Methods and arrangements for improving flow of cryogenic fluids

Abstract

Operating fluid flows in a gas circulating subsystem that comprises a room temperature part (211), an inbound line (203), and an outbound line (204) Respective cryogenically cooled sections of said inbound line (203) and said outbound line (204) are cooled below 100 K. Operating fluid is fed in an inbound direction from the room temperature part (211) towards a cold part (201, 2102, 2103) of a cold source. Operating fluid is also drawn in an outbound direction from a cold part (202, 2102, 2103) of the cold source towards said room temperature part (211). One or more mechanical micropumps (401, 503, 1001, 1002, 1601, 1602, 1802, 1902, 1903) are used in the respective cryogenically cooled section of the inbound line (203) and / or the outbound line (204) to affect the flow of operating fluid in the inbound and / or outbound direction, respectively.

Description

[0001] METHODS AND ARRANGEMENTS FOR IMPROVING FLOW OF CRYOGENIC FLUIDS

[0002] FIELD OF THE INVENTION

[0003] The invention is related to the technical field of cryogenic cooling systems . In particular the invention is related to ways in which flows of cryogenic fluids can be improved at low temperatures, such as below 100 K or, in particular, below 10 K.

[0004] BACKGROUND OF THE INVENTION

[0005] Cryogenic cooling systems are intricate pieces of machinery designed to cool a target region or payload volume down to very low temperatures and maintain such conditions for desired periods of time . The payload to be cooled may contain e . g. a scientific experiment, a quantum computer, a measurement setup, and / or something else, the correct operation of which may require temperatures in the order of only some kelvins or even well below one kelvin. A cryogenic cooling system may also be called a cryostat . In some sources, the designation cryogenic cooling system is used for just that subsystem of a cryostat that produces the low temperatures, while the cryostat is additionally said to comprise other subsystems like mechanical support, vacuum pumping, radiation shielding, cabling, and the like . In this text the terms cryostat and cryogenic cooling system are used as synonyms of each other, possibly including an interpretation that a cryostat may be somewhat simpler, like a vacuum can with a single cold source (mechanical cooler or bath of liquid cryogen) , while a cryogenic cooling system may be more elaborate with one or more outer cold sources for pre-cooling and one or more inner cold sources (such as dilution refrigerators for example) to reach the coldest temperatures .Fig. 1 is a simplified schematic illustration of a cryogenic cooling system equipped with a dilution refrigerator and a mechanical pre-cooler . The outermost structure is a vacuum can 101, which is shown with dashed lines in fig. 1. The topmost flange 102 is the lid of the vacuum can. The room temperature stage 103 of the mechanical pre-cooler is attached thereto . The first stage 104 of the mechanical pre-cooler is attached to a first flange 105 and the second stage 106 of the mechanical pre-cooler is attached to a second flange 107. The first and second flanges may be called the 50 K flange and the 4 K flange for example, reflecting their approximate temperatures during operation.

[0006] Further below there are more flanges, like the still flange 108 to which the still 109 of the dilution refrigerator is attached. In fig . 1 the mixing chamber 110 of the dilution refrigerator is attached to the base temperature flange 111. Reference designator 112 illustrates the target region at which the payload is to be refrigerated. A payload is frequently referred to as the sample . There may be extending structures called cold fingers thermally coupled to the mixing chamber 110, with which the target region for the payload may be brought also elsewhere than on the flange (if any) on which the mixing chamber 110 is installed.

[0007] Cylindrical, flat-bottomed radiation shields, which are not shown in fig. 1 for graphical clarity, are typically attached to the flanges in a nested configuration, in order to keep radiated heat from surrounding, higher-temperature parts from reaching the colder parts inside . The structure may comprise other, intermediate flanges like a so-called 100 mK flange between the still flange 108 and the base temperature flange 111. Aligned apertures may exist in the flanges to provide a so-called line-of-sight port to the target region 112. If the system is equipped with a sample changer, a load lock can be attached to a gate valve in the top flange102 where a cover 113 is seen in fig. 1. One or more elongate probes may be provided to move a sample holder, initially placed in the load lock, through the aligned apertures into place at the target region 112. Parts and subsystems related to sample changing are not shown in fig. 1 for maintaining simplicity.

[0008] Many aspects of efficient cooling with a dilution refrigerator or other cold source in a cryostat are related to the ways in which the operating fluid or other kind of cryogen flows through the gas circulating subsystem and / or other circulation channels . An important part of the subsystems for circulating fluid cryogens consists of the pumps that are outside the vacuum chamber, in the room temperature environment . In the case of a dilution refrigerator a compressor is used, typically at least during cool-down, to maintain appropriate pressure and flow rate in the inbound line in which 3He (helium-three) flows through various cooling stages before entering the mixing chamber . Suction pumps, such as turbo pumps and / or Roots pumps, are used to draw 3He from the still through the outbound line, from which the 3He is then circulated back in. Other kinds of channel and pump arrangements exist in other kinds of cryostats . It would be desirable to find novel ways of improving the flow of 3He and other cryogenic fluids at various parts of the cryostat, for example to get higher cooling power without having to invest in expensive additional pumps of the known kind.

[0009] SUMMARY

[0010] An obj ective is to present methods and arrangements that enable more efficient operation of a cryostat . Another obj ective is to present such methods that are widely applicable in cryostats and cryogenic cooling systems of different dimensions and cooling powers . A yet further obj ective is to ensure that efficient operation of a dilution refrigerator can be ensured withsolutions that are mechanical robust, reliable in operation, and possible to implement and operate at reasonable cost .

[0011] These and further advantageous obj ectives are achieved by using mechanical micropumps to affect the flow of operating fluid in one or more parts of a gas circulating subsystem.

[0012] According to a first aspect, there is provided an arrangement that comprises one or more cold sources, each said cold source comprising one or more cold parts . One or more gas circulating subsystems comprise a respective room temperature part, a respective inbound line for feeding operating fluid in an inbound direction from the respective room temperature part towards a respective cold part among said one or more cold parts, and a respective outbound line for drawing operating fluid in an outbound direction towards the respective room temperature part . At least one inbound line and at least one outbound line of said one or more gas circulating subsystems comprise respective cryogenically cooled sections configured to be cooled to temperatures below 100 K in operation. The arrangement comprises one or more mechanical micropumps in the respective cryogenically cooled section of the respective inbound line and / or the respective outbound line, configured to affect the flow of operating fluid in the inbound and / or outbound direction, respectively.

[0013] According to an embodiment, one or more of said mechanical micropumps are configured to pump operating fluid further in the inbound and / or outbound direction, respectively. This involves at least the advantage that pressure differences between parts of the gas circulation subsystem can be made to serve better the aim of efficient cooling.

[0014] According to an embodiment, among said one or more cold sources is a dilution refrigerator that comprises a mixing chamber and a still among its cold parts .Among said one or more gas circulating subsystems is then a gas circulating subsystem of said dilution refrigerator, called the first gas circulating subsystem in the following. The inbound line of the first gas circulating subsystem is configured to feed operating fluid in an inbound direction from the room temperature part of the first gas circulating subsystem towards said mixing chamber . The outbound line of the first gas circulating subsystem is configured to draw operating fluid in an outbound direction from said still towards said room temperature part of the first gas circulating subsystem. One or more of said one or more mechanical micropumps are located in the respective cryogenically cooled section of the inbound line and / or the outbound line of the first gas circulating subsystem, respectively. This involves at least the advantage that efficient operation of the dilution refrigerator can be ensured with solutions that are mechanical robust, reliable in operation, and possible to implement and operate at reasonable cost .

[0015] According to an embodiment, the arrangement comprises a Joule-Thomson impedance as a part of said inbound line and a mechanical microcompressor preceding said Joule-Thomson impedance in said inbound direction, configured to increase pressure of the operating fluid at entry into said Joule-Thomson impedance . Said mechanical microcompressor is one of said one or more mechanical micropumps . This involves at least the advantage that a larger reduction in the temperature of inbound operating fluid can be achieved in the Joule-Thomson impedance .

[0016] According to an embodiment, the inbound line comprises a 4 K thermalization point for thermalizing the operating fluid flowing in said inbound direction to a temperature of essentially 4 K. Said mechanical microcompressor and said Joule-Thomson impedance may then both be located after said 4 K thermalization pointin said inbound direction. This involves at least the advantage that advantageous initial conditions can be achieved for the operating fluid entering the Joule-Thomson impedance .

