Single-stage Cryocooler Research Articles

Experimental performance of a modified Collins cycle cryocooler for in-space applications

This paper presents initial experimental results of a modified-Collins-cycle cryocooler designed for space cooling applications. A high efficiency and lightweight single-stage cryocooler design to provide 100 W of cooling at temperatures at 90 K is discussed. With multi-staging, cooling to 4 K is possible. A single-stage room-temperature floating piston-machine is experimentally demonstrated with helium as the working fluid. Cryocooler construction materials and control system methodologies are discussed.

Design Consideration and Conductor Selection of a Low AC Loss HTS REBCO Magnet Carrying High Currents at 20 K and 40 K

AC loss in high temperature superconductor coils have been frequently studied, however, mostly for AC power ap-plications at 77 K, rather than specifically for high current but low frequency AC superconducting magnet at 20-40 K. Due to their easy operation and Helium shortage, more HTS magnet systems employ conduction-cooling with cryocoolers. The HTS magnets are known for high stability and likely tolerate high AC loss, but it is unclear what is the maximum AC frequency assuming that cry-ocooler has limited capability (a few hundred Watts) for the 20-40 K temperature range. This paper will specifically study AC loss in a simple HTS dipole but with three conductor/cable options using simulations, (1) 12 mm wide tape, (2) two parallel 6 mm wide tapes, and (3) 6/2 (six 2 mm strands) Roebel cables. It has been found that the magnet at 5 Hz generates 200 – 400 W AC loss at 20 K or 40 K, potentially be cooled by two single stage cryocoolers. The 6/2 Roe-bel cable based magnet may allow higher frequency (6-8 Hz) due to its transposition and narrower conductor width.

Forces and Stresses in the Windings of a Superconducting Fault Current Limiter

This paper presents the design of a Superconducting Fault Current Limiter (SFCL) and calculation results of forces and stresses in the windings of a resistive fault current limiter. The design of the fault current limiter consists of two parallelly connected and magnetically coupled windings, cooled by a single stage cryocooler. Magnetically compensated windings made of HTS tape give a very low voltage on the limiter at a nominal current. Limitation of the short-circuit time and the value of the maximum initial fault current reduces the thermal and dynamic effects of the passage of a fault current. Using devices which limit the value of a fault current can lower the level of required short-circuit capacity of the elements in a system. However, selected means of fault currents limitation must maintain the power quality standards. A perfect fault current limiter is required to have substantial impedance in fault conditions and zero impedance at work currents. Such requirements are met by a SFCL. An increase of current caused by the occurrence of a fault current results in the transition of the superconducting material from the superconducting state into the resistive state. This increases the impedance of short-circuit loop, allowing the fault current value to decrease. During a short-circuit, the forces generated from the short-circuit current also act on the limiter windings. Short-circuit current causes stresses in the superconducting tape. Exceeding the permissible stress value results in an irreversible reduction in the critical current of the superconducting tape. Calculations of the forces and stresses in the HTS tape for the maximum value of the short-circuit current were carried out using the finite element method. The constructed limiter was tested and the winding design ensures that the tape stresses are at a safe level even for short-circuit currents.

Determining the temperature-dependent London penetration depth in HTS thin films and its effect on SQUID performance

The optimum design of high-sensitivity Superconducting Quantum Interference Devices (SQUIDs) and other devices based on thin high-temperature superconductor (HTS) films requires accurate inductance modeling. This needs the London penetration depth λ to be well defined, not only at 77 K, but also for any operating temperature, given the increasingly widespread use of miniature low-noise single-stage cryocoolers. Temperature significantly affects all inductances in any active superconducting device, and cooling below 77 K can greatly improve device performance; however, accurate data for the temperature dependence of inductance and λ(T) for HTS devices are largely missing in the literature. We report here inductance measurements on a set of 20 different thin-film YBa2Cu3O7−x SQUIDs at 77 K with thickness t = 220 or 113 nm. By combining experimental data and inductance modeling, we find an average penetration depth λ(77)=391 nm at 77 K, which was independent of t. Using the same methods, we derive an empirical expression for λ(T) for a further three SQUIDs measured on a cryocooler from 50 to 79 K. Our measured value of λ(77) and our inductance extraction procedures were then used to estimate the inductances and the effective areas of directly coupled SQUID magnetometers with large washer-style pickup loops. The latter agrees better than 7% with experimentally measured values, validating our measured value of λ(77) and our inductance extraction methods.

