Cryogenic systems for superconducting devices

Cryogenic systems for superconducting devicesSteven W.Van Sciver *National High Magnetic Field Laboratory,Florida State University,1800E.Paul Dirac Drive,Tallahassee,FL 32310,USAAbstractThe principles entering into the selection of cryogenic systems for superconducting magnet applications are reviewed.Types of refrigeration systems,operating temperature ranges and e ciency are all issues that must be evaluated in selecting the most appropriate cryogenic system.Speci®c examples of magnet systems currently under development are discussed in the context of these principles.Ó2001Elsevier Science B.V.All rights reserved.Keywords:Cryogenics;Superconducting magnets;Cryocoolers1.IntroductionSafe and reliable cryogenic refrigeration is a requirement for the operation of all supercon-ducting devices from electronics to magnet sys-tems.In general,such devices require temperatures below 80K,a temperature that can be achieved using one of several boiling cryogens or closed-cycle refrigerators.Further,a wide range of cool-ing powers,from microwatts to kilowatts at low temperature,are required depending on applica-tion.Cooling systems for superconducting devices have undergone steady development since the ®rst liquefaction of helium and the discovery of su-perconductivity by H.Kamerlingh Onnes in the early 1900s.Today,a wide range of cryogenic technologies tailored to speci®c applications are available to superconducting system designers.All superconducting devices share the need for su cient refrigeration to overcome their low temperature heat loading.This loading typically comes in two forms:(1)heat leaks from the sur-roundings and (2)internal heat generation in the device.In addition,normally the refrigeration sys-tem needs to bring the superconducting device from ambient temperature to its low operating temperature in a reasonable length of time.The present paper is a survey of the cryogenic refrigeration options available for superconduct-ing magnet systems.Superconducting magnets span a wide range of sizes and shapes depending on application and thus their cooling systems also vary.The three main approaches to magnet cool-ing are:1.batch ®lling with a boiling cryogen such as liq-uid helium or liquid nitrogen where the heat is extracted by the evaporation of the liquid,2.closed cycle refrigeration that circulates cryo-gen through the magnet and removes heat at a lique®er/refrigerator located outside the mag-net vessel,Physica C 354(2001)129±135*Corresponding author.Tel.:+1-850-644-0998;fax:+1-850-644-0867.E-mail address:vnsciver@ (S.W.Van Sci-ver).0921-4534/01/$-see front matter Ó2001Elsevier Science B.V.All rights reserved.PII:S 0921-4534(01)00013-23.cryocooler cooling that extracts heat by direct contact between the magnet and a cold surface maintained by a specialized refrigeration cycle.Particular selection of a cryogenic system for a superconducting magnet depends on a number of factors.First among these is the required operat-ing temperature.With the early high®eld metallic superconductors,NbTi(T c 9K)and Nb3Sn (T c 18K),the only option is a helium system operating near the normal boiling point of LHe (NBP 4:2K).However,with the development of useful high temperature superconducting ceramic wires and tapes,it is now possible to operate mag-nets at considerably higher temperatures,possibly as high as the NBP of LN2(77K).These devel-opments have also led to innovative magnet sys-tems including magnets that are cooled only by direct contact with a cryocooler operating any-where from4to80K.Why is it advantageous to operate a supercon-ducting magnet at higher temperature?From the cryogenic viewpoint,the answers are obvious. Higher temperature implies higher thermodynamic e ciency of the refrigeration system.A quantita-tive measure of this e ciency is given by the co-e cient of performance(COP),which is a direct measure of the amount of ambient power needed to supply a given amount of low temperature re-frigeration.Speci®cally,the COP is de®ned as COP g T h=T cÀ1where the quantity in parentheses is the thermo-dynamically ideal Carnot factor and the g>1is a measure of the non-ideality of the system.For a moderate size helium system operating at10%of Carnot,the COP is about750W/W.The e ciency factor,g,also increases with refrigerator size[1]. High e ciency implies a smaller capital investment and operating cost for a given low temperature heat load.Another advantage to a higher operating tem-perature for a superconducting magnet comes from the increased thermal capacity of the system. Although the critical current in superconductors decreases with increasing temperature,the thermal capacity of the magnet strongly increases to the