[0017] According to an embodiment, the inbound line comprises a 4 K thermalization point for thermalizing the operating fluid flowing in said inbound direction to a temperature of essentially 4 K. The mechanical microcompressor may then be located before said 4 K thermalization point in said inbound direction and said Joule-Thomson impedance may be located after said 4 K thermalization point in said inbound direction. This involves at least the advantage that any possible heat load imposed by the mechanical microcompressor may be absorbed by the refrigerator arrangement that maintains the 4 K temperature .

[0018] According to an embodiment, said mechanical microcompressor is a piezoelectric compressor and comprises an inlet, an outlet, and a chamber between said inlet and outlet; a piezoelectrically actuatable membrane as one limiting surface of said chamber; one or more inlet orifices connecting said inlet to said chamber; one or more outlet orifices connecting said chamber to said outlet; one or more elastically deformable inlet valve flaps covering said one or more inlet orifices on the side of the chamber; and one or more elastically deformable outlet valve flaps covering said one or more outlet orifices on the side of the outlet . This involves at least the advantage that a structure of well-known kind and reliable operation can be used.

[0019] According to an embodiment, the arrangement comprises a plurality of mechanical microcompressors at a same location of said inbound line, coupled in serial and / or parallel configuration, and a control arrangement coupled to said plurality of mechanical microcompressors and configured to selectively operate said plurality of mechanical microcompressors for implementing acontrollable flow impedance in said inbound line . This involves at least the advantage of offering a flexible way of controlling the flow conditions in the inbound line .

[0020] According to an embodiment, said one or more mechanical micropumps comprise at least one piezoelectric fan pump in said outbound line, configured to increase flow rate of gaseous operating fluid in said outbound direction at the respective location. This involves at least the advantage that the efficiency of further pump means in the room temperature part can be enhanced .

[0021] According to an embodiment, a fan element in said piezoelectric fan pump comprises an elastically deformable fan member fixedly attached at a first part thereof, and a piezoelectric actuator configured to generate, when subj ected to an alternating actuating voltage, resonant oscillations in said fan member to make a non-fixed second part thereof oscillate back and forth in a direction transverse to said outbound direction. This involves at least the advantage that a relatively simple structure can be used.

[0022] According to an embodiment, a piezoelectric fan pump comprised in said one or more piezoelectric fan pumps comprises a plurality of fan elements distributed in a pipe section that forms part of the outbound line . This involves at least the advantage that a significant effect on the flow of operating fluid can be achieved using relatively small and simple functional parts .

[0023] According to an embodiment, said piezoelectric fan pump is in the cryogenically cooled section of the outbound line after the still in said outbound direction. This involves at least the advantage that a significant improvement can be achieved in the efficiency of suction pumps that draw gaseous operating fluid out of the still and into the room temperature part .According to an embodiment, the arrangement comprises a heat exchanger in which a part of the inbound line comes into thermal coupling with operating fluid flowing in said outbound line . At least one of said one or more piezoelectric fan pumps may then be located before said heat exchanger in said outbound direction, for increasing flow of operating fluid in that part of the outbound line in which said thermal coupling takes place . This involves at least the advantage that the rate at which thermal energy is transferred between the opposite flows in the heat exchanger can be improved.

[0024] According to an embodiment, the arrangement comprises one or more vibration elements in the respective cryogenically cooled section of the inbound line and / or the outbound line, configured to generate turbulence in the flow of operating fluid at the respective part of the inbound line and / or the outbound line . This involves at least the advantage that the inherent thermally insulating property of laminar flows can be partly or completely avoided.

[0025] According to an embodiment, at least one of said one or more vibration elements is located inside a respective section of the inbound line or outbound line . This involves at least the advantage that a relatively significant amount of turbulence can be generated with relatively simple actuating element (s) .

[0026] According to an embodiment, at least one of said one or more vibration elements is located in a wall part of a respective section of the inbound line or outbound line . This involves at least the advantage that the need to draw operating voltage connectors inside the respective section of the inbound or outbound line can be avoided.

[0027] According to an embodiment, among said one or more cold sources is a closed-loop fluid refrigeration system with a vessel of liquid cryogen as its cold part . Among said one or more gas circulating subsystems maythen be a gas circulating subsystem of said closed-loop fluid refrigeration system, called the second gas circulating subsystem in the following. The inbound line of the second gas circulating subsystem may be configured to feed operating fluid in an inbound direction from the room temperature part of the second gas circulating subsystem towards the vessel of liquid cryogen. The outbound line of the second gas circulating subsystem may be configured to draw operating fluid in an outbound direction from said vessel of liquid cryogen towards said room temperature part of the second gas circulating subsystem. At least one of the one or more mechanical micropumps may be located in the respective cryogenically cooled section of the inbound line and / or the outbound line of the second gas circulating subsystem and configured to affect the flow of operating fluid in the inbound and / or outbound direction, respectively. This involves at least the advantage that efficient operation of the closed-loop fluid refrigeration system can be ensured with solutions that are mechanical robust, reliable in operation, and possible to implement and operate at reasonable cost .

[0028] According to a second aspect, there is provided a method for making operating fluid flow in a gas circulating subsystem that comprises a room temperature part, an inbound line, and an outbound line . The method comprises cooling respective cryogenically cooled sections of said inbound line and said outbound line below 100 K, feeding operating fluid in an inbound direction from the room temperature part towards a cold part of a cold source, and drawing operating fluid in an outbound direction from a cold part of the cold source towards said room temperature part . The method comprises using one or more mechanical micropumps in the respective cryogenically cooled section of the inbound line and / or the outbound line to affect the flow of operating fluid in the inbound and / or outbound direction, respectively.According to an embodiment, the method comprises using the one or more mechanical micropumps to pump operating fluid further in the inbound and / or outbound direction, respectively. This involves at least the advantage that pressure differences between parts of the gas circulation subsystem can be made to serve better the aim of efficient cooling.

[0029] According to an embodiment, the method comprises using one or more of said one or more mechanical micropumps to increase pressure of the operating fluid at entry into a Joule-Thomson impedance in said in-bound direction. This involves at least the advantage that a larger reduction in the temperature of inbound operating fluid can be achieved in the Joule-Thomson impedance .

[0030] According to an embodiment, the method comprises implementing a controllable flow impedance in said inbound line by selectively operating a plurality of mechanical microcompressors in the inbound line . This involves at least the advantage of offering a flexible way of controlling the flow conditions in the inbound line .

[0031] According to an embodiment, the method comprises increasing flow rate of gaseous operating fluid in said outbound direction by operating at least one piezoelectric fan pump in said outbound line . This involves at least the advantage that the efficiency of further pump means in the room temperature part can be enhanced .

[0032] According to an embodiment, the method comprises generating turbulence in the flow of operating fluid at one or more parts of the inbound line and / or the outbound line by operating one or more vibration elements in the respective cryogenically cooled section of the inbound line and / or the outbound line . This involves at least the advantage that the inherent thermally insulating property of laminar flows can be partly or completely avoided.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings :

[0034] Figure 1 is a schematic illustration of a cryostat,

[0035] figure 2a illustrates a dilution refrigerator, figure 2b illustrates an alternative form of a part of the dilution refrigerator of fig. 2a,

[0036] figure 3 is a phase diagram of 3He, figure 4 illustrates a dilution refrigerator, figure 5 illustrates a dilution refrigerator, figure 6 illustrates a piezoelectric pump in suction phase,

[0037] figure 7 illustrates a piezoelectric pump in pump phase,

[0038] figure 8 illustrates the use of a plurality of pumps in series or in parallel,

[0039] figure 9 illustrates the operating principle of an example of a piezoelectric fan pump,

[0040] figure 10 illustrates a dilution refrigerator, figure 11 illustrates an example of using piezoelectric fan pumps,

[0041] figure 12 illustrates an example of using piezoelectric fan pumps,

[0042] figure 13 illustrates an example of using piezoelectric fan pumps,

[0043] figure 14 illustrates an example of using piezoelectric fan pumps,

[0044] figure 15 illustrates an example of using piezoelectric fan pumps with ring-shaped membranes,

[0045] figure 16 illustrates a dilution refrigerator,figure 17 illustrates effects of laminar and turbulent flow of operating fluid,

[0046] figure 18 illustrates causing turbulence with a vibrating member,

[0047] figure 19 illustrates causing turbulence with a vibrating member,

[0048] figure 20 illustrates an example of using piezoelectric fan pumps,

[0049] figure 21 illustrates a cryostat with circulation subsystems for liquid cryogens, and

[0050] figure 22 illustrates a cryostat with cooling down to about 1 K.

