Title of Invention

A METHOD FOR ACHIEVING AND MAINTAINING CRYOGENIC COOLING FOR A CRYOGENIC COOLING SYSTEM AND THE COOLING SYSTEM

Abstract

A method and apparatus for providing cryogenic cooling to HTS devices (24), in particular those that are used in high-voltage electric power applications. The method involves pressurizing liquid cryogen (46, 48) to above one- atmospheric pressure to improve its dielectric strength, while sub-cooling the liquid cryogen to below its saturation temperature in order to improve the performance of the HTS components (24) of the device. An apparatus (10) utilizing such a cooling method consists of a vessel that contains a pressurized gaseous orvogen region (44) and a sub-cooled liquid cryogen bath, a liquid cryogen heating (52) coupled with a gaseous cryogen venting scheme (30) to maintain the pressure of the cryogen to a value in a range that corresponds to optimum dielectric strength of the liquid cryogen, and a cooling system that maintains' the liquid cryogen (46, 48) at a temperature below its boiling point to improve the performance of HTS materials (24) used in the device (10). Fig No. 6

Full Text

BACKGROUND [0002] The InwntioD relates generally to a cryc^oiic cooling system for hifj^ ttmpnalure supenxMiducttr (HTS) devices and more patticuleriy to a cryogenic cooling syston fiff HTS deinces having high-voltage electric povfet i^licadons. 10003] There exists HTS cooling systems dut use the pn^>eTtie5 of Uquid nitrogen to. ■ achjew ciyogenic cooling. Nonnally, liquid nitrogm is used at one atmospheric pressure (O.I MPa) where its operating temperature (boiling point) is at 77 degrees Kelvin. However, since the critical cunent density of HTS materials improves significantly at tonperatures lower than 77K, methods have been devel(^>ed to reduce the temperature ofthe liquid nitrogen by manipuiating its operUingenviroament. Fig. 1 is a p(pressure)-T(tenqMntture) diagram showing the relationship amongst the p. T and the three phases (solid, liquid and vapor/gas) of a typical substance. For nitrogen, die 'triple Point" is about 63.1SK at U,S3kPa. This shows t^ reducing the pressure of liquid nitrogen its botlmg point temperatuie cm be lowered to about 63K below which solid nitrogen would form. One example of using such pn^xities of liquid nitrogen to achieve lower operating temperature is provided bi US Patent 5,477,693. It describes a roediod of using vacuum pump to pump die gaseous (Utrogen region in a ctyogm containmoit vessel (cfTostat) that ccmtains bodi die liquid and gaseous nitrogen. Pumphig reduces the pressure of the liquid nitrogen bodi dierefore reducing its ten^ieradire (boilmg point) to below 77K. The perfonnance of die superconductcv, namely its critical current level. Is dien signiflcantly improved. [0004] Even tiiough die prior art increases tiie performance of HTS materials by lowering the boiling teiiq>erature of liquid nitFOgen Arou^ lowering its pressure, it is in the expense of significantly degrading the dielectric strength of liquid nitrogoi and as a consequence such cooling systems are not suitable for high-voltage HTS applications. Typically, liquid cryogen based cooling systems for hi^-voltage HTS devices rely in large degree on the dielectric properfies of the liquid cryogen as the main electrical insulation medium. Thoe are two major foctcnrs that influence the dielecbic [»operties of liquid nitrogen. One is tiie intrinsic dielectric strength of liquid nitrogen that is pressure dc^)«ident. Fig. 2 shows the dielectric strengtii of liquid nitrogen as a iimctioo of pressure. The strengtti decreases sharply when the pressure is below one atmospheric pressure (O.lMPa) while the optimum value resides in the range of between 0.3MPa and 0.5MPa. The other major fector is the bubbles that occur in die liquid nitrogen. Bubbles, especially large size bubbles, tend to reduce the dielectric strength of liquid nitrogen. Bubbles will be generated whm objects submerged in liquid nitrogen are heated to above the boiling temperature of liquid nitrogen. Lowered boiEii^ point in liquid nitrogen will &us make bubble generation more easily. Therefore metiiod of lowering liquid nitrogen tempentture by lowering its pressure will have negative impact on both bdors that govern the dielectric strength of liquid nitrogen. Cooling systems based on such and similar aiq>roached are therefore ill suited for high-voltage