point where it is comparable with that of the liquid cryogen.Table1lists properties of several com-mon cryogens compared to that of copper.Note that the volumetric heat capacity of copper is comparable to that of liquid nitrogen at80K. Thus,although the magnet may not achieve as high an operating current or magnetic®eld if it operates at higher temperature,it may be more thermally stable due to the larger heat capacity of the winding.This potential advantage is most ev-ident in high temperature superconductors.2.Refrigeration/cooling technologiesAs discussed above,there are basically three methods in common use for cooling supercon-ducting magnets.The®rst and most common ap-proach is to batch®ll the magnet system vessel with cryogen,usually liquid helium.The magnet will thus be immersed in an isothermal bath and the heat deposited will go into evaporation of the liquid.Higher temperature cryogens have the de-sirable feature of larger heats of vaporization,see Table1.However,the operating temperature is restricted over a narrow range and therefore may not be suitable for many magnets.Many superconducting magnets are parts of systems and therefore should be integrated with a suitable closed-cycle refrigerator.Until recently, the most common approach to closed-cycle re-frigeration for magnets is to use a helium lique®er/ refrigerator to deliver liquid to the magnet and recover vapor.For the most part,such systems areTable1Thermal properties of selected cryogenic¯uids compared to that of copperFluid/material property LHe@4.2K LNe@30K LN2@77K Copper RRR 100Heat capacity/volume(kJ/m3K)570220016500.9@4.2K,240@30K,1750@77K Latent heat/volume(kJ/m3)2600104,000160,000N/AThermal conductivity(W/m K)0.0270.1130.14660@4.2K,2100@30K,540@77K 130S.W.Van Sciver/Physica C354(2001)129±135based on a recuperative Collins refrigeration cycle, shown in Fig.1.Recuperative cycles use a steady ¯ow of helium that passes through a combination of refrigerator components:compressor,heat ex-changers,expansion engines and/or Joule±Thom-son expansion valves to ultimately supply the cold helium.Such a system is rather complexin design and requires experience to operate and maintain. Recuperative lique®ers/refrigerators are moderate to large in size(10W to10s of kW at4.5K),have relatively high thermodynamic e ciencies(10±30%)and system costs in the range of a few $105±$107.They are capital intensive and require considerable technical support.In applications, the largest such systems are in operation at particle accelerator laboratories.For small scale applications,cryogenic technol-ogy has made considerable recent advances mostly in the area of closed cycle regenerative cryocoolers. The principal di erence between recuperative and regenerative cycles has to do with the¯uid¯ow.In recuperative cycles,the¯ow is steady and can be described by equilibrium thermodynamics laws.At any point in time,a given location in the¯ow stream has a thermodynamic state given by its local temperature and pressure.Therefore,the re-frigeration cycle can be optimized by evaluating the thermodynamic conservation equations for the components that make up the cycle.On the other hand,the¯ow in a regenerative cryocooler oscil-lates intermittently exchanging heat with the re-generator.In fact,many cryocoolers depend on non-equilibrium conditions to exist for operation. Thus,the thermodynamics of a regenerative cryo-cooler is much more di cult to model and opti-mize.There are many cryocooler cycles of interest to applications,but in current context of supercon-ducting magnet systems only two will be consid-ered,the Gi ord±McMahon(GM)cycle[2]and the ori®ce pulse tube(OPT)cryocooler[3].The GM cryocooler is the most common cryocooler in use today,while the OPT cryocooler is still under development.Schematic diagrams of these two cycles are shown in Fig.2.The principal compo-nents of a GM cryocooler are the compressor, displacer,rotary valve and regenerator.In this case,the rotary valve switches the¯uid¯ow back and forth through the regenerator and displacer. The OPT cryocooler replaces the displacer with a straight tube connected to a bu er volume through in this case two ori®ces.The ori®ces shift the phase between the¯uid velocity and pressure to optimize therefrigeration. Fig.1.Collins recuperative cryogeniccycle.In both cases,the regenerator stores the refrig-eration.It is designed much like a heat exchanger except that it uses materials that have large heat capacities at low temperatures.Conventional GM cryocoolers that are used for cryopump cooling and as shield coolers in bath cooled magnets use lead (Pb)or phosphor bronze as the regenerator material and have useful cooling power down to about 30K in single stage units and 15K in