[0051] DETAILED DESCRIPTION

[0052] In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the present disclosure may be placed. It is understood that other aspects may be utilised, and structural or logical changes may be made without departing from the scope of the present disclosure . The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the present disclosure is defined by the appended claims .

[0053] For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures . On the other hand, for example, if a specific apparatus is described based on functional units, a corresponding method may include a step performing the described functionality, even if such step is not explicitly described or illustrated inthe figures . Further, it is understood that the features of the various example aspects described herein may be combined with each other, unless specifically noted otherwise .

[0054] In this description, the term cold source is used to designate a system that is designed and operable to refrigerate at least a part of the inside of a cryostat to cryogenic temperatures . Examples of cold sources include, but are not limited to, pulse tube refrigerators, dilution refrigerators, Joule-Thomson refrigerators, and closed-loop fluid refrigeration systems .

[0055] In this description, the term gas circulation subsystem is used to designate a system that is designed and operable to circulate gaseous or at least partly gaseous substances between a room temperature part and a cold part of a cold source . In some cases, a gas circulation subsystem may constitute an integral part of a cold source . As a non-limiting example, proper operation of a dilution refrigerator necessitates circulating operating fluid, which is partly gaseous and partly in liquid form, through a large number of parts of the dilution refrigerator . A gas circulation subsystem thus constitutes an important part of the dilution refrigerator . As another non-limiting example, a closed-loop fluid refrigeration system comprises, as its name indicates, a closed loop in which at least partly gaseous operating fluid can be circulated through channels and vessels in which the operating fluid may appear in various relative amounts of liquid and gaseous phases .

[0056] Fig. 2a illustrates an example of how certain flows of operating fluid can be arranged in an arrangement that comprises a dilution refrigerator . Parts of the dilution refrigerator shown in fig. 2a comprise the mixing chamber 201 and the still 202. Schematically shown with dashed triple lines are some of the cold stages of a cryostat in which the dilution refrigeratoris located. Each such cold stage can comprise, for example, a corresponding flange or arrangement of flanges in the cryostat . Shown in fig. 2a are the 4.2 K stage which may be thermally coupled to the coldest stage of a respective mechanical refrigerator, the 1 K stage on which the still 202 is located, the 100 mK stage below the still stage, and the 10 mK stage on which the mixing chamber 201 is located. The temperatures and cold stages shown here should be construed as examples and not limiting the use of other temperatures and / or other cold stages .

[0057] The arrangement comprises a gas circulation subsystem for the dilution refrigerator . A room temperature part of the gas circulation subsystem is not shown in fig. 2a otherwise than being schematically represented with reference designator 211. An inbound line 203 is provided for feeding operating fluid in the inbound direction from the room temperature part 211 towards the mixing chamber 201. An outbound line 204 is provided for drawing operating fluid in the outbound direction from at least the still 202 towards the room temperature part . Depending on the construction of the dilution refrigerator, also the route provided for drawing operating fluid in the outbound direction from the mixing chamber 201 towards the still 202 may be considered as a part of the outbound line . The operating fluid in the dilution refrigerator is, characteristically to dilution refrigerators, a mixture of helium isotopes helium-4 and helium-3 that occur in various concentrations and various proportions of liquid and gaseous phases in different parts of the gas circulation subsystem.

[0058] Both the inbound line 203 and the outbound line 204 comprise respective cryogenically cooled sections configured to be cooled to temperatures below 100 K in operation. In fig. 2a, the sections shown above the 4.2 stage represent the warmer ( 100 K ... 4.2 K) parts of therespective cryogenically cooled sections . Considering just the cryogenically cooled sections in this description is well-founded as the properties and behaviour of the operating fluid at low temperatures (and at the pressures encountered in the cryogenically cooled sections) differ significantly from those encountered in the room temperature part of the gas circulation subsystem. Parts of the cryostat that are used to maintain the cryogenically cooled sections of the inbound and outbound lines 203 and 204 at temperatures below 100 K in operation may comprise various refrigerator devices, including but not being limited to one or more mechanical refrigerators such as pulse tubes, Gifford-McMahon coolers, Stirling coolers, Joule-Thomson coolers, turbine based coolers (Turbo-Brayton) , or the like .

[0059] Heat exchangers can be used at various parts of the inbound and outbound lines 203 and 204 for utilizing the cold outbound flow of operating fluid to cool the inbound flow. Shown in fig. 2a are a Joule-Thomson impedance and heat exchanger 205 where the inbound line 203 and the outbound line 204 come together above the still, a heat exchanger section 206 of the inbound line 204 going through the still 202 , a continuous heat exchanger 207 between the 1 K and 100 mK stages, and a step heat exchanger 208 at or close to the 100 mK stage .

[0060] Other parts shown separately in fig. 2a are two flow impedances 209 and 210 along the inbound line 203 : one before the still 202 and one after it in the flowing direction of operating fluid in the inbound line 203.

[0061] Fig. 2b shows an alternative implementation of that part of the inbound line 203 that passes the still 202 . In fig. 2b, the heat exchanger section 206 of the inbound line 204 does not go through the still 202 but has just a thermally conductive coupling to the still 202. As such, for the following description it is of little importance whether the inbound line goes through the still, has only a thermally conductive coupling tothe still, or is subj ected to an appropriate amount of cooling otherwise . Also, it should be noted that in a practical implementation there may be more than one inbound line, and the inbound line ( s) may have branching and / or convergence points at various locations along its or their length.

[0062] The effect of a Joule-Thomson impedance and heat exchanger such as the one shown with reference designator 205 in fig. 2a can be considered in more detail with reference to the enthalpy vs . pressure diagram of helium-3 in fig. 3. The horizontal axis in fig.

[0063] 3 represents pressure and the vertical axis represents enthalpy. The numbered curves indicate temperature in kelvins, and the inverse drop-formed line 301 delimits a liquef action / evaporation region. Inside the liquefac-tion / evaporation region, the lower end of the vertical section of each temperature curve represents a point where the helium fluid is 100 % liquid, and the upper end of the vertical section represents a point where the helium fluid is 100% gas .

[0064] The dashed line through points A, B, C, and D in fig. 3 represents active cooling of a sample of helium-3 in an example case . As shown by the horizontal section B -> C of the dashed line, isenthalpic lowering of pressure, such as throttling through an orifice in a thermally insulating system, cools the helium-3 if the initial conditions are right . As a comparison, line E -> F shows an isenthalpic expansion from different initial conditions, warming the helium-3 instead of cooling it .

[0065] Within the area of interest in fig. 3, the longer one can make the horizontal section B -> C, the larger decrease in temperature can be achieved. In other words, taken the proper initial conditions, one should aim at the largest possible decrease in pressure in the Joule-Thomson impedance . In prior art solutions, the pressure of the operating fluid entering the Joule-Thomson impedance is generated with one or more compressors in the room temperature part of the gas circulating subsystem. However, the pressure the compressors can generate is limited by the compressors themselves and the connecting tubes . Inherent impedances of the tubing will reduce the initial pressure at low temperature where the expansion pressure difference should be maximized.

[0066] According to a novel idea, it is possible to increase the pressure difference in a Joule-Thomson impedance by using a mechanical micropump in a cryogenically cooled section of the inbound line 203. Such mechanical micropump may then be configured to affect the flow of operating fluid in the inbound direction in a way that results in a pressure difference that is larger than without the mechanical micropump . Considered in the inbound direction, a mechanical micropump can be used as a compressor before the Joule-Thomson impedance (possibly coupled in series or otherwise integrated with a heat exchanger) to increase the pressure of operating fluid at the input of the Joule-Thomson impedance . Additionally, or alternatively, a mechanical micropump can be used as a suction pump after the Joule-Thomson impedance to decrease the pressure at the output of the Joule-Thomson impedance .

