CRYOGEN-FREE COOLING APPARATUS

Abstract

The disclosure relates to a cryogen-free cooling apparatus for cooling a sample, comprising a vacuum chamber, a first cooling device which is configured to generate a first temperature in the vacuum chamber to provide a main thermal bath, a second cooling device, which is in connection with a sample stage on which a sample is to be arranged, wherein the second cooling device is a solid state cooler which is configured to provide a second temperature to the sample stage, and wherein the second temperature is different from the first temperature, and a sample loading device which is configured to change the sample while operating the first cooling device and the second cooling device, wherein the sample stage is held in the vacuum chamber by a plurality of first fibers of low thermal conductivity such that the sample stage is thermally decoupled from the main thermal bath.

Claims

1.-13. (canceled)

14. A cryogen-free cooling apparatus for cooling a sample, comprising a vacuum chamber providing a main thermal insulation vacuum; a sample stage arranged in the main thermal insulation vacuum and fluidly immersed in the main thermal vacuum, the sample stage being configured to directly couple to a sample loaded onto the sample stage by a sample loading device; a first cooling device arranged in the main thermal insulation vacuum, the first cooling device being configured to generate a first temperature in the vacuum chamber to provide a main thermal bath; and a second cooling device arranged in the main thermal insulation vacuum, the second cooling device being in connection with the sample stage, wherein the second cooling device is configured to provide a second temperature to the sample stage, and wherein the second temperature is different from the first temperature.

15. The cryogen-free cooling apparatus of claim 14, further including the sample loading device configured to change the sample on the sample stage inside the main thermal insulation vacuum.

16. The cryogen-free cooling apparatus of claim 15, wherein the sample loading device is configured to load the sample directly into the main thermal insulation vacuum.

17. The cryogen-free cooling apparatus of claim 14, further including a first mechanical suspension holding the sample stage in the vacuum chamber and in the main thermal insulation vacuum, wherein the first mechanical suspension is of low thermal conductivity such that the sample stage is thermally decoupled from the main thermal bath.

18. The cryogen-free cooling apparatus of claim 17, wherein the first mechanical suspension has a thermal conductivity of less than 0.1 W/(Km).

19. The cryogen-free cooling apparatus of claim 17, wherein the first mechanical suspension is provided by a plurality of first fibers or wires.

20. The cryogen-free cooling apparatus of claim 19, wherein the first fibers or wires have a thickness of less than 0.1 mm.

21. The cryogen-free cooling apparatus of claim 19, further including a strain mechanism configured to apply tension to the plurality of first fibers or wires.

22. The cryogen-free cooling apparatus of claim 14, further including a sample stage locking device configured to mechanically lock the sample stage in place.

23. The cryogen-free cooling apparatus of claim 22, wherein the sample stage locking device is configured to mechanically lock the sample stage in place during loading of the sample.

24. The cryogen-free cooling apparatus of claim 22, wherein the sample stage locking device includes one or more shafts configured to be inserted in corresponding holes of the sample stage to mechanically lock the sample stage in place.

25. The cryogen-free cooling apparatus of claim 22, wherein the sample stage locking device does not touch the sample stage in an unlocked state.

26. The cryogen-free cooling apparatus of claim 22, wherein the sample stage locking device is configured to provide a thermal link to the main thermal bath in a locked state.

27. The cryogen-free cooling apparatus of claim 14, further including: a sample radiation shield which surrounds the sample stage; and a second mechanical suspension holding the sample radiation shield in the vacuum chamber and in the main thermal insulation vacuum, wherein the second mechanical suspension is of low thermal conductivity such that the sample radiation shield is thermally decoupled from the main thermal bath.

28. The cryogen-free cooling apparatus of claim 27, wherein the second mechanical suspension is provided by a plurality of second fibers.

29. The cryogen-free cooling apparatus of claim 14, wherein the second temperature provided by the second cooling device is lower than the first temperature provided by the first cooling device.

30. The cryogen-free cooling apparatus of claim 14, wherein the first temperature provided by the first cooling device is in the range of 2K to 4K.

31. The cryogen-free cooling apparatus of claim 14, wherein the second temperature provided by the second cooling device is 300 mK or less, or 50 mK or less.

32. The cryogen-free cooling apparatus of claim 14, wherein the first cooling device is a mechanical cooling device and the second cooling device is a solid state cooler.