HTS applications. BRIEF DESCRIPTION Briefly, in accordance with the present invention, a metiiod is provided for designing a liquid-cryogen-based cryo^nic cooling system for HTS devices that have flie chaiactwistics of lower operating temperature of liquid wyogen to improve the critical current densi^ of HTS materials while at tiie same thne substantially in boiling temperature and within its sub-cooled temperature range using cryocooling means. Applying such methodology, in accordance with one emtjodiment of the present invmtion, there is provided a cyrcgenic cooling system having an inner vessel, at least one HTS element, and an outer vessel. The space between the outer and inner vessel is maintained under a vacuum and multi-layer insulation (MU) material is used to sunound the inner vessel to provide it with hernnal insulation to the radiation heat load. The inner vessel is housed inside the outer vessel and stores liquid ciyogo). Above the liquid cryogen region there is a gaseous region of the ciyogen and is pressurized above oae absolute atomo^eric pressure. XJquid heating and gas ventmg means aie in place to control and maintain the pressure within the mner vessel. To address die high-voltage msulation issue of this cryogenic cooling systnn, a buclcet or similar cmifiguration made of dielectric materials is employed surrounding the HTS and throu^out cryostat to ensure adequate high-vo]tage msutadon. In addition, screens with smalt mesh sizes are deployed througout liquid cryogen regions to breakdown large-size bubbles generated during device operation. Another feame of this cryogenic cooling system is a tiiermal transfer plate tfiat is disposed inside the inner vessel around the circumference to divide the liquid cryogen into two regions. The r^on below the plate is sub-cooted to a temp^atme thai improves the performance of HTS. The region above the plate is a buffer region where a temperature transition occurs between the boundary of the liquid and gas regions and the boundary of the buffer region and the sub-cooled liquid region. The thermal transfer plate also couples the heat from both the temperature transition buffer region and the sub-cooled region to a cooling means such as a cryogenic refrigerator (cryocooler). TTie cryocooler is employed to maintain the temperature of the region below the plate to withm the range of the sub-cooled liquid temperature range, Irom the boiling temperature at the pressure, to the triple point temperabire of the liquid ciyogen. DRAWINGS [0005] Thest and other features, aspects, and advantages of the present invention will become better understood wbm fte following detailed description is read with leference to the accompanying drawings in wliich like characters represent like parts ftrougfaout the drawing wherein; (OOOti) Fig. 1 is a typical p-T diffprani showing phase changes of a substance under various pressure and temperaluie re^es. [0007] Fig. 2 is a relationship between the dielectric strength of liquid nitrogen and die absolute pressure it is undei'. [OOOS] KG. 3 is an illustration of one embodiment of fte cryogenic cooling system of die present invention. [0009] FIG. 4 is a schematic diagram of die states of die cryc^en used in one embodiment of the cryogenic cooiing system of die presem invention. [OOIO] FIG. S is a graph showing the thickness of the liquid nitrogen thetmal-gradient-layer (TGL) under various heat input loads, for cases where the liquid nitrogen is mostly in a stagnant state. [OOIL] FIG. 6 is a graph showing the relationship of the liquid nitrogen TGL diickness vs. various heat loads in the vapor and TGL regions, for cases where the liquid nitrogen is mostly in a stfagnant state. DETAILED DESCRIPTION [0012] The present invention generally relates to a uyogenic cooling systems for HTS device that have high-voltage applications even diough it can also be applied to HTS devices that have other general purposes. The method of providing such a cryogenic cooling system includes maintahing a pressurized ciyogen region that comprises a liquid as well as gaseous region, to above one absolute atmospheric pressure. The method further involves maintaining temperature of part or all of the liquid cryogea regions to below its boiling temperature (sub-cooled) using cooling means such as a cryogenic refrigerator (ciyocooler). [0013] Briefly, in accordance -with the present invention, a method is