Cryogenic cooling system with combined mechanical cooler and dilution refrigerator

Abstract

A cryogenic cooling system comprises a vacuum chamber (101), a plurality of thermal stages (105, 106, 107, 503, 504) cooled to respective temperatures during operation, and a gastight sleeve (301, 401), at least a portion of which protrudes in the vacuum chamber (101). Inside said gastight sleeve (301, 401) are one or more cooling stages (501, 502) of a first cooling module (302). At least a first part (505, 506) of a second cooling module is inside said gastight sleeve (301, 401). A first thermally conductive coupling (508, 509, 510) exists between a first cooling stage (501, 502) and said first part (505, 506) of said second cooling module. A second thermally conductive coupling (515, 516, 521) exists between a first thermal stage (105, 106, 107) and a respective part (304, 305) of said second cooling module.

Description

[0001] CRYOGENIC COOLING SYSTEM WITH COMBINED MECHANICAL COOLER

[0002] AND DILUTION REFRIGERATOR

[0003] FIELD OF THE INVENTION

[0004] The invention is generally related to the cooling of cryostats . In particular, the invention is related to structural solutions and refrigeration mechanisms that enable cooling a cryostat efficiently, with reasonable consequences in structural complexity .

[0005] BACKGROUND OF THE INVENTION

[0006] Early cryostats were cooled with baths of liquid cryogens , such as liquid nitrogen and liquid helium . Later, mechanical cooling devices such as Stirling cryocoolers , Gifford-McMahon coolers , Pulse Tube Refrigerators ( PTRs ) , and Joule-Thomson coolers have been introduced to implement so-called cryogen-free cooling . I f the core part of the cryostat comprises a further cooling system such as a dilution refrigerator, which only becomes operative at temperatures at and below about 4 K, the required pre-cool ing may be made with for example a PTR . In a typical case , the PTR has two cool ing stages , of which the first stage is used to achieve a temperature around 40 K-70 K and the second stage precools the still of the dilution refrigerator to the required 3 K-4 K level .

[0007] Conventional arrangements of mechanical coolers and dilution refrigerators have certain drawbacks . For example , in cryostats that require large amounts of cooling power it may prove necessary to install and use several cooler devices , which takes up considerable space . It would also be desirable to be able to accurately control the available cooling power so that advantageously low temperatures can be reached with reasonable use of operating power and not too compl icated machinery . Another drawback encountered in at least some systems is that it is difficult to make them compatible with UHV (Ultra High Vacuum) conditions that may be needed in the payload space of the cryostat .

[0008] SUMMARY

[0009] It is an obj ective to present a cryogenic cooling system and a cooling method that offer reasonable base temperatures and accurate control of cooling power . Another obj ective is to combine effective cooling with only a reasonable increase in structural complicatedness . A yet further obj ective is to present cooler solutions that are or can be made compatible with UHV conditions .

[0010] These and further advantageous obj ectives are achieved by packing essential parts of a mechanical refrigerator and a dilution refrigerator within a common gastight sleeve .

[0011] According to a first aspect , there is provided a cryogenic cooling system that comprises a vacuum chamber and, inside said vacuum chamber, a plurality of thermal stages configured to be cooled to respective temperatures during operation . The cryogenic cooling system compri ses a gastight sleeve , at least a portion of which protrudes in the vacuum chamber . Inside said gastight sleeve are one or more cooling stages . The cryogenic cooling system comprises a second cooling module , at least a first part of which is inside said gastight sleeve . A first thermally conductive coupling exists between a first cooling stage among said cooling stages and said first part of said second cooling module that is inside said gastight sleeve . A second thermally conductive coupling exists between a first thermal stage among said plurality of thermal stages and a respective part of said second cooling module .

[0012] According to an embodiment , said first cooling module is a mechanical refrigerator . This involves at least the advantage that relatively low temperatures can be achieved inside the gastight sleeve with well-known and reliable technology .

[0013] According to an embodiment , said second cooling module i s a dilution refrigerator , parts of which comprise a still , a mixing chamber, an inbound line for making operating fluid flow in a direction towards said mixing chamber, and an outbound line for making operating fluid flow in a direction outwards from said mixing chamber . This involves at least the advantage that very low temperatures can be achieved at the coldest parts of the cryogenic cooling system .

[0014] According to an embodiment , said first thermal stage is a mixing chamber flange of said cryogenic cooling system and said second thermally conductive coupling is a coupling between said mixing chamber and said mixing chamber flange . This involves at least the advantage that very low temperatures can be achieved at the coldest thermal stage ( s ) of the cryogenic cooling system .

[0015] According to an embodiment , the cryogenic cooling system comprises a third thermally conductive coupling between said still and a sti ll flange that constitutes one thermal stage among said plurality of thermal stages , different from said mixing chamber flange . This involves at least the advantage that a low temperature achievable at the still of the dilution refrigerator can be utili zed to keep parts of an electronic circuit , a scientific experiment , or other payload in the cryogenic cooling system at a corresponding low temperature .

[0016] According to an embodiment , said mixing chamber is outside said gastight sleeve . This involves at least the advantage that making a thermally conductive coupling between the mixing chamber and a possible payload to be cooled by the cryogenic cool ing system is relatively easy . According to an embodiment also said still is outside said gastight sleeve . This involves at least the advantage that making a thermally conductive coupling between the still and a possible payload to be cooled by the cryogenic cooling system is relatively easy .

[0017] According to an embodiment , said mixing chamber and said still are inside said gastight sleeve . This involves at least the advantage that the gastight sleeve can be used to make a relatively self-contained module within the cryogenic cooling system .

[0018] According to an embodiment , the cryogenic cooling system comprises a detachable attachment of said first cooling module and said second cooling module in said gastight sleeve for selectively removing said first cooling module and said second cooling module from said gastight sleeve without compromising vacuum in said vacuum chamber . This involves at least the advantage that servicing the first and / or second cooling modules is easy and the vacuum chamber with the gastight sleeve attached can be subj ected to UHV baking or simi lar treatment without having to ris k damaging the f irst or second cooling module .

[0019] According to an embodiment , the cryogenic cooling system comprises , inside said gastight sleeve , one or more thermally conductive couplings between said inbound line and said first cooling module for cooling the operating fluid flowing through said inbound line during operation . This involves at least the advantage that the first cooling module may have an important supporting function in the operation of the dilution refrigerator .

[0020] According to an embodiment , the cryogenic cooling system comprises , inside said gastight sleeve , one or more thermally conductive couplings between said inbound line and said outbound line for exchanging heat between operating fluid flowing in said inbound line and operating fluid flowing in said outbound line . This involves at least the advantage that the operating fluid may be made to reach an advantageous ly low temperature along its route .