33. The cryogen-free cooling apparatus of claim 14, wherein the second cooling device is a magnetic cooler, an adiabatic demagnetization refrigerator, a barocaloric refrigerator, a multi-stage cooling device or a thermoelectric cooler.

34. The cryogen-free cooling apparatus of claim 14, wherein the sample stage includes: an electrical connector configured to establish an electrical connection to the sample; an optical connector configured to provide an optical access to the sample; at least one magnetic field sensor; at least one temperature sensor; or a heater.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Following, embodiments are disclosed with reference to figures. Here show:

[0050] FIG. 1a a schematic view of a cooling apparatus,

[0051] FIG. 1b a cooling apparatus,

[0052] FIG. 2 a sample stage including a sample radiation shield,

[0053] FIG. 3 a sample stage locking device.

[0054] FIG. 4 a mechanical heat switch,

[0055] FIG. 5 a working principle of the heat switch of FIG. 4,

[0056] FIG. 6 a schematic drawing of a suspension mechanism of the heat switch of FIG. 4,

[0057] FIG. 7 a superconducting magnet,

[0058] FIG. 8 a power connector for the magnet of FIG. 7,

[0059] FIG. 9 a frontal view of the combined magnet and power connector,

[0060] FIG. 10 a schematic view of a salt pill,

[0061] FIG. 11 an overview of the salt pill of FIG. 10, and

[0062] FIG. 12 a schematic view of another embodiment of a salt pill.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0063] FIG. 1a shows a schematic drawing of a cooling apparatus. Low temperatures (e.g. 4 K) are provided by a first cooling device 100, e.g. a pulse tube cooler, inside a vacuum vessel 101. A sample can be introduced into the vacuum vessel 101 using a sample loading device 102. The sample is loaded onto a sample stage 4, which can be locked in place by a sample stage locking device 105 (e.g. during loading of the sample). The sample stage 4 is surrounded by a sample magnet 104 used to provide a magnetic field at the position of the sample. A second cooling device can be one or multiple ADR stages 106. There may be any number between one and eight ADR stages 106. Multiple ADR stages 106 can be combined for multi-stage operation. Each ADR stage 106 comprises a heat switch 107, a magnetic refrigerant 108 and a magnet 109, e.g. a superconducting magnet. The first ADR stage 106 is connected to the first cooling device by a high thermal conductivity connection 103 through the heat switch 107. The subsequent ADR stages 106 are connected to each other through their respective heat switches 107. The final ADR stage 106 is coupled to the sample stage 4.

[0064] Depending on the number of built-in ADR stages the following main operating modes may be realized:

[0065] Single stage operation using a single ADR stage: The refrigerant is magnetized by applying current to the ADR stage's magnet while the heat switch between the main thermal bath and the ADR stage is closed. The heat created by magnetization is absorbed by the main thermal bath. Upon full magnetization and relaxing to the main thermal bath's temperature, the single ADR stage is isolated from the main thermal bath by opening the heat switch and demagnetized by removing the magnetic field. The temperature of the ADR stage and the sample stage connected to it is thereby reduced to a lower temperature depending on the magnetocaloric material used. After the cooling power of the refrigerant is depleted, the system warms up to the temperature of the main thermal bath and the refrigerant has to be remagnetized. This kind of operation is also called single-shot operation.

[0066] Multi-stage operation using two or more ADR stages: All ADR stages are magnetized while the respective heat switches are closed. Upon full magnetization of all stages, the heat switch between the main thermal bath and the first ADR stage is opened and the first ADR stage is demagnetized, hence its temperature is lowered. Once all ADR stages reach the final temperature of the first ADR stage, the next heat switch is opened and the remaining ADR stages are cooled to the final temperature of the next ADR stage. This procedure is repeated for all available ADR stages. Alternatively, any lower ADR stage can be isothermally magnetized at the temperature of a preceding ADR stage. After the cooling power of the final ADR stage is depleted, the system warms up to the temperature of the main thermal bath (e.g. 4 K) and the ADR stages have to be remagnetized.

[0067] Continuous operation using three or more ADR stages: The ADR stages are magnetized in a cascading manner. While the last ADR stage is used for temperature control of the sample stage, it is regenerated by the preceding ADR stage(s) at a defined temperature to keep the sample temperature stable. Once the last ADR stage is regenerated, the preceding ADR stage is regenerated by its own preceding ADR stage. This proceeds up to the first ADR stage, which is connected to the main thermal bath.

