Refrigeration for Superconductors

Refrigeration for Superconductors RAY RADEBAUGH Invited Paper Temperatures in the range of 0.05 to 80 K are required for most One of the earliest applications appeared about 50 years ago applications of superconductors. Refrigeration powers range from for cooling infrared sensors to about 80 K to enhance the ca- fractions of a watt for many electronic applications to kilowatts for pability of night vision for the military. To date, over 140 000 some large magnet and power applications. This paper reviews the various types of refrigeration methods currently available to meet Stirling cryocoolers have been produced for tactical mili- the needs of various applications of superconductors. The methods tary applications [1]. Refrigeration powers vary from about covered include mainly the gas cycles, which are divided into 0.15 to 1.75 W, which are appropriate for many high-tem- the recuperative types (steady flow), such as the Joule–Thomson, perature superconductor (HTS) electronic applications. The Brayton, and Claude cycles, and the regenerative types (oscillating use of HTS microwave filters for cellular phone base sta- flow), such as Stirling, Gifford–McMahon, and pulse tube cycles. Methods for reaching millikelvin temperatures are briefly men- tions has become the largest application of HTS, with more tioned as well. The operating principles of the various methods are than 4000 systems now in the field. Refrigeration powers described, and the advantages and disadvantages of each are given are around 6 W at 77 K. In the last decade or so, the de- to help the user understand which approach may work best for a sire for night-vision surveillance and missile detection from particular application. All cryogenic refrigeration methods have satellites has prompted extensive research on improved cry- a common set of problems that have hindered many applications of superconductors. These problems and recent developments to ocoolers to meet the stringent requirements for space use. overcome some of these problems are discussed. The use of HTS cables for superconductor power applications requires considerably higher refrigeration powers. Keywords—Applications, cooling, cryocoolers, cryogenics, superconducting devices, superconducting magnets, superconducting These applications include motors, generators, transformers, power systems, superconductors. fault-current limiters (FCL) and transmission lines. Fig. 1 shows a map of all the application areas for cryocoolers on the plane of refrigeration power versus temperature. The I. INTRODUCTION superconducting applications are shown as crosshatched A. Applications of Cryocoolers regions in this figure. Cooling of superconducting electronics, magnets, and Cryopumps are the largest commercial application for cry- power systems is the main application of cryocoolers dis- ocoolers and produce the very clean vacuums required by the cussed in this paper. However, it is useful to note that semiconductor industry in fabricating circuits with extremely cryocoolers are used in a very wide range of applications. It narrow linewidths. Cryopumps contain a charcoal bed cooled is important to understand some of these other applications to about 15 K with a Gifford–McMahon (GM) cryocooler because coolers developed for those cases may be adapted that pumps all gases and leaves no trace of oil contamina- for cooling superconductors with little or no modifications. tion in the vacuum space. These two-stage cryocoolers cry- The sharing of cryocooler development with other applica- otrap water vapor on baffles cooled to about 80 K with the tions is important in reducing the often significant problem first stage in order to prevent loading of the charcoal with the of the cost of cryocoolers. water vapor. During the peak of semiconductor production, Table 1 lists some of the more common applications of about 20 000 cryopumps per year were produced, but this has cryocoolers. Cryocoolers are required for a wide and ex- declined some in the last few years. panding variety of applications as cryocoolers are improved. The largest application of superconductors is for magnets in magnetic resonance imaging (MRI) systems. About 1000 systems per year are being produced. Magnetic fields Manuscript received December 19, 2003; revised April 21, 2004. of 1.5–3 T are typical for these magnets, with the higher The author is with the Cryogenic Technologies Group, National In- fields providing better resolution. Liquid helium is generally stitute of Standards and Technology, Boulder, CO 80305 USA (e-mail: [email protected]). used in cooling these NbTi coils to 4 K. Cryocoolers are Digital Object Identifier 10.1109/JPROC.2004.833678 used in these cryostats in a few different ways. Originally, U.S. Government work not protected by U.S. copyright. PROCEEDINGS OF THE IEEE, VOL. 92, NO. 10, OCTOBER 2004 1719 Table 1 Some Common Applications of Cryocoolers Table 2 Potential Cryocooler Problems A large demand now exists for high-field (around 20 T) superconducting magnets for use in nuclear magnetic resonance (NMR) spectrometers for investigating the structure of macromolecules, such