Small Cryocoolers Research Articles

A Thirty-year History of Superconducting Microwave Devices and Fundamental Studies Thereof

Thirty years have passed since the discovery of high-temperature superconductors (HTS), and practical applications of HTS equipment and devices are moving into full-swing. From the beginning of their discovery, the use of HTS in microwave devices has been recognized as one of their most practical applications. This is because the surface resistance of HTS is about 1/1000 of that of pure copper at 1 GHz and 77 K. From early on, researchers have recognized that high performance of microwave passive devices could be realized by utilizing the characteristics of HTS. In addition, the size of HTS microwave devices is relatively small, and they can be cooled by a small cryocooler. Furthermore, we were able to establish high-quality HTS thin-film fabrication technology essential for producing high performance microwave devices in a short period of time. In view of the above, I would like to present an overview of the past thirty years of superconducting microwave devices and fundamental studies thereof.

A tissue snap-freezing apparatus without sacrificial cryogens

Molecular technologies in cancer diagnosis require a fresh and frozen tissue, which is obtained by means of snap-freezing. Currently, coolants such as solid carbon dioxide and liquid nitrogen are used to preserve good morphology of the tissue. Using these coolants, snap freezing of tissues for diagnostic and research purposes is often time consuming, laborious, even hazardous and not user friendly. For that reason snap-freezing is not routinely applied at the location of biopsy acquisition. Furthermore, the influence of optimal cooling rate and cold sink temperature on the viability of the cells is not well known. In this paper, a snap-freezing apparatus powered by a small cryocooler is presented that will allow bio-medical research of tissue freezing methods and is safe to use in a hospital. To benchmark this apparatus, cooldown of a standard aluminum cryo-vial in liquid nitrogen is measured and the cooling rate is about -25 K/s between 295 K and 120 K. Sufficient cooling rate is obtained by a forced convective helium gas flow through a gap formed between the cryo-vial and a cold surface and is therefore chosen as the preferred cooling method. A conceptual design of the snap-apparatus with forced flow is discussed in this paper.

Long-Term Operation of the Superconducting Triplet Quadrupoles With Small Cryocoolers for BigRIPS In-Flight Separator and RI-Beam Delivery Line at RIKEN

We report long-term operation of the superconducting triplet quadrupoles (STQs) with small cryocoolers for the BigRIPS in-flight separator and RI-beam delivery line. Each STQ is cooled by liquid helium bath cooling method with a small stand-alone cryogenic system in which a Gifford-McMahon (GM)/Joule-Thomson cryocooler and a GM cryocooler are mounted on the cryostat. While the GM/Joule-Thomson cryocooler recondenses evaporating helium gas in the helium vessel, the GM cryocooler cools the 70 K shield and current leads by conduction. We operate 21 STQ systems forming beam lines for more than five years. We report on how we maintain cryocoolers without warming up the magnets. Operational experiences including troubles in the long-term operation are also reported.

A Device for Measuring High-TC Superconductor Transition Temperature with Electric Transport and AC Susceptibility Methods Based on Small Cryocooler Cooling

A device for measuring high-TC superconducting transition temperature in a small cryocooler is described which allows a samples temperature to fall steadily from room temperature to 45K. This cryocooler not only meets the high-TC superconductors measurement requirements, but also yields accurate measurement results under ambient conditions. In this device, two measurement methods for measuring high-TC superconducting transition temperature are applied: the electric transport and the AC susceptibility. The former adopts a four-wire method to eliminate the contact resistance between the leads and the sample. During the measurement process the current reversal method for datum processing is applied to eliminate the thermoelectric voltages; this ensures the repeatability error less than 0.4K in the measurement of the superconducting transition temperature. The AC susceptibility method adopts a lock-in amplifier to provide an AC voltage signal for the primary coil, and then lock-in amplifier is used to measure the electrical signals of the secondary coil according to the AC magnetic susceptibility values. The advantage of this device is that by merely changing the position of the sample in the vacuum chamber, the R-T curve and the χ-T curve of a superconducting sample in the same temperature range can be automatically recorded, which provides reliable data for high-TC superconductor transition curve measurement.