[0021] According to an embodiment , said one or more thermally conductive couplings between said inbound line and said outbound line comprise at least one of : a step heat exchanger, a counterflow heat exchanger . This involves at least the advantage that well-known and reliable technology can be used to implement said thermally conductive couplings .

[0022] According to an embodiment , at least a part of said outbound line cons ists of a portion of free space inside said gastight sleeve . This involves at least the advantage that a relatively large cross section may be offered for the flow of operating fluid in the outbound line , with a correspondingly low flow impedance .

[0023] According to an embodiment , there are one or more filler pieces made of a thermally insulating material and filling a respective part of free space inside said gastight sleeve . This involves at least the advantage that unwanted thermal conduction through any gaseous substance inside the sleeve can be reduced .

[0024] According to an embodiment , the cryogenic cooling system comprises a fourth thermally conductive coupl ing between a f irst , warmer cooling stage among said cooling stages of the first cooling module and a respective thermal stage among said plurality of thermal stages , and / or a fifth thermally conductive coupling between a second, colder cooling stage among said cooling stages of the first cooling module and a respective thermal stage among said plurality of thermal stages . This involves at least the advantage that the first cooling module can be used to support cooling of thermal stages external to the gastight sleeve .

[0025] According to an embodiment , at least one of said fourth and fifth thermally conductive couplings is comprised in the cryogenic cooling system and is a controllable coupling applicable of being selectively made thermally conductive and thermally insulating . This involves at least the advantage that the extent to which the first cooling module is loaded by heat loads outs ide the gastight sleeve can be controlled, with consequent advantages in making the first cooling module reach desired temperatures .

[0026] According to an embodiment , the cryogenic cooling system comprises a third cooling module outside said gastight sleeve , coupled to cool at least one of said plurality of thermal stages . This involves at least the advantage that the cooling of parts inside the sleeve and outside the sleeve can be made relatively independent of each other, with consequent advantages in the optimi zed utili zation of available cooling power .

[0027] According to an embodiment , the cryogenic cooling system comprises a gas handling system configured to pump gaseous substances out of said gastight sleeve . This involves at least the advantage that a thermally insulating vacuum can be generated inside the gastight sleeve .

[0028] According to a second aspect , there is provided a cryogenic cooling apparatus comprising a first cooling module and a second cooling module . The cryogenic cooling apparatus comprises a gastight sleeve enclosing said first cooling module and said second cooling module . Outside said gastight sleeve is a gastight coupling interface for gastightly connecting said gastight sleeve to an opening in an external vacuum chamber in a way that makes at least a portion of the gastight sleeve protrude in the vacuum chamber . Thermal coupling means are provided for thermally coupling predetermined parts of said first and second cool ing modules to respective thermal stages inside said external vacuum chamber once the gastight sleeve is in place protruding in the vacuum chamber . BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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 :

[0030] Figure 1 illustrates a cryogenic cooling system, figure 2 illustrates thermally conductive couplings to a mechanical refrigerator, figure 3 illustrates a cryogenic cooling system, figure 4 illustrates a cryogenic cooling system, figure 5 illustrates thermal couplings in a cryogenic cooling system, figure 6 illustrates thermal couplings in a cryogenic cooling system, figure 7 illustrates thermal couplings in a cryogenic cooling system, figure 8 illustrates thermal couplings in a cryogenic cooling system, and figure 9 illustrates a cryogenic cooling module .

[0031] DETAILED DESCRIPTION

[0032] 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 .

[0033] 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 il lustrated 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 in the 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 .

[0034] Fig . 1 is a schematic cross section of a cryogenic cooling system that comprises a vacuum chamber 101 as the outermost shell-like structure shown in fig . 1 . Inside the vacuum chamber 101 are a plurality of nested radiation shields shown with reference designators 102 , 103 , and 104 . Al so ins ide the vacuum chamber 101 are a plural ity of thermal stages . A thermal stage is a piece of hardware configured to be cooled to a respective temperature during operation . The vacuum inside the vacuum chamber serves to provide thermal insulation between the thermal stages inside the vacuum chamber . The temperature of the thermal stages can be deterministically maintained at a predetermined low temperature during operation through use of one or more refrigerators thermally coupled thereto . Basically, the top plates of each of the radiation shields 102 , 103 , and 104 can be considered thermal stages . Additionally, shown inside the innermost radiation shield 104 in fig . 1 are two further thermal stages 106 and 107 to which no respective radiation shields are shown to be attached .

[0035] Three refrigerators are shown in fig . 1 . Somewhat more generally the term cooling module could be used . The term refrigerator i s typically taken to mean an apparatus where active refrigeration occurs inside , due to physical phenomena like actively maintained phase conversions , pressure changes , isotope dilution, energetic electron flow, and / or the li ke . The term cool ing module can additionally cover systems that cause a cooling effect through heat exchange and convection, for example by making refrigerated coolant flow in a pipeline that passes through a region to be cooled so that the coolant absorbs heat therefrom and releases the heat elsewhere at a more distant location . In general , a cooling module is a subsystem of a cryogenic cooling system, operable for generating and maintaining a unidirectional net flow of thermal energy in the sense that the cooling effect thereof is a result of physical phenomena maintained by, and characteristic of , parts belonging to that subsystem . Examples of cooling modules include , but are not limited to, mechanical refrigerators such as pulse tubes , Gifford-McMahon coolers , and Joule-Thomson coolers ; compressor-based coolers that circulate operating fluid in a closed cycle containing evaporation and condensation stages ; dilution refrigerators where helium-3 is made to cross a phase boundary; thermoelectric refrigerators such as Peltier devices ; and solid-state microrefrigerators such as electrocaloric coolers or NI S refrigerators , S INI S refrigerators , or Sm-S refrigerators based on normal metal - insulator - superconductor tunnel j unctions , superconductor - insulator - normal metal - insulator - superconductor tunnel j unctions , or semiconductor - superconductor tunnel j unctions .

[0036] Mechanical refrigerators are used as one example of cooling modules in fig . 1 . There are two mechanical refrigerators 108 and 109 , each with two cooling stages . At the time of writing this text , a typical example of a mechanical refrigerator is a pulse tube with one , two , or three cooling stages . Both mechanical refrigerators in f ig . 1 have a f irst (warmer ) cooling stage and a second ( colder) cooling stage . Pulse tubes of this kind may be configured to maintain their first cooling stage at a temperature around 50 K and their second cooling stage at a temperature around 4 K during operation . These numerical values are examples used for illustration, and other temperatures could be involved . The cooling power at which a pulse tube or other kind of mechanical refrigerator can absorb heat at each of its cooling stages depends on the structure and dimensioning of the mechanical refrigerator in question . In the example shown in fig . 1 , both mechanical refrigerators 108 and 109 have their first cooling stage thermally coupled to the thermal stage at the top of the first radiation shield 102 and their second cooling stage thermally coupled to the thermal stage at the top of the second radiation shield 103 .

