Cryostat incorporating high efficiency cryocooling system

Abstract

A novel and useful cryostat that incorporates a high efficiency hybrid cryocooling system suitable for use in a quantum computer. A high efficiency Joule-Thomson (J-T) expansion (evaporative) circuit is integrated with a two-stage cryocooling system such as a Gifford-McMahon (G-M) cryocooler. The J-T expansion circuit integrated with the 2-stage cryocooling system is located entirely inside the vacuum chamber of the cryostat except for the electric motor that drives the J-T compressor stage which is external. The J-T compressor first stage is thermally anchored to the G-M first stage at 35K and driven by the external electric motor, the J-T condenser second stage is thermally anchored to the G-M second stage at 3.5K, and the J-T evaporator third stage is cooled to a target temperature of less than IK. The internal 'cold' pump performs the function of reducing the pressure on the helium reservoir while consuming less energy minimizing the size of the pump. One or more magnetic clutch mechanisms provide thermal isolation between the external motor and the inside of the vacuum chamber. Driving and driven magnets are coupled across a non-magnetic membrane such as sapphire.

Description

[0001] 12485.0110

[0002] CRYOSTAT INCORPORATING HIGH EFFICIENCY CRYOCOOLING SYSTEM

[0003] REFERENCE TO PRIORITY APPLICATION

[0004] This application claims the benefit of U.S. Provisional Application No. 63 / 687,266, filed August 26, 2024, entitled “High Efficiency Cryocooling System” incorporated herein by reference in its entirety.

[0005] FIELD OF THE DISCLOSURE

[0006] The subject matter disclosed herein relates to the field of cryostats and more particularly relates to a cryostat that incorporates a high efficiency cryocooling system suitable for use in a quantum computer.

[0007] BACKGROUND OF THE INVENTION

[0008] Cryocoolers and cryostats are essential technologies in the field of cryogenics, which deals with the production and maintenance of very low temperatures, typically below 120 K (- 153°C or -243°F). These devices enable a wide range of applications, from scientific research in quantum computing and superconductivity to practical uses in medical imaging (e.g., MRI machines), infrared sensors for astronomy, and cooling superconducting electronics. While cryocoolers focus on active cooling, cryostats provide the insulated environment to sustain those low temperatures.

[0009] A cryocooler is a mechanical refrigeration system designed to achieve cryogenic temperatures, generally below 123 K (-150°C). Unlike standard refrigerators, cryocoolers operate in extreme cold where quantum effects and material behaviors change dramatically. To reach these conditions, cryocoolers often utilize liquid cryogens like helium or nitrogen, which liquify at relatively low temperatures. Many are closed-cycle designs, which recirculate the working fluid (often helium gas), avoiding the need for replenishment and enabling “dry” operation. Common types include Stirling, Gifford-McMahon (G-M), pulse-tube, and loule- Thomson (J-T) cryocoolers, each suited to different temperature ranges, sizes, and efficiency needs.

[0010] Cryocoolers rely on thermodynamic cycles involving compression, expansion, heat exchange, and sometimes regeneration to remove heat from a target area. The basic principle is to use a working fluid (typically a gas like helium) to absorb heat at low temperatures and reject it at higher temperatures, often through mechanical work. 12485.0110

[0011] A cryostat is an insulated container or vessel designed to maintain objects, fluids, or systems at cryogenic temperatures, isolating them from ambient heat. Modern “dry” cryostats incorporate closed-cycle cryocoolers to reach liquid helium temperatures (around 4 K) without consumable cryogens, making them more user-friendly and cost-effective.

[0012] Currently, the world is on the brink of a quantum revolution and to make this a reality, quantum computers with millions of qubits will be required. One approach to realize this is by developing quantum computers based on existing commercial silicon semiconductor technologies, as this quantum technology offers sufficient scalability with a reasonable footprint and energy requirement.

[0013] When using semiconductor technologies, efficient cryostats capable of maintaining a temperature of less than IK while dissipating up to 1W of energy from the semiconductor quantum processor are necessary.

[0014] Some existing cryostats include external circulation to reach temperatures less than 3K. These systems employ an external pump that acts on the helium within the system to reduce pressure, and thus also the temperature. The pump, however, also pulls cold helium out of the system where it warms in an external dump before being recycled into the system and recooled. In this so called ‘warm-cycle’, the helium is warmed and must be re-cooled. This constitutes additional work in the system that serves no purpose.

[0015] For example, existing Joule-Thompson cryostats achieve cooling below 4K by using an external helium pump. The pump reduces the pressure on a helium reservoir, thereby reducing the temperature. Because the pump is external, the helium is warmed to room temperature and must be cooled significantly before it is able to liquify again at 4K. It is noted that the temperature of the helium does not go straight from room temperature to 4K when reentering the cryostat.

[0016] Although such systems with external pumps can provide sufficient cooling power for quantum computing it has the disadvantage of a large form factor and is very energy inefficient and is therefore very expensive to operate.

[0017] Existing cryogenic cooling systems have been designed to refrigerate workloads down to below IK, but their size and power requirements have not generally been seen as design constraints, and have not been minimized.

[0018] It is thus desirable to have a fully internal helium cycle, thereby enabling the cryostat to be small, power efficient, and robust. This enables air-cooled quantum computer systems that fit inside a regular server rack that prioritize thermal efficiency and size. Currently available cryocoolers and cryostats, however, face issues related to performance, reliability, 12485.0110 and application-specific demands. Connecting the cryocooler to a quantum device in a cryostat is problematic, as thermal links can introduce heat leaks or mechanical stress. Miniaturization for cryocoolers (e.g., for quantum chips) while achieving sub-lK temperatures reliably in compact forms is difficult. Additional challenges of current cryostats include vibration, efficiency, and integration issues.

[0019] 12485.0110

[0020] SUMMARY OF THE INVENTION

[0021] The present invention is a novel and useful cryostat that incorporates a high efficiency hybrid cryocooling system suitable for use in a quantum computer. A high efficiency Joule- Thomson expansion (evaporative) circuit is integrated with a two-stage cryocooling system such as a Gifford-McMahon (G-M) cryocooler. The hybrid system is suitable for use in a high efficiency cryogenic cooling system for quantum computers. The Joule-Thomson (J-T) expansion circuit integrated with the 2-stage cryocooling system is located entirely inside the vacuum chamber of the cryostat except for the electric motor that drives the J-T compressor stage which is external to the vacuum chamber. The first stage of the J-T circuit (i.e. the compressor stage) is thermally anchored to the G-M first stage at 35K and driven by the external electric motor, the J-T condenser second stage is thermally anchored to the G-M second stage at 3.5K, and the J-T evaporator third stage is cooled to a target temperature of less than IK.

[0022] The cryostat replaces the typically external (warm) pump with an internal cold pump that performs the same function of reducing the pressure at the liquid helium reservoir. Because the helium never exits the system, it only reaches a temperature of around 40 Kelvin, significantly lowering energy consumption and minimizing the size of the pump. By removing the large external pump, the form factor of the system is also significantly reduced.