provided for designing a liquid-ctyogen-based ci^ogenic cooling system for HTS devices tliat have (he characteristics of lower operating temperature of liquid cryogcn to improve the critical current densi^ of HTS materials while at tiie same time substantially increasii^ the dietectric strengtti of flic liquid cryogen, making such a cryogenic cooling system suitable for high-voltage implications. Such a mediod comprises the steps of maintaining a pressurized aryogea witfiin the ciyogen containment vessel that contains both liquid and gaseous regJMis of the cjyogcn. It further includes steps of maintaining the temperature of a portion or all of the liquid cryogen at and below its boiling temperature and vvidiin ia sub-cooled temperature rsnge using ciyocooting means. (0014] Applying such methodology, in accordance with one embodiment of the presmt invenrion, there is provided a cyrogenic cooling system having an inner vessel, at least one HTS clement, and an outer vessel. The space between the outer and liner vessel is maintained under s vacuum and multi-layer insulation (MLl). the material is used to surround the inner vessel to provide it with thermal insulation to the radiation heat load. The inner vessel is housed inside the outer vessel and stores liquid cryogen. Above the liquid ciyogen region there is a gaseous region of the ciyogn and is pressurized above one absolute atmospheric pressure. Liquid heating and gas venting means are in place to control and maintain the pressure within the inner vessel. Heating boils liquid cryogen and evaporates to gaseous space thus increasing the pressure, Venting releases gaseous cryogen to the outside atmosphere flhs reducing the pressure within the vessel. Such heating and venting process can be controlled by an automated monitoring and feedback system. As discussed earlier, bubbles, especially large size bubbles, tend to degrade the dielectric strength of liquid ciyogen. Bubbles can be generated when objects submerged in liquid cryogen get heated to above its boiling temperature. Pressurization raises the boiling temperature of the liquid cryogen. Raised boiling point will make bubble generation more difficult thus improving the dielectric properties of the liquid cryogen. To further address the high-voltage insulation issue of this cryogenic cooling system, a bucket or similar configuration made of dielectric materials can be employed surrounding the HTS and throughout ciyostat to ensure adequate high-voltage insulation. In addition, screens with small mesh sizes can be deployed throghut liquid cryogen regions to breakdown larger-Size bubbles if tiiey were generated during device operation. AooQier feature of tiijs exogenic cooling system is a thermaJ transfer plate fiiat is disposed inside the inner vessel around the circumference to divide the liquid cryogen into two tegkins. The region below Ae plate is sub-cooled to a temperature that improves flie performance of HTS. The region above the plate is a buffer region where a temperature transition occurs between the boundary of the liquid and gas regions and the boundary of the buffer region and Ae sub-cooled liquid region. The thermal transfer plate also couples the heat from boft the temperature tension buffer region and the sub-cooled region to a cooling means such as a cryogenic refrigerator (ciyocooler). The cryocooler is employed to maintain the temperature of the region below the plate to within the range of the sub-cooled liquid temperature range, from the boiling temperature at the pressure, to the triple point temperature of the liquid crygen. If the liquid crygen is sub-cooled to below its triple point temperature, solid cryogen will been to form which may or may not be a desired result. In the case when sub-cooling is achieved through the use of a cryocooler, such a practice is not desired since at or below die triple point temperature, solid cryogen will form around the interface to the cryocooler and significantly degrade the cooling performance of the cryocooler. (0M5J One embodimmt of the apparatus of present invention is illustrated in Figure 3. A cryogenic cooling system 10 of the present invention comprises an outer containment vessel 12, an inner containment vessel IS adapted to be contained inside the outer vessel 12, a venting pott 30 pneumatically coupled to the inner vessel, a high-voltage bushing 14 electrically and mechanically