[0067] Fig. 4 shows an arrangement that is in many ways like that of fig. 2a earlier, with similar parts shown with the same reference designators . The arrangement of fig. 4 comprises a mechanical micropump 401 in a cryogenically cooled section of the inbound line 203, configured to affect the flow of operating fluid in the inbound direction. In particular, the mechanical micropump 401 is configured to pump operating fluid further in the inbound direction. A Joule-Thomson impedance and heat exchanger 205 is provided as a part of the inbound line 203. The mechanical micropump 401 is a mechanical microcompressor preceding the Joule-Thomson impedancein the inbound direction, configured to increase the pressure of the operating fluid at entry into the Joule-Thomson impedance .

[0068] The location of the mechanical microcompressor close to the input of the Joule-Thomson impedance means that the pumped operating fluid will meet only a small amount of inherent flow impedance before entering the Joule-Thomson impedance . Thus, even a relatively small increase in pressure created with the mechanical microcompressor may enable achieving a significantly larger cooling effect on the inbound operating fluid in the Joule-Thomson impedance .

[0069] Also shown in fig. 4 is the so-called 4 K thermalization point 402 for thermalizing the operating fluid flowing in the inbound direction to a temperature of essentially 4 K (4.2 K in fig. 4 ) . In the embodiment of fig . 4 , both the mechanical microcompressor 401 and the Joule-Thomson impedance and heat exchanger are located after the 4 K thermalization point in the inbound direction. This is not an essential requirement . Fig. 5 illustrates an alternative embodiment, in which a mechanical microcompressor 503 is located before the 4 K thermalization point 402 in the inbound direction and the Joule-Thomson impedance and heat exchanger 205 are located after the 4 K thermalization point 402 in the inbound direction. It is also possible to combine the embodiments of figs . 4 and 5, so that there would be at least two mechanical microcompressors for the purpose explained above, one before and one after the 4 K thermalization point 402. Additionally, or alternatively, at least one mechanical microcompressor may be located at the 4 K thermalization point 402.

[0070] Placing a mechanical microcompressor before the 4 K thermalization point as shown in fig. 5 (or placing one exactly at the 4 K thermalization point) involves the advantage that the actively cooled thermal stage (the 4.2 K stage in fig. 5) between the mechanicalmicrocompressor and the Joule-Thomson impedance can be used to absorb any additional heat generated by the mechanical microcompressor . A downside is, on the other hand, that the section of the inbound line 203 between the mechanical microcompressor and the Joule-Thomson impedance is longer, causing more flow impedance and thus weakening the desired pressure-increasing effect at the input of the Joule-Thomson impedance .

[0071] Physical dimensions of the inbound line 203 at its cryogenically cooled sections are typically very small . Additionally, as in all cryogenically cooled applications, all thermal loading of the cooling system should be minimized. This means that for the purposes explained above with reference to figs . 3 - 5, a mechanical microcompressor with suitably small dimensions, small power requirement, and high efficiency should be found.

[0072] A mechanical microcompressor fulfilling these requirements can be produced utilizing a piezoelectric compressor as shown in cross section in figs . 6 and 7. Fig. 6 shows the intake stroke and fig. 7 shows the compression stroke . The piezoelectric compressor of figs . 6 and 7 comprises an inlet 601, an outlet 602, and a chamber 603 therebetween. A piezoelectrically actuatable membrane 604 is provided as one limiting surface of the chamber 603. One or more inlet orifices 605 connect the inlet 601 to the chamber 603 in a way that allows operating fluid to flow from the inlet 601 to the chamber 603. One or more outlet orifices 606 connect the chamber 603 to the outlet 602 in a way that allows operating fluid to flow from the chamber 603 to the outlet 602. One or more elastically deformable inlet valve flaps 607 are provided, covering the one or more inlet orifices 605 on the side of the chamber 603. Correspondingly, one or more elastically deformable outlet valve flaps 608 are provided, covering said one or more outlet orifices 606 on the side of the outlet 602.Applying a first operating voltage value to the piezoelectrically actuatable membrane 604 makes it bulge outwards from the chamber 603, as shown in fig. 6. The resulting decrease in the internal pressure of the chamber 603 draws operating fluid from the inlet 601 through the inlet orifice (s) 605 to the chamber 603, elastically deforming the inlet valve flap (s) 607 in a way that opens a flow passage for the operating fluid to flow. Applying a second operating voltage value to the piezoelectrically actuatable membrane 604 makes it bulge inwards to the chamber 603, as shown in fig. 7. The resulting increase in the internal pressure of the chamber 603 forces operating fluid from the chamber 603 through the outlet orifice (s) 606 to the outlet 602, elastically deforming the outlet valve flap (s) 608 in a way that opens a flow passage for the operating fluid to flow. Repeating the intake stroke of fig . 6 and the compression stroke of fig. 7 over and over again produces a net effect of pumping operating fluid further in the direction from left to right in figs . 6 and 7.

[0073] As the required mechanical parts are relatively simple in form and relatively few in numbers, it is possible to make the piezoelectric compressor small, with characteristic dimensions in the scale of only few millimetres or even less than one millimetre . Using voltage values to operate the piezoelectrically actuatable membrane causes very little dissipation of electric current, which means generating only little waste heat during operation.

[0074] In the description of figs . 6 and 7 above, it was assumed that the inlet valve flap (s) 607 and outlet valve flap (s) 608 are elastically deformable but operationally passive, only reacting to pressure differences between the inlet 601, chamber 603, and outlet 602, respectively. It is also possible to use operationally active - like piezoelectrically (or otherwise) actuatable - valve members in a mechanical microcompressor .Such an approach involves the advantage that the flowing operating fluid does not need to use a portion of its kinetic energy to elastically deform the valve members . As a result, the flow impedance caused by the mechanical microcompressor can be made smaller .

[0075] In an alternative embodiment of a mechanical microcompressor, valves or valve flaps of the kind described above are not needed at all . Instead, there may be a series of chambers or chamber sections, at least some of which are limited by a respective piezoelectri-cally actuatable membrane . By operating the piezoelec-trically actuatable membranes at suitable relative phasing, it is possible to make the resulting phased variations in increasing and decreasing chamber volume create a net effect that advances the flow of gaseous medium through the series of chambers or chamber sections .

[0076] Irrespective of whether valve members are operationally passive or active, it is possible to use a mechanical microcompressor (or a plurality of mechanical microcompressors) to implement a controllable flow impedance in the inbound line 203. Basically, this may means selectably setting a mechanical microcompressor (or at least some of a plurality of mechanical microcompressors) in a state of larger or smaller flow impedance . It may also mean the use of a plurality of mechanical microcompressors in series and / or in parallel and making a selected subset of them actively pump operating fluid, so that the larger the proportion of inactive ones, the larger the net flow impedance caused.

[0077] Fig. 8 illustrates two examples where a plurality of mechanical microcompressors 801, 802 is provided at a same location of the inbound line . In the left example, the plurality of mechanical microcompressors 801 are coupled in a serial configuration, and in the right example, the plurality of mechanical microcompressors 802 are coupled in a parallel configuration. More elaborate implementations could involve mechanicalmicrocompressors in both serial and parallel configurations . A control arrangement 803 is coupled to the plurality of mechanical microcompressors 801, 802 and configured to selectively operate them for implementing a controllable flow impedance in the inbound line . The control arrangement 803 is labelled as a pump and / or impedance control in fig. 8, as the same controlling entity from which control signals come to the mechanical microcompressors can control both their pumping activity and their use as components of a controllable flow impedance . The principle of fig. 8 can be applied in any kinds of inbound lines of any or all embodiments described in this text .

[0078] Mechanical microcompressors, like the piezoelectric compressor of figs . 6 and 7, are well suited for affecting the flow of operating fluid in the cryogenically cooled section of the inbound line where the operating fluid may occur in the gaseous phase, the liquid phase, or a mixed gaseous / liquid phase; and where the density and pressure of the operating fluid is relatively high compared to densities and pressures encountered in the cryogenically cooled section of the outbound line . In significant parts of the last-mentioned, the operating fluid occurs largely in the gaseous phase and while its extremely low temperature may mean a reasonable density for a gas, its pressure is relatively low. For this reason, mechanical micropumps of other kind may be needed for appropriately affecting the flow of operating fluid in the cryogenically cooled section of the outbound line .