[0023] The cryostat also employs one or more magnetic clutch assemblies to provide thermal isolation and / or vacuum sealing between the external motor and the inside of the vacuum chamber. Driving and driven magnets are coupled across a non-magnetic membrane such as sapphire.

[0024] Note that alternative methods of reaching low temperatures of approximately IK and below also use a Joule-Thomson stage using helium. In these methods, however, the helium is recompressed outside of the vacuum system which involves much larger compressors and much more work to recompress and re-cool the helium back down from near ambient temperatures.

[0025] Several advantages of the present invention include significantly increased thermal efficiency and significantly smaller size meaning that air-cooled, desktop or server rack scale cryocoolers can be constructed that plug into a regular utility supply (e.g., 120 / 240V 20A supply) to serve as accessible and scalable quantum computer platforms. Another advantage is that the cryostat of the present invention requires significantly lower volumes of helium used in the system which is especially beneficial given the very high cost of He3. 12485.0110

[0026] This, additional, and / or other aspects and / or advantages of the embodiments of the present invention are set forth in the detailed description which follows; possibly inferable from the detailed description; and / or learnable by practice of the embodiments of the present invention.

[0027] There is thus provided in accordance with the invention, a cryostat, comprising a vacuum chamber, a 2-stage cryocooler within said vacuum chamber having a first cooling stage and a second cooling stage, said second cooling stage at a lower temperature than said first cooling stage, a Joule-Thomson (J-T) expansion circuit within said vacuum chamber having a hot end compressor first stage located alongside and thermally anchored to said cryocooler first cooling stage, a condenser second stage located alongside and thermally anchored to said cryocooler second cooling stage, and a J-T cold end evaporator orifice thermally anchored at a third stage, said third stage at a temperature lower than said condenser second stage.

[0028] There is also provided in accordance with the invention, a cryostat, comprising a vacuum chamber, a 2-stage cryocooler within said vacuum chamber having a first stage and a second stage, a Joule-Thomson (J-T) cryocooler circuit within said vacuum chamber having a first stage, a second stage, and a third stage, a drive motor located outside said vacuum chamber, and a magnetic clutch operative to couple rotational torque from said external drive motor to said J-T first stage through a vacuum tight, magnetically permeable membrane incorporated in said vacuum chamber.

[0029] There is further provided in accordance with the invention, a cryostat, comprising a vacuum chamber, a Gifford-McMahon (G-M) 2-stage cryocooler having a first cooling stage and a second cooling stage within said vacuum chamber, a Joule-Thomson (J-T) expansion circuit located entirely within said vacuum chamber having a hot end compressor first stage located alongside and thermally anchored to said G-M cryocooler first cooling stage, a condenser second stage located alongside and thermally anchored to said G-M cryocooler second cooling stage, a J-T cold end evaporator orifice thermally anchored at a third stage, said third stage at a temperature lower than said condenser second stage, and a drive motor located external to said vacuum chamber, and a magnetic clutch operative to couple rotational torque from said external drive motor to said J-T compressor first stage through a vacuum tight magnetically permeable membrane affixed to said vacuum chamber. 12485.0110

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Fig. l is a diagram illustrating a isometric view of an example cryostat;

[0032] Fig. 2 is a diagram illustrating a side sectional view of a first example 2-stage cryocooler and 3 -stage Joule-Thomson (J-T) refrigerator circuit;

[0033] Fig. 3 is a block diagram illustrating a first example 2-stage cryocooler and the gas flow path in the 3 -stage Joule-Thomson (J-T) refrigerator circuit;

[0034] Fig. 4 is a block diagram illustrating a first example 2-stage cryocooler and 3-stage Joule-Thomson (J-T) refrigerator circuit and associated heat shields;

[0035] Fig. 5 is a block diagram illustrating the components and related gas flow path in an example cryostat constructed in accordance with the present invention;

[0036] Fig. 6 is a diagram illustrating an example multi-turbo compressor assembly incorporated in the 3-stage Joule-Thomson (J-T) refrigerator circuit;

[0037] Fig. 7 is a diagram illustrating a side sectional view of a second example 2-stage cryocooler and 3-stage Joule-Thomson (J-T) refrigerator circuit with an external upper heat exchanger;

[0038] Fig. 8 is a diagram illustrating a side sectional view of a second example 2-stage cryocooler and 3-stage Joule-Thomson (J-T) refrigerator circuit with an internal upper heat exchanger;

[0039] Fig. 9 is a block diagram illustrating a second example 2-stage cryocooler and the gas flow path in the 3-stage Joule-Thomson (J-T) refrigerator circuit;

[0040] Fig. 10 is a block diagram illustrating a second example 2-stage cryocooler and 3-stage Joule-Thomson (J-T) refrigerator circuit and associated heat shields;

[0041] Fig. 11 is a diagram illustrating a side sectional view of a second example 2-stage cryocooler and 3-stage Joule-Thomson (J-T) refrigerator circuit with an external motor drive and magnetic clutch assembly;

[0042] Fig. 12A is a diagram illustrating a perspective view of a motor outside the vacuum chamber and associated magnetic clutch assembly;

[0043] Fig. 12B is a diagram illustrating an example magnetic clutch drive of the present invention in more detail;

[0044] Fig. 13 is a diagram illustrating the lower heat exchanger and J-T second and third stage;

[0045] Fig. 14 is a diagram illustrating the lower heat exchanger in more detail; and

[0046] Fig. 15 is a block diagram illustrating the example cryostat including initial gas fill at time of manufacture. 12485.0110

[0047] DETAILED DESCRIPTION

[0048] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be understood by those skilled in the art, however, that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

[0049] Among those benefits and improvements that have been disclosed, other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying figures. Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely illustrative of the invention that may be embodied in various forms. In addition, each of the examples given in connection with the various embodiments of the invention which are intended to be illustrative, and not restrictive.

[0050] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings.

[0051] The figures constitute a part of this specification and include illustrative embodiments of the present invention and illustrate various objects and features thereof. Further, the figures are not necessarily to scale, some features may be exaggerated to show details of particular components. In addition, any measurements, specifications and the like shown in the figures are intended to be illustrative, and not restrictive. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

[0052] Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 12485.0110

[0053] Any reference in the specification to a method should be applied mutatis mutandis to a system capable of executing the method. Any reference in the specification to a system should be applied mutatis mutandis to a method that may be executed by the system.

[0054] Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The phrases “in one embodiment,” “in an example embodiment,” and “in some embodiments” as used herein do not necessarily refer to the same embodiment s), though it may. Furthermore, the phrases “in another embodiment,” “in an alternative embodiment,” and “in some other embodiments” as used herein do not necessarily refer to a different embodiment, although it may. Thus, as described below, various embodiments of the invention may be readily combined, without departing from the scope or spirit of the invention.

[0055] In addition, as used herein, the term “or” is an inclusive “or” operator, and is equivalent to the term “and / or,” unless the context clearly dictates otherwise. The term “based on” is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. In addition, throughout the specification, the meaning of “a,” “an,” and “the” include plural references. The meaning of “in” includes “in” and “on.”