coupled to the inner vessel 18, and a cryocooler 20 that is therm&lly and mechanically coupled to the inner vessel. Th& high-voltage bulling 14 can be used to supply ejeclric current to HTS 24 and is connected to the outside higb-voitage power sources such as an electric power grid. HTS 24 is coiq)led to a HTS siqjport 32, which in turn is coupled to a thermal transfer medium 26. A copper ring 36 is mounted along the circumference of the bner vessel and is securely afBxed to a thermal transfer medium 26. An inner vessel support 34 is coupled to the inner vessel 18. HTS 24 may also be the HTS assembly of a matrix fault current limiter (MFCL) as described by US patert application 2003/0021074A1, assigned to the assignee of the presmt inveotjon and herein incorporated by reference [0016] 7%e space between the outer 12 and inner 18 vessel is mamtained under vacuum and rautti-Iayer insulation (MLI) material 22 is used to surround the inner vessel 18 to provide it with thermal insulation to the radiatition heat load. fOOlT] An inner vessel vmting port 30 provides gas~venting means for inner vessel 18 to reduce the gas pressure in inner vessel 18. Additionally, an auxiliary gas evaporation heater 52 may be employed to heat and boil liquid ciyogen to increase the pressure of the inner vessel 18. these two aspects of tbe ciyostat form the basis of the pressure control mechanism of the present invention in achievbg an oplunat pressure level of inner vessel 18, as is further described herein. [0018] The size of tbe inner vessel 18 can be determined to provide adequate cooling capacity to meet cooling requirements for the HTS 24. {0019] Tbe inner vessel 18 houses cryogen that has a liquid as well as a gaseous region. In one exemplary embodiment the cryogen is nitrogen and is pressurized at OJMPa in order to achieve the optimum dielectric stpsngth of liquid nittogen per Fig. 2. Bubbles, especially large-size bubbles in the liquid nitrogen could degrade its dielectric strength. Bubble generates when heat generated in tTTS 24 causes its temperature to be above the boiling temperature of the liquid nitrogen it submei;ges in. Increasing the [mssure in a cryostat also mcreases the boiling temperature of the liquid nitrogen. When the nitrogen pressure is mtuntatned si 03MPa, the boiling temperature of liquid nitrogen is elevated to 88K compared to the 77K at O.lMPa. This makes the bubble generation more diflicult therefore improves the electrical insulation properties of tfie liquid cryogen. In addition, in order to prevent electric insulation breakdown betweoi HTS 24 and the inner vessel 18, HTS 24 is surrounded by a dielectric medium 38 diat acts an elechic insulation baiiler. Otho' measures of 'mprov'aig the high-voltage insution of the oyogenic cooling system includede, placing buckets, tubes, boxes or screens or similar objects made from dielectric materials in a methed configuration to breakdown the size of bubbles if they were generated donas the device operatitm. The cell dimoisions of the mesh structure or apertures are selected to be sufficiently small so that any bubbles penetrating the screen will become small enough so that they will not cause substantial degradation of dielectric strength of liquid nitrogen and will not cause any voltage insulation breakdown within HTS 24 and its surrounding environment In one exemplary embodiment the screen apertures have a diameter in a range up to S millimeters. [0020] At 0.3MPa pressure, the surface temperature at the liquid and gaseous nimigen boundary 42 is the boiling (saturation) temperature of the boiling liquid nitrogen which is 88K. The liquid nitrogen region is further divided into two regions by a thennal transfer medium 26. The liquid region below the plate 26 is a sub-cooled zone 48 while above the plate 26 is a thennal buffer region 46. The temperature of the sub-cooled region 48 is maintained at about 65K by a cryocooler 20. HTS 24 is submerged in a sub-cooled liquid cryogen region. Because of the lowered operating temperature {65K), the perfonnance of the HTS 24 namely Its critical current density level is significantly improved. The CTyocooIer may be a closed-cycle cryocooler, which is selected from the group including a Gifford-McMahon refrigerator or a pulse-tube refrigerator or a combination of both refrigerator systems. [0021] There will be a temperature transition from 88K at die liquid/gas sur&ce 