[0068] For all above mentioned operating modes, the magnetization heat of each individual ADR stage may also be used to achieve heating of the last ADR stage and the sample stage attached to it.

[0069] FIG. 1b shows a drawing of an embodiment of a cooling apparatus. It comprises a mechanical two-stage first cooling device 110, e.g. a pulse tube cooler, which is mounted in a vacuum vessel 112. The vacuum vessel 112 has multiple openings 111, which may serve as feedthroughs for wiring, optical fibers, or transmission shafts. A first radiation shield 114 is suspended from the top of the vacuum vessel 112 by low thermal conductivity struts 113. The first radiation shield 114 is thermally anchored to a first stage of the first cooling device 110. A second radiation shield 122 is thermally anchored to a second stage of the cooling device 110 by a thermal bus 118 and suspended from the top of the first radiation shield 114 using low thermal conductivity struts 113. The second radiation shield 122 surrounds a second cooling device, which, in this embodiment, is a multi-stage ADR. In the interest of clarity only two magnets, 123a and 123b, and a heat switch 120 are shown. The magnets may be mounted such that their main magnetic field is oriented parallel (magnet 123a) or perpendicular (magnet 123b) with respect to the wall of the vacuum vessel 112. A refrigerant is arranged inside a magnet bore 124. It may be suspended inside the magnet bore 124 on thin strands of fiber, e.g. Aramid fibers (e.g. Kevlar), to minimize the thermal leak. The sample stage (see FIG. 2 for details) comprises an ultra-low temperature stage 119 and a sample radiation shield 116. The ultra-low temperature stage 119 is cooled by the second cooling device and may be mechanically secured using a sample stage locking device 121 to allow for higher forces and torques on the ultra-low temperature stage 119 during loading of a sample. Therefore, a sample can be loaded directly into the running system without warm up or disassembly. The sample is loaded directly into the insulation vacuum to avoid heat leaks caused by a second vacuum vessel inside the apparatus. In order to avoid radiation heat leaks, the loading slot can be closed when not loading a sample, e.g. using baffles mounted onto the first radiation shield 114 and the second radiation shield 122. The ultra-low temperature stage 119 is mounted inside of the sample radiation shield 116 by means of a mechanical suspension of low thermal conductivity. The sample stage is mounted inside a sample magnet 117 by means of high modulus fibers of low thermal conductivity, e.g. Polyaramid fibers. The sample magnet 117 is attached to the vacuum vessel 112 by low thermal conductivity struts. The sample magnet 117 can either be made from a conventional low temperature superconductor (e.g, cooled to 4 K) or from a high temperature superconductor (e.g. cooled to 40 K).

[0070] FIG. 2 shows the sample stage 4. The sample stage 4 comprises a sample radiation shield 116, inside of which the ultra-low temperature stage 119 is arranged. One or more samples may be arranged on the ultra-low temperature stage 119 The sample radiation shield 116 is mounted inside the vacuum vessel by rings 12, 13 using a plurality of thin (e.g. 0.02 mm thick) high modulus fibers (e.g. Aramid fiber like Kevlar) and thermally anchored to a higher temperature ADR stage. The ultra-low temperature stage 119 is mounted inside the radiation shield 116 using a strain mechanism 14 which is used to apply tension to thin (e.g. 0.02 mm thick) high modulus fibers (e.g. Aramid fibers like Kevlar) running from the strain mechanism 14 through holes 15, 16 to the ultra-low temperature stage 119.

[0071] A sample holder 17 can be moved from outside the cooling apparatus through an opening of the sample radiation shield 116 by a removable external manipulator (not shown). Thermal contact to the ultra-low temperature stage 119 and mechanical stabilization of the ultra-low temperature stage 119 is established by first introducing shafts 25 (see FIG. 3) through holes 18 and thereby locking it. Once locked, the manipulator is used to push the sample holder 17 onto a spring 19. Alternatively, a screw can be used to hold the sample holder 17 on the ultra-low temperature stage 119. The external manipulator is then removed. To minimize heat flow into the ultra-low temperature stage 119, the sample stage lock is then opened.

[0072] The sample radiation shield 116 has the form of a cylinder. The sample radiation shield 116 has three holes 18 which are arranged in a common plane. The holes 18 are evenly separated from each other. The sample radiation shield 116 may be made of a material having a high thermal conductivity, e.g. high-purity metals such as copper, silver, and gold. The shield may additionally be covered with a superinsulation foil to further reduce the radiation heat.