[0056] A diagram illustrating a isometric view of an example cryostat is shown in Figure 1. The cryostat, generally referenced 10, comprises a structural manifold 14 (also referred to as a frame, base, or support manifold), vacuum chamber 12, 2-stage cryocooler 16, 3-stage Joule- Thomson (J-T) expansion circuit 13 having an electric compressor drive motor 11 located outside the vacuum chamber, and a quantum PCB assembly 18 only the top of which is shown.

[0057] The cryostat functions to minimize heat transfer through conduction, convection, and radiation using multilayer insulation, vacuum jackets, and radiation shields. The main components include insulation, cooling, and integration mechanisms. A vacuum environment reduces conduction and convection, while insulation (e.g., gold plating, aluminized Mylar layers, etc.) blocks radiation. The cooling mechanism involves a controlled flow of cryogen (e.g., helium) which evaporates to provide cooling, balanced against heat input from a target circuit which is a quantum PCB assembly and various electrical circuits or heaters for temperature control. The cryocooler can attach to cold fingers or plates that conduct heat away from the target circuit. Temperature sensors (e.g., thermocouples or diodes) and heaters allow precise control down to millikelvin stability. Integration includes the cryostat housing the target circuit or device, with ports for electrical cables, leads, etc., optical access, vacuum pumps, and one or more thermal anchors. 12485.0110

[0058] In general, the cryostat of the present invention replaces the compressor (i.e. the warm pump) that is typically external to the vacuum chamber with a compressor (i.e. the cold pump) that is inside the vacuum chamber. Both pumps perform the same function by reducing the pressure on the helium reservoir. In the present system, however, the helium never leaves the system and is maintained at the first stage temperature of approximately 35 to 40 Kelvin. Placing the compressor first stage inside the vacuum chamber has the advantage of significantly lowering energy consumption as well as minimizing the size of the pump required. In addition, removing the large external pump significantly reduces the overall form factor of the system as well.

[0059] In particular, a high efficiency Joule-Thomson (J-T) expansion or evaporative circuit integrated with a two-stage cryocooling system, such as a Gifford-McMahon (G-M) system, etc. is disclosed that is suitable for use in a high efficiency cryogenic cooling system and cryostat in a quantum computer. G-M type cryocoolers feature a mechanical drive in which a cooling agent (i.e. the heat storage material) moves back and forth inside a cylinder to repeat adiabatic expansion of the refrigerant (helium) to generate a chilled temperature. G-M type cryocoolers are similar to Stirling type coolers but use a displacer and valves to control gas flow, separating the compression stage (often at room temperature) from the expansion stage. This allows for larger cooling capacities and temperatures as low as 4K.

[0060] In one embodiment, the invention supports the application of a Joule-Thomson (J-T) expansion cooling circuit that is added to a Gifford-McMahon (G-M) 2-stage cryocooling system to form a hybrid cooling system. The J-T circuit is located entirely inside the vacuum chamber of a cryostat with the drive motor located external to the vacuum chamber. Joule- Thomson cryocoolers rely on the J-T effect, whereby gas expands through a throttle (e.g., a porous plug, orifice, or valve), which provides cooling due to intermolecular forces.

[0061] It is appreciated by one skilled in the art that alternative 2-stage cryocooling systems may be used such as a Stirling refrigerator, pulse-tube refrigerator, or a second J-T expansion circuit.

[0062] For illustration purposes only, the example cryostats disclosed herein use a commercially available 2-stage G-M cryocooler such as the RDK-101 manufactured by Sumitomo Heavy Industries, Ltd., Tokyo, Japan. In one embodiment, the J-T first stage compressor is thermally anchored to the first G-M cooling stage at 35K, the second stage condenser is at the G-M second cooling stage at 3.5K, and the third stage evaporator is at a third stage at a typical target temperature of less than IK. 12485.0110

[0063] A diagram illustrating a cross sectional view of a first example 2-stage cryocooler integrated with a 3-stage Joule-Thomson (J-T) refrigerator circuit is shown in Figure 2. The example cryostat, generally referenced 20, comprises a 2-stage cryocooler 24 having a first stage cold head 25 affixed to a first stage platform 26 and second stage cold head 27 affixed to a second stage platform 28, structural manifold 22, a 3-stage Joule-Thomson (J-T) refrigerator circuit that includes a hot end compressor first stage 34 thermally anchored to the first stage platform, a condenser second stage 36, and a cold end evaporator third stage 42 affixed to a third stage platform 46 to which a quantum printed circuit board (PCB) 44 is thermally anchored. Note that the platforms are preferably constructed from a material of high thermal conductivity such as copper.

[0064] In operation, the helium is cycled between a gas and a liquid between the bottom of the lower evaporation tube 39 and the top of the upper evaporation tube 38. In particular, cooled liquid helium that emerges from the end of the lower reverse flow heat exchanger tube chills the cold head 42 via evaporative cooling to the target cryogenic temperature of less than IK. The low pressure evaporated helium gas generated from evaporative cooling from the cold head 42 rises through upper and lower evaporator tubes 38 and 39 to the compressor first stage 34 where it is compressed to a high pressure gas which is forced through He line (i.e. tube) 30 to the second stage of the 2-stage cryocooler 24 which is at a typical temperature of 4K. The gas is forced through internal channels in a cone shaped liquefier 29 where it condenses to a liquid and sent to the J-T second stage 36 via He line (i.e. tube) 32. The liquid helium then travels down a spiral reverse flow heat exchanger tube 40 within lower evaporator tube 39 to the cold head 42 where it is forced to exit through an orifice (not shown) which provides the evaporative cooling of the cold head to the target temperature of less than IK. A quantum PCB 44 assembly is thermally anchored to a third stage platform 46 to which the cold head 42 is also thermally anchored.

[0065] A block diagram illustrating a first example 2-stage cryocooler and the gas flow path in the 3-stage Joule-Thomson (J-T) refrigerator circuit is shown in Figure 3. The example cryostat, generally referenced 60, comprises a vacuum chamber 70 and 2-stage G-M cryocooler 62 having a first stage 77 at approximately 35K and a second stage 79 at approximately 3.5K. Integrated with the 2-stage G-M cooler is a 3-stage J-T expansion circuit 75 driven by an electric drive motor 64 located outside the vacuum chamber 70. Rotational torque is coupled from the motor to the turbo compressor first stage of the J-T circuit via a magnetic clutch including driving magnet 68 at the bottom of drive shaft 66, driven magnet 72 at the top of drive shaft 74. A major benefit of using a magnetic clutch is to break the thermal coupling 12485.0110 between the heat generating electric motor and the compressor in the J-T circuit which lies inside the vacuum chamber.

