An experimental method is described for determining the low-temperature heat capacity (C(p)) of mg-sized powder samples using the Quantum Design "Physical Properties Measurement System" (PPMS). The powder is contained in an Al pan as an ∼1 mm thick compressed layer. The sample is not mixed with Apiezon N grease, as compared to other methods. Thus, it is not contaminated and can be used for further study. This is necessary for samples that are only available in tiny amounts. To demonstrate the method various samples, all insulating in nature, were studied including benzoic acid, sapphire and different silicate minerals. The measurements show that the method has an accuracy in C(p) to better than 1% at T above 30-50 K and ±3-5% up to ±10% below. The experimental procedure is based on three independent PPMS and three independent differential scanning calorimetry (DSC) measurements. The DSC C(p) data are used to slightly adjust the PPMS C(p) data by a factor CpDSC/CpPPMSat298K. This is done because heat capacities measured with a DSC device are more accurate around ambient T (⩽0.6%) than PPMS values and is possible because the deviation of PPMS heat capacities from reference values is nearly constant between about 50 K and 300 K. The resulting standard entropies agree with published reference values within 0.21% for the silicates, by 0.34% for corundum, and by 0.9% for powdered benzoic acid. The method thus allows entropy determinations on powders with an accuracy of better than 1%. The advantage of our method compared to other experimental techniques is that the sample powder is not contaminated with grease and that heat capacity values show less scatter at high temperatures.
We describe the design of a reusable Indium wire seal which has a small profile and is leak tight to better than 1x10(-10) std. cc/sec. from room temperature down to approximately mK. The pressure necessary to deform the Indium wire o-ring is provided by a screw-cap mating to threads on the outside of the cylindrical volume to be sealed.
The objective for the research was to determine how the heat
transfer and pressure drop characteristics changed in a stacked,
wire-screen regenerator, if the screens which were used to construct the
matrix were rolled to decrease their thickness, thereby reducing the
volume of the regenerator. Reducing the “dead volume” for
regenerative refrigeration cycles was the ultimate motivation for this
research. The results show the compactness factor (j<sub>H</sub>/f) is
adversely affected by rolling. Flattening the screens by 15, 30, or 50%,
does not cause a new mechanism for heat transfer to be introduced to the
matrix, but it does increase the total pressure drop. Rolling the
screens causes a decrease in both wetted surface area and pore size,
hence, surface phenomena like heat transfer are reduced, but, the
inertial pressure drop increases. The effectiveness of the regenerator
is also reduced by rolling the screens
A saturation vapor pressure equation, p(T), is an essential component in the 3He state equation currently under development. The state equation is valid over the range 0.01–20 K with pressures from 0 to the melting pressure or 15 MPa. The vapor pressure equation consequently must be valid from 0.01 K to the critical temperature. This paper surveys available 3He critical temperature and pressure measurements, leading to new recommended critical values of 3.3157 K and 114603.91 Pa. The ITS-90 temperature scale is defined by the 3He vapor pressure from 0.65 to 3.2 K. A new vapor pressure equation is developed for the interval from the upper end of the T90 scale to this newly defined critical point, employing a mathematical form in which the second derivative d2p/dT2 diverges in agreement with scaling laws at the critical point. Below 0.65 K, an empirical vapor pressure expression is adopted, consistent with a theoretical expression valid in the limit T → 0. These two new components are fitted to be piecewise continuous with the EPT-76 p(T) scale rather than the ITS-90 T(p) scale between 0.65 and 3.2 K. Probable deviations between this vapor pressure scale and PLTS-2000 melting pressure–temperature scale are recognized, but not reconciled.
This paper describes a refrigerator which employs both the dilution of 3He in 4He and 3He or 4He evaporation cycles, to contribute to the total heat load. The refrigerator operates over the temperature range 0.028–4.2 K, which can be covered using a simple change-over method in the external gas connections. Using the evaporation cycle, the region above 0.75 K can be reached, which is inaccessible using dilution. In the joint mode when both the evaporation and dilution cycles are in use, the total cooling power of the refrigerator at 0.4 < T < 0.75 K can be much higher than the cooling power of the 3He-4He dilution cycle alone. The refrigerator allows the study of temperature dependence of the heat gain caused by recondensing of a superfluid film and the heat conduction of the gas.