Thermal analysis and cryogenics of the Baby-IAXO magnet

Baby-IAXO is a new helioscope, a demonstration version of the proposed full-size IAXO, the International Axions Observatory. It is currently under design and once installed it will search via the Primakoff effect for axions or axion-like particles (ALPs) originating in the Sun. Axions can transform into photons in the presence of a significant magnetic field, and then be detected. The superconducting magnet for Baby-IAXO comprises two 10 m long racetrack coils, spaced by 0.8 m, operated in a quadrupole configuration and generating an average magnetic field of 2.1 T in the two 700 mm diameter free bores for axion-to-photons decay positioned in between the coils. Cool down and operational cryogenic requirements are handled by a combination of single and double stage cryocoolers. For the cool down of the 18.5 t cold mass within 22 days and for current leads cooling, the use of powerful single stage cryocoolers is of paramount importance. The cooling power is distributed across the cold mass and thermal shield using helium gas flow enforced by cryocirculators. The design of the 10 kA conduction cooled current leads is presented as well, since at 70 K these represent the dominant heat load. The cryogenic system allows for a constant heat load of 1.2 kW at the first stage and 8.5 W at the 4.2 K cold mass. Alternatively, the magnet may be operated in persistent mode, thereby reducing the heat load and the number of cryocoolers. It requires a more complex cooling circuit, implementation of a 10 kA persistent mode switch, and a delicate balance of energizing and operational heat loads.

Thermally Activated ReBCO Switches for Charging High-Current Magnets

Aiming at design improvements for the International Axion Observatory, alternative options for powering of the 20-kA magnet are under consideration. Use of high-Temperature superconductors in a transformer-rectifier system cooled by a single stage cryocooler at 40 K-50 K level is a promising option, which allows to avoid massive movable bus bars, current leads, and related services. The overall efficiency of such a magnet powering system is determined by the characteristics of high-current switches, which have to operate in a fast synchronous manner. We studied the performance of thermally activated superconducting switches made of adjusted 4-mm-wide ReBCO tapes, using etching of the tapes in a FeCl-3 bath to increase their resistance in normal state and to prepare low-mass heaters perfectly matching the tape surface. First experimental results on the 'off' state performance and time constants of the switches, when toggling between 'on' and 'off' states, are presented. Finally, we discuss a conceptual design of an all-ReBCO rectifier system comprising a superconducting transformer and thermally activated switches, which can be scaled up for charging large magnet systems like particle detector magnets where slow charging is feasible.

Miniature Piezoelectric Compressor for Joule-Thomson Cryocoolers

Joule-Thomson (JT) cryocoolers operate with a continuous flow of the working fluid that enters the cooler at a high pressure and leaves it at a lower pressure. Ideally, the temperature of the outgoing fluid equals the temperature of the entering fluid. JT cryocoolers that operate with pure refrigerants require high pressure of a few tens of MPa where the low pressure is usually around 0.1MPa. Circulation of the working fluid in such cases requires high pressure ratio compressors that evidently have large dimensions. JT cryocoolers can operate with much lower pressure ratios by using mixed-refrigerants. Cooling from 300K to about 80K in a single stage cryocooler normally requires a pressure ratio of about 1:25.In the present research a miniature compressor driven by piezoelectric elements is developed in collaboration between Rafael and the Technion. This type of compressor has the advantage of improved long life compared to other mechanical compressors, very low vibrations, and silent operation. In the current case, the design goal of the intake and discharge pressures has been 0.1 and 2.5MPa, respectively, with a flow rate of 0.06g/s. The compressor has two compression stages; 1:5 and 5:25.Several configurations have been considered, fabricated, and tested. The performance of the last configuration approaches the desired specification and is presented in the current paper together with the design concept.

Current Leads for Conduction-Cooled Magnets at 20–30 K

Cryogenic current leads are designed and tested for conduction-cooled superconducting magnets operating at 20-30 K. These magnets are under development in Korea for electrical machinery with MgB2 wires or YBCO tapes. Since a single-stage cryocooler is employed, the current leads are made of metal over the entire length. Different sizes of copper conductor are fabricated for 80-100 A level and experimentally tested with a commercial Gifford-McMahon cryocooler. Temperature is measured at the coldhead and several axial locations along the conductor in order to estimate the cryogenic load. A geometric condition to minimize the cooling load is clearly demonstrated by the experiment. An analytical model is also presented to explain the optimal condition, taking into account the properties of copper and the refrigeration capacity of cryocooler.