42, to the 65K at the heat transfer plate 26. There are liquid evaporation and gas condensalton process simultaneously occurring along the liquid/gas boundary 42 where an equilibrium state will ultimately form if the HTS device is operating at its steady state and the heat input into the cryostat and cooling by the cryocooier reaches equilibrium. The liquid nitrogen in region 46 could be in a mostly stagnant state or in a turbulent flowing regime depending on the heat load and pattern that exist in this region. The thermal buffer region 46 therefore isolates the sub-cooled region 48 from the events within the region 46. [0022] In this example, the thermal transfer medium 26 is made of copper, which has very good thermal conduction properties and has apertures along its surface (not shown) to facilitate the heat transfer between two two liquid nitrogen regions as well as the heat transfer from fliese two regions to the cryocooler 20. even though the thermal transfer plate 26 is not required to achieve the cryogenic cooling system under present invention, its presence will signincantly improve the thermal transfer characteristics of such a system. The fliermal transfer medium 26 may be a plate, ring, bar or similar congurations, such thermal transfer medium made of copper or similar metal for facilitating transfsr of heat from the ctyogen regins to the cryocooling means. {0023] In summaiy, the present invention has several features that more suitable for high-volt^e triplications while at the same time can improve the performaoce of the HTS materials. Pressurization of cryogen can put the ciyogen at its most optimum dielectric strength while sub-coolng the liquid ciyogen region where HTS resides increases the critical current density of the HTS matertals. [0024] Next The case is described where liquid cryogen in the thermal buffer region or thermal gradient level (TGL) 46 region of the cryogenic cooling system of present invention is in a mostly sysnant state. Sudi an environment can exist if the overall heat leak into the TGL is relativejy low and there Is Hale or no convective heat transfer taking place within ibis region. The exemplary embodiment assumes liquid nitrogen as a cooling medium and is pressurized at 0.3MPa absolute (under which the boilmg temperature of liquid nitnjgen is about 88K), and the sub-cooled liquid nitrogen region is at about 65K. Again, referring to Fig. 3 for an exemplary system composition. The beat transfer mechanism from the liquid surface 42 to ttie thermal transfer medium 26 is described as follows. Any heat that flows mto gas area 44 will raise the temperstun of the gas if it is not iaunediately^ansfenedoui of the gaseous region. At the gas/liquid mterface 42, the gas is condensed at the surface of the ciyogen. The beat of ctmdcnsation is then transferred by thermal conduction through TGL 46 to die sub-cooled liquid nitrogen region 48 that is maintained by oyocooler 20. The thickness of TGL 46 and its sur&ce area, defined by copper ring 36, determines the amount of transfirable heat through the layer since the upper tempnatuie (88 degrees Kelvin) and lower temperahire (65 degrees Kelvin) are eSecdveiy set. If die heat input is greater than the set heat conduction value for a certain TGL 46 thickness, the excess beat evaporates the cryogcn and reduces the TGL thickness, thus Increasing the heat transfer rate until a new equilibrium js reacbed. If the heat input is less than the heat conduction value Arough the TGL 46, there will be a net coadensation mcreasing the TGL thickness. The result is that for a certain heat load from the sur&ce 42 to heat transfer medium 26, an optimum equilibrium TGL thickness (LopO will develop. The time dependence of the layer thickness "L" development is given as the TGL increase by condensation minus TGL decrease by evaporation by the heat load "Q", expressed modiematically as: (00251 dL/dt = k X (S/L) x AT x l/{So) - Qf(Sa), wherein, k ■= tiiermal conductivity of the liquid cryogM (for liquid niftt)gen, k = 1,5 m Watt/cm/Kelvin); 10026) wherein, S - sur&ce area of the TGL (idA x lOtf cm^ for tSie case vMtxti surface 43 diam^er is 100 cm); [0027] wherein, AT = tonperature difference between iqiper and lower boundaries of the TGL (88K - fiSK = 23 Kelvin); [0028] and whwcin, a = latent heat or condensation heat of gas/liquid cryogen (for nitrogen, a = 162 Joule/cm'). [0029} Hie optimum thickness