[0079] Fig. 9 illustrates one example of another kind of mechanical micropump . The apparatus shown in fig . 9 is a piezoelectric fan pump or, more accurately, a fan element 901 of a piezoelectric fan pump .

[0080] The fan element 901 shown in fig . 9 comprises an elastically deformable fan member 902, fixedly attached at a first part thereof . In fig. 9, the fixedattachment is shown with reference designator 903, clamping one end of the elastically deformable fan member 902 and holding it steady. In the embodiment of fig.

[0081] 9, the fan member 902 is a flat piece of material that remains sufficiently elastic also at cryogenically cooled temperatures, yet with high enough Q factor to enable resonant oscillations at high enough frequencies . As an example, the fan member 902 may be a piece of a metal sheet, such as stainless steel sheet . Other possible materials for the elastically deformable fan member 902 include but are not limited to other metals, such as brass, copper, aluminium, and titanium; plastics such as polyamide; ceramics such as aluminium oxide and aluminium nitride, and others . In the embodiment of fig.

[0082] 9, the fan member 902 is essentially rectangular in form, but this is just an illustrative example and the fan member 902 could have different forms, like semicircular, polygon with rounded corners, or the like .

[0083] The fan element 901 comprises a piezoelectric actuator 904 configured to generate, when subj ected to an alternating actuating voltage, resonant oscillations in the fan member 902. The generated resonant oscillations should be of a kind that make a non-fixed part of the fan member 902 oscillate back and forth. In the embodiment of fig. 9, the piezoelectric actuator 904 comprises two flat pieces of piezoelectric material attached to opposite side surfaces of the fan member 902. It would also be possible to have a piezoelectric actuator on only one side . An alternating actuating voltage conducted to the piezoelectric actuator 904 makes the deformable fan member 902 oscillate between the upwards curved form shown in the top part of fig. 9 and the downwards curved form shown in the bottom part of fig.

[0084] 9.

[0085] Fig. 10 illustrates some examples of locations where piezoelectric fan pumps can be used, with general reference to an arrangement of the kind originally shownin fig. 2a earlier . In the embodiment of fig. 10, there is a first piezoelectric fan pump 1001 in the cryogenically cooled section of the outbound line 204 after the still 202, when considered in the outbound direction. The first piezoelectric fan pump 1001 is quite close to the still 202, before the outbound line 204 passes the 4.2 K stage of the system. A second piezoelectric fan pump 1002 is also located in the cryogenically cooled section of the outbound line 204 after the still 202, but further away, in the section of the outbound line 204 between the 4.2 K and 50 K stages of the system. Both these locations are examples, and a practical system could have a piezoelectric fan pump at any or both of them, and / or at some other location along the outbound line 204. A diameter of a pipe section in the outbound line 204 is typically larger than 1 cm, and may be in the order of 10 cm or more .

[0086] As already mentioned above, there are differences between the inbound flow and outbound flow of operating fluid in a dilution refrigerator . In those sections of the outbound line 204 where the use of piezoelectric fan pumps is considered here, the operating fluid is essentially in gaseous form and its pressure is relatively small . Piezoelectric fan pumps may be used to improve the flow rate of gaseous operating fluid by placing the fan elements so that the back-and-forth oscillating movement of the respective elastically deformable fan member (s) takes place in a direction essentially transverse to the outbound direction.

[0087] Fig. 11 shows an example of a piezoelectric fan pump in a pipe section 1120 of an outbound line of a gas circulating subsystem that may be used as a part of a dilution refrigerator . The piezoelectric fan pump of fig. 11 comprises a total of twelve fan elements 1101 to 1112 distributed in the pipe section 1120, arranged on three transverse levels along the local longitudinal direction of the outbound line . Fan elements 1101 to1104 are on a first level, fan elements 1105 to 1108 are on a second level, and fan elements 1109 to 1112 are on a third level . The outbound direction, i . e . the flowing direction of operating fluid, is upwards in fig. 11. Each fan element has the non-fixed part of its fan member directed generally in the flow direction of operating fluid, so that the back-and-forth oscillations generated with the respective piezoelectric actuator will make said non-fixed part oscillate in a direction generally transverse to the outbound direction.

[0088] Figs . 12 to 14 show alternative embodiments where piezoelectric fan pumps are used in a pipe section of the outbound line . In the embodiment of fig. 12, a plurality of fan elements are distributed in a pipe section so that each of them is on a different level 1201 to 1208 along the local longitudinal direction of the outbound line . The fan elements constitute a spiralling pattern in fig. 12, so that at each consecutive level, the respective fan element is displaced from that on the previous level by an angle around the central axis of the pipe section. The embodiment of fig. 13 is otherwise similar to that of fig . 11, but the fan elements 1301 to 1312 have their respective fan members directed straight upwards, i . e . exactly in the flow direction of the gaseous operating fluid, and their respective fixing parts designed so that operating fluid may flow on both sides of the respective fan member .

[0089] The embodiment of fig. 14 is otherwise similar to that of fig. 11, but there are differences between fan elements 1401 - 1412 in how the respective fan member is directed. For example, on the topmost level in fig.

[0090] 14, fan elements 1402 and 1403 have the plane defined by their fan member directed tangentially to the cylindrical form of the pipe section 1120, while fan elements 1401 and 1404 have the plane defined by their fan member directed radially to the cylindrical form of the pipe section 1120. On the next level, the roles of the fanelements regarding the direction of their respective fan members are switched.

[0091] The embodiments shown in figs . 11 to 14 can be combined in various ways, for example so that in a spiralling distribution like that in fig. 12, every second fan element has its fan member directed differently like in f ig . 14 .

[0092] Fig. 15 illustrates another possible form of a piezoelectric fan pump 1501. The elastically deformable fan member 1502 in the piezoelectric fan pump 1501 has the form of a ring-shaped band. One edge of the ringshaped band is fixedly attached to the walls of a pipe section 1520 of the outbound line, as shown with reference designator 1503 in fig. 15. A piezoelectric actuator 1504 on a surface of the ring-shaped band may be subj ected to an alternating actuating voltage, causing resonant oscillations in the fan member 1502 as shown with dashed lines in fig. 15 : the non-fixed edge of the elastically deformable fan member 1502 oscillates back and forth essentially in he transverse direction of the pipe section 1520. Hence, the resonant oscillations take place in a direction transverse to the outbound direction of the operating fluid flow in the outbound line .

[0093] The oscillating frequency used in a piezoelectric fan pump should be selected high enough to make the non-fixed part (s) of the respective elastically deformable fan member (s) move fast enough, considering the general dimensions of the system and the properties (temperature, density, pressure) of the operating fluid the flow of which is to be affected. As a rough estimate, the velocity of gas molecules is about kT per each degree of freedom, where k is the Boltzmann constant and T is the temperature in kelvins . At cryogenically cooled temperatures of the kind encountered in the outbound line below the 50 K stage, a characteristic estimate for the velocity of helium atoms is in the order of 100 metres per second. Assuming that the gaseous medium could beat or close to its molecular limit, the free end of an elastically deformable fan member of a piezoelectric fan pump should be made to move at roughly similar velocities . In figs . 11 to 15, assuming that the diameter of the illustrated pipe section is in the order of 10 cm, the piezoelectric fan pumps could be driven with an actuating voltage having its frequency in the order of at least some kilohertz, preferably some tens of kilohertz, or even up to a megahertz . However, if the gaseous medium is not at or close to the molecular limit but rather closer to the viscous limit, significantly lower frequencies, down to only few Hz may be enough.

[0094] Advantages may be gained by using mechanical micropumps in the outbound line because the pumps used in gas circulating subsystems of conventional dilution refrigerators to draw operating fluid out of outbound lines are very expensive . Pump types used for these purposes include at least Roots pumps and turbo pumps . For a Roots pump, when used for this purpose, its pumping capacity may be directly proportional to the pressure at its inlet . If mechanical micropumps along the outbound line could, say, triple the pressure of operating fluid at the inlet of an array of Roots pumps in the room-temperature part, the number of Roots pumps needed to establish and maintain a certain required flow of circulated operating fluid could be reduced by a factor of three . For turbo pumps, the association between inlet pressure and pumping capacity is not as straightforward as for Roots pumps, but similar savings in pumping hardware required in the room temperature part may nevertheless be achieved.