[0066] The first stage is cooled to the G-M first stage temperature (e.g., 35K). The compressor stage may comprise any combination of radial 76 and / or axial 78 compressor blades or blade assemblies. The turbo compressor wheel / system and / or axial compressor blade system is located inside the vacuum system. It may comprise a single turbo compressor wheel that combines axial compression with centrifugal compression in a single wheel, or a sequence of turbo compressor wheels where the output of one wheel is fed into the input of the next wheel as described in Figure 6 infra. The compressor inside the vacuum system can also be an axial compressor, or sequence of axial compressors where the output of one axial compressor wheel is fed into the input of the next axial compressor wheel. Further, the compressor may comprise a molecular drag stage. The compressor inside the vacuum system can also be a combination of turbo (radial) compressor, axial compressor, and molecular drag pump stage. Note that in one embodiment, the compressor spins at a relatively high RPM, e.g., 50-100k, but requires relatively low torque to create the required pressure differential.

[0067] In one embodiment, a single turbo compressor sits inside a corresponding compressor shroud at the top of the evaporator tube, and a series of axial compressor wheels are arranged on a separate but magnetically coupled shaft or shafts further down the evaporator tube in order to more effectively move the evaporated helium at the Joule-Thomson valve along the evaporator tube while compressing it.

[0068] In operation, the first stage functions to compress the helium gas to generate high pressure output which then enters the upper high temperature reverse flow heat exchanger (not shown) within the evaporation tube 80 illustrated by gas flow arrows 81. The G-M second stage 79 at a temperature of 3.5K liquifies the high pressure helium gas where it then enters the lower low temperature reverse flow heat exchanger (not shown) illustrated by gas flow arrows 83. Finally, the high pressure helium, is forced through an orifice or expansion nozzle 82 in the final J-T expansion stage (i.e. third stage) and through evaporative cooling reaches the target temperature of less than IK. A quantum PCB assembly 87 is thermally anchored to the third stage. The helium gas generated from evaporation flows upwards via the low pressure generated by the compressor blades as illustrated by gas flow arrows 85 to complete the helium cycle.

[0069] In one embodiment, the electric motor 64 that drives the compressor first stage of the J-T expansion circuit is controlled by a motor control circuit 61. A soft start mechanism 63 within the control circuit provides a slow ramp-up of the motor. The function of the turbo 12485.0110 compressor soft start mechanism is to provide a slow ramp-up in rotational speed of the compressor drive motor or motors outside the vacuum chamber in order that the start-up drag of magnetic coupling or couplings does not cause the magnetic drive to slip. If full torque is reached too quickly, the magnetic coupling can slip and the torque transmitted across the magnets drops to near zero as the magnets are no longer coupled. The torque transmitted across the window is actually provided by the slight axial offset between the driving magnets and the driven magnets, and there exists a maximum torque that if exceeded the magnetic drive will slip. Note that this axial offset can be measured, controlled and visualized by use of magnetic sensors positioned close to each of the coupled pairs of magnets.

[0070] In one embodiment, the one or more drive shafts for the compression system are fabricated from stainless steel. Numerous grades of stainless steel exhibit very low thermal conductivity, especially at cryogenic and near cryogenic temperatures, such as austenitic stainless steels, high-nickel austenitic alloys, ferritic stainless steels, and duplex stainless steels. Fabricating the compressor drive shaft(s) from thin walled stainless steel tubes allows the realization of thermally resistant (i.e. poor heat conduction) and light weight turbo (radial) and axial compressor assemblies.

[0071] A block diagram illustrating a first example 2-stage cryocooler and 3-stage Joule- Thomson (J-T) refrigerator circuit and associated heat shields is shown in Figure 4. The example cryostat, generally referenced 90, comprises a vacuum chamber 120, 2-stage G-M cryocooler 92 having a first stage 94 at approximately 35K and a second stage 96 at approximately 3.5K. Integrated with the 2-stage G-M cooler is a 3-stage J-T expansion circuit driven by an electric drive motor 91 located outside the vacuum chamber 120. Rotational torque is coupled from the external motor 91 to the turbo compressor first stage 108 of the J-T circuit via a magnetic clutch including magnets 102, 104 separated by a non-magnetic membrane 106 such as a sapphire window. A drive shaft 100 couples rotation torque from the motor to the turbo compressor 108. A structural manifold 98 functions as a mounting frame for the various components of the cryostat. The J-T circuit further comprises upper high temperature reverse flow heat exchanger spiral tube 110 externally wrapped around upper evaporator tube 130, high pressure helium tube 114 connected to the J-T second stage 112 where the now liquified helium it enters the lower low temperature reverse flow heat exchanger spiral tube 116 within lower evaporator tube 132 and exits at the expansion nozzle 118 to achieve the target temperature of less than IK at the cold head 128.

[0072] The cryostat 90 comprises several heat shields to limit thermal radiation including the vacuum chamber outer heat shield 120, heat shield 122 (35K) enclosing both the G-M cooler 12485.0110 and J-T circuit, heat shield 124 (4K) enclosing the two second stages and J-T third stage, and heat shield 126 (<1K) enclosing the quantum PCB and J-T circuit third stage. Note that the heat shields may be gold plated as gold is highly effective at reflecting infrared radiation, the primary form of heat transfer in a vacuum environment. By gold plating the heat shields, the amount of radiative heat absorbed by the sensitive components within the cryostat is drastically reduced due to the low emissivity of gold, ensuring they remain at their required operating temperature. Low emissivity is desired for heat shields to minimize the heat radiated from warmer sections in the cryostat to colder parts.

[0073] A block diagram illustrating the components and related gas flow path in an example cryostat constructed in accordance with the present invention is shown in Figure 5. The example cryostat, generally referenced 140, comprises vacuum chamber 141, 2-stage G-M cryocooler 142 having a first stage 146 at approximately 35K and a second stage 148 at approximately 4K. Integrated with the 2-stage G-M cooler is a 3-stage J-T evaporation or expansion circuit 157 driven by an electric drive motor 168 located outside the vacuum chamber. Rotational torque is coupled from the external motor to the turbo compressor first stage 150 of the J-T circuit via a magnetic clutch which include magnets 170, 172 separated by a non-magnetic membrane (not shown) such as a sapphire window.

[0074] The first stage incorporates one or more radial and / or axial blade assemblies 176 that compress the helium gas sucked up through the evaporator tube 161. In this example, a 2-stage turbo compressor 152 is shown but alternative compressor implementations are also contemplated without departing from the scope of the invention. The heated compressed helium gas, which is too warm to be a liquid, is cooled to the temperature of the G-M first stage 146 and then flows through He line 156 to the G-M second stage 148 where it is then cooled to a temperature low enough for the helium gas to condense to a liquid. The liquid helium then flows through a lower low temperature reverse flow heat exchanger 160 that lies within the evaporator tube 161 and exits at the expansion nozzle 164 to typically create a pool of liquid helium and achieving the target temperature of less than IK at the cold head 166. A quantum PCB assembly 167 is thermally anchored to the cold head 166. helium at room temperature (300K) is supplied to the closed J-T expansion circuit at the time of manufacture via a supply tank 144, control valve(s) 162, and He line 174. The J-T circuit is first evacuated and then the valve(s) are configured to permit filling the circuit with helium which is then sealed where the helium is retained in the closed J-T expansion circuit.