[0037] As an example of a third cooling module in fig . 1 , a dilution refrigerator 110 is shown . It extends through the two outer radiation shields 102 and 103 and has its still 111 thermally coupled to the thermal stage shown with reference designator 105 . The mixing chamber 112 of the dilution refrigerator 110 is thermally coupled to the innermost thermal stage 107 . The intermediate thermal stage 106 therebetween can be the so-called 100 mK stage that is thermally coupled to an intermediate part of the dilution refrigerator between its still 111 and mixing chamber 112 . A schematically represented gas handling system 113 is configured to provide all the gas handling functions required for the appropriate operation of the dilution refrigerator 110 .

[0038] The operating principle of a dilution refrigerator requires there to be a system of conduits for circulating operating fluid . While the cooling effect produced with a dilution refrigerator requires a mixture of the helium-3 and helium-4 isotopes , of these the circulation of especially helium-3 in the system of conduits is of importance . One or more inbound lines are provided for making operating f luid f low in the direction towards the mixing chamber 112 , while one or more outbound lines are provided for making operating fluid flow in the direction outwards from the mixing chamber 112 . The isotope composition of the circulated operating fluid may be different at various parts of the system of conduits , as dictated by the operating principle of a dilution refrigerator . The operating fluid flowing through the inbound line ( s ) is pre-cooled on the way, typically utili zing one or more of the available mechanical refrigerators or cooling modules of some other kind .

[0039] Fig . 2 illustrates a mechanical refrigerator 109 with a first cooling stage 201 and a second cooling stage 202 . A conduit 203 shown in fig . 2 belongs to a dilution refrigerator and forms a part of an inbound line for making operating fluid flow in a direction towards a mixing chamber (not shown in fig . 2 ) . As an alternative , the conduit 203 could belong to a cool ing module of some other kind, like a cooling module in which an inbound line was used to make operating fluid flow in a direction towards a heat exchanger used to absorb heat from a payload region .

[0040] There are three thermal couplings of the conduit 203 to parts of the mechanical refrigerator 109 in fig . 2 . At 204 , a relatively short section of the conduit 203 is in a thermally conductive connection with the first cooling stage 201 of the mechanical refrigerator 109 . At 205 , a relatively long section of the conduit 203 is in a thermally conductive connection with a section of the mechanical refrigerator 109 that extends between its cooling stages 201 and 202 . At 206 , a section of the conduit 203 goes through a block of thermally conductive material connected to the second cooling stage 202 of the mechanical refrigerator 109 .

[0041] Referring back to fig . 1 , the parallel use of two similarly coupled mechanical refrigerators 108 and 109 increases the cooling power that is available for cooling the thermal stages to which the cooling stages of the mechanical refrigerators 108 and 109 are coupled . However, it also takes up a relatively large proportion of the hori zontal cross-sectional area of the cryogenic cooling system . In particular, those parts of the refrigerators 108 , 109 , and 110 that are in the room temperature environment on the topside of the vacuum chamber 101 take up a lot of space . Also , as the lowest temperature to which the operating fluid in the inbound line can be pre-cooled is only the common temperature of the respective thermal stage and not any lower, it may happen that the dilution refrigerator 110 cannot reach its very lowest possible base temperatures and / or its highest pos sible cooling power at the base temperature .

[0042] Fig . 3 is a schematic cross section of a cryogenic cool ing system that resembles that of fig . 1 in many aspects , like in comprising a vacuum chamber 101 and inside it a plurality of thermal stages conf igured to be cooled to respective temperatures during operation . As in fig . 1 , the vacuum inside the vacuum chamber 101 serves to provide thermal insulation between the thermal stages inside the vacuum chamber . As a dif ference to fig . 1 , the cryogenic cooling system of fig . 3 comprises a gastight sleeve 301 , at least a portion of which protrudes in the vacuum chamber 101 . As the word sleeve indicates , the gastight s leeve 301 is a hollow, generally cylindrical structure . It is gastightly closed at both ends : the upper end that is located outside the vacuum chamber 101 and the inner end located inside the vacuum chamber 101 . Inside the gastight s leeve 301 are one or more of the cooling stages of a first cooling module 302 , which in fig . 3 is a mechanical refrigerator . In the embodiment of fig . 3 , the whole mechanical refrigerator with its two cool ing stages is inside the gastight sleeve 301 . Inside the vacuum chamber 101 , the gastight sleeve 301 may protrude through one or more of the thermal stages and / or the radiation shields , for which purpose there are respective openings in such thermal stages and / or radiation shields .

[0043] It is advantageous to make the gastight sleeve 301 of a material that , in addition to being gastight , has low thermal conductivity at cryogenically cooled temperatures . An example of such a material is stainless steel . It is also possible to make at least some portions of the gastight sleeve of one or more multi-layered or otherwise composite-like materials , so that the various component materials therein contribute to desired physical characteristics like structural stability, low thermal conductivity, gastightness , and the like . Low thermal conductivity of the sleeve material is particularly important in cases where the sleeve material is in physical contact with two or more such parts of the cryogenic cooling system that are to be held at different temperatures during operation .

[0044] A gas handling system 303 is configured to pump gaseous substances out of the gastight sleeve 301 . While there may be also some more sophisticated uses of the gas handling system 303 as described later in this text , for the time being it i s suff icient to assume that the gas handling system 303 is capable of creating and maintaining a thermally insulating vacuum inside the gastight sleeve 301 . Hence , during operation there are essentially no such gaseous substances inside the gastight sleeve 301 that could make a thermal shortcut of significance between two parts of different temperatures , like between the first and second cooling stages of the mechanical refrigerator 302 . The cryogenic cooling system of fig . 3 comprises a dilution refrigerator as its second cooling module . Some parts of the dilution refrigerator are inside the gastight s leeve 301 , while in thi s embodiment the mixing chamber 304 and the still 305 are outside the gastight sleeve 301 . There is a thermally conductive coupl ing between the mixing chamber 304 and one of the thermal stages (here : the coldest thermal stage 107 ) . More generally, the configuration in fig . 3 may be characteri zed so that there is a thermally conductive coupling between at least one of the thermal stages of the cryogenic cooling system as a whole and a respective part of the second cooling module , which in fig . 3 is the dilution refrigerator .