[0095] Mechanical micropumps in the outbound line may be used also to improve the operation of heat exchangers in which a part of the inbound line comes into thermal coupling with operating fluid flowing in the outbound line . Fig. 16 illustrates an arrangement of the general kind described earlier with reference to figs . 2a and2b, with corresponding parts shown with the same reference designators . Heat exchangers where a part of the inbound line 203 comes into thermal coupling with operating fluid flowing in the outbound line 204 include, for example, the Joule-Thomson impedance and heat exchanger 205 above the still 202 and a continuous heat exchanger 207 between the 1 K and 100 mK stages . One mechanical micropump 1601, for example a piezoelectric fan pump, is located before the continuous heat exchanger 207 in the outbound direction. Another mechanical micropump 1602, for example a piezoelectric fan pump, is located before the Joule-Thomson impedance and heat exchanger 205 in the outbound direction. Other possible locations for mechanical micropumps in the outbound direction are shown in the same way as in fig. 10. Any or all such mechanical micropumps may appear in a practical system. The role of such mechanical micropumps, if used, is to increase the flow of operating fluid in that part of the outbound line 204 in which the respective thermal coupling takes place .

[0096] Increasing flow and affecting pressure are not the only ways in which advantages may be gained by using mechanical micropumps . Additionally, or alternatively, mechanical micropumps can positively affect the flow of operating fluid by generating turbulence . To provide some background, fig. 17 illustrates schematically two sections of a flow channel . In the first section 1701, the flow of fluid is laminar, while in the second section 1702 the flow of fluid is turbulent . If the fluid flowing in the channel were to exchange thermal energy with the walls (and / or through the walls) of the channel, the turbulent conditions on the right lead to better efficiency. In a laminar flow like that on the left in fig.

[0097] 17, a boundary layer of fluid flowing closest to the wall of the channel may limit the heat exchange, while a turbulent flow like that on the right in fig. 17 enhances convective heat transport in the fluid.Figs . 18 and 19 illustrate parts 1801 and 1901 of a cryogenically cooled section of a fluid flow line, which may be an inbound line or an outbound line in a gas circulation subsystem of a dilution refrigerator . The arrangement of which the dilution refrigerator is part comprises a vibration element 1802, 1902 , and / or 1903 in the inbound or outbound line, configured to generate turbulence in the flow of operating fluid at the respective part of the inbound or outbound line . In the embodiment of fig. 18 , at least one vibration element 1802 is located inside a respective section 1801 of the inbound or outbound line . In the embodiment of fig. 19, at least one vibration element 1902 and / or 1903 is located in a wall part of the respective section 1901 of the inbound or outbound line . If the respective wall part is part of a heat exchanger and, consequently, separates the operating fluid flows in the inbound and outbound lines, the vibration element 1902 or 1903 may generate turbulence both in the operating fluid flowing in the inbound line and in the operating fluid flowing in the outbound line .

[0098] Fig. 20 illustrates an example embodiment in which a heat exchanger comprises a part 2001 of an inbound line spiralling along the walls of a part 2002 of an outbound line . The part 2001 of the inbound line is a narrow pipe made of a material of low thermal conductivity at cryogenically cooled temperatures at least in the axial ( fluid flow) direction, such as stainless steel . The pipe may be thermally coupled to additional structures for increasing the surface area in contact with the operating fluid flowing in the outbound line . In fig. 20, the spikes 2003 extending from the pipe towards the central axis of the part 2002 of the outbound line represent such additional structures . Additional structures used for this purpose could have a different outer appearance, like planar surfaces for example .Vibration elements are provided in the part 2002 of the outbound line for generating turbulence in the flow of operating fluid. In the embodiment of fig.

[0099] 20, the vibration elements are piezoelectrically operated fan elements 2004, 2005, 2006, 2007, and 2008 of the general kind that were described earlier with reference to figs . 9, 11, 12, and 14. They are placed in the part 2002 of the outbound line so that each fan element is before the respective section of the part 2001 of the inbound line, considered in the outbound direction. The fan elements 2004, 2005, 2006, 2007, and 2008 may have a dual function in the embodiment of fig.

[0100] 20; they may generate turbulence in the outbound flow of operating fluid for more efficient heat exchange with the inbound flow, and they may generally improve the flow of operating fluid in the outbound direction, contributing to the advantageous effects explained earlier with reference to figs . 10 to 16.

[0101] Fig. 21 illustrates an example of an arrangement, particularly a cryogenic cooling system that could be characterized as a cryostat with wet cooling. Inside the cryostat is an inner vacuum chamber 2101 which, as its name suggests, is a gastight container that may house one or more payloads to be kept at cryogenically cooled temperatures during operation. Surrounding the inner vacuum chamber 2101, in the embodiment of fig. 21, is the so-called helium bath 2102, which is essentially a vessel designed to hold liquid cryogen (here : liquid helium) during operation. Surrounding the helium bath 2102 is the so-called nitrogen bath 2103, which is another vessel designed to hold liquid cryogen (here : liquid nitrogen) during operation. The vessels for liquid cryogens may have vacuum-insulated walls to reduce the propagation of heat from outside to inside of the vessel . In some wet-cooled cryostats, only one vessel for liquid cryogen may be sufficient .There are two cold sources of the closed-loop fluid refrigeration type in fig. 21 : one with nitrogen and another with helium as operating fluid. The nitrogen bath 2103 constitutes a cold part of the first closed-loop fluid refrigeration system. A gas circulating subsystem comprises one or more pumps 2104 configured to draw out the evaporating gaseous nitrogen, a recondenser 2105 configured to refrigerate and recondense the gaseous nitrogen, as well as circulation channels 2106, 2107, and 2108 for maintaining the circulation of collected gaseous nitrogen back into the nitrogen bath 2103 in liquid form. Similarly, the helium bath 2102 constitutes a cold part of the second closed-loop fluid refrigeration system. A gas circulating subsystem for helium comprises one or more pumps 2109 configured to draw out the evaporating gaseous helium, a recondenser 2110 configured to refrigerate and recondense the gaseous helium, as well as circulation channels 2111, 2112, and 2113 for maintaining the circulation of collected gaseous helium back into the helium bath 2102 in liquid form.

[0102] At least a part of each gas circulation subsystem in fig. 21 constitutes a respective room temperature part . Typically, at least the pumps 2104 and 2109 are at room temperature . The channels 2106 and 2111 configured to direct evaporating gaseous operating fluid in the outbound direction towards the pumps constitute the respective outbound lines . Similarly, the channels 2108 and 2113 configured to direct refrigerated and recondensed operating fluid in the inbound direction into the respective baths constitute the respective inbound lines . All sections of the outbound and inbound lines that are inside the outermost thermal insulation of the cryostat may count as respective cryogenically cooled sections as, due to their thermal coupling with the fluid cryogens, they are at respective low temperatures during operation. Depending on how the recondensers 2105and 2110 are implemented and where their parts are located in relation to other parts of the arrangement, it is possible that also at least some of the channels 2107 and 2112 shown in fig. 21 as preceding the recondensers 2107 and 2112 may count as cryogenically cooled sections of respective inbound lines . The same applies to parts that would be principally included in the recondensers 2105 and 2111 themselves in the schematic representation of f ig . 21.

[0103] One or more mechanical micropumps could be placed in in any of the outbound lines 2106 and / or 2111, particularly in their respective cryogenically cooled section (s) , for affecting the flow of operating fluid. The placing and operating principle of such mechanical micropumps could follow the principles of any of figs .

[0104] 11 to 15, for example . If such mechanical micropumps are configured to pump operating fluid further in the outbound direction, a sufficient overall flow rate of operating fluid in the respective gas circulation subsystem may be achieved with fewer requirements to the main pump (s) 2104 and / or 2109.

[0105] One or more mechanical micropumps could be placed in any of the inbound lines of an arrangement of the kind shown in fig. 21. The placing and operating principle of such mechanical micropumps could follow the principles of any of figs . 4 to 8, for example . Such mechanical micropumps could have a role in pumping operating fluid further in the inbound direction, for example . Additionally, or alternatively, such mechanical micropumps could have a role in generating turbulence in the flow of operating fluid at the respective part of the respective inbound line, for example .