The Design of Cryogen-Free Cryostat for Pulsed High Magnetic Field Based on GM Cryocooler

Based on a small GM cryocooler, a cryogen-free insert cryostat for physics experiment in a pulsed high magnetic field has been designed. The GM cryocooler is used as the cooling source. The sample is cooled by liquid helium for the low temperatures and by helium gas for the high temperatures. The inner diameter of the cryostat is 8 mm and the outer diameter of the tail is 16 mm, thus it can be inserted into a 60 T pulsed magnet. Experimental results show that the temperature can be controlled from 1.5 K to 300 K easily. In the sample region, stability of the temperature is obtained: ±0.1 K at 20 K-300 K, ±0.2 K at 5 K-20 K and ±0.02K at 1.5 K-5 K. The system may be used for electrical transport and magnetization measurements in a pulsed high magnetic field.

Compact terahertz passive spectrometer with wideband superconductor-insulator-superconductor mixer

We developed a compact terahertz (THz) spectrometer with a superconductor-insulator-superconductor (SIS) mixer, aiming to realize a portable and highly sensitive spectrometer to detect dangerous gases at disaster sites. The receiver cryostat which incorporates the SIS mixer and a small cryocooler except for a helium compressor has a weight of 27 kg and dimensions of 200 mm × 270 mm × 690 mm. In spite of the small cooling capacity of the cryocooler, the SIS mixer is successfully cooled lower than 4 K, and the temperature variation is suppressed for the sensitive measurement. By adopting a frequency sweeping system using photonic local oscillator, we demonstrated a spectroscopic measurement of CH(3)CN gas in 0.2-0.5 THz range.

Effect of Regenerator Entrance Shape on Performance of Compact Pulse Tube Cryocooler

Pulse tube cryocoolers are more attractive than other small cryocoolers because they have no moving parts in the expander and thus have higher reliability. However, Stirling type pulse tube cryocooler needs further improvement on regenerator. The entrance areas at the regenerator ends of the typical pulse tube cryocooler are smaller than the cross-sectional area of the regenerator matrix. So, the working gas flow in the regenerator has non-uniform velocity distribution with the expansion and contraction flow. They cause the excessive pressure loss and reheat loss. In this study, the effect of the regenerator entrance shape on the performance of compact pulse tube cryocooler was examined experimentally. Additionally, the analytical method for losses in the regenerator using experimental data is suggested. Experimentally and analytically, the effect of after-cooler end at the regenrator has been bigger than that of cold-head end.

Radiation Heat Loads to Superconducting Quadrupoles for BigRIPS In-Flight Separator at RIKEN

The BigRIPS fragment separator in the RIKEN RIBF project is characterized by fourteen superconducting triplet quadrupoles (STQ's) with large apertures and high-field gradients. While the five STQ's on the first stage of the BigRIPS are cooled by a liquid helium cryogenic plant, the nine STQ's on the second stage are cooled by small cryocoolers on their cryostat. This first stage serves to produce and separate RI beams, so that the first two STQs are exposed to the high radiation from the production target and beam dumps, which causes radiation heat loads as well as radiation damages. Measurements of the radiation heat loads to the cryostats of the first two STQs during commissioning operations of the RIBF facility are reported.

Practical Operation of Cryogen-Free Programmable Josephson Voltage Standards

Cryogen-free operation is rapidly becoming the preferred implementation of most superconducting electronics systems including programmable Josephson voltage standard (PJVS) systems. There are strong operational incentives for using the smallest possible cryocooler in order to minimize acoustic noise, system footprint, and power consumption. In addition, Nb/NbxSi1-x/Nb junction technology, which operates near 4 K, offers better yield than NbN/TiNx/NbN technology, which can operate at 8.5 K, thus making lower temperature operation near 4 K desirable. As junction density increases, however, self-heating of the junctions can create significant thermal gradients between the arrays and coldhead. Thus careful design of the overall system is required to maintain acceptable operating margins. We have developed a calorimetric measurement technique to characterize the system variables and used it to evaluate several different PJVS configurations. This technique uses the PJVS subarrays as both heat sources and temperature sensors, in conjunction with a time gated measurement technique, to characterize the thermal response of the system. A passive thermal filter incorporating a Pb thermal mass is used to reduce the temperature oscillations of the cryocooler. Our results suggest that, with appropriate system design, operation of a practical 10 V PJVS on a small (nominally 100 mW capacity at 4.2 K) cryocooler may be possible.