In order to design efficient low temperature devices, one must often know the thermal conductivity of materials from which the devices will be built. Other researchers have reported measurements on useful materials, but the thermal conductivity of many of the materials used by low temperature physicists at Cornell has not been published. The purpose of this work is to report the thermal conductivity of some of them.
An empirical method is presented to calculate magneto-resistance (MR) and a corresponding temperature shift of the well known TVO temperature sensors in the ranges from 0.1 K to approximately 10 K and up to 8 T. Features of MR, depending on the temperature and magnetic field, are discussed. The maximum positive MR-values, depending on the magnetic field, correspond to the temperature of T ≈ 0.67 K: 12.5% at 8 T. At the fixed magnetic field the zero MR-value is reached both by increasing or decreasing the temperatures with respect to 0.67 K. For example, at the magnetic field of 4 T, the zero MR-value appears at T ≈ 0.2 K and T ≈ 6 K. A constant moderate negative relative MR of 19%, which does not depend on the magnetic field from 2 to 8 T, is revealed at T ≈ 0.1 K. In the ranges from 1.5 to 4.4 K and from 0 to 9 T, in particular, the temperature shift has a linear dependence on B-value and inverse proportionality on the dimensionless sensitivity S. A brief explanation of the behavior of TVO sensors under magnetic fields is presented as well.
A.c. and impulse dielectric strengths of cryogenic liquid under atmospheric pressure-boiling state were investigated experimentally, with reference to the behaviour of a thermally induced bubble, which might be generated at quenching condition of immersed-cooling superconducting devices, using a parallel plane electrode configuration. The experimental results with LN2 as the liquid show that the bubble shape under electric field stress depends significantly on the applied voltage waveform. That is, the thermal bubble under a.c. stress varies complicatedly in its shape according to the sinusoidal variation in voltage and vibrates violently, whereas the effect of impulse stress on the boiling phenomenon is negligible. With a.c. voltage, the breakdown voltage falls suddenly from liquid breakdown voltage near to one of the saturated gases at a threshold heater power of boiling onset.In constrast to this, with impulse voltage the reduction of breakdown voltage with heater power is gradual and the time to breakdown depends on the existence of thermal bubbles. These breakdown characteristics are attributed to the bubble behaviour in the presence of electric field.
A design optimization procedure of a 0.1-ton/day active magnetic regenerative (AMR) hydrogen liquefier model is described. The liquefier is proposed for the industrial liquid hydrogen market with overall efficiency being the primary measure of performance. This performance is described here in terms of particle size, bed length, and inter-stage temperature. Efficiency comparable to larger gas cycle plants is predicted. The magnetic liquefier may be modified to operate as a two-stage magnetic refrigerator between 77 and 20 K with high efficiency. The paper describes an optimization method as applied to the design of a two-stage AMR hydrogen liquefier and presents the associated results. A five-parameter optimization process is performed since there are five changeable parameters; the low- and high-stage particle sizes, the low- and high-stage bed lengths, and the inter-stage temperature. Model results are presented and compared with experimental results of an actual liquefier.
The critical temperature and the electromagnetic properties of NbTi-Cu composites with ≈9.4 × 106 very fine superconducting filaments, prepared by the multiple compaction process, were measured. The corresponding critical current density, volume pinning force, effective transverse resistivity, hysteresis and eddy current losses at different field amplitudes and frequencies up to 50 Hz, were determined and some of these quantities were compared with theoretical calculations including proximity, size and surface effects. The Jc(d) dependence on the filament diameter, d, is explained by the change of the upper critical field, Bc2, with Tc, where the Tc(d) dependence is taken from experimental values. The unexpected very high critical current densities at low fields (> 1011 A m−2 at B ≲0.5 T) and the maximum volume pinning force () obtained at unusually low fields of B ≈ 1 T were explained theoretically by the existence of a one-dimensional flux line lattice in the filaments. The magnetization curves, the a.c. losses, the ratio of JcM from magnetization and JcT from resistive measurements, as well as the theoretical calculations show that the first compaction level strands behave like monofilaments with strong surface current contribution. This is mainly due to the proximity effect which renders the copper between the filaments to be essentially superconducting up to applied fields of ≲5 T. To eliminate the increase of hysteresis losses due to this effect, some highly resistive material, e.g. cupronickel, should be introduced at the basic compaction of the filaments. A peculiar minimum on the magnetization curves appears that is not caused by flux jumps on reversing the magnetic field direction.