The Cryogenics of a Thermosiphon-Cooled HTS MRI Magnet—Assembly and Component Testing

The team at GE Global Research presents cryo assembly and component test results of a high-temperature superconducting (HTS) limb size magnetic resonance imaging (MRI) scanner using Sumitomo's DI-BSCCO tape conductor, under an NIH research grant. The goal is to investigate the thermosiphon behavior for different MRI operating modes, validating the cryogenic robustness of this cooling approach and its performance limits. The magnet is indirectly cooled using cooling tubes with liquid neon filling and a single-stage cryocooler for reliquefying.

Detrapping and retrapping of free carriers in nominally pure single crystal GaP, GaAs, and 4H–SiC semiconductors under light illumination at cryogenic temperatures

We report on extremely sensitive measurements of changes in the microwave properties of high purity nonintentionally-doped single-crystal semiconductor samples of gallium phosphide, gallium arsenide, and 4H–silicon carbide when illuminated with light of different wavelengths at cryogenic temperatures. Whispering gallery modes were excited in the semiconductors while they were cooled on the coldfinger of a single-stage cryocooler and their frequencies and Q-factors measured under light and dark conditions. With these materials, the whispering gallery mode technique is able to resolve changes of a few parts per million in the permittivity and the microwave losses as compared with those measured in darkness. A phenomenological model is proposed to explain the observed changes, which result not from direct valence to conduction band transitions but from detrapping and retrapping of carriers from impurity/defect sites with ionization energies that lay in the semiconductor band gap. Detrapping and retrapping relaxation times have been evaluated from comparison with measured data.

Design and Results of a YBCO Magnet for a 95 GHz Gyrotron Application

The use of high-temperature superconductors (HTS) for cryogen-free gyrotron magnets will greatly reduce the power requirement and overall physical size of the system. Because some gyrotron systems are mounted on a vehicle and need to be mobile, the refrigeration system must be as lightweight and compact as possible. Cryomagnetics previously built a BSCCO magnet based on the design of a NbTi magnet. The limiting factors with this magnet were the cost of the BSCCO material and the excessive weight due to the necessity of field-shaping iron. Since it is projected that the cost of 2G YBCO tape will become significantly lower than BSCCO, and the critical current will also increase, the next generation of these magnets will mitigate those problems. The first objective was to study quench behavior in HTS coils. Cryomagnetics has also designed, built and successfully tested a 3.57 T magnet wound with 2G YBCO tape. This is the first full-scale magnet of its kind built using 2G YBCO tape. It consists of 10 double-pancakes operating at 140 A and 20 K. A single-stage cryocooler is used to cool the magnet. Because the performance of YBCO material is improving quickly, future magnets will be able to operate at lower current, higher temperature and will be lighter weight.

The design and performance of a Bi-2223/Ag magnet cooled by a single-stagecryocooler

The design and fabrication of a split coil Bi-2223/Ag tape superconducting magnet cooledby a single-stage cryocooler is presented. The magnet consists of six double-woundpancakes impregnated by epoxy resin with inner diameter 50 mm and outer diameter98 mm. Critical currents of individual pancakes, double pancakes and the whole magnet aremeasured, calculated and discussed. The magnet’s performance was tested at liquid heliumand nitrogen temperatures as well as at solid nitrogen temperatures and cooled by anAL230 single-stage Gifford-McMahon cycle cryorefrigerator. The measured magnetcurrents are compared with those estimated by calculations using short sample data.

Ac Loss and Thermal Runaway of 1 T Cryocooler-Cooled Oxide Superconducting Pulsed Coil

We fabricated a 1 T cryocooler-cooled oxide superconducting pulsed coil. It is a 16-layer solenoidal coil wound with an interlayer-transposed 4-strand parallel conductor composed of Bi-2223 multifilamentary flat wires. The insulated strands were transposed only between layers to make all strands inductively equivalent. To suppress the temperature rise due to the AC loss, the aluminum nitride (AlN) plates were arranged between layers as heat drains. The pulsed coil was cooled down to 30 K by a single stage cryocooler. It can be operated continuously at 40 K in the 1 T triangular wave operation at 1 Hz. First we studied the temperature dependence of thermal runaway current of the coil under the application of direct transport current. The runaway current was higher than the critical current, which was defined as the current where the electric field of 10/sup minus;4/ V/m was generated on average over the whole length of the conductor. Next we studied the temperature variation of the coil in a sinusoidal operation of 1 Hz. The temperature increment was much more gradual than that in the DC operation. In this paper, we discuss the thermal properties of the cryocooler-cooled oxide superconducting pulsed coil, taking into account the heat generation of AC loss and flux-flow loss and the cooling capacity of a cryocooler.