C02 パルス管冷凍機性能に及ぼす蓄冷器出入口形状の影響(パルス管冷凍機,熱音響機器及び関連要素と応用システム(1))

Pulse tube cryocoolers are more attractive than other small cryocoolers because they have no moving parts in the expander and thus have higher reliability. Stirling type pulse tube cryocooler need further improvement, on especially, regenerator. The working gas flow in the regenerator becomes a flow field with non-uniform velocity distribution accompanying the expansion and contraction flow. They cause the excessice pressure loss and reheat loss. We examined the effect of the regenerator entrance shape on the performance of pulse tube cryocooler experimentally. The effect of after-cooler end was bigger than that of cold-head end.

Improvement of the helium circulation system

We have developed an efficient cooling system to collect evaporated helium gas, cool it down to liquid and return it into a dewar using a small cryocooler without making any disturbing noise to the MEG. A very special transfer tube that passes the liquid helium, the low temperature helium gas (about 5 K) and the relatively high temperature helium gas (about 40 K) has been developed. Also, a new refiner to collect contamination gases is developed which enables longer continuous operation of the system.

C152 液体水素容器の低ボイルオフ化の研究(OS-16 先端技術に関わる熱交換過程I)

Conversion into Hydrogen Energy Society is advanced focusing on the application to a fuel cell electric vehicle. As volume and weight density of liquid hydrogen are large, it is the method which was most excellent as the storage method of hydrogen. However, in order to store liquid hydrogen stably over a long period of time, decreasing the loss of energy, development of an efficient small cryocooler becomes important. This paper reports the research about improvement in the refrigeration efficiency of cryocoolers.

The integration of liquid cryogen cooling and cryocoolers with superconducting electronic systems

The need for cryogenic cooling has been a critical issue that has kept superconducting electronic devices from reaching the market place. Even though the performance of many of the superconducting circuits is superior to silicon electronics, the requirement for cryogenic cooling has put the superconducting devices at a serious disadvantage. This paper discusses the process of refrigerating superconducting devices with cryogenic liquids and small cryocoolers. Three types of cryocoolers are compared for vibration, efficiency and reliability. The connection of a cryocooler to the load is discussed. A comparison of using flexible copper straps to carry the heat load and using heat pipe is shown. The type of instrumentation needed for monitoring and controlling the cooling is discussed.

Construction and tests of a heart scanner based on superconducting sensors cooled by small stirling cryocoolers

At the University of Twente, a heart scanner has been designed and constructed that uses superconducting devices (superconducting quantum interference devices (SQUIDs)) to measure the magnetic field of the heart. A key feature is the elimination of liquid cryogens by incorporating cryocoolers. In the design, two coolers are operated in counter-phase to reduce the mechanical interference. In addition to the application of ferromagnetic shields around the compressors, the magnetic cooler interference is reduced by placing the SQUID magnetometers coplanar with respect to the coolers. In this way, the cooler noise was reduced to a level below the intrinsic sensor noise: 0.16 pT/√Hz. A temperature of 60 K was realised with a cool-down time of about 2 h. The corresponding heat load to the coolers is roughly 0.9 W. Magnetocardiograms were recorded inside a magnetically shielded room.

Design and performance of the HESSI cryostat

The High Energy Solar Spectroscopic Imager (HESSI) spacecraft, to be launched in July 2000, will be used to observe the Sun with the finest angular and energy resolutions ever achieved from a few keV to hundreds of keV. The spacecraft will use an array of nine germanium (Ge) detectors, each 7.1 cm in diameter and 8.5 cm long, operating at 75 K. The detectors are mounted in a cryostat on a common coldplate, and cooled by a small Stirling-cycle cryocooler. This paper describes the design of the cryostat, special accommodations for the Ge detectors, interfaces with the cryocooler, and thermal performance of the engineering test unit.