Breakdown phenomena initiated by thermally generated bubbles in liquid helium are investigated experimentally. The process is considered to be of importance for the design of electrical insulation of superconducting devices. The results of the investigations show that the boiling processes are significantly affected by the application of an electrostatic field and that the bubble shape varies complicatedly with the magnitude of electric field, heater power for generation of bubble and the arrangement of electrode system. Thus the breakdown voltage of liquid helium in a parallel plane gap depends on those experimental conditions. It is argued that the breakdown strength of saturated liquid helium at atmospheric pressure in the presence of thermal bubbles becomes lower than that of saturated helium vapour at the same pressure.
A study has been made of the influence of carbon and aluminium additions on the mechanical properties under uniaxial tensile loading and by Charpy V notch impact tests at room temperature and −196°C on the Fe-24 % Mn alloys. It is concluded that the Fe-24 % Mn-5 % Al-0.2 % C appears as a new nickel-free iron-based alloy which is particularly interesting for cryogenic applications. In these alloys, both additions of carbon and aluminium contribute to the stability of the austenitic phase by suppressing the γ → ϵ martensitic transformation of the binary Fe-24 % Mn and to the solution hardening of the manganese-rich austenitic alloy.
We present measurements of the low-temperature thermal conductivity of a number of polymeric and composite materials from 0.3 to 4 K. The materials measured are Vespel SP-1, Vespel SP-22, unfilled PEEK, 30% carbon fiber-filled PEEK, 30% glass-filled PEEK, carbon fiber Graphlite composite rod, Torlon 4301, G-10/FR-4 fiberglass, pultruded fiberglass composite, Macor ceramic, and graphite rod. These materials have moderate to high elastic moduli making them useful for thermally-isolating structural supports.
We have determined the magnetoresistance of RuO2-based resistors (Scientific Instruments (SI) RO-600) between 0.05 and 0.3 K in magnetic fields up to 8 T. The magnetoresistance is negative around 0.5 T and then becomes positive at larger fields. The magnitude of the negative magnetoresistance increases rapidly as the temperature is lowered, while that of the positive magnetoresistance has smaller temperature dependence. We have also examined the temperature dependence of the resistance below 50 mK in zero magnetic field. It is described in the context of variable-range hopping (VRH) conduction down to 15 mK. Hence, the resistors can be used as thermometers down to at least 15 mK.
Results are presented of the Kapitza conductance between annealed and electropolished ‘clean” copper samples and liquid He4 in the temperature range 0.3 K to 1.3 K. A description is given of the apparatus and techniques developed to enable in situ high vacuum treatment of the sample surface, without an excessive heat load on the He3 refrigerator during measurements of the Kapitza conductance. The experimental results of this work are in excellent agreement with previous results, measured at higher temperatures, for similarly prepared surfaces. The temperature dependence of the measured heat transfer coefficient shows a clear maximum at 1 K. This new result is in qualitative agreement with the theoretical treatment of the Kapitza conductance in terms of acoustic theory, modified to include phonon attenuation and impedance matching. However the large magnitude of the heat transfer coefficient, suggests that some alternative mechanism may dominate the Kapitza conductance, even to ‘clean’ surfaces.
Ultracold neutrons can be produced by exciting superfluid liquid helium with cold neutrons. A 3 m long horizontal cryostat, dubbed Mark 3000, which was designed and operated to create this process is described.
A calorimeter of high sensitivity for measurements of the specific heat of thin films in the temperature range 0.6–4.4 K is described. A Be foil serves as the substrate. The special arrangement allows measurements on highly disordered metallic films produced by quench-condensation on the cooled substrate.