[0045] In the embodiment of fig . 3 , those parts of the dilution refrigerator that are inside the gastight sleeve 301 comprise at least ( the warmest ) parts of the inbound and outbound lines , the purpose of which is to make operating fluid flow in the directions towards the mixing chamber 304 and outwards from the mixing chamber 304 respectively . As an example , an inbound line may have one or more thermal couplings with parts of the mechanical refrigerator 302 , following a similar approach as in fig . 2 , for example . More examples of what roles the parts of the dilution refrigerator that are inside the gastight sleeve 301 may have , and what kind of thermal ly conductive couplings can be made , are described in detail later in this text .

[0046] Fig . 4 shows a cryogenic cooling system that is otherwise similar to that of fig . 3 but now the mixing chamber 304 and the still 305 of the dilution refrigerator are inside the gastight sleeve 401 .

[0047] The mere purpose of using a further cooling module , li ke a dilution refrigerator for example , in a cryogenic cooling system is to create and maintain a so- called base temperature that is lower than what could be achieved and maintained with the first cooling module . I f the further cooling module is a dilution refrigerator like in figs . 3 and 4 , the base temperature may be in the order of only a few millikelvins at a region where some kind of payload like a quantum processing circuit , a scientific experiment , or the like can be installed . For this reason, there i s provided a thermally conductive coupling between the mixing chamber 304 and the respective thermal stage 107 of the cryogenic cooling system .

[0048] Whether or not there is need for a payload space that can be kept at an intermediate temperature , like the temperature of the stil l for example , depends on the case . In both cases of figs . 3 and 4 , there may be a thermally conductive coupling between the still 305 and another thermal stage 105 of the cryogenic cooling system . The same goes for the so-called 100 mK stage 106 which is shown in both figs . 3 and 4 : if one is needed, there may be a thermally conductive coupling between it and a respective part of the dilution refrigerator between the mixing chamber 304 and the still 305 .

[0049] An intermediate variant between the embodiments of figs . 3 and 4 could be one in which the mixing chamber 304 of the dilution refrigerator is outside the gastight sleeve but the sti ll 305 is inside the gastight sleeve .

[0050] The use of a gastight sleeve 301 or 401 that encloses at least parts of the first and second cooling modules involves a number of principal advantages . One of them is related to savings in space . As the first and second cooling modules are structurally intertwined with each other, it is not necessary to reserve separate real estate for both of them in the cross section perpendicular to the principal direction of the cryostat . The principal direction means the direction in which the main temperature gradient is oriented between the thermal stages inside the cryostat ; in a typical graphical representation like that used in figs . 3 and 4 the principal direction is the vertical direction . The increased free space can be used to install other neces sary functionalities , like cabling, connectors , and / or visual ports that may be required by the payload that the cryogenic cooling system is cooling .

[0051] Another advantage is the possibility of dedicating a first cooling module , like a mechanical refrigerator for example , solely for pre-cooling purposes that benefit the second cooling module , which may be a dilution refrigerator for example . In both fig . 3 and fig . 4 , there is another mechanical refrigerator 108 for maintaining the outer radiation shields 102 and 103 at their desired temperatures during operation . Depending on the s i ze of the system and the heat loads that will be encountered, it may be advisable to dedicate more than one such further cooling modules for this purpose . Instead of or in addition to pulse tubes or other mechanical refrigerators capable of reaching cryogenic temperatures , one may use other kinds of cooling modules like compressor-based cooling or circulation of liquid coolant .

[0052] Another advantage comes in the form of somewhat more lenient requirements for the gastightness and vacuum compatibility of parts inside the gastight sleeve . As an example, the conduits , heat exchangers , and other parts of a dilution refrigerator require relatively delicate structures , which may be difficult to make completely gastight even in the first place and in which minor leaks may develop during use as a result of repeated circulation through large temperature ranges . Additionally, or alternatively, it could be desirable to use materials such as plastics , and / or manufacturing methods such as 3D printing, to manufacture some of the parts , while strict requirements of vacuum compatibility may have previously impeded the use of such materials and / or manufacturing methods . I f such parts are inside the gastight sleeve , possible leaks will not contaminate the vacuum in the main vacuum chamber . The gastight sleeve itself may be simpler as a structure and thus easier to make gastight in a way that ensures also maintaining the gastightness after longer periods of use and after many rounds of circulating between operating temperatures and higher temperatures like room temperature . As an example , the gastight sleeve may consist of one or more cylindrical sections and end plates , made of stainless steel and / or some other suitable materials and attached together by brazing or welding . During operation, the level of vacuum inside the gastight sleeve does not necessarily need to be as high as in the surrounding vacuum chamber, and the possible occurrence of impurities does not matter that much . For thi s reason, it is not necessary to require as strict compatibil ity with high or ultra-high vacuum conditions of those materials that are only used inside the gastight sleeve .

[0053] Further advantages may be gained if there is a detachable attachment of the first cooling module and the ( respective parts of the ) second cool ing module in said gastight sleeve for selectively removing them from the gastight sleeve without compromising vacuum in the vacuum chamber . Removing the first cooling module and the ( respective parts of the ) second cooling module from their attachments in the gastight sleeve may be needed for example to do some servicing . Further, if the removability concerns all such parts of the mechanical refrigerator and dilution refrigerator that would be susceptible to high temperatures ( in the range of 100 to 300 degrees centigrade ) , they can be removed for the time it takes to bake the vacuum chamber 101 in preparation for UHV (ultra-high vacuum) operation . UHV conditions are not needed in all applications , but if they are , it is advantageous to be able to bake the inside of the vacuum chamber 101 at such elevated temperatures for the required period, which may be in the order of 24 to 48 hours . In case it is considered advantageous to protect the removable parts of the first and second cool ing modules from atmospheric contamination like water also in their removed configuration, it is possible to equip the gastight sleeve with a load lock system . An example of a load lock system could comprise a gate valve approximately at the level of the top plate of the vacuum chamber 101 and an upwards protruding extension of the gastight sleeve above the gate valve . The system could comprise one or more elongate probes with which one could li ft the removable parts of the f irst and second cooling modules above the gate valve , after which the gate valve would be closed . After the baking of the main vacuum chamber, the gate valve could then be opened, and the removable parts of the cooling modules could be lowered to their operating positions inside that part of the gastight sleeve that protrudes in the vacuum chamber .