[0106] An arrangement where a closed-loop fluid refrigeration system constitutes a cold source is not necessarily based on large vessels where refrigeration takes place through evaporation in a bath of liquid cryogen. An alternative form of a closed-loop fluidrefrigeration system is one where operating fluid is circulated primarily in liquid form and refrigeration takes place conductively in heat exchanger (s) installed at the obj ect (s) to be cooled. Mechanical micropumps could be used in the cryogenically cooled sections of inbound and outbound lines of such closed-loop fluid refrigeration system, for example to enhance turbulence in the flow of operating fluid and / or to improve the flow rate at selected parts of the closed-loop fluid refrigeration system.

[0107] Fig. 22 illustrates an example of how certain flows of operating fluid can be arranged in an arrangement that comprises a closed-loop fluid circulation system that in many ways resembles the upper stages of a dilution refrigerator . Parts that are similar to e . g. the dilution refrigerator shown in fig. 2a comprise a still 2202 , an inbound line 2203, and an outbound line 2204. Of the inbound line 2203 and outbound line 2204, it must be noted that they only extend down to the still 2202 ; there are no such further parts that in a dilution refrigerator would extend to a mixing chamber . Additionally, the arrangement of fig. 22 comprises cold stages, like the 50 K and 4 .2 K stages, cooled with an appropriate cold source, like the respective stages 502 and 503 of a pulse tube refrigerator . Each such cold stage can comprise, for example, a corresponding flange or arrangement of flanges in the cryostat .

[0108] Among the cold stages is also a 1 K stage on which the still 2202 is located. Calling the vessel on the 1 K stage a still may be slightly misleading, as in the cryostat of fig. 22 its purpose is merely to act as a heat exchanger where thermal energy present at the 1 K stage continuously evaporates some of the operating fluid and is transported away in the form of the latent heat of the gaseous evaporation product (s) .

[0109] The arrangement of fig. 22 comprises a gas circulation subsystem for the closed-loop fluid circulationsystem. A room temperature part of the gas circulation subsystem is not shown in fig. 22 otherwise than being schematically represented with reference designator 211. The inbound line 2203 is provided for feeding operating fluid in the inbound direction from the room temperature part 211 towards the still 2202. The outbound line 2204 is provided for drawing operating fluid in the outbound direction from the still 2202 towards the room temperature part 211.

[0110] Both the inbound line 2203 and the outbound line 2204 comprise respective cryogenically cooled sections configured to be cooled to temperatures below 100 K in operation. In fig. 22, the sections shown above the 4.2 stage represent the warmer (100 K ... 4.2 K) parts of the respective cryogenically cooled sections . Parts that are used to maintain the cryogenically cooled sections of the inbound and outbound lines 2203 and 2204 at temperatures below 100 K in operation may comprise various refrigerator devices, including but not being limited to one or more mechanical refrigerators such as pulse tubes, Gifford-McMahon coolers, Stirling coolers, Joule-Thomson coolers, turbine based coolers (TurboBrayton) , or the like .

[0111] Heat exchangers can be used at various parts of the inbound and outbound lines 2203 and 2204 for utilizing the cold outbound flow of operating fluid to cool the inbound flow. Shown in fig. 22 is a Joule-Thomson impedance and heat exchanger 205 where the inbound line 2203 and the outbound line 2204 come together above the still . Other exemplary parts shown separately in fig. 2a include a flow impedance 209 along the inbound line 2203, before the still 2202 in the flowing direction of operating fluid in the inbound line 2203.

[0112] Mechanical micropumps may be used in the inbound and / or outbound lines 2203 and 2204 of fig. 22 , for achieving at least some of similar effects as those already described in association with the otherembodiments above . Example locations for mechanical micropumps in the inbound line 2203 include, but are not limited to, before the Joule-Thomson impedance and heat exchanger 205 in the inbound direction (see reference designator 401) , between the 50 K and 4.2 K stages (see reference designator 503) , and before the flow impedance 209 (see reference designator 2201 ) . Example locations for mechanical micropumps in the outbound line 2204 include, but are not limited to, before the Joule-Thomson impedance and heat exchanger 205 in the outbound direction ( see reference designator 1602 ) , before the 4.2 K stage in the outbound direction (see reference designator 1001 ) , and between the 4.2 K and 50 K stages in the outbound direction (see reference designator 1002 ) .

[0113] Conceptually, the arrangement of fig. 22 has similarities with that of fig. 21, even if the operating principle of the arrangement of fig. 22 is not based on any massive baths of liquid cryogen. It comprises a closed-loop fluid refrigeration system with a vessel 2202 of liquid cryogen as its cold part . It comprises a gas circulating subsystem, the inbound line 2203 of which is configured to feed operating fluid in an inbound direction from the room temperature part 211 of the gas circulating subsystem towards the vessel 2202 of liquid cryogen. The outbound line 2204 of the gas circulating subsystem is configured to draw operating fluid in an outbound direction from the vessel 2202 of liquid cryogen towards the room temperature part 211 of the gas circulating subsystem. One or more mechanical micropumps are located in the respective cryogenically cooled section of the inbound line 2203 and / or the outbound line 2204 of the gas circulating subsystem and configured to affect the flow of operating fluid in the inbound and / or outbound direction, respectively.

[0114] Examples of operating fluids for use in closed-loop fluid refrigeration systems include, but are notlimited to, helium-4, helium-3, nitrogen, hydrogen, neon, argon, and / or mixtures of these .

[0115] It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways . The invention and its embodiments are thus not limited to the examples described above, instead they may vary within the scope of the claims .

Claims

CLAIMS1. An arrangement comprising:- one or more cold sources, each said cold source comprising one or more cold parts (201, 202, 2102, 2103, 2202 ) , and- one or more gas circulating subsystems, comprising: -- a respective room temperature part (211, 2104, 2109) ,-- a respective inbound line (203, 2108, 2113, 2203) for feeding operating fluid in an inbound direction from the respective room temperature part (211, 2104, 2109) towards a respective cold part (201, 202, 2102, 2103, 2202 ) among said one or more cold parts, and -- a respective outbound line (204, 2106, 2111, 2204 ) for drawing operating fluid in an outbound direction towards the respective room temperature part (211, 2104, 2109) ;wherein at least one inbound line (203, 2108, 2113, 2203) and at least one outbound line (204, 2106, 2111, 2204 ) of said one or more gas circulating subsystems comprise respective cryogenically cooled sections configured to be cooled to temperatures below 100 K in operation,characterized in that the arrangement comprises one or more mechanical micropumps (401, 503, 1001, 1002, 1601, 1602, 1802, 1902, 1903) in the respective cryogenically cooled section of the respective inbound line (203, 2108, 2113, 2203) and / or the respective outbound line (204, 2106, 2111, 2204 ) , configured to affect the flow of operating fluid in the inbound and / or outbound direction, respectively.

2. An arrangement according to claim 1, wherein one or more of said mechanical micropumps (401, 503, 1001, 1002, 1601, 1602 ) are configured to pump operating fluid further in the inbound and / or outbound direction, respectively.

3. An arrangement according to any of claims 1 or 2, wherein:- among said one or more cold sources is a dilution refrigerator that comprises a mixing chamber (201 ) and a still (202 ) among its cold parts;- among said one or more gas circulating subsystems is a gas circulating subsystem of said dilution refrigerator, called the first gas circulating subsystem in the following,- the inbound line (203) of the first gas circulating subsystem is configured to feed operating fluid in an inbound direction from the room temperature part (211 ) of the first gas circulating subsystem towards said mixing chamber (201 ) , and-- the outbound line (204 ) of the first gas circulating subsystem is configured to draw operating fluid in an outbound direction from said still (202 ) towards said room temperature part (211 ) of the first gas circulating subsystem;wherein one or more of said one or more mechanical micropumps (401, 503, 1001, 1002, 1601, 1602, 1802, 1902, 1903) are located in the respective cryogenically cooled section of the inbound line (203) and / or the outbound line (204 ) of the first gas circulating subsystem, respectively.