[0073] FIG. 3 shows a sample stage locking device 121. The sample stage locking device 121 is used to absorb forces and torques during loading of the sample holder 17 on the ultra-low temperature stage 119 and during removing of the sample holder 17 from the ultra-low temperature stage 119. In order to avoid heat leaks, the sample stage locking device 121 shall not touch the sample stage 4 in an unlocked status. The sample stage locking device 121 has to work at low temperature (e.g. 4 K) and may not produce excessive heating when operated. Self-lubricating bearings may be used for the shafts and lever joints to decrease friction and, consequently, decrease heating during operation. Self-lubricating bearings may be made, e.g. of Teflon, PEEK, or Vespel.

[0074] The sample stage locking device 121 comprises an inner region 26. The sample stage 4 (see FIG. 2) can be locked by turning a taper bearing 21 by an outside shaft. This results in rotation of a main shaft 22, which in turn moves a ring 28. The ring 28 moves toggle levers 24 resulting in a movement of the shafts 25 towards the inner region 26, fixing the sample stage 4 in place. A stop element 27 provides a stopping contact. The ring 28 is mounted in a self-lubricating shell 29 to facilitate movement at low temperatures. The whole assembly is mounted on a main ring 23 made of a high thermal conductivity material. The ring may be made of a variety of materials, e.g. aluminum, brass, or copper. All movable parts need to be machined such that movement is still possible even at low temperatures.

[0075] FIG. 4 shows one embodiment of a mechanical heat switch for connecting the ADR stages. To make the ADR principle work, one needs to thermally decouple the ADR stages from each other when magnetizing or demagnetizing, i.e. warming or cooling, respectively. Therefore, a heat switch is needed to disconnect two thermal baths, which may or may not be at the same temperature. The switching process is driven mechanically; hence a mechanical contact is a thermal one. However, it is possible to attach the mechanical heat switch assembly to a higher temperature stage (e.g. 4 K) and to operate the switch between two lower temperature stages while minimizing the thermal leak to the low temperature stages. This enables the usage of the heat switch in ultra-low temperature ADR stages.

[0076] A thermal contact can be established between a first thermal bath 30 and a second thermal bath 31, which can be of the same or of different temperatures. The first thermal bath 30 is fixed in a first suspension 32 by means of mechanical suspension of low thermal conductivity. The first suspension 32, in turn, is held by a base plate 33. The second thermal bath 31 is fixed in a second suspension 34, which is attached to the first thermal bath 30. Minimal thermal conductance between the second thermal bath 31 and the second suspension 34 is achieved by anchoring the second suspension at the first thermal bath using very thin (e.g. 0.02 mm thick) high modulus fibers (e.g. Aramid fibers like Kevlar). The fibers are tightened by a strain mechanism 35 The setup allows the temperature of the base plate 33 to be different from the temperatures of first and second thermal baths 30, 31, while producing a minimal heat leak through the fiber suspension.

[0077] FIG. 5 shows the principle of the mechanical heat switch between the first thermal bath 30 and the second thermal bath 31. A connection between a first thermally conductive part 40 and a second thermally conductive part 41 can be established by turning a pulling shaft 42, which results in a contact component 43 moving towards the first thermally conductive part 40 and thereby pressing a flexible part 44 of the first thermally conductive part 40 against the second thermally conductive part 41. Disconnecting can be achieved by turning or releasing the switching shaft 42 in the other direction, which results in the contact component 43 moving towards the second thermally conductive part 41, allowing the flexible part 44 to relax and assume its former position. The switching shaft 42 is mounted in a self-lubricating bearing 45 (e.g. made of PEEK—Polyether ether ketone). The first and second thermally conductive parts 40, 41 each are suspended at two points 46 to allow for 3-dimensional adjustment regarding each other.

[0078] FIG. 6 shows a suspension mechanism of the heat switch of FIG. 4. A first thermal bath 50 and a second thermal bath 51 are suspended in a first suspension 52 and a second suspension 53, respectively, by fibers (e.g. Aramid fibers). The fibers are glued to a glue contact 54 which is arranged inside a screw 55 inside the first suspension 52. The fibers can be tightened by screwing the screw 55 out of the first suspension 52. Initial tension is supplied by a spring washer 56. The suspension may exhibit a three-fold rotational symmetry around the central components 50 and 51.