[0054] A yet further advantage may come in the form of a pos sibility to lower the power consumption of the mechanical refrigerator or other kind of first cooling module once the operating temperatures have been reached . As the first cooling module inside the sleeve does not (necessarily) need to take part in absorbing any heat that loads the thermal stages of the cryostat during operation, it may suffice to run it at a lower power level after the initial cool -down of all parts has been completed . This may help to achieve a smaller overall carbon footprint of the whole cryogenic cooling system over its useful life . The cooling power of cooling modules is also typically dependent on the temperature achieved, with less cooling power available at the very lowest temperatures . I f the first cooling module inside the s leeve does not need to take part in absorbing any heat that loads the thermal stages of the cryostat during operation, it may be possible to run its coldest parts at lower temperature , even if that meant that the first cooling module could then offer only a smaller cooling power at such a lower temperature .

[0055] Fig . 5 illustrates schematically some possible thermal couplings between parts of a cryogenic cooling system . In fig . 5 it i s assumed that the f irst cool ing module is a mechanical refrigerator with two cooling stages 501 and 502 . Also , it is assumed that the second cooling module is a dilution refrigerator that comprises a sti ll 305 , a mixing chamber 304 , an inbound line 505 for making operating fluid flow in a direction towards the mixing chamber 304 , and an outbound line 506 for making operating fluid flow in a direction outwards from said mixing chamber 304 .

[0056] On the right in fig . 5 is the lower part of the mechanical refrigerator, with its first cooling stage 501 and second cooling stage 502 held at temperatures in the order of 50 K and 4 K, respectively, during operation . As in all other examples in this text , these temperatures are used only for illustrative purposes , and other temperatures could be involved . The 50 K flange 503 and the 4 K flange 504 of the cryogenic cooling system are also shown .

[0057] Following the principle explained earlier in this text , the cryogenic cooling system may comprise , inside the gastight s leeve , one or more thermally conductive couplings between the inbound l ine 505 and the first mechanical refrigerator for cooling the operating fluid flowing through the inbound line 505 during operation . Examples of such thermally conductive couplings in fig . 5 are :

[0058] - a thermal ly conductive coupling 507 between the inbound line 505 and an upper part 520 of the mechanical refrigerator, above its first cooling stage 501 ,

[0059] - a thermal ly conductive coupling 508 between the inbound line 505 and the first cooling stage 501 , - a thermal ly conductive coupling 509 between the inbound line 505 and the lower part 519 of the mechanical refrigerator, between its first cooling stage 501 and second cool ing stage 502 ,

[0060] - a thermally conductive coupling 510 between the inbound line 505 and the second cool ing stage 502 .

[0061] To implement the thermally conductive couplings mentioned above in practice , one may apply the principles shown earlier with reference to fig . 2 , for example .

[0062] Inside the gastight sleeve there may be one or more thermally conductive couplings between the inbound line 505 and the outbound line 506 for exchanging heat between operating fluid flowing in the inbound line 505 and operating fluid flowing in the outbound line 506 . Examples of such thermally conductive couplings in fig . 5 are :

[0063] - a thermal ly conductive coupling 511 between the inbound and outbound lines between the mixing chamber 304 and the still 305 ,

[0064] - a thermal ly conductive coupling 512 between the inbound line 505 and the still 305 (which conceptually forms a part of the outbound line ) ,

[0065] - a thermal ly conductive coupling 513 between the inbound and outbound lines between the first and second cooling stages 501 and 502 of the mechanical refrigerator,

[0066] - a thermal ly conductive coupling 514 between the inbound and outbound lines above the first cooling stage 501 of the mechanical refrigerator .

[0067] Thermally conductive couplings between the inbound line 505 and the outbound line 506 may compri se , for example , one or more step heat exchangers , one or more counterflow heat exchangers , or any combination of these .

[0068] Due to the chaining of thermally conductive couplings , like those shown with reference designators 507 and 514 for example , it may be said that a cryogenic cooling system with a configuration like that in fig . 5 may also comprise one or more thermally conductive couplings between the outbound line 506 and the first cooling module .

[0069] Also shown in fig . 5 are possible thermally conductive couplings 515 , 516 , and 521 between the mixing chamber 304 and the mixing chamber flange 107 ; between the still 305 and the still flange 105 ; and between intermediate parts of the dilution refrigerator and the 100 mK flange 106 , respectively . Any or all of these could be characteri zed as thermally conductive couplings between a thermal stage among the plurality of thermal stages in the cryogenic cooling system and a respective part of the second cooling module , which in fig . 5 is the dilution refrigerator . As described already above , the thermal stages meant here are those between which the vacuum ins ide the vacuum chamber serves to provide thermal insulation .

[0070] Fig . 6 is a more practically oriented illustration of a way to implement the principles explained above with reference to fig . 5 . Simi lar to fig . 5 , the first cooling module is a mechanical refrigerator with two cooling stages 501 and 502 . These are located inside a gastight sleeve 401 that is assumed to protrude inside a vacuum chamber . The mixing chamber flange 107 , the still flange 105 , the 4K flange 504 , and the 50K flange 503 on the right in fig . 6 belong to a plurality of thermal stages inside the vacuum chamber . Also similar to fig . 5 , the second cooling module is a dilution refrigerator that comprises a still 305 , a mixing chamber 304 , an inbound line 505 for making operating fluid flow in a direction towards the mixing chamber 304 , and an outbound line 506 for making operating fluid flow in a direction outwards from said mixing chamber 304 .

[0071] In fig . 6 , the mixing chamber 304 is thermally coupled to a bottom plate 601 of the sleeve 401 . A pliable section of thermally conductive material provides a thermally conductive coupling 515 between the bottom plate 601 of the sleeve and the mixing chamber flange 107 . In the main part of fig . 6 , the bottom plate 601 constitutes a part of the gastight enclosure provided by the sleeve 401 . It is also possible to place the mixing chamber 304 on a separate plate or flange inside the sleeve and to use a separate bottom plate to close the lower end of the sleeve , as in the alternative shown below the main part of fig . 6 . Such a solution could help to isolate the mixing chamber 304 from any mechanical vibration that could propagate through the walls of the sleeve . Mechanical vibration may originate from the mechanical refrigerator, for example . In such a case , a pliable section of thermally conductive material could be used ( also ) between the plate or flange supporting the mixing chamber and the wall or other structural element of the sleeve , as indicated with the two sections 515 in the lower part of fig . 6 .

[0072] A counterflow heat exchanger implements the thermally conductive coupling 511 between the inbound and outbound lines between the mixing chamber 304 and the still 305 . I f the system comprises a 100 mK flange, it could be thermally coupled to a suitable part of the counterflow heat exchanger or to an intermediate section of the conduits if there are multiple heat exchangers at this level of the system .