4. An arrangement according to claim 3, comprising :- a Joule-Thomson impedance as a part of said inbound line (203) of the first gas circulating subsystem, and - a mechanical microcompressor (401, 503) preceding said Joule-Thomson impedance in said inbound direction, configured to increase pressure of the operating fluid at entry into said Joule-Thomson impedance, wherein said mechanical microcompressor (401, 503) is one of said one or more mechanical micropumps .5 . An arrangement according to claim 4 , wherein :- the inbound line of the first gas circulating subsystem comprises a 4 K thermali zation point ( 402 ) for thermali zing the operating fluid flowing in said inbound direction to a temperature of essentially 4 K, and- said mechanical microcompressor ( 401 ) and said Joule-Thomson impedance are both located after said 4 K thermali zation point ( 402 ) in said inbound direction .6 . An arrangement according to claim 4 , wherein :- the inbound line of the first gas circulating subsystem comprises a 4 K thermali zation point ( 402 ) for thermali zing the operating fluid flowing in said inbound direction to a temperature of essentially 4 K, and- said mechanical microcompressor ( 503 ) is located before said 4 K thermali zation point ( 402 ) in said inbound direction and said Joule-Thomson impedance is located after said 4 K thermali zation point ( 402 ) in said inbound direction .7 . An arrangement according to any of claims 4 to 6 , wherein said mechanical microcompressor ( 401 ) is a piezoelectric compressor ( 401 , 503 ) and comprises :- an inlet ( 601 ) , an outlet ( 602 ) , and a chamber ( 603 ) between said inlet ( 601 ) and outlet ( 602 ) ,- a piezoelectrically actuatable membrane ( 604 ) as one limiting surface of said chamber ( 603 ) ,- one or more inlet ori fices ( 605 ) connecting said inlet ( 601 ) to said chamber ( 603 ) ,- one or more outlet ori fices ( 606 ) connecting said chamber ( 603 ) to said outlet ( 602 ) ,- one or more elastically deformable inlet valve flaps( 607 ) covering said one or more inlet ori fices ( 605 ) on the side of the chamber ( 603 ) , and- one or more elastically deformable outlet valve flaps ( 608 ) covering said one or more outlet ori fices ( 606 ) on the side of the outlet ( 602 ) .8 . An arrangement according to any of claims 4 to 7 , comprising :- a plurality of mechanical microcompressors ( 801 , 802 ) at a same location of said inbound line ( 203 ) of the first gas circulating subsystem, coupled in serial ( 801 ) and / or parallel ( 802 ) configuration, and- a control arrangement ( 803 ) coupled to said plurality of mechanical microcompressors ( 801 , 802 ) and configured to selectively operate said plurality of mechanical microcompressors ( 801 , 802 ) for implementing a controllable flow impedance in said inbound line ( 203 ) of the first gas circulating subsystem .9 . An arrangement according to any of the preceding claims , wherein said one or more mechanical micropumps comprise at least one piezoelectric fan pump ( 1001 , 1002 , 1601 , 1602 ) in the respective outbound line ( 204 ) , configured to increase flow rate of gaseous operating fluid in outbound direction at the respective location .10 . An arrangement according to claim 9 , wherein a fan element ( 901 , 1501 ) in said piezoelectric fan pump ( 1001 , 1002 , 1601 , 1602 ) comprises :- an elastically deformable fan member ( 902 , 1502 ) fixedly attached ( 903 , 1503 ) at a first part thereof , and- a piezoelectric actuator ( 904 , 1504 ) configured to generate , when subj ected to an alternating actuating voltage , resonant oscillations in said fan member ( 902 , 1502 ) to make a non- fixed second part thereofoscillate back and forth in a direction transverse to said outbound direction.

11. An arrangement according to claim 10, wherein a piezoelectric fan pump comprised in said one or more piezoelectric fan pumps ( 1001, 1002, 1601, 1602 ) comprises a plurality of fan elements ( 1101— 1112, 1301-1312, 1401-1412 ) distributed in a pipe section ( 1120) that forms part of the respective outbound line (204 ) .

12. An arrangement according to any of claims 9 to 11, when depending on at least claim 3, wherein said piezoelectric fan pump ( 1001, 1002, 1602 ) is in the cryogenically cooled section of the outbound line of the first gas circulation subsystem (204 ) after the still (202 ) in the outbound direction.

13. An arrangement according to any of claims 9 to 12, when depending on at least claim 3, comprising a heat exchanger (205, 207 ) in which a part of the inbound line (203) of the first gas circulating subsystem comes into thermal coupling with operating fluid flowing in said outbound line (204 ) of the first gas circulating subsystem, wherein at least one ( 1601, 1602 ) of said one or more piezoelectric fan pumps is located before said heat exchanger (205, 207 ) in the outbound direction, for increasing flow of operating fluid in that part of the outbound line (204 ) of the first gas circulating subsystem in which said thermal coupling takes place .

14. An arrangement according to any of the preceding claims, comprising one or more vibration elements ( 1802, 1902, 1903) in the respective cryogenically cooled section of the respective inbound line (203) and / or outbound line (204 ) , configured to generate turbulence in the flow of operating fluid at therespective part of the respective inbound line (203) and / or outbound line (204 ) .

15. An arrangement according to claim 14, wherein at least one ( 1802 ) of said one or more vibration elements is located inside a respective section of the respective inbound line (203) or outbound line (204 ) .

16. An arrangement according to any of claims 14 or 15, wherein at least one ( 1902, 1903) of said one or more vibration elements is located in a wall part of a respective section of the respective inbound line (203) or outbound line (204 ) .

17. An arrangement according to any of the preceding claims, wherein:- among said one or more cold sources is a closed-loop fluid refrigeration system with a vessel (2102, 2103, 2202 ) of liquid cryogen as its cold part;- among said one or more gas circulating subsystems is a gas circulating subsystem of said closed-loop fluid refrigeration system, called the second gas circulating subsystem in the following,- the inbound line (2108, 2113, 2203) of the second gas circulating subsystem is configured to feed operating fluid in an inbound direction from the room temperature part (211, 2104, 2109) of the second gas circulating subsystem towards the vessel (2102, 2103, 2202 ) of liquid cryogen, and-- the outbound line (2106, 2111, 2204 ) of the second gas circulating subsystem is configured to draw operating fluid in an outbound direction from said vessel (2102, 2103, 2202 ) of liquid cryogen towards said room temperature part (211, 2104, 2109) of the second gas circulating subsystem;wherein at least one of the one or more mechanical micropumps is located in the respective cryogenicallycooled section of the inbound line (2108, 2113, 2203) and / or the outbound line (2106, 2111, 2204 ) of the second gas circulating subsystem and configured to affect the flow of operating fluid in the inbound and / or outbound direction, respectively.

18. A method for making operating fluid flow in a gas circulating subsystem that comprises a room temperature part (211 ) , an inbound line (203) , and an outbound line (204) , the method comprising:- cooling respective cryogenically cooled sections of said inbound line (203) and said outbound line (204 ) below 100 K,- feeding operating fluid in an inbound direction from the room temperature part (211 ) towards a cold part (201, 2102, 2103) of a cold source, and- drawing operating fluid in an outbound direction from a cold part (202, 2102, 2103) of the cold source towards said room temperature part (211 ) ;characterized in that the method comprises :- using one or more mechanical micropumps (401, 503, 1001, 1002, 1601, 1602, 1802, 1902, 1903) in the respective cryogenically cooled section of the inbound line (203) and / or the outbound line (204 ) to affect the flow of operating fluid in the inbound and / or outbound direction, respectively.

19. A method according to claim 18, comprising using the one or more mechanical micropumps (401, 503, 1001, 1002, 1601, 1602 ) to pump operating fluid further in the inbound and / or outbound direction, respectively.

20. A method according to claim 19, comprising using one or more of said one or more mechanical micropumps to increase pressure of the operating fluid at entry into a Joule-Thomson impedance in said inbound direction.

21. A method according to any of claims 18 to 20, comprising implementing a controllable flow impedance in said inbound line (203) by selectively operating a plurality of mechanical microcompressors ( 801, 802 ) in the inbound line (203) .

22. A method according to any of claims 18 to 21, comprising increasing flow rate of gaseous operating fluid in said outbound direction by operating at least one piezoelectric fan pump ( 1001, 1002, 1601, 1602 ) in said outbound line (204 ) .

23. A method according to any of claims 18 to 22, comprising generating turbulence in the flow of operating fluid at one or more parts of the inbound line (203) and / or the outbound line (204 ) by operating one or more vibration elements ( 1802, 1902 ) in the respective cryogenically cooled section of the inbound line (203) and / or the outbound line (204 ) .

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