The properties of some RuO2 thick film resistors, Philips type RC-01, have been studied between 15 mK and 4 K in magnetic fields up to 5 T. Resistors with room temperature resistance values between ≈ 1.0 and 4.7kΩ appear to have sufficient sensitivity, dR/RdT, for them to be used as secondary thermometers for temperatures < 4 K Thick film resistors have several advantages over carbon composition resistance thermometers which are used more generally, e.g. a better reproducibility, an easier method of mounting on metal surfaces and a lower heat capacity. However, in contrast to what has been published so far about thick film resistors from other manufactures, the type RC-01 shows a considerable magnetoresistance, which is of the same order as that found for carbon resistors.
We present here a double stage adiabatic demagnetization apparatus for specific heat measurements down to 0·015°K. Great care has been taken to avoid heat leaks which have been reduced to less than 1 erg/min. To cool salts, sample, and shields from 80°K to 1·3°K, a heat exchanger using the circulation of He has been built. The principal thermometer is a 1 g sphere of cerium magnesium nitrate (CMN), the susceptibility of which is measured with a Cryotronics mutual inductance bridge. A primary current divider has been added to decrease the stray heating of the specimen holder by the primary field below 1°K and to increase the sensitivity during the calibration. The thermal equilibrium between the sample and the thermometer has been studied. A specific heat as low as 70 μJ at 70 mdeg has been measured. The precision is discussed.
Four commercial Sunpower M87N Stirling-cycle cryocoolers will be used to extend the lifetime of the Alpha Magnetic Spectrometer-02 (AMS-02) experiment. The cryocoolers will be mounted to the AMS-02 vacuum case using a structure that will thermally and mechanically decouple the cryocooler from the vacuum case. This paper discusses modifications of the Sunpower M87N cryocooler to make it acceptable for space flight applications and suitable for use on AMS-02. Details of the flight model qualification test program are presented.AMS-02 is a state-of-the-art particle physics detector containing a large superfluid helium-cooled superconducting magnet. Highly sensitive detector plates inside the magnet measure a particle’s speed, mass, charge, and direction. The AMS-02 experiment, which will be flown as an attached payload on the International Space Station, will study the properties and origin of cosmic particles and nuclei including antimatter and dark matter.Two engineering model cryocoolers have been under test at NASA Goddard since November 2001. Qualification testing of the engineering model cryocooler bracket assembly including random vibration and thermal vacuum testing was completed at the end of April 2005. The flight cryocoolers were received in December 2003. Acceptance testing of the flight cryocooler bracket assemblies began in May 2005.
The enthalpy-pressure (H-p) diagram of He3 was determined by experiment. All important data for the operation of a continuous He3 refrigerator working on the Joule-Thomson principle can be derived from this diagram. In particular, the achievable refrigerating capacity at preselected operating conditions may be immediately ascertained. The inversion curve of He3 for T ⩽ 4.17 K was determined from the H-p diagram and is in fair agreement with the data for T ⩾ 4 K already known.
An inductive superconducting fault current limiter protects power system by limiting the amplitude of fault current by the inductance of its dc reactor. Therefore, it is very important to design the dc reactor of high critical current prior to fabrication. At first, the optimal design parameters were calculated by using finite element method and then the superconducting dc reactor for 1.2 kV/80 Arms inductive superconducting fault current limiter was designed by considering the conduction-cooling characteristics. Moreover, the design, fabrication and conduction-cooling method of the superconducting dc reactor were introduced. Actually, the superconducting dc reactor was fabricated and cooled down to 20 K by using GM cryocooler. Finally, the short-circuit test was performed and the experimental results were discussed.
The Kapitza conductance and thermal conductivity of ofhc-copper, niobium, ultra high purity aluminium, and of the aluminium alloy 6061 A1 have been measured in the temperature range from 1.3 to 2.1 K, yielding both quantities in the same steady state experiment. The temperature dependence of the Kapitza conductance, ho, was between T3.3 and T4.6 for the different samples, which is higher than the most frequently observed T3 dependence. The magnitude of ho for both ofhc-copper and aluminium agrees well at 1.9 K with an empirical prediction, but for niobium it is a factor of two to four lower than the value predicted. At 1.9 K, ho is higher by a factor of two for an annealed and chemically polished niobium sample than for an untreated sample. The thermal conductivity measured from ofhc-copper and 6061 A1 as in good agreement with the value calculated from the resistivity of these materials and the Wiedemann-Franz law. The measured thermal conductivity obtained for an annealed niobium sample is a factor of 2.8 higher than the highest published value.