[0073] The thermally conductive coupling 512 between the inbound line 505 and the still 305 , which conceptually forms a part of the outbound line , may be implemented like in previously known dilution refrigerators . The stil l 305 is thermally coupled to a section 602 of thermally conductive material inside the sleeve 401 . A thermally conductive coupling 516 from the section 602 of thermally conductive material is made through another pliable section of thermally conductive material . Also here it is possible to additionally or alternatively place a pliable section of thermally conductive material inside the sleeve , as shown in fig . 6 .

[0074] A thermally conductive coupling 510 between the inbound line 505 and the second cooling stage 502 is shown as a heat-exchanging section of the inbound l ine thermally coupled to the second cool ing stage 502 . Another pliable section of thermally conductive material provides a thermally conductive coupling 517 between the thermally conductive support plate 603 of the second cooling stage 502 inside the sleeve on one hand and the 4K flange 504 on the other hand . Also here it is possible to additionally or alternatively place a pliable section of thermally conductive material inside the sleeve , as shown in fig . 6 .

[0075] Adj acent to the lower part of the mechanical refrigerator, there is a counterflow heat exchanger thermally coupled to the intermediate section of the mechanical refrigerator, implementing both the thermally conductive coupling 509 between the inbound line 505 and the lower part 519 of the mechanical refrigerator and the thermally conductive coupling 513 between the inbound and outbound lines between the first and second cooling stages 501 and 502 of the mechanical refrigerator .

[0076] A thermally conductive coupling 508 between the inbound line 505 and the first cooling stage 501 is shown as another heat-exchanging section of the inbound line thermally coupled to the first cooling stage 501 . A further pliable section of thermally conductive material provides a thermally conductive coupling 518 between the thermally conductive support plate 604 of the first cooling stage 501 inside the sleeve on one hand and the 50K flange 503 on the other hand . Also here it is possible to additionally or alternatively place a pliable section of thermally conductive material inside the sleeve , as shown in fig . 6 .

[0077] In all embodiments where a pliable section of thermally conductive material is used to make a thermally conductive connection that simultaneously precludes the propagation of mechanical vibrations , other kinds of couplings could be used . Examples of such other kinds of coupl ings include but are not l imited to gasgap couplings , fluid couplings , bellows filled with fluid, and heat pipes .

[0078] Adj acent to the upper part of the mechanical refrigerator, there is a counterflow heat exchanger thermally coupled to the corresponding intermediate section of the mechanical refrigerator, implementing both the thermally conductive coupling 507 between the inbound line 505 and the upper part 520 of the mechanical refrigerator and the thermally conductive coupling 514 between the inbound and outbound lines above the cooling stage 501 of the mechanical refrigerator .

[0079] Any part of the wall s of the s leeve 401 could be made to include a section of bellows . As an example , the section of the vertical walls of the sleeve between the level of the 4K flange 504 and the level of the still flange 105 could compri se bellows . By using bellows , propagation of mechanical vibrations along the walls of the s leeve may be impeded . It is particularly advantageous to isolate the mixing chamber 304 and possibly also the still 305 from any kind of mechanical vibrations . Bellows or corresponding vibration-attenuating gas-tight structural elements could also be used at those parts where the sleeve is connected to the main vacuum chamber of the cryogenic cool ing system . I f the bellows are of the kind that allow significant extending and contracting, it is poss ible to utili ze the bellows even as means that enable temporarily retracting ( at least part of ) the hardware inside the sleeve from their normal location inside the vacuum chamber .

[0080] Fig . 7 illustrates schematically some possible thermal couplings between parts of another cryogenic cool ing system . Many parts of the i llustration in f ig . 7 are similar to those of fig . 5 and hence carry the same reference designators . The following description concentrates on those parts that are different from fig . 5 .

[0081] As a significant difference , in fig . 7 at least a part of the outbound line 506 consists of a respective portion of free space inside the gastight sleeve . Here , the expression free space means a space that in the hori zontal direction ( or more generally : in the direction perpendicular to the general orientation of operating fluid flow in the outbound line ) is del imited by structures of the sleeve rather than by some conduits or other separate structures inside the sleeve .

[0082] One such portion of free space is shown as a sleeve part 701 above the still 305 but ( at least mostly) below the second cooling stage 502 of the mechanical refrigerator . Another such portion of free space is shown as a sleeve part 702 that is ( at least mostly) located between the first and second cooling stages 501 and 502 of the mechanical refrigerator . Yet another such portion of free space is shown as a sleeve part 703 that is ( at least mostly) above the f irst cooling stage 501 of the mechanical refrigerator . In each case , the respective thermally conductive coupling 704 , 705 , or 706 means the thermal coupling between the portion of operating fluid flowing in the direction towards the mixing chamber 304 through the respective portion of the inbound line 505 and the portion of operating fluid flowing in the direction out of the mixing chamber 304 through the respective portion of the outbound line 506 .

[0083] The sleeve parts 701 , 702 , and 703 are shown as separate items in fig . 7 . Thi s does not neces sarily mean that they would be strictly separate in a practical implementation . Rather, they may be sections of a larger, essentially continuous portion of free space inside the sleeve so that one such sleeve part j ust continues directly into the subsequent such sleeve part .

[0084] Fig . 8 is a more practically oriented illustration of a way to implement the principles explained above with reference to fig . 7 . Simi lar to a number of the preceding figures , the first cooling module is a mechanical refrigerator with two cooling stages 501 and 502 . These are located inside a gastight sleeve 401 that is assumed to protrude inside a vacuum chamber . The mixing chamber flange 107 , the still flange 105 , the 4K flange 504 , and the 50K flange 503 on the right in fig . 8 belong to a plurality of thermal stages inside the vacuum chamber . The second cooling module is again a dilution refrigerator that comprises a still 305 , a mixing chamber 304 , an inbound line 505 for making operating fluid flow in a direction towards the mixing chamber 304 , and an outbound line 506 for making operating fluid flow in a direction outwards from said mixing chamber 304 .

[0085] In fig . 8 , the bottom 801 of the mixing chamber 304 constitutes also the lower end of the sleeve 401 . A pliable section of thermally conductive material provides a thermally conductive coupling 515 between the mixing chamber 304 and the mixing chamber flange 107 . Here and at all other parts of fig . 8 where a pliable section of thermally conductive material is mentioned it is also possible to use the alternative kinds of vibration-isolating thermally conductive couplings described above with reference to fig . 6 .

[0086] A counterflow heat exchanger implements the thermally conductive coupling 511 between the inbound and outbound lines between the mixing chamber 304 and the still 305 . I f the system comprises a 100 mK flange , it could be thermally coupled to a suitable part of the counterflow heat exchanger or to an intermediate section of the conduits if there are multiple heat exchangers at this level of the system .