[0075] A diagram illustrating an example multi-turbo compressor assembly incorporated in the 3-stage Joule-Thomson (J-T) refrigerator circuit is shown in Figure 6. The example compressor 12485.0110 assembly, generally referenced 180, comprises four radial compressor blades 188, 190, 192, 194 arranged as two pairs of back to back compressor blades. The lower half of the magnetic clutch mechanism 182 is shown coupled to shaft 184 that drives the compressors. In operation, low pressure (LP) helium enters the lower compressor 188 which sits at the top of the evaporator tube 181shown as dashed arrow 191. It is drawn in from the low pressure created by the compressors. Each of the compressors provides a certain compression ratio with the combination of all four providing four times the compression each individual compressor blade provides. The first compressor 188 radially compresses the helium gas and it is forced out the sides through channels provided in the housing 186 (i.e. machined). The channels guide the helium to the input of the second compressor blade 190 as shown by dashed arrow 183. Since all four compressor blades spin in the same direction, the individual blades in compressors 190 and 194 may be oriented in opposite directions from the blades in compressors 188 and 192.

[0076] Compressor blade 190 functions to radially compress the helium gas where it exits through machined channels to the input of compressor blade 192 as shown by dashed arrow 185. Compressor blade 192 functions to radially compress the helium gas further where it exits the sides as shown by dashed arrow 187 and is input to the fourth and final stage of the assembly, compressor blade 194, which functions to radially compress the helium gas where it exits as high pressure (HP) output 189.

[0077] Placing the pairs of radial compressor blades back to back provides the advantage of eliminating the backside friction that normally is present if the backside of each radial compressor blade was exposed.

[0078] In a second example embodiment, a Joule-Thomson expansion circuit sits entirely inside a 2-stage cryocooler, e.g., Gifford-McMahon, within the vacuum chamber of the cryostat. It operates with the compressor thermally anchored at the G-M first stage (~35K), the condenser thermally anchored at the G-M second stage (~4K). The J-T valve at the bottom of the evaporator tube is thermally anchored at a J-T third stage (~600mk), and back to the compressor thermally anchored at the G-M first stage at the top of the evaporator tube.

[0079] The thermal difference between the hot end (i.e. the compressor) and the cold end (i.e. the J-T valve) is 35K-1K = ~34K. This is in contrast to a J-T system that is added to a G-M cryocooler where the compressor sits outside of the vacuum chamber. In this case, the thermal difference between the hot end and the cold end is 300K-1K = -299K, which requires significantly more power and a much larger compressor unit.

[0080] Note that the maximum theoretical thermal efficiency of a heat engine is given by its Carnot efficiency: 12485.0110

[0081] 17max= 1 — — (1)

[0082] ‘H where rj is the Carnot efficiency, Tc is the absolute temperature of the cold reservoir, and TH is the absolute temperature of the hot reservoir. For the above example: external compressor: TH= 300; Tc= 1; r / max= 0.0033 (2) internal compressor: TH= 35; Tc= 1; rmax= 0.029 (3)

[0083] Locating the compressor inside the vacuum chamber thus improves the efficiency by a factor of approximately 8.8 or greater.

[0084] In one embodiment, helium is used as the condensing / evaporating medium inside the J-T system. In one embodiment, the cryocooler uses the helium 4 (He4) isotope. In another embodiment, the more expensive but more thermally effective helium 3 (He3) isotope is used. A combination of the two may also be used.

[0085] In addition, at least two reverse flow heat exchangers are disclosed that are situated inside and / or outside the evaporator tube. In operation, the upper (warmer) heat exchanger prewarms the evaporated He gas before it reaches the compressor at the top of the evaporation tube, while at the same time pre-cooling the gas that exits the compressor. The lower (cooler) heat exchanger pre-warms the evaporated He gas after it leaves the cold head in contact with the J-T valve, while at the same time pre-cooling the gas that enters the J-T valve.

[0086] A diagram illustrating a side sectional view of a second example 2-stage cryocooler and integrated 3 -stage Joule-Thomson (J-T) refrigerator circuit with an external upper heat exchanger is shown in Figure 7. The example cryostat, generally referenced 200, comprises a 2-stage cryocooler 202 having a first stage cold head 211 affixed to a first stage platform 212 and second stage cold head 222 affixed to a second stage platform 223, structural manifold 208, an integrated 3-stage Joule-Thomson (J-T) refrigerator circuit that includes a hot end compressor first stage 214 thermally anchored to the first stage platform, a condenser second stage 218 thermally anchored to the second stage platform, and a cold end evaporator third stage 234 affixed to a third stage platform 230 to which a quantum printed circuit board (PCB) assembly 232 is thermally anchored. Rather than having a helium line from the first to second stages as in Figure 2, in this second example the hot compressed helium gas enters an upper reverse flow heat exchanger 216 that runs external and is thermally bonded to the evaporator tube 224.

[0087] In operation, the helium is cycled between a gas and a liquid between the bottom of the lower evaporation tube 226 and the top of the upper evaporation tube 224. In particular, cooled liquid helium that emerges from the end of the lower reverse flow heat exchanger tube 228 12485.0110 within the lower evaporator tube chills the cold head 234 via evaporative cooling to the target cryogenic temperature of less than IK which reaches the quantum PCB 232 via the third stage platform 230.

[0088] The low pressure helium gas generated from evaporative cooling from the cold head 234 rises through upper and lower evaporator tubes 226, 224 to the compressor first stage 214 where it is compressed to a high pressure gas which then enters the upper reverse flow heat exchanger 216. Arriving at the second stage of the 2-stage cryocooler 202 which is at a typical temperature of 4K, the gas is forced through internal channels machined or molded into a cone shaped liquefier 218 where it is sufficiently cold to condense to a liquid. The helium then enters the lower reverse flow heat exchanger tube 228 within the lower evaporator tube 226 to the cold head 234 where it is forced to exit through an orifice (not shown) which provides the evaporative cooling of the cold head to the target temperature of less than IK. A quantum PCB 232 is thermally anchored to a third stage platform 230 to which the cold head is also thermally anchored.

[0089] A diagram illustrating a side sectional view of a second example 2-stage cryocooler and 3-stage Joule-Thomson (J-T) refrigerator circuit with an internal upper heat exchanger is shown in Figure 8. The example cryostat, generally referenced 240, comprises a 2-stage cryocooler 242 having a first stage cold head 251 affixed to a first stage platform 252 and second stage cold head 262 affixed to a second stage platform 260, structural manifold 248, an integrated 3-stage Joule-Thomson (J-T) refrigerator circuit that includes a hot end compressor first stage 254 thermally anchored to the first stage platform, a condenser second stage 258 thermally anchored to the second stage platform, and a cold end evaporator third stage 274 affixed to a third stage platform 270 to which a quantum printed circuit board (PCB) assembly 272 is thermally anchored.