[0079] To homogenously magnetize or demagnetize the refrigerant of an ADR stage (e.g. a salt pill), a superconducting magnet may be used. The magnetic field profile provided by the magnet should be constant over the entire volume of the magnetocaloric material which serves as a refrigerant. In this embodiment a triple Helmholtz design of the primary coil of the magnet was chosen such as to minimize field gradients. The dimensions of the magnet fit the geometry of the salt pill perfectly. To reduce stray fields along the coil axes to manageable levels, a single cylindrical compensation coil was designed around the primary coil.

[0080] FIG. 7 shows a schematic cut along the axes of a superconducting magnet. An aluminum primary coil-former 60 is designed in a triple Helmholtz geometry. The coil-former 60 may also be made of stainless steel. The Helmholtz design comprises three coil parts 63, 64, 65 which form the primary coil. The first coil part 63 and the second coil part 65 are identical. In the center between the first and the second coil part 63, 65, a third coil part 64 is arranged. The third coil part 64 has a different winding number than first and second coil parts 63, 65. The primary coil parts 63, 64, 65 are wound on the primary coil-former 60 and mechanically supported by spacers 80 (FIG. 9) assembled in a compensation coil-former 62 around it. The compensation coil-former 62 may, e.g., be made of aluminum or stainless steel. A single wire of NbTi:Cu multifilament superconductor material in a Cu-Matrix can be used to wind the primary coil parts 63, 64, 65. NbTi:Cu multifilament superconductor material may also be used for the compensation coil 66 but wound in a direction opposite to the winding of the first, second and third coil parts 63, 64, 65.

[0081] To drive the whole magnet with only one power supply, special joints are needed to link the different coils with each other. FIG. 8 shows a schematic view of this principle, in the geometry chosen seven solder joints 70 are mechanically suspended in the compensation coil-former 62 by an aluminum support 71, but with the requirement to be electrically isolated from it using aluminum oxide spacers 72.

[0082] The magnet assembly as a whole is shown in FIG. 9. The compensation coil former 62 is used as mechanical anchor of the setup, which is usually kept at a temperature of 4 K to cool the NbTi:Cu material to its superconducting phase. The solder joints 71 as well as the primary coil-former 60 are attached to the compensation coil-former 62. The primary coil-former 60 is supported by spacers 80 made of aluminum or stainless steel. A special suspension 81 makes it possible to use the magnet assembly horizontally, e.g. in a cryostat combined with a sample load mechanism.

[0083] in ADR technology, the refrigerant may be provided in form of a so-called salt pill. The salt pill is used to store magnetocaloric materials and to thermally connect them to a thermal bus. Following, a salt pill design is disclosed which provides optimal filling factor as well as very high thermal conductivity. Simultaneously, the thermal bus can be connected on both sides of the salt pill to allow for flexibility in cooling device design.

[0084] FIG. 10 shows a salt pill suitable for the cooling device disclosed herein. The salt pill can also be used in other cryostats based on the ADR principle. The salt pill comprises a main thermal bus 90, which can be connected to other components on both sides of the salt pill. Thermal matrices 92a, 92b, each made from a single piece of high conductivity copper, are welded to the main thermal bus 90 Thermal matrices 92a, 92b can comprise slits to reduce eddy-current heating during magnetization or demagnetization of the ADR refrigerant. The salt pill is enclosed by a stainless steel case 94, welded to the main thermal bus 90. After introducing the magnetocaloric material into the case 94 (usually by growing crystals inside), the case 94 is sealed by welding a stainless steel lid 91 to the case 94.

[0085] FIG. 11 shows holes 95 in the thermal matrix 92a used to introduce the magnetocaloric material.

[0086] FIG. 12 shows another embodiment of a salt pill suitable for the cooling device. The salt pill can be used with other ADR-based cryostats. The salt pill comprises a main thermal bus 90 made of high thermal conductivity copper or silver. A stainless steel container 94 is welded to the main thermal bus 90 It can be sealed by welding the stainless steel lid 91 to the main thermal bus 90. Inside the container 94, a solid magnetocaloric material 93 is interspersed with high conductivity copper or silver plates 92 providing thermal coupling. The plates 92 are welded directly to the main thermal bus 90 during assembly.

[0087] The features disclosed in the specification, the claims and the figures can be relevant for the implementation of embodiments either alone or in arbitrary combination with each other.