of the TQL is realized when dl^dt [0030] The graph in Figure 5 shows calculated data wherein the relationship of the time it takes to reach an equilibrium thickness of the TGL to various heat loads. Figure 5 illustrates a plot 60 of the time depen [0031] The resulting process is a cnse sys^m. This implies that the parameter control^ such as temperature. pressure and ciyogen level arc not very sensitive to variation over time. One important result from this analysis is that for the lOO-watt case, the optimum TGL thickness is only a few centimeters. The trend of decreased TGL thickness with increasing heat load leads to die conclusion that with increased heat loads, the TGL is getting more sensitive to variation in operating parameters and moves the system into e less stable operating regime. [0032J The previously described embodiments of the present invention have many features including a pressurized ciyogen gaseous region and a sub-cooled liquid region, a heating and venting scheme to maintain the pressure, a bubble size control mechanism, and a cooling means that maintain cryogen at a temperature at or below its boiling point within a sub-cooled temperature range. The characteristics and effects of all these features make the cryogenic cooling system of present invention more advantageous for use in high-voltage HTS applications. [00331 While only certain features of Ae invention have been illustrated and described herein, many modifications and changes wilt occur to those skilled in the art It is, therefore, to be understood that die appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. In addition, when describing the present invention, nitrogen, in liquid and gaseous phase, was mentioned as a otogenic medium. It is also to be understood that other ctyogens can be used in place of nitrogen in the cryogenic cooling system of present mvention. WE CLAIM: 1. A method for achieving and maintaining cryogenic cooling for a cryogenic cooling system (10) having a cryogen containment vessel (18) that stores cryogen in a liquid state and a gaseous state, and having at least one superconductor (24), the method comprising the steps of: maintaining a pressurized cryogen region (44) within the cryogen containment vessel (18); and maintaining the temperature of a portion of the liquid cryogen within a liquid cryogen region (48) at and below its boiling temperature using sub-cooling means (20). 2. The method of cryogenic cooling as claimed in claim 1, where in it comprises the step of maintaining the pressure of the cryogen to above one absolute atmospheric pressure, in order to improve the dielectric strength of the cryogen. 3. The method of cryogenic cooling as claimed in claim I, wherein it comprises the step of heating and boiling the liquid cryogen to increase the pressure of the gaseous cryogen region (44). 4. The method of cryogenic cooling as claimed in claim 3, wherein the step of heating and boiling the liquid cryogen comprises the step of heating the liquid cryogen in the liquid cryogen region (46). 5. The method of cryogenic cooling as claimed in claim I, wherein it comprises the step of venting gaseous cryogen to reduce the pressure of the gaseous cryogen region (44). 6. The method of cryogenic cooling as claimed in claim 5, wherein the step of venting gaseous cryogen comprises the use of a venting port (30) on the cryoger containment vessel (1S). 7. The method of cryogenic cooling as claimed in claim 1, wherein the cryogen containment vessel (18) is housed in an outer vessel (12) that is adapted to maintain a vacuum, 8. The method of cryogenic cooling as claimed in claim 7, wherein the outer vessel (12) contains a saturated liquid cryogen that provides sub-cooling means (20) to the liquid cryogen contained in the inner vessel (18). 9. The method of cryogenic cooling as claimed in claim 1, wherein the sub-cooling means (20) is a closed-cycle cryocooler. 10. The method of cryogenic cooling as claimed in claim 9, wherein the closed-cycle cryocooler is a GitTord-McMahon refrigerator, 11. The method of cryogenic cooling as claimed in claim 9, wherein the closed-cycle cryocooler is a pulse-tube refrigerator. 12. The cryogenic cooling system (10) as claimed in claim 1, wherein the sub-cooling means (20) comprises a saturated liquid cryogen within an outer vessel (12), the saturated liquid cryogen sub-cools the liquid cryogen contained in the inner vessel (13). 