[0090] Operation of the cryostat 240 is similar to that of cryostat 200 described in Figure 7 with the difference being that the upper reverse flow heat exchanger tube 256 is internal to the upper evaporator tube 264. The low pressure helium gas generated from evaporative cooling from the cold head 274 rises through lower and upper evaporator tubes 266, 264 to the compressor first stage 254 where it is compressed to a high pressure gas which then enters the internal upper reverse flow heat exchanger 256. Arriving at the second stage platform 260 which is at a typical temperature of 4K, the gas is forced through internal channels machined or molded into a cone shaped liquefier or thermalizer cone 258 where it is sufficiently cold to condense to a liquid. The liquid helium then enters the lower reverse flow heat exchanger tube 268 within the lower evaporator tube to the cold head 274 where it is forced to exit through an 12485.0110 orifice (not shown) which provides the evaporative cooling of the cold head to the target temperature of less than IK. The quantum PCB assembly 272 is thermally anchored to a third stage platform 270 to which the cold head is also thermally anchored.

[0091] A block diagram illustrating a second example 2-stage cryocooler and the gas flow path in the 3-stage Joule-Thomson (J-T) refrigerator circuit is shown in Figure 9. The example cryostat, generally referenced 280, comprises a vacuum chamber 296 and 2-stage G-M cryocooler 282 having a first stage cold head at a first stage 308 at approximately 35K and a second stage cold head 314 at a second stage 310 at approximately 3.5K. Integrated with the 2-stage G-M cooler is a 3-stage J-T expansion circuit 293 driven by an electric drive motor 282 located outside the vacuum chamber 296. Rotational torque is coupled from the motor to the turbo compressor first stage of the J-T circuit via shafts 284, 290 and associated magnetic clutches. The upper magnetic clutch includes driving magnet 286 at the bottom of motor shaft 284 and driven magnet 288 at the top of drive shaft 290. The lower magnetic clutch includes driving magnet 292 at the bottom of drive shaft 290 and driven magnet 294 at the top of the compressor shaft.

[0092] As described supra, in this second example, the helium circuit is entirely contained within the evaporator tube 298. The use of a lower magnetic clutch assembly allows the helium to be completely isolated at low temperature inside the vacuum chamber. Without the lower magnetic clutch assembly, the helium would reach the upper magnetic coupling (which may be at room temp depending on configuration), thermally shorting the helium cycle to much higher temperature than desired.

[0093] In operation, the first stage 308 is cooled to the G-M first stage temperature (e.g., 35K) and is thermally anchored to the J-T compressor stage which functions to compress the helium gas where it travels through pathways or channels 320. The compressor stage may comprise any combination of radial 300 and axial 302 compressor blades or blade assemblies. The first stage functions to compress the helium gas to generate high pressure output which then enters the upper high temperature reverse flow heat exchanger (not shown) within the evaporator tube 298 illustrated by gas flow arrows 297. The G-M second stage 310 at a temperature of 3.5K is thermally anchored to the J-T condenser stage functions to liquify the high pressure helium gas within pathways or channels 318 molded or machined into the condenser stage. It then enters the lower low temperature reverse flow heat exchanger (not shown) illustrated by gas flow arrows 301. Finally, the high pressure helium, is passed through an orifice or expansion nozzle 316 in the final J-T expansion stage (i.e. third stage) and through evaporative cooling reaches the target temperature of less than IK at the cold head 306 to which the quantum PCB assembly 12485.0110

[0094] 304 is thermally anchored. The helium gas generated from evaporation flows upwards via the low pressure generated by the compressor blades as illustrated by gas flow arrows 299 to complete the helium cycle.

[0095] The electric motor 282 that drives the compressor first stage of the J-T expansion circuit is controlled by a motor control circuit 281. A soft start mechanism 283 within the control circuit provides a slow ramp-up of the motor. The function of the turbo compressor soft start mechanism is to provide a slow ramp-up in rotational speed of the compressor drive motor or motors outside the vacuum chamber in order that the start-up drag of magnetic coupling or couplings does not cause the magnetic drive to slip. If full torque is reached too quickly, the magnetic coupling can slip and the torque transmitted across the magnets drops to near zero as the magnets are no longer coupled. The torque transmitted across the window is actually provided by the slight axial offset between the driving magnets and the driven magnets, and there exists a maximum torque that if exceeded the magnetic drive will slip. Note that this axial offset can be measured, controlled and visualized by use of magnetic sensors positioned close to each of the coupled pairs of magnets.

[0096] In an example embodiment, the cooling power of the cryostat with the integrated inline J-T expansion circuit as taught by the present invention is as follows: the G-M first stage provides approximately 5W at 35K, the G-M second stage provides approximately 0.1W at 3.5K, and the J-T third stage provides approximately lOmW at IK.

[0097] A block diagram illustrating a second example 2-stage cryocooler and 3-stage Joule- Thomson (J-T) refrigerator circuit and associated heat shields is shown in Figure 10. The example cryostat, generally referenced 330, comprises a vacuum chamber 354, 2-stage G-M cryocooler 334 having a first stage 346 at approximately 35K and a second stage 348 at approximately 3.5K. Integrated with the 2-stage G-M cooler is a 3-stage J-T expansion circuit driven by an electric drive motor 332 located outside the vacuum chamber 354. Rotational torque is coupled from the external motor to the turbo compressor first stage 340 of the J-T expansion circuit via one or more magnetically coupled drive shafts. In this example, a magnetic clutch on either end of drive shaft 338 couples rotational torque from the motor to the J-T compressor first stage via upper magnetic clutch including driving and driven magnets 331, 333, respectively, separated by non-magnetic membrane 362 (e.g., sapphire window) and a lower magnetic clutch including driving and driven magnets 363, 367, respectively, separated by non-magnetic membrane 365. As described supra, the helium circuit is entirely contained within the evaporator tubes. The use of a lower magnetic clutch assembly allows the helium to be completely isolated at low temperature inside the vacuum chamber. 12485.0110

[0098] A structural manifold 336 functions as a mounting frame for the various components of the cryostat. The J-T circuit further comprises upper high temperature reverse flow heat exchanger spiral tube 342 that is located either external or internal to upper evaporator tube 341. The helium liquifies in the J-T condenser second stage 344 thermally anchored to the G- M second stage 348 after which it enters the lower low temperature reverse flow heat exchanger tube 350 within lower evaporator tube 349 and exits at the expansion nozzle 352 to achieve the target temperature of less than IK at the cold head 351.

[0099] The cryostat 330 comprises several heat shields to limit thermal radiation including the vacuum chamber heat shield 354, heat shield 356 enclosing both the G-M cooler and J-T circuit, heat shield 358 enclosing the two second stages and J-T third stage, and heat shield 360 enclosing the quantum PCB and J-T circuit third stage. In an alternative embodiment, heat shield 360 encloses only the quantum PCB but is thermally connected to the J-T circuit third stage. Note that the heat shields may be gold plated as gold is highly effective at reflecting infrared radiation.