Regenerative cryocoolers that employ 4He as working fluid can only reach a lowest temperature of about 2 K. This limitation can be overcome by the use of 3He as working fluid. Here we report on the performance of a two-stage pulse tube cooler that consists of two parallel stages with independent gas circuits. The pressure oscillation in each stage is generated by means of a separate compressor in combination with a rotary valve. With 4He in both stages, the minimum no-load temperature of the 2nd stage was 2.23 K, and cooling powers of 50 W at 53 K and 380 mW at 4.2 K were simultaneously available at electrical input powers of 4.54 and 1.45 kW to the 1st and 2nd stage, respectively. Using 3He as working fluid in the 2nd stage, a minimum stationary temperature of 1.27 K has been achieved, which is, up to now, the lowest temperature obtained by regenerative cryocoolers. At an electrical input power of 1.3 kW, the 2nd stage provides a cooling power of 42 mW at 2.0 K and 518 mW at 4.2 K. With 3He, at the same operating condition, the cooling power at 4.2 K was found to be larger than with 4He.
A new tapered resonator was introduced in the thermoacoustic-Stirling heat engine (TASHE) to explore its potential of achieving higher pressure ratio. With average pressure of 1.5 MPa and heating power of 3 kW, a high pressure ratio above 1.40 was obtained, which is very beneficial to be a powerful driving source of pulse tube cryocoolers or thermoacoustic refrigerators. Moreover, a relatively low onset temperature of 73 °C was also observed that showed the feasibility of its application in the field of using low quality heat sources.
The molar volume of pure liquid has been determined by a capacitance-measurement technique in the temperature range from 50 to 1000 mK and the pressure range from 0 to 1.57 MPa. For any practical purposes, the obtained data are fitted with an appropriate formula based on the Landau's phonon–roton model. Our empirical formula can reproduce V40(T,P) within the accuracy of .
Following pioneer work a decade ago at CEA/CEN-Grenoble, France we have designed and constructed a static magnetic refrigerator with a capacity of about 20 W, operating on a quasi-Carnot cycle between 1.8 and 4.5 K. The active core, which contains 10.1 kg of single-crystal gadolinium-gallium garnet (GGG), is magnetized and demagnetized by a 3.5 T pulsed-field, low-loss superconducting magnet operating at 4.5 K. Thermal switching to the cold and warm sources is produced by alternatively flushing the core with liquid helium from the 1.8 and 4.5 K baths, by means of alumina-piston and -cylinder displacer pumps. Precise modelling of thermodynamic properties of GGG, as well as detailed analysis of the different sources of irreversibility, allow to estimate the available refrigeration power and optimize the operating cycle.
A tandem magnetic refrigerator has been developed for continuous operation between 4.2 and 1.8 K. This paper presents the design, computer simulation and construction of the prototype machine. The regenerative concept is employed in the magnetic refrigerator to obtain a cascaded magnetic Carnot cycle effect along the temperature axis from the cold end to the warm end. Entropy pumping action from the cold heat reservoir to the warm heat reservoir occurs in the active magnetic regenerator. The inherent sources of irreversibility in regenerative magnetic refrigerators, i.e. heat capacity imbalance between magnetic refrigerant and heat transport medium, helium entrainment and dead volume effect, have been minimized by design optimization of the system components. The magnetic system of the tandem refrigerator has two virtually identical units, each consisting of a gadolinium gallium garnet (GGG; Gd3Ga5O12) magnetic core, a superconducting magnet, a warm end heat exchanger and a cold end heat exchanger. These components are united by a cryogenic displacer which shuttles the heat transport medium, subatmospheric 3He gas, between the two units. The prototype of the designed magnetic refrigerator has operated continuously, producing a net refrigeration rate of 12 mW per magnetic core at 1.8 K.
Two devices operational with Teslas NbTi coils have been built in order to undertake developmental programmes in superconducting magnets for fusion and high field research. The inner bore for experiments is in the 300 mm range.This paper describes the cryostat system which enables superfluid helium at 1.8 K and atmospheric pressure to be used. Some constructional details concerning the main components are given.The result of the cryogenics tests are finally presented: a fully automated operation at 1.8 K with a rather small standard pump and a liquid helium consumption less than 4 lh -t-1 fed at 4.2 K.