A Versatile Laboratory Electromagnet With HTS Coils

We report the development of a laboratory electromagnet utilizing high temperature superconducting (HTS) coils which is designed to generate fields in excess of 3 Tesla, but with power consumption and size significantly below that possible with copper-wound coils for an equivalent useful field volume. The components are complete and the system is currently being assembled. The electromagnet is based around a compact iron yoke with four HTS double pancake coils, employing a total of 1.6 km of first generation HTS (Bi-2223/Ag composite) wire, to generate the field across an air-gap of 50 mm. The electromagnetic design was tailored to take into account the anisotropic in-field performance of the HTS conductor, requiring a minimization of the radial field component at the coils while retaining sufficiently open access for installation of the components of a vibrating sample magnetometer with which this magnet will be used. The HTS coils are conduction cooled with a single-stage cryocooler that delivers approximately 27 W of cooling power at the design operating temperature of 35 K; HTS current leads are employed to reduce the thermal load at the coils. In the present configuration the coils are housed in a single cryostat, but the design is easily adaptable to provide for a number of specific application requirements. Contrary to the generally held view that HTS magnets are still too expensive, this project demonstrates that utilization of HTS conductor is now an attractive option in a number of application areas.

Improved high-T/sub c/ superconductor sampler circuits using Josephson transmission line buffers

We report on an increase of the measurement bandwidth of a high-T/sub c/ superconductor sampler circuit. The circuit composes a waveform measurement system, in which it is cooled by a compact single-stage cryocooler and operates at a temperature of 35-40 K. In the sampler circuit, there exists a sampling jitter due to thermal fluctuation in the critical current of the pulse generator trigger junction. To reduce the jitter, one can overdrive the trigger junction. However, leakage current into the adjacent sampler element due to parasitic inductance is an obstacle to the use of a larger trigger current. We attempted to suppress the leakage current by inserting a Josephson transmission line (JTL) buffer into the sampler circuit. It was found that a trigger current two-times larger than that of the conventional circuit became possible with the addition of a four-stage JTL and that the frequency at which the measured amplitude decreases by 3 dB increased from 7 GHz to 14 GHz. This result suggests that the jitter was suppressed in the sampler circuit with the JTL buffer.

Experimental results of the model coil for cooling design of a 1 T cryocooler-cooled pulse coil for SMES

The authors have been developing high-Tc superconducting coils for SMES applications. Their primary goal is to make a HTS coil which is cooled to 40 K by a single-stage cryocooler and continuously operated at 1 Hz with a field amplitude of 1 T. The coil has heat drains of AlN plates to remove heat because of AC losses. They made a cooling model coil system to study the effective arrangement of the heat drains. The system consisted of a model coil using Cu conductors, current leads and a cryocooled system. The test coil was divided into three sections in different arrangement of heat drains. The model coil was daubed with a high thermal conductivity epoxy resin to improve thermal contact resistance between the conductors and AlN plates. They tested the coil by Joule heating which was equal to AC losses. They measured the temperature distribution in the coil and the temperature difference between Cu conductors and AlN plates. The temperature difference was measured between 0.2 K and 0.7 K. The results will be applied to the 1 T HTS coil design.

High-field warm-bore HTS conduction cooled magnet

A 7.25 T laboratory magnet utilizing Bi 2223 conductor has been designed, built, and installed at the Naval Research Laboratory. Operating at 21 K at full field, the coil provides field homogeneity of /spl plusmn/1% in a 2-inch warm-bore. The system is conduction cooled with a pair of Leybold single stage cryocoolers that allow cooldown in less than 36 hours and allow extended fast ramp operation. Operation at a total refrigerator input power of 6 kW is facilitated by the use of ASC Cryosaver/sup TM/ HTS current leads. The fully integrated system consists of the magnet, cryogenic system, control and protection system and power supply. This paper presents information on the magnet design, construction and subsequent testing.