[0087] The thermally conductive coupling 512 between the inbound line 505 and the still 305 , which conceptually forms a part of the outbound line , may be implemented like in previously known dilution refrigerators . A structural part of the still 305 ( or some suitably coupled intermediate part ) has a thermally conductive coupling 516 to the still flange 105 through another pliable section of thermally conductive material .

[0088] Above the still 305 , a part of the outbound line 506 consists of a portion of free space inside the gastight sleeve 401 . A thermally conductive coupling 704 is shown between the portion of operating fluid flowing in the direction towards the mixing chamber 304 through the respective portion of the inbound l ine 505 and the portion of operating fluid flowing in the direction out of the mixing chamber 304 through the free space inside the sleeve 401 , which free space constitutes the respective portion of the outbound line 506 .

[0089] A similar arrangement is repeated at the levels of the lower part 519 and the upper part 520 of the mechanical refrigerator . The inner diameter of the sleeve is shown to be larger than at those sections that are closer to the mixing chamber 304 and the still 305 . By selecting the inner diameters of sections of the sleeve , it is possible to set certain geometric boundary conditions that have a role in making the pressure of the operating f luid in the outbound l ine achieve suitable values . Additionally or alternatively, one or more sections of the sleeve 401 may comprise one or more filler pieces made of a thermally insulating material . Such filler piece may fill a respective part of free space inside the gastight sleeve 401 and thus affect the effective cross section that is available for the operating fluid to flow in the outbound line . I f filler pieces are used, they may also have a role in helping to maintain temperature gradients in the outbound flow of operating fluid, as they reduce the cross section through which heat from the upper parts of the outbound line may be conducted towards the lower parts .

[0090] Reference designator 802 shows a component that could be a Joule-Thomson impedance , a flow impedance , or a throttling valve . Such components are typically used in the inbound line to control the flow of operating fluid . In fig . 8 , said component is shown in the inbound line between the heat exchangers that implement the thermally conductive couplings 704 and 512 . Additionally, or alternatively, such components could be used above the heat exchanger that implements the thermally conductive coupling 704 and / or between the heat exchangers that implement the thermally conductive couplings 512 and 511 .

[0091] Above , it was repeatedly assumed that there may be thermally conductive couplings between one or more of the cooling stages of the mechanical refrigerator ( or, more generally, the first cooling module ) and respective thermal stages in the cryogenic cooling system : see thermally conductive couplings 517 and 518 . It should be noted that such thermally conductive couplings are not necessary, at least if the mechanical refrigerator in question ( or, more generally, the first cooling module ) is dedicated solely to the pre-cooling of the dilution refrigerator ( or, more generally, the second cooling module ) .

[0092] I f such thermally conductive couplings do exist, one or more of them may be a controllable coupling applicable of being selectively made thermally conductive and thermally insulating . As an example , such controllability can be utili zed so that the controllable coupling ( s ) in question may be held thermally conductive for the time it takes to pre-cool the whole cryogenic cooling system ( close ) to the operating temperatures , but subsequently made thermally insulating . Thus , the mechanical refrigerator ( or, more generally, the first cooling module ) could contribute to the pre-cooling stage , shortening the time it takes to reach the operating temperatures , but subsequently use the whole of its available cooling power to effectively pre-cool the operating fluid and / or other parts of the dilution refrigerator ( or, more generally, the second cooling module ) . This may enable the dilution refrigerator ( or, more generally, the second cool ing module ) reach and maintain a lower base temperature than if some of the cooling power of the mechanical refrigerator ( or, more generally, the first cooling module ) would be continuously directed to other parts of the cryogenic cooling system .

[0093] Referring back to f igs . 3 and 4 , the gas handling subsystem that is schematically shown with reference designator 303 may be a part of the cryogenic cooling system but that is not mandatory . It is possible to manufacture the entity consisting of the gastight sleeve 301 or 401 and the cool ing module parts inside it as a standalone component the inside of which is pumped into vacuum as a part of its manufacturing, after which the gastight sleeve 301 or 401 is sealed so that no further pumping of gases is needed . This applies to al l embodiments described in this text .

[0094] Fig . 9 illustrates a cryogenic cooling apparatus , a characteristic feature of which is that it can be used as an essentially self-contained subsystem of a larger cryogenic cooling system . While not separately shown in fig . 9 , the cryogenic cool ing apparatus of fig . 9 may comprise a first cooling module , such as a mechanical refrigerator for example , and a second cooling module such as a dilution refrigerator for example . The cryogenic cooling apparatus comprises a gastight sleeve that consists of a body portion 901 and a top cover portion 902 . These can be gastightly connected to each other as shown with reference designator 903 so that the resulting gastight enclosure encloses both the first and second cooling modules . Outside the gastight sleeve is a gastight coupling interface for gastightly connecting the gastight s leeve to an external vacuum chamber 101 , as shown with reference designator 908 , in a way that makes at least a portion of the gastight sleeve protrude in the vacuum chamber 101 . Such a gastight coupling interface may comprise a suitable connection flange , for example .

[0095] Inside the gastight sleeve of fig . 9 , there are shown attachments for the mixing chamber and still of a dilution refrigerator and the lower and upper stages of a mechanical refrigerator, with reference designators 904 , 905 , 906 , and 907 respectively . From these , thermal couplings 515 , 516 , 517 , and 518 can be made to the respective thermal stages 107 , 105 , 504 , and 503 inside the vacuum chamber 101 and outside the gastight s leeve once the cryogenic cooling apparatus has been installed to its place in the cryogenic cool ing system . As mentioned earl ier in this text , the thermal couplings 517 and 518 can be controllable thermal couplings that can be selectively made thermally conductive and thermally insulating . As also mentioned earlier, the vacuum inside the vacuum chamber serves to provide thermal insulation between the thermal stages inside the vacuum chamber .

[0096] A gas handling subsystem 303 is schematically shown in fig . 9 for at least pumping gaseous substances out of the gastight sleeve once closed . I f needed, the gas handling subsystem 303 may comprise also functions used to circulate operating fluids through some of or all the cooling modules inside the gastight sleeve during operation .

[0097] A cryogenic cooling apparatus that has the general principal structure of fig . 9 can be used for example so that one makes the gastight connection 908 with the top cover portion 902 and at least the most heat- sensitive parts of the cryogenic cool ing modules still removed . The inside of the vacuum chamber 101 can then be baked for UHV operation . After a cool-down period, one may install the ( remaining parts of the ) cooling modules , close the top cover portion 902 , use the gas handling subsystem 303 to pump the inside of the gastight sleeve to vacuum, and then start operating the cryogenic cooling modules . Alternatively, if the gastight connection 908 allows temporarily withdrawing the cryogenic cooling apparatus from the vacuum chamber 101 , one may do so for the duration of the UHV baking while the cooling modules are already inside , and then move the cryogenic cooling apparatus back into its operating position when the UHV baking has been completed .