The Large Hadron Collider's superconducting magnets are cooled by superfluid helium at 1.8 K and housed in cryostats that minimise the heat inleak to this temperature level by extracting heat at 70 K and 5 K. In the first generation of prototype cryostats, the radiative heat to the 1.8 K temperature level accounted for 70% of the total heat inleak. An alternative to enhance the cryostat thermal performance incorporates a thermalised radiation screen at 5 K. In order to avoid contact between the 5 K radiation screen and the cold mass, insulators are placed between both surfaces. Sets of commercial fibre glass nets (spacers) are insulator candidates to minimise the heat inleak caused by any accidental contact between the two temperature levels. A model to estimate their performance is presented. A set-up to thermally characterise them has been designed and is also described in the paper. Finally, results as a function of the number of nets forming the spacer, the boundary temperatures and the compressive force in the spacer are presented.
A simple device is described, presently used for magnetization measurements, which, when placed in a liquid helium dewar, enables any temperature, T, between 2 and 1000 K to be regulated to within 0.05 K. The reproducibility of the temperature is better than 0.1 ± 2 × 10−3T (K). The relative homogeneity of the temperature over 50 mm is ≈ 3 × 10−3. It is possible to sweep automatically over the complete temperature range in one day. Only one thermometer (iron-rhodium) is needed for regulation as well as measurement of the temperature. The outer diameter of the device in the sample region is 26 mm. The corresponding useful diameter for the sample is then 6.5 mm. Helium consumption does not exceed 0.15 dm3 h−1, even when regulating at high temperatures. The power radiated onto a helium bath is calculated when a radiation shield is cooled at one end only. A simple expression giving the evaporator pressure as a function of the room temperature parameters is derived, allowing the lowest achievable temperature to be deduced.
A miniature pulse tube refrigerator reaching temperatures below 100K has been developed. In this paper the test results of the refrigerator are reported. The refrigerator is a orifice pulse tube refrigerator. It is based on a miniature Stirling refrigerator. It consists of a compressor, a cold head and. a reservoir. The features are high operating frequency and compact size. The lowest temperature is achieved to 98K.
Young's moduli of the epoxy-resin matrix material used in NEMA-designation G-10CR and G-11CR fibreglass-cloth-reinforced composites were measured dynamically and semi-continuously between ambient and liquid-nitrogen temperatures. Both materials exhibit regular temperature behaviour, showing large Young's-modulus changes, about 125 and 50%, respectively. Internal friction decreased about 80% during cooling to liquid-nitrogen temperature (76 K). The different thermoelastic coefficients together with different internal friction reflect different internal structures in the two materials.
Young's moduli were determined dynamically for two fibreglass-cloth-epoxy composites in the warp, fill, and normal directions between room temperature and liquid-nitrogen temperature. Dynamic internal friction relates inversely to dynamic modulus in the studied materials. The experimental arrangement consisted of a Marx three-component oscillator at frequencies between 40 and 90 kHz.
An 11 T liquid helium-free superconducting magnet designed at 6 K in vacuum using high temperature superconducting current leads was developed. The coil was conductively cooled down from room temperature to 4.1 K in 40 h by two 4 K GM-cryocoolers. In a performance test, the coil temperature rose to 6.8 K for the inner Nb3Sn coil and 5.9 K for the outer NbTi coil, while sweeping the field at 5 A min−1. A central field of 10.7 T in a 52 mm room temperature bore was generated at an operating current of 149 A. Holding the field at 10.5 T was achieved continuously for 24 h at a constant coil temperature of 4.8 K.
The cyclic fatigue life of stable austenitic 20Cr-16Ni-6Mn-0.14N steel in two melts, obtained by different techniques, was studied in a wide range of stress amplitudes, in vacuum, at 293 and 11 K. The influence of grain size on the cyclic yield stress was formulated at both temperatures. The effects of a vacuum and low temperatures on the fatigue characteristics and the crack growth micromechanism were also investigated.