two stage units.This performance has been improving recently with the introduction of new materials.Interest in achieving liquid helium temperature with cryocoolers has inspired materials develop-ment for improved cryocooler performance.In particular,rare earth metals and compounds such as Nd,ErNi,HoCu 2,have magnetic ordering anomalies at low temperatures and associated large peaks in the speci®c heat.Fig.3displays the low temperature volumetric heat capacity of a se-lection of these materials [4].Thus,modern re-generators grade the materials into layers from the warm to cold end in order to achieve the highest volumetric heat capacity at the local operating temperature.The speci®c bene®t of this innovation has been the achievement of useful cooling powerto 20K in single stage and 4K in two stage commercial cryocoolers.3.Cryogenics for superconducting magnet systems Cryogenics is a necessary but often undesirable part of a superconducting system because it in-creases cost,reduces reliability and increases complexity.Clearly,the most desirable cryogenic system would have minimum impact on the oper-ation of the magnet.As discussed above,there are essentially three approaches to cooling of su-perconducting magnets:bath cooling in a liquid cryogen,closed-cycle cooling with a recuperative refrigerator/lique®er and direct cooling with a closed-cycle regenerative cryocooler.Each ap-proach has unique advantages that can meet a variety of magnet cooling requirements.To sum-marize the capability of each approach,we will consider the application to a particular magnet system.The best example of bath cooled magnet systems are those developed for the magnetic resonance imaging (MRI)industry.Thousands of MRI magnet systems are installed in health facil-ities worldwide with almost no visible evidence of cryogenics.This is accomplished through use of an optimized low-loss cryostat design including pos-sibly a cryocooler for refrigerating the thermal mercial MRI magnet systems are now to the point where they normally require only annual cryogenic service mostly for topping o the liquid cryogen.The magnet systems provide steady ®elds with a thermal isolation su cient to reduce liquid helium consumption to a minimum (usually less than 1W at 4.2K).Most often this level of helium consumption can only be achieved with the use of persistent current magnets with demount-able current leads.Similar magnet technology is in place for reciprocating magnetic separation (RMS)equipment,many of which are located in the Brazilian rainforest as shown in Fig.4.Clearly as with MRI magnet systems,this technology is best suited for unattended operation.For larger high current magnet systems that experience regular ®eld and current variations,it is necessary to provide active cooling byconnectingFig.3.Volumetric speci®c heat of common regenerative ma-terials [4].132S.W.Van Sciver /Physica C 354(2001)129±135the magnet vessel to a closed-cycle refrigerator.For large magnets using metallic superconductors,the refrigerator of choice is a Collins-type machine that delivers low temperature helium to the mag-net and recovers the cold return ¯ow to optimize the thermodynamic cycle.There are many exam-ples of this approach in high energy particle ac-celerators,nuclear fusion experiments and others.At the National High Magnetic Field Laboratory (NHMFL),we operate a large superconducting solenoid as part of our 45-T hybrid magnet,(Fig.5)[5].This magnet stores about 115MJ at full ®eld of 14T with an operating current of 10kA.The system is cooled with a pair of Collins-cycle re-frigerators with a total combined capacity of 400W at 4.5K (150l/h as lique®ers).An additional feature of this system is its 1.8K operating tem-perature in a bath of super¯uid helium.However,this system is not particularly user friendly.During operation the magnet and its cryogenic system are under constant automated monitoring with tech-nical support on call should a problem arise.The third option for superconducting magnet cooling has developed over the past 10years through the advent of two enabling technologies.First,there has been steady progress in regenera-tive cryocooler technology to the point where now one can obtain GM and OPT cryocoolers that can deliver greater than 1W of cooling at 4K.The other enabling technology is high temperature superconductor current leads.The combination of these two technologies has allowed for direct conduction cooling of NbTi and Nb 3Sn magnets.Such magnets do not require liquid cryogens for their operation and can be fully automated for cooldown,magnet energizing and safe operation.A number of such systems are in industrial appli-cation today for magnetic processing.For exam-ple,Cryomagnetics &Cryomech