[0098] Additionally, or alternatively, the approach explained above allows swapping a piece of hardware located inside the sleeve without having to warm up and open the whole cryostat . This makes it possible to e . g . take out some hardware for servicing and / or to replace some hardware if it is malfunctioning or if a corresponding piece of hardware but with different performance specifications is needed .

[0099] A large system where cryogenically cooled temperatures are used may comprise one or more vacuum chambers and one or more sleeve-enclosed refrigerator systems , coupled to each other in various ways .

[0100] It is obvious to a person skil led 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. A cryogenic cooling system, comprising:- a vacuum chamber (101) ,- inside said vacuum chamber (101) , a plurality of thermal stages (105, 106, 107, 503, 504) configured to be cooled to respective temperatures during operation, wherein the vacuum inside the vacuum chamber (101) serves to provide thermal insulation between the thermal stages (105, 106, 107, 503, 504) inside the vacuum chamber,- a gastight sleeve (301, 401) , at least a portion of which protrudes in the vacuum chamber (101) , and- inside said gastight sleeve (301, 401) , one or more cooling stages (501, 502) of a first cooling module (302) ; characterized in that the cryogenic cooling system comprises :- a second cooling module, at least a first part (505, 506) of which is inside said gastight sleeve (301, 401) ,- a first thermally conductive coupling (508, 509, 510) between a first cooling stage (501, 502) among said cooling stages and said first part (505, 506) of said second cooling module that is inside said gastight sleeve (301, 401) , and- a second thermally conductive coupling (515, 516, 521) between a first thermal stage (105, 106, 107) among said plurality of thermal stages and a respective part (304, 305) of said second cooling module.

2. A cryogenic cooling system according to claim 1, wherein said first cooling module (302) is a mechanical refrigerator.

3. A cryogenic cooling system according to any of claims 1 or 2, wherein said second cooling module is a dilution refrigerator, parts of whichcomprise :- a still (305) ,- a mixing chamber (304) ,- an inbound line (505) for making operating fluid flow in a direction towards said mixing chamber (304) , and- an outbound line (506) for making operating fluid flow in a direction outwards from said mixing chamber (304) .

4. A cryogenic cooling system according to claim 3, wherein:- said first thermal stage is a mixing chamber flange (107) of said cryogenic cooling system, and- said second thermally conductive coupling is a coupling (515) between said mixing chamber (304) and said mixing chamber flange (107) .

5. A cryogenic cooling system according to claim 4, comprising a third thermally conductive coupling (516) between said still (305) and a still flange (105) that constitutes one thermal stage among said plurality of thermal stages, different from said mixing chamber flange.

6. A cryogenic cooling system according to any of claims 3 to 5, wherein said mixing chamber (304) is outside said gastight sleeve (301) .

7. A cryogenic cooling system according to claim 6, wherein also said still (305) is outside said gastight sleeve (301) .

8. A cryogenic cooling system according to any of claims 3 to 5, wherein said mixing chamber (304) and said still (305) are inside said gastight sleeve (401) .

9. A cryogenic cooling system according to any of claims 1-5 or 8, comprising a detachable attachment of said first cooling module (302) and said second cooling module in said gastight sleeve (401) for selectively removing said first cooling module (302) and said second cooling module from said gastight sleeve (401) without compromising vacuum in said vacuum chamber (101) .

10. A cryogenic cooling system according to any of claims 3 to 9, wherein the cryogenic cooling system comprises, inside said gastight sleeve (301, 401) one or more thermally conductive couplings (507, 508, 509, 510) between said inbound line (505) and said first cooling module (302) for cooling the operating fluid flowing through said inbound line (505) during operation.

11. A cryogenic cooling system according to any of claims 3 to 9, wherein the cryogenic cooling system comprises, inside said gastight sleeve (301, 401) one or more thermally conductive couplings (511, 512, 513, 514, 704, 705, 706) between said inbound line (505) and said outbound line (506) for exchanging heat between operating fluid flowing in said inbound line (505) and operating fluid flowing in said outbound line (506) .

12. A cryogenic cooling system according to claim 11, wherein said one or more thermally conductive couplings (511, 512, 513, 514, 704, 705, 706) between said inbound line (505) and said outbound line (506) comprise at least one of: a step heat exchanger, a counterflow heat exchanger.

13. A cryogenic cooling system according to any of claims 3 to 12, wherein at least a part of said outbound line (506) consists of a portion of freespace (701, 702, 703) inside said gastight sleeve (301, 401) .

14. A cryogenic cooling system according to any of the preceding claims, comprising one or more filler pieces made of a thermally insulating material and filling a respective part of free space inside said gastight sleeve (301, 401) .

15. A cryogenic cooling system according to any of the preceding claims, comprising at least one of :- a fourth thermally conductive coupling (518) between a first, warmer cooling stage (501) among said cooling stages of the first cooling module (302) and a respective thermal stage (503) among said plurality of thermal stages,- a fifth thermally conductive coupling (517) between a second, colder cooling stage (502) among said cooling stages of the first cooling module (302) and a respective thermal stage (504) among said plurality of thermal stages.

16. A cryogenic cooling system according to claim 15, wherein at least one of said fourth (518) and fifth (517) thermally conductive couplings is comprised in the cryogenic cooling system and is a controllable coupling applicable of being selectively made thermally conductive and thermally insulating.

17. A cryogenic cooling system according to any of the preceding claims, comprising a third cooling module (108) outside said gastight sleeve (301, 401) , coupled to cool at least one of said plurality of thermal stages.

18. A cryogenic cooling system according to any of the preceding claims, comprising a gas handlingsystem (303) configured to pump gaseous substances out of said gastight sleeve (301, 401) .

19. A cryogenic cooling apparatus, comprising a first cooling module (302) and a second cooling module; characterized in that the cryogenic cooling apparatus comprises :- a gastight sleeve (401) enclosing said first cooling module (302) and said second cooling module,- outside said gastight sleeve (401) , a gastight coupling interface for gastightly connecting (908) said gastight sleeve (401) to an opening in an external vacuum chamber (101) in a way that makes at least a portion of the gastight sleeve (401) protrude in the vacuum chamber (101) , and- thermal coupling means for thermally coupling predetermined parts of said first and second cooling modules to respective thermal stages inside said external vacuum chamber once the gastight sleeve is in place protruding in the vacuum chamber (101) , wherein the vacuum inside the vacuum chamber (101) serves to provide thermal insulation between the thermal stages inside the vacuum chamber.

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