In order to improve the cooling performance of pulse tube cooler (PTC) at 20–40 K, hybrid regenerators are often employed. In this paper a three-layer regenerator, which consists of woven wire screen, lead sphere and Er3Ni is optimized to enhance the cooling performance and explore the lowest attainable refrigeration temperature for a single-stage PTC. The efforts focus on the temperature range of 80–300 K, where woven wire screens are used. Theoretical and experimental studies are carried out to study the metal material and the mesh size effect of woven wire screens on the performance of the single-stage G-M type PTC. A lowest no-load refrigeration temperature of 11.1 K was obtained with an input power of 6 kW. The PTC can supply 17.8 W at 20 K and 39.4 W at 30 K, respectively.
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.
The aim of this paper is to provide an overview of a recent study carried out—within the framework of the European Fusion Program—to design a 12.5 T superconducting dipole. By focusing on the CICC based design option, the overall design procedure is presented. In particular, the 2D optimization of the dipole cross section is described including the magneto-static analysis of the winding and iron yoke, the mechanical analysis of the conductor jacket, insulation and outer cylinder, the conductor hot spot analysis, etc. As far as the thermo-hydraulic design is concerned, simulations of nominal as well as offset operating conditions (e.g., magnet quench) are presented with emphasis on their role played in the overall magnet design. For example, diagrams reporting the helium heat removal capabilities, pressure drop, mass flow, etc. are shown and their usefulness as guidance for the magnet designer described.
A strongly pinned (TI/Pb)-(Sr/Ba)-CaCuO bulk superconductor of TI-(1223) phase with a single TI-O plane has been developed with a magnetic Jc over 43 000 A cm−2 at 77 K and about 600 000 A cm−2 at 20 K and 1 T. Single crystals of TI-(1223) have been prepared and used for investigations of magnetization hysteresis anisotropy using SQUID. When the field was applied parallel to the c axis, a high field peak related to inhomogeneous stoichiometry and properties across the specimen was observed in the magnetization hysteresis loop around 1 T at 77 K. Ag-sheathed tapes with TI-(1223) core were fabricated by the drawing-rolling method. The transport Jc was 25 000 A cm−2 in the absence of a magnetic field, and stayed - 500 A cm−2 up to 8 T at 77 K.
We have carried out systematic studies on the critical current, magnetic irreversibility line and magnetic relaxation for a c-axis orientated powdered (Tl0.5Pb0.5)Sr2Ca2Cu3O9 (Tl-1223) sample. This single TlO layer compound, in comparison with the double TlO layer compound Tl2Ba2Ca2Cu3O10 (Tl-2223), exhibited a high irreversibility line, less temperature and magnetic field dependent critical currents and a smaller flux creep rate. The substantially greater flux pinning ability of Tl-1223 can be understood in terms of its structural characteristics; in Tl-1223, there is only one insulating TlO layer between superconducting CuO2 blocks while there are two in Tl-2223. The increase in the number (and distance) of insulating layers weakens the Josephson coupling between CuO2 blocks along the c-axis and the results in a decrease in the value of Jc. This decoupling between blocks causes the vortex lattice to break into pancake-like vortices which are easily thermally activated.
The fracture toughness and tensile behaviour of arc-melted and hot-rolled Fe-12Ni alloys containing up to 4 atomic percent reactive metal additions of Al, Nb, Ti, or V were determined at 77 K. Cryogenic toughness was improved up to 7.5 times that of binary Fe-12Ni, depending on the reactive metal, its concentration, and annealing temperature.
Thermomechanical processing (TMP) was evaluated as a method of strengthening normally tough iron-12 nickel-reactive metal alloys at cryogenic temperatures. Five iron-12 nickel alloys with reactive metal additions of aluminium, niobium, titanium, vanadium, and aluminium plus niobium were investigated. The primary evaluation was based on the yield strength and fracture toughness at 77 K. At 77 K, a yield strength of 1.3 GPa (187 ksi) with a corresponding KIcd toughness of 243 MPa m (221 ksi in) was achieved with an iron-12 nickel-0.5 aluminium alloy which had undergone a 300 K rolling followed by annealing at 800 to 900 K. This represents the highest combination of cryogenic strength and toughness observed in iron-nickel alloys.
