of four identical satellites will contain a telescope and instruments for imaging and spectroscopic analysis of hard and soft X-rays. The X-ray Microcalorimeter Spectrometer (XMS) instrument will use arrays of microcalorimeters to detect X-rays with energies from 0.2 to 6 keV. To achieve the desired energy resolution of 2 eV, the detectors will be cooled to 50 mK. The projected cooling power requirement is approximately 5 μW. In order to meet the mission lifetime requirement of 6 years (with a goal of 10 years), a mechanical cryocooler will be used to provide a heat sink for the low temperature cooler. In this paper we discuss the development of a continuous adiabatic demagnetization refrigerator (CADR) that meets the cooling requirements of the XMS instrument. The four-stage CADR presently being tested can operate continuously at 50 mK using a 4.2 K helium bath as a heat sink. Its cooling power of 6 μW at 50 mK exceeds the XMS requirement, and its peak heat rejection rate is less than 7 mW. Details of the CADR''s design and operation, as well as the development program leading up to a flight-qualified instrument, are discussed. [Copyright &y& Elsevier]

Space pulse-tube cryocoolers require electronics to control the cooling temperature and self-induced vibration. Other functions include engineering diagnostics, telemetry and safety protection of the unit against extreme environments and operational anomalies. The electronics must survive the harsh conditions of launch and orbit, and in some cases severe radiation environments for periods exceeding 10 years. A number of our current generation high reliability radiation hardened electronics units have been launched and others are in various stages of assembly or integration on a number of space flight programs. This paper describes the design features and performance of our next generation flight electronics designed for the STSS payloads. The electronics provides temperature control with better than +/-50 mK short-term stability. Self-induced vibration is controlled to low levels on all harmonics up to the 16th. A unique active power filter limits peak-to-peak reflected ripple current on the primary power bus to less than 3% of the average DC current. The 3 kg unit is capable of delivering 180 W continuous to NGST''s high-efficiency cryocooler (HEC). [Copyright &y& Elsevier]

The control of cryogenic temperatures is usually accomplished by a passive stage, exploiting the combined effect of a thermal mass connected to a thermal resistance; by an active control, often of a PID type, based on the combination of a dedicated sensor, a heater and a controller; or by a combination of the two. Such a system typically uses a controlled stage that is isolated from the source of the fluctuations by a thermal isolator. Controlled insertion of heat into this stage counters the temperature fluctuations reaching the stage. Inherent to this type of system is the insertion of heat into the controlled stage that eventually reaches the cold end of the cooler, reducing the net heat lift available. The larger the thermal isolation, the smaller the reduction of the net heat lift, but with the attendant increase in the interface temperature. Any scheme that can reduce the penalty associated with the loss of heat lift or the temperature offset would be attractive in terms of cooler performance. If the cooler system has a recuperative heat exchanger between the coldest heat sink and a higher temperature precooler, a different approach can be used. In this paper we describe a novel control approach capable of passively damping low frequency fluctuations, requiring minimal reduction of cooler heat lift and minimal temperature increase of the cold end interface. This alternative scheme is based on the idea of controlling the temperature of a section of the recuperative heat exchanger between the coldest precooler and the cold end of the cooler and it has been tested on a 20 K hydrogen sorption JT cooler. [Copyright &y& Elsevier]