[0100] A diagram illustrating a side sectional view of a second example 2-stage cryocooler and 3 -stage Joule-Thomson (J-T) refrigerator circuit with an external motor drive and magnetic clutch assembly is shown in Figure 11. The example cryostat, generally referenced 370, comprises a vacuum chamber 376, 2-stage G-M cryocooler 372 having a first stage with cold head 392 thermally anchored to a first stage platform 390 at approximately 35K and a second stage with cold head 393 thermally anchored to a second stage platform 391 at approximately 3.5K. Integrated with the 2-stage G-M cooler is a 3-stage J-T expansion circuit driven by an electric motor 374 located external to the vacuum chamber 376. Rotational torque is coupled from the external motor to the turbo compressor first stage 399 of the J-T expansion circuit via one or more drive shafts 371 coupled via magnetic clutches. In this example, a magnetic clutch on either end of drive shaft 371 couples rotational torque from the motor to the J-T compressor first stage via a first pair of driving and driven magnets separated by a non-magnetic membrane 397 (e.g., sapphire window) and a second pair of driving and driven magnets separated by nonmagnetic membrane 398. The compressor 399 may comprise a multistage radial and / or axial blade assembly.

[0101] A structural manifold 373 functions as a mounting frame for the various components of the cryostat. The J-T circuit further comprises upper high temperature reverse flow heat exchanger spiral tube 395 that is located either external or internal to upper evaporator tube 396 (external is shown) which also may comprise an internal radiation shield lining. The helium liquefies in the J-T condenser second stage 394 having one or more internal channels 12485.0110 or passages 375 and thermally anchored to G-M second stage 393 and the second stage platform 391 after which it enters the lower low temperature reverse flow heat exchanger tube 384 within lower evaporator tube 382 and exits at the expansion nozzle 386 to achieve the target temperature of less than IK at the cold head 388 which is thermally anchored to the quantum PCB assembly (not shown) through a third stage platform (not shown).

[0102] The cryostat 330 comprises several heat shields to limit thermal radiation including the vacuum chamber heat shield 376, heat shield 378 enclosing both the G-M cooler and J-T circuit, and heat shield 380 enclosing the quantum PCB and J-T circuit third stage. Note that the heat shields may be gold plated as gold is highly effective at reflecting infrared radiation.

[0103] A diagram illustrating a perspective view of a motor outside the vacuum chamber and associated magnetic clutch assembly is shown in Figure 12A. In an example embodiment, the magnetic clutch mechanism deployed in the cryostat provides a non-contact vacuum feed through of the motor torque to the J-T expansion circuit compressor. In one embodiment, the compressor drive motor or motors 402 are positioned outside the vacuum chamber 402 so that they can be more effectively cooled while not dumping their operational heat into the cryosystem. Further, a magnetic clutch is used to transmit the rotational torque or effort while crossing from the outside (i.e. atmospheric) pressure regime through to the low pressure or near-vacuum regimes inside the vacuum chamber. In one embodiment, a driving rare-earth magnet is attached to the motor shaft outside the vacuum chamber and magnetically coupled to a corresponding driven rare-earth magnet 414 attached to a coupling drive shaft 416 or the compressor drive shaft itself both inside the vacuum chamber, separated by a magnetically non permeable membrane 412 such as a sapphire window that is sealed to the vacuum chamber and covers a hole through it. The vacuum chamber itself can incorporate the magnetically non permeable membrane by local thinning of the vacuum chamber itself, attaching an off the shelf commercially available window assembly, or by welding, gluing or otherwise attaching an appropriate vacuum-tight membrane. One or more bearings 418 are provided to support the coupled drive shaft.

[0104] The magnetic clutch assembly also provides non-contact thermal isolation whereby one or more additional drive shafts are axially connected that reach further down into the Joule- Thomson evaporation tube, by magnetically coupling any additional co-axial shafts as needed, with gaps between the magnetically coupled elements crossing the near-vacuum conditions inside the evaporation tube, thereby enabling control of thermal conduction along the drive shafts as shown in Figures 9, 10, and 11. 12485.0110

[0105] A diagram illustrating an example magnetic clutch drive of the present invention in more detail is shown in Figure 12B. In one embodiment, the driving and driven magnets are rare-earth type magnets that are configured to fit in the ends of the drive shafts. Driving magnet 403 has a circular shape and is formed to fit within the hollow portion of the end of drive shaft 401. In addition, the driving and driven magnets are magnetized such that the orientation of the magnetic field is horizontal rather than vertical which is indicated in the figure by dark and light halves of each magnet. A non-magnetic membrane such as a sapphire window 405 is affixed to the vacuum chamber 407 or other additional chamber therewithin. The driven magnet 409 is also horizontally magnetized and fits entirely within the top of the hollow drive shaft 411.

[0106] A diagram illustrating the lower reverse flow heat exchanger and J-T second and third stage is shown in Figure 13. The lower reverse flow heat exchanger 440 normally is positioned within the lower evaporator tube 436. For illustration purposes, it is shown partially removed from the lower evaporator tube. In operation, helium gas liquifies at the J-T condenser second stage 432 and travels down the spiral heat exchanger tube 440 to the cold head 438 at the bottom of the lower evaporator tube 436. The second stage flanges 432, 434 are physically connected and thermally anchored to the second stage platform such as the second stage platform 260 (Figure 8). The vanes or steps 442 of the spiral heat exchanger tube function to minimize the radiation of heat upwards to the second and first stages through the upper evaporation tube 430.

[0107] A diagram illustrating the lower heat exchanger in more detail is shown in Figure 14. The lower reverse flow heat exchanger, generally referenced 450, comprises an elongated metal tube 452 in the shape of a spiral. The spiral tube is configured to be placed inside the lower evaporator tube. The liquified helium travels down the spiral tube which is cooled by the rising helium gas generated from evaporation at the bottom of the third stage cold head. Liquid helium emerges under pressure from the orifice 458 at the end of the tube and collects in a pool in the cold head. Heat from the quantum PCB causes the liquid helium to evaporate which lowers the temperature to the target value of IK or less. The vanes or steps 454 of the spiral are attached to the tube via clamping arms 456. The vanes function to (1) force the helium gas to take a spiral path which aids in cooling as well as to (2) minimize the radiation of heat upwards to the second and first stages. Note that the lower portion of the tube near the orifice has been straightened out for illustration purposes only and is normally coiled within the evaporator tube. 12485.0110

[0108] A block diagram illustrating the example cryostat including initial gas fill at time of manufacture is shown in Figure 15. The example cryostat, generally referenced 460, comprises vacuum chamber 464, vacuum pump 474, 2-stage G-M cryocooler 476, 3-stage J-T evaporation circuit 511, and a helium fill mechanism 481 all mounted to the structural manifold 462. The 2-stage G-M cryocooler includes a first stage cold head 490 affixed to a first stage platform 498 and a second stage cold head 492 affixed to a second stage platform 500. The 3- stage J-T expansion circuit 511 is integrated with the 2-stage G-M cryocooler and includes compressor first stage 518 driven by an electric drive motor 478 located outside the vacuum chamber, a reverse flow heat exchanger 496 within evaporative tube 494, second stage 520 which liquifies the helium, and a third stage thermally anchored to a third stage platform 502. Rotational torque is coupled from the motor to the radial / axial compressor via shaft 510 and magnetic clutches which include driving and driven magnets 504, 508, respectively, separated by non-magnetic window 506, and driving and driven magnets 512, 516, respectively, separated by non-magnetic window 514.