13. The method of cryogenic cooling as claimed in claim t, wherein it comprises the step of maintaining the pressure of the cryogen to raise the boiling point of the cryogen and therefore raising the temperature under which the cryogen generates bubbles. 14. The method of cryogenic cooling as claimed in claim 1, wherein it comprises the step of maintaining an optimum thickness of the thermal gradient layer (46) in the case of a stagnant liquid cryogen. wherein the optimum thickness of such thermal gradient layer (46) is expressed by the equation k x S x (AT)/Q. wherein "S" is the surface area of the thermal gradient layer (46). and wherein 'ΔT" is the temperature difference across the thermal gradient layer (46), and wherein "k" is the thermal conductivity of the cryogen in the thermal gradient layer (46), and wherein "Q" is the heat input to the thermal gradient layer (46) through the boundary surface between the thermal gradient layer (46) and the gaseous regions(44). 15. A cyrogenic cooling system (10) having an inner vessel (18), at least one high temperature superconductor (24). and an outer vessel (12), the inner vessel (18) adapted to be contained inside the outer vessel (12) and adapted to store pressurized cryogen in a liquid state and a gaseous slate, the cooling system comprising: liquid heating means (52) for boiling off liquid cryogen in order to increase the pressure at the gaseous region (44); gas venting means (30) for releasing gas in order to reduce the pressure at the gaseous region (44); and cryogenic cooling means (20) for maintaining a portion of the liquid cryogen within liquid cryogen region (48) within a sub-cooled temperature range that is at and below its boiling temperature. 16. The cryogenic cooling system (10) as claimed in claim 15, wherein the outer vessel (12) is a vacuum vessel. 17. The cryogenic cooling system (10) as claimed in claim 15, wherein the outer vessel (12) contains a saturated liquid cryogen that provides sub-cooling means to the liquid cryogen bath contained in the inner pressure vessel (18). 18. The cryogenic cooling system (10) as claimed in claim 15, wherein the cryogenic cooling is a closed-cycle cryocooler. 19. The cryogenic cooling system (10) as claimed in claim 18, wherein the closed-cycle cryocooler is selected from the group comprising a Gifford-McMahon refrigerator and a pulse tube refrigerator. 20. The cryogenic cooling system (ID) as claimed in claim 18, wherein the closed-cycle cryocooler comprises a close-cycle refrigerator and a sub-cooled liquid cryogen housed in an outer vessel (12). 21. The cryogenic cooling system (10) as claimed in claim 15, wherein it comprises a thermal transfer medium (26) in a plate, ring, or bar configuration, such thermal transfer medium (26) is made of copper and also copper for facilitating transfer of heat from the cryogen regions to the cryogenic cooling means (20). 22. The cryogenic cooling system (10) as claimed in claim 15, wherein it comprises a dielectric medium, wherein the dielectric medium encapsulates the high temperature superconductor (24). 23. The cryogenic cooling system (10) as claimed in claim 22. wherein the dielectric medium is a wire mesh, wherein the mesh has apertures no larger than 5 millimeters to facilitale the reduction of the sizes of bubbles in the liquid cryogen regions (46,48). 24. The cryogenic cooling system (10) as claimed in claim 15, wherein it comprises a thermal transfer plate (26) disposed inside the inner vessel (18) for coupling thermal heat within the liquid cryogen regions (46, 48). 25. The cryogenic cooling system (10) as claimed in claim 15, wherein it comprises multi-layer insulation (22) surrounding the inner vessel (18) for reducing the radiation heat leak into the inner vessel (!8). 26. The cryogenic cooling system (10) as claimed in claim 24, wherein it comprises a bi-metal interface coupled to the thermal transfer plate (26) for facilitating the transfer of heat to the cryogenic cooling means (20). 27. The cryogenic cooling system (10) as claimed in claim 15, wherein it comprises a vacuum space and corresponding means to maintain the vacuum space, for the interface between the inner vessel (18) and the cryogenic cooling means (20) independent of the vacuum space of the outer vessel (12) and the corresponding means to maintain the vacuum space.

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