as proteins, in solutions. These magnets are cooled with liquid helium. Cryocoolers have not been used, even for reliquefaction, in these systems because their vibration can degrade the signal quality. Improved cry- Fig. 1. Map of cryocooler applications in plane of refrigeration ocoolers with less vibration, such as pulse tube refrigerators, power versus temperature. Superconductivity applications are may lead to the future use of cryocoolers in this application. shown as crosshatched regions. Refrigeration powers for the many superconducting magnets and RF cavities used in large-scale particle accelera- GM cryocoolers were used for cooling 20 K shields to tors for high-energy physics experiments are typically tens of significantly reduce the boiloff of the liquid helium. More kilowatts at 4.2 K, but some systems employ 2 K sections for recently, 4 K GM cryocoolers of typically 1–1.5 W are the enhanced heat transfer of superfluid helium. These large used to reliquefy the helium boiloff or they are used for helium liquefaction plants use either the Brayton or Claude conduction cooling the magnets in a dry system. Pulse tube cycle and have many stages of expansion turbines to attain cryocoolers have just recently been used for this application, high efficiency. particularly for head imagers, where low vibration is needed In general, low-temperature superconducting (LTS) sys- to prevent distortion of the MRI signal. In low-field MRI tems must be cooled to about 4–10 K, which requires at least systems that employ permanent magnets (0.1–0.5 T) the use two stages of refrigeration and much more power input for of HTS radio frequency sensor coils can greatly increase the the same refrigeration power compared with that required for signal-to-noise ratio, making them more competitive with HTS systems that may operate in the temperature range of the superconducting magnet systems. 30–80 K. One-stage systems can reach temperatures down 1720 PROCEEDINGS OF THE IEEE, VOL. 92, NO. 10, OCTOBER 2004 Table 3 Properties of Several Cryogenic Fluids to about 30 K, but their efficiency becomes quite small for be more serious in some types of cryocoolers than in others. temperatures below about 40–50 K. These differences will be discussed later. B. Refrigeration Requirements and Potential Cryocooler C. Open and Closed Refrigeration Systems Problems Many of the problems associated with cryocoolers may Fig. 1 shows the required temperatures and refrigeration be bypassed when using liquid cryogens to provide the powers for many of the applications discussed here. Other re- necessary cooling. Such a practice is very common in a lab- quirements for the refrigerators used to cool superconducting oratory environment, where cryogens are readily available systems have to do with making the refrigeration system “in- and trained personnel are present to periodically transfer visible” to the user. That is, communication from the refrig- the cryogens. However, for most practical applications, the eration system to the outside world via such mechanisms as requirement of periodic cryogen transfer makes the system vibration, noise, input power, heat rejection, electromagnetic quite “visible” and undesirable to the user, especially in interference (EMI), maintenance, failure, cost, etc., should be remote applications. The use of liquid cryogens may be minor when compared with that of the total system. For ex- desirable from the standpoint of providing thermal buffering ample, if the refrigerator costs only about 10% of the total with varying heat loads and for enhancing heat transfer. In superconducting system package and most failures occur at such cases the best refrigeration system may be a cryocooler locations other than in the refrigerator, then the user is not so that liquefies a gas such as nitrogen, neon, or helium, which aware of the presence of the refrigerator and can feel com- is then circulated passively or actively throughout the su- fortable with its use in the system. Cryocoolers often have perconducting system. The boiloff gas is returned to the problems meeting the ideal system requirements, and users cryocooler (liquefier) to be reliquefied. Such systems are should be aware of these limitations. Fortunately, research closed, and there is no need to replenish the cryogen. The and development on cryocoolers has been leading to signifi- use of a cryocooler to liquefy the helium boiloff in a MRI cant improvements in all of the problem areas. Thus, the po- system is a good example of such a closed cryogen system. tential user also should be aware of the latest status of cry- In these systems, the cryogen is merely the heat transfer ocooler performance when making decisions on the use of medium, and the user never needs to deal with the cryogen. cryocoolers. Attributes of cryocoolers that may present prob- These closed systems are usually more desirable in practical lems in various applications are listed in Table 2. applications than open systems where the liquid cryogen In small electronic systems for terrestrial applications, must be trucked in from a remote liquefier.

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