[0109] The cryostat also comprises a removable quantum PCB assembly 471 which includes a stacked arrangement of first, second, and third stage platform sections 499, 501, 503, respectively, that fit into corresponding openings in the first, second, and third stage platforms 498, 500, 502, respectively. The quantum PCB 472 is thermally anchored to the third stage platform section 503. Heat shield 470 encloses the quantum PCB to thermally isolate it. Additional heat shields are attached to the platforms including heat shield 468 attached to the second stage platform 500 and heat shield 466 attached to the first stage platform 498.

[0110] The helium fill mechanism 481 comprises a source of He3 and / or He4 supplied in a tank or canister 482, valves 484, mass flow control (MFC) device 480, and gas flow lines 488, 486. As described supra, the J-T expansion circuit is a closed system and is filled with helium to operate properly. At the time of manufacture, the contents of the closed J-T circuit must first be evacuated and then replaced with the helium gas (e.g., He3, He4, He3 / He4 mix, etc.). The MFC, which is an electrically controlled regulator, and associated valves are controlled and configured to first evacuate the contents of the J-T circuit by opening the contents to the vacuum created by the vacuum pump 474 via gas line 486. Once emptied, the J-T circuit is filled with helium by reconfiguring the valves to allow helium to flow from the tank, through the MFC to the J-T circuit via gas line 488. An example vacuum pump suitable for use with the cryostat of the present invention is the HiPace 80 Neo turbomolecular vacuum pump manufactured by Pfeiffer Vacuum Inc., Nashua, New Hampshire, USA. 12485.0110

[0111] Those skilled in the art will recognize that the boundaries between logic and circuit blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures may be implemented which achieve the same functionality.

[0112] Any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermediary components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality.

[0113] Furthermore, those skilled in the art will recognize that boundaries between the above described operations are merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.

[0114] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0115] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first,” “second,” etc. are used to arbitrarily distinguish between the elements such terms 12485.0110 describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. As numerous modifications and changes will readily occur to those skilled in the art, it is intended that the invention not be limited to the limited number of embodiments described herein. Accordingly, it will be appreciated that all suitable variations, modifications and equivalents may be resorted to, falling within the spirit and scope of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

12485.0110CLAIMS1. A cryostat, comprising: a vacuum chamber; a 2-stage cryocooler within said vacuum chamber having a first cooling stage and a second cooling stage, said second cooling stage at a lower temperature than said first cooling stage; a Joule-Thomson (J-T) expansion circuit within said vacuum chamber having: a hot end compressor first stage located alongside and thermally anchored to said cryocooler first cooling stage; a condenser second stage located alongside and thermally anchored to said cryocooler second cooling stage; and a J-T cold end evaporator orifice thermally anchored at a third stage, said third stage at a temperature lower than said condenser second stage.

2. The cryostat according to claim 1, wherein said J-T circuit comprises a first reverse flow heat exchanger located between said compressor first stage and said condenser second stage and a second reverse flow heat exchanger located between said condenser second stage and said evaporator orifice third stage.

3. The cryostat according to claim 1, wherein said J-T compressor comprises one or more radial and / or axial compressor blades.

4. The cryostat according to claim 1, further comprising a drive motor external to and magnetically coupled through said vacuum chamber to said J-T compressor first stage.

5. The cryostat according to claim 1, wherein said compressor first stage is within a temperature range of 20 to 80K, said condenser second stage is within a temperature range of 2.1 to 8K, and said third stage is within a temperature range lOOmK to 2K.

6. A cryostat, comprising: a vacuum chamber; a 2-stage cryocooler within said vacuum chamber having a first stage and a second stage; a Joule-Thomson (J-T) cryocooler circuit within said vacuum chamber having a first stage, a second stage, and a third stage;12485.0110 a drive motor located outside said vacuum chamber; and a magnetic clutch operative to couple rotational torque from said external drive motor to said J-T first stage through a vacuum tight, magnetically permeable membrane incorporated in said vacuum chamber.

7. The cryostat according to claim 6, wherein said J-T circuit comprises a first reverse flow heat exchanger located between said compressor first stage and said condenser second stage and a second reverse flow heat exchanger located between said condenser second stage and said evaporator valve third stage.

8. The cryostat according to claim 6, wherein said J-T compressor comprises one or more radial and / or axial compressor blades.

9. The cryostat according to claim 6, wherein said vacuum tight magnetically permeable membrane comprises a sapphire window.

10. The cryostat according to claim 6, wherein said magnetic clutch comprises a plurality of magnetically coupled co-axial drive shafts to maximize thermal isolation.

11. The cryostat according to claim 10, wherein said plurality of co-axial drive shafts comprise stainless steel.

12. The cryostat according to claim 6, further comprising a compressor soft start mechanism to ramp up rotational speed of said J-T first stage compressor to prevent slippage of and control said magnetic clutch mechanism.

13. A cryostat, comprising: a vacuum chamber; a Gifford-McMahon (G-M) 2-stage cryocooler having a first cooling stage and a second cooling stage within said vacuum chamber; a Joule-Thomson (J-T) expansion circuit located entirely within said vacuum chamber having: a hot end compressor first stage located alongside and thermally anchored to said G-M cryocooler first cooling stage; a condenser second stage located alongside and thermally anchored to said G- M cryocooler second cooling stage;12485.0110 a J-T cold end evaporator orifice thermally anchored at a third stage, said third stage at a temperature lower than said condenser second stage; and a drive motor located external to said vacuum chamber; and a magnetic clutch operative to couple rotational torque from said external drive motor to said J-T compressor first stage through a vacuum tight magnetically permeable membrane affixed to said vacuum chamber.

14. The cryostat according to claim 13, wherein said J-T circuit comprises a first reverse flow heat exchanger located between said compressor first stage and said condenser second stage and a second reverse flow heat exchanger located between said condenser second stage and said evaporator valve third stage.

15. The cryostat according to claim 13, wherein said J-T compressor comprises one ormore radial and / or axial compressor blades.

16. The cryostat according to claim 13, wherein said vacuum tight magnetically permeable membrane comprises a sapphire window.

17. The cryostat according to claim 13, wherein said magnetic clutch comprises a plurality of magnetically coupled co-axial drive shafts to maximize thermal isolation.

18. The cryostat according to claim 17, wherein said plurality of co-axial drive shafts comprise stainless steel.

19. The cryostat according to claim 13, further comprising a compressor soft start mechanism to ramp up rotational speed of said J-T first stage compressor to prevent slippage of and control said magnetic clutch mechanism.

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