Low thermal leakage coaxial cable for HTS devices

A high temperature superconductor (HTS) filter has been developed recently. It is cooled by a small cryocooler and is operated at 70 Kelvin (K) in a vacuum chamber. Coaxial cables are used for the high frequency transmission line between the filter in lower temperature and other circuits at room temperature. Because HTS filters are typically used in low-loss applications, there is an important subject to optimize electrical loss and thermal leakage in the transmission line. The usual semirigid coaxial cables have a small amount of loss; they conduct too much outside heat into the HTS filter. We have developed our own coaxial cables to optimize both electrical and thermal performance. The outer conductor of this new coaxial cable was made of plated thin copper film and was designed to have an optimum thickness of 0.005 mm. SMA connectors are fitted carefully to both ends of the coaxial cable. The new coaxial cable that is 1.68 mm in diameter and 100 mm in length is evaluated. The evaluation result found the electrical insertion loss to be 0.175 dB at 2.0 GHz and thermal leakage to be 48 mW from 300 K to 77 K. We have also developed an HTS receiving filter subsystem for a CDMA base station using our own coaxial cables. We obtained excellent results.

Development of the Los Alamos solid-state optical refrigerator

Laser-induced cooling of a solid by net anti-Stokes fluorescence, first experimentally demonstrated in 1995, can be the basis of a new type of cryocooler, an optical refrigerator. This article describes the physics and design issues of a practical optical refrigerator for operation at 77 K. In particular, the Los Alamos Solid-State Optical Refrigerator (LASSOR) which we are developing would have an operating efficiency comparable to commercial small cryocoolers, be completely vibration-free and operate for years without maintenance.

Cryogenic design of a high- T c SQUID-based heart scanner cooled by small Stirling cryocoolers

A heart scanner based on high- T c SQUIDs is currently under development at the University of Twente. It is intended to be used in standard clinical environments without a magnetically shielded room. In order to make the application simple to use, the SQUIDs will be cooled by small cryocoolers, thus realizing a turnkey apparatus. The aimed field resolution is 50 fT RMS Hz − 1 2 in a measuring band of 0.1–100 Hz. The mechanical cooler interference is reduced by incorporating two coolers and operating them in counter phase. The magnetic cooler interference is reduced by positioning the coolers and the SQUIDs in a coplanar arrangement, and by separating the SQUIDs from the cold tips with a solid conducting thermal interface. A design is presented in which a temperature of 55 K is expected with a cool-down time of less than 1 h.

A space-qualified experiment integrating HTS digital circuits and small cryocoolers

High temperature superconductors (HTS) promise to achieve electrical performance superior to that of conventional electronics. For application in space systems, HTS systems must simultaneously achieve lower power, weight, and volume than conventional electronics, and meet stringent space qualification and reliability requirements. Most effort to date has focused on passive RF/microwave applications. However, incorporation of active microwave components such as amplifiers, mixers, and phase shifters, and on-board high data rate digital signal processing is limited by the power and weight of their spacecraft electronic and support modules. Absence of data on active HTS components will prevent their utilization in space. To validate the feasibility in space of HTS circuits and components based on Josephson junctions, we need to demonstrate HTS circuits and critical supporting technologies, such as space-qualified packaging and interconnects, closed-cycle cryocooling, and interface electronics. This paper describes the packaging, performance, and space test plan of an integrated, space-qualified experimental package consisting of HTS Josephson junction circuits and all the supporting components for NRL's high temperature superconductor space experiment (HTSSE-II). Most of the technical challenges and approaches are equally applicable to passive and active RF/microwave and digital electronic components, and this experiment will provide valuable validation data.

小型冷凍機の移動体通信への応用

The recent development of high-temperature superconducting (HTS) filters for the receiver front-end of mobile telecommunication base stations and small cryocoolers is described in this paper. The merits of the HTS filters are small size, low noise and sharp skirt. One of these filters combined with a cryogenic low-noise amplifier contributes greater noise reduction to the receiver and improves the Carrier/Noise power ratio (C/N) of the transmission. The HTS filters, having high quality factors, are also effective for reducing out-of-band interference. On the other hand, cryogenic cooling is an essential and important technology for HTS filters. A reliable, low-cost, small cryocooler is required. In the USA, HTS filter systems have been field-tested using prototype equipment with small cryocoolers.