The operation of large superconducting devices at temperatures above that of liquid helium has received sporadic consideration for many years. The main advantage of such a mode of operation has been considered to be the reduced expense of refrigeration, as obviously it can be cheaper to operate a device at 13 K than at 4.2 K. A relatively large superconducting solenoid has been put into operation in this laboratory to evaluate feasibility of such operation, and we find that it is a simple and convenient technique indeed, with the unexpected characteristic of enhanced stability of the niobium-tin superconductor.
Advances in sorption refrigeration at Aerojet Electronic Systems Division have resulted in concept designs which offer significant performance improvements. A 2 W, 137 K sorption refrigerator has been constructed which is the first cooler to incorporate these advanced concepts. This cooler is currently undergoing performance testing. Powdered charcoal is used as the sorbent and methane as the refrigerant. Expansion is accomplished using a passive Joule-Thomson valve. The test results are presented here and a comparison is made to the performance predicted by a detailed second-order analytical model.
A dynamic method for specific heat measurement within a temperature range from 4 to 150°K is described, which allows a direct recording of (dT/dt) proportional to 1/c against the temperature T. The measurements are not mean values over a temperature interval ΔT; in this manner it is possible to determine anomalies of the specific heat occuring in narrow temperature intervals.
Ti 15V–3Cr–3Sn–3Al, sometimes referred to as Ti 15-3-3-3 or “Magic Titanium”, is a candidate material for components requiring high mechanical strength and low thermal conductivity at cryogenic temperatures. The electrical resistance of Ti 15-3-3-3 was measured between 230 mK and room temperature, and the thermal conductivity between 230 mK and 7.7 K. A superconducting transition was observed at TC = 3.89 ± 0.01 K. Below the superconducting transition temperature, the thermal conductivity was fitted to a function of the form λ(T<TC)=α·T·e-β·TC/T, where α = 0.043 ± 0.002 W/(m K2) and β = 0.27 ± 0.01. Above TC, the thermal conductivity of Ti 15-3-3-3 was fitted to a function of the form λ(T > TC) = γ · Tδ, where and δ = 0.4 ± 0.05. The thermal conductivity of Ti 15-3-3-3 is compared with other materials commonly used for the construction of thermally isolating support structures. Ti 15-3-3-3 is shown to exhibit one of the lowest ratios of thermal conductivity to mechanical strength and is thus particularly well suited for such applications.
This paper describes a cryocooler cooled NbTi superconducting magnet system. The technical features of this magnet system are a 4K-Gifford-McMahon (GM) refrigerator using magnetic regenerator material and a high-Tc Bi2Sr2CaCu2Oy superconducting current lead. The NbTi coil was directly cooled by the 4K-GM refrigerator without liquid helium and it took about 21 hours for the NbTI coil to be cooled from room temperature to below 4 K. The stable magnetic field of 6 T at 3.5 K and the maximum magnetic field of 6.45 T were obtained in the 180 mm room temperature bore.
An extensive metallurgical study is presented which is intended to explain variations in the mechanical properties of Ni18 200 grade maraging steel in various product forms and orientations. Fracture toughness and Charpy impact values are found to decrease with decreasing temperature and be dependent on product form, specimen orientation, and metallurgical condition. Fatigue crack growth rates are dependent on temperature only. Fractographic analysis reveals that the decrease in toughness at -170 C is not associated with cleavage-type fracture morphology. Those specimens exhibiting low fracture toughness at room temperature or -170 C are found to have a significantly larger number of titanium-rich particles associated with dimple formation on the fracture surface.
The miniature Joule-Thomson open-cycle cryostat and gas supply system used with the Mariner 6 and 7 infrared spectrometers are described. Long-term storage of 5 500 lbf in-2g (1 lbf in−2 = 6 895 N m−2) hydrogen and 6 000 lbf in−2 g nitrogen during the five month Earth to Mars cruise was utilized, culminating in the release and regulated expansion of these ultra pure gases through the cryostat during the 1969 Martian flyby to provide 22 K liquid hydrogen cooling for an Hg:Ge infrared semiconductor detector. Design considerations, handling and operating procedures, as well as the final performance of the gas supply system are discussed.