have developed a 9T,32mm warm bore NbTi conduction cooled by a 4K pulse tube cryocooler Fig.6.Such systems require relatively little technical support for oper-ation and can provide steady or slowly varying magnetic ®elds in moderate size volumes.The two principal disadvantages of conduction cooled low temperature superconducting magnets are that the magnet must be extremely well thermally insulated to stay under the allowable 4K heat removal ca-pacity of about 1W.This requirement often es-tablishes a limit to the size and operating current of conduction cooled LTS magnets as well as on the cool down time.Also,these magnets are in-herently unstable due to small heat capacity at 4K and thus must be operated with care to minimize AC loss and other disturbances that might quench the magnet.Many of the problems of scaling up conduction cooled magnet systems are alleviated when con-sidering HTS magnet systems operating at signif-icantly higher temperatures.At high temperatures the heat capacity of the magnet is much larger than its value at 4K.For example,at 30K the speci®c heat of magnet materials is about 250times that at 4K.This feature enhances the ther-mal stability of the magnet and allows larger AC loss,faster ramp and more reliable operation.InFig.5.45T hybrid magnet cryostat at theNHMFL.Fig.4.Two 500mm bore,5T reciprocating ore separator magnets in the Brazilian rain forest.S.W.Van Sciver /Physica C 354(2001)129±135133addition,since HTS magnets operate at 30K or above,the cryocooler technology is signi®cantly easier.Currently,commercial single stage GM cryocoolers with cooling powers in excess of 100W at 30K are rger cooling power and greater thermal capacity should allow for the de-velopment of larger conduction cooled magnet systems.Still one inherent disadvantage of direct cooling only is that the magnet has essentially no thermal store to permit continued operation should the cryocooler fail.Reliability can be built into the system by providing duplicate compo-nents (cold heads,compressors,etc.)but there are still limits to how well the system can be main-tained during a cooling system breakdown.In an e ort to provide magnets systems that still use cryocoolers for refrigeration but also can continue operation for some period of time,hybrid systems (which include liquid cryogen and cryo-coolers)have been developed.This approach was ®rst developed for MRI magnet systems,where the small liquid helium boil-o can be recondensed using a 4K two stage GM cryocooler to achieve essentially a zero-helium loss system.This concept has also been developed for conduction cooled LTS magnets where a small liquid helium reservoir is used to increase stability of the magnet [6].For HTS magnet systems that operate at higher temperatures,e.g.T 30K,it may similarly be desirable to provide a thermal reservoir to main-tain the system cold and operational even if the cryocooler is in service.This could be achieved by utilizing a small contained volume of liquid neon in direct contact with the superconducting magnet and cooled with a single stage cryocooler.Such a device would have signi®cant advantages over conventional direct cooling mostly in terms of re-liability of operation.4.SummaryThe goal of any superconducting magnet cryo-genics system should be to have minimum impact on the operation of the magnet.To achieve this goal,the cryogenic system must bring the magnet to its operating temperature,reliably maintain it at that temperature for as long as necessary.It also needs to deliver this condition at modest cost.At present most superconducting magnet cryogenic systems do not meet these requirements.For the most part,the systems are too capital intensive,require too much maintenance and technical sup-port.However,the recent combined development of high performance cryocoolers,HTS current leads and HTS magnets have allowed the appli-cation of considerably simpler cryogenic systems.Future magnet systems should bene®t from these technologies.AcknowledgementsThe National High Magnetic Field Laboratory is supported by the National Science Foundation and the State of Florida under grant DMR-9527035.The author also expresses appreciationtoFig.6.Conduction cooled 9T,32mm warm bore supercon-ducting NbTi magnet cooled by a 4K pulse tube cryocooler (Cryomagnetics,Inc.).134S.W.Van Sciver /Physica C 354(2001)129±135D.Richards of Carpco/Otokumpu and C.Wang of Cryomech,Inc.for technical information.References[1]R.A.Burns,M.A.Green,Adv.Cryog.Engng.43(1998)1661.[2]H.O.McMahon,W.E.Gi ord,Adv.Cryog.Engng.5(1960)354.[3]R.Radebaugh,J.Zimmerman,D.Smith,Adv.Cryog.Engng.31(1986)779.[4]ng,H.Hafner,C.Heiden,Adv.Cryog.Engng.43(1998)1573.[5]ler,et al.,IEEE 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