Still chamber, dilution refrigerator, system and method

Abstract

Disclosed is a still chamber for a dilution refrigerator. The still chamber comprises a base region for accommodating a liquid mixture of helium-3 and helium-4, an outlet for directing helium vapor evaporated from the liquid mixture into a still pumping line and one or more cooling elements between the base region and the outlet for recondensing helium-4 from the helium vapor.

Description

[0001] Still chamber, dilution refrigerator, system and method

[0002] FIELD

[0003] The present invention relates to dilution refrigeration .

[0004] BACKGROUND

[0005] A dilution refrigerator, which may here refer to a helium-3 (He-3 ) / helium-4 (He-4 ) dilution refrigerator in particular, is a cryogenic device that can provide continuous cooling at temperatures below 1 Kelvin, for example to a temperature of 10-100 mK or below.

[0006] A still chamber forms an essential component of the dilution refrigerator since it is there where removal of helium-3 from a He-3 / He-4-mixture takes place to facilitate cooling at a mixing chamber of the dilution refrigerator . Improving the working of the still chamber is sought to yield benefits in the overall performance of the dilution refrigerator .

[0007] SUMMARY

[0008] According to a first aspect, a still chamber for a dilution refrigerator comprises a base region for accommodating a liquid mixture of helium-3 and helium-4 and an outlet for directing helium vapor evaporated from the liquid mixture into a still pumping line . Advantageously, the still chamber may comprise one or more cooling elements between the base region and the outlet for recondensing helium-4 from the helium vapor . This allows increasing the relative amount of helium-3 in the helium vapor with respect to helium-4.

[0009] In an embodiment, the one or more cooling elements comprises one or more openings for recondensing helium-4 from the helium vapor passing through the one or more openings . This allows the cooling to be effected to the helium vapor while minimizing the need for redirection of flow of the helium vapor from the base region to the outlet .

[0010] In an embodiment, the one or more cooling elements comprises one or more cooling grids comprising the one or more openings . This allows an increased surface area for heat exchange between the one or more cooling elements and the helium vapor while minimizing the need for redirection of flow of the helium vapor from the base region to the outlet .

[0011] In an embodiment, the one or more cooling elements comprises one or more tubular conduits defining the one or more openings . This allows an increased surface area for heat exchange between the one or more cooling elements along a flow direction of the helium vapor .

[0012] In an embodiment, the one or more cooling elements are arranged for recondensing helium-4 at a temperature of 0.7-1. 2 Kelvin, preferably 0.7-0, 9 Kelvin.

[0013] In an embodiment, the still chamber is arranged to accommodate the liquid mixture at the base region at a temperature of 0. 6-1.5 Kelvin.

[0014] In an embodiment, the one or more cooling elements comprise one or more electric coolers, in particular one or more normal-superconducting tunnel junctions .

[0015] In an embodiment, the still chamber com-prises a helium-3 circulation channel for facilitating transfer of helium-3 for cooling the one or more cooling elements .

[0016] In an embodiment, the one or more cooling elements have a first surface facing the base region and a second surface facing the outlet and the one or more cooling elements are arranged to transfer heat from the second surface to the first surface . This allows the one or more cooling elements to recondense helium-4 from the helium vapor while heating the liquid mixture of helium-3 and helium-4 for increasing evaporation. In an embodiment, the still chamber comprises one or more temperature sensors for measuring temperature at the one or more cooling elements and / or at the liquid mixture .

[0017] In an embodiment, the still chamber comprises one or more heaters coupled to the one or more cooling elements for directly heating the one or more cooling elements . This allows the operation of the still chamber to be continued in a situation where the ambient temperature in the still chamber has decreased for mitigating flow of helium vapor to the outlet .

[0018] In an embodiment, the still chamber is arranged to recondense the helium-4 from the helium vapor back into the liquid mixture . This allows increasing the relative amount of helium-4 in the liquid mixture with respect to helium-3, thereby facilitating improved cooling of the dilution refrigerator with the still chamber .

[0019] According to a second aspect, a dilution refrigerator comprises the still chamber according to the first aspect or any of its embodiments, including any combination of embodiments .

[0020] According to a third aspect, a system comprises the dilution refrigerator according to the second aspect claim and an external cooling system arranged to be coupled to the dilution refrigerator for cooling the one or more cooling elements . The one or more cooling elements can thus be cooled independently from the internal cooling cycle of the dilution refrigerator, e . g. by a separate helium-3 circulation. External cooling as referred herein does not involve helium flow, including the helium vapor and the liquid mixture, through the still chamber as a heat transfer medium but is instead provided separately from the internal cooling of the dilution refrigerator .

[0021] According to a fourth aspect a method for a dilution refrigerator is disclosed. The method comprises, within a still chamber of the dilution refrigerator, cooling helium vapor evaporating from a liquid mixture of helium-3 and helium-4 for recondensing helium-4 from the helium vapor . This can be performed using an external cooling separate from the internal cooling of the dilution refrigerator .

[0022] In effect, the present solutions allow distilling helium-4 of the liquid mixture by successive evaporation and condensation. Helium-3 of the liquid mixture is conversely evaporated and removed from the liquid mixture .

[0023] The present solutions can allow increased still pressures for the dilution refrigerator . In general, such increased pressures could lead to increased relative amount of helium-4 vapor in the helium vapor when the still chamber is pumped and also to stability and other problems in the dilution refrigerator . Typically, in dilution refrigerators still temperature and pressure is set as high as possible for a high-flow situation, as long as system stays stable and cooling power does not weaken. In practice limit has been somewhere between 0.9 - 1 K where relative amount of helium-4 vapor in the helium vapor stays below 25% at the outlet of the still chamber .

[0024] The solutions allow increasing pressure at the still chamber while maintaining a decreased relative amount of helium-4 vapor in the helium vapor by distilling helium-4 out of the helium vapor at the outlet of still chamber . One or more additional cooling elements such as a cold distillation surface can be arranged at the still chamber for condensing more helium-4 than helium-3. Enthalpy of incoming circulating helium-3 cannot necessarily be used effectively for this without evaporating helium-3 and thus increasing risk of unstable flow. One implementation is to have the cooling elements cooled by an external helium-3 circulation and a pot to provide energy to condense helium-4. The present solutions all increase of temperature at the still chamber higher than typically, i . e . without the cooling element (s) . For example, the temperature of the liquid mixture may be increased above IK, for example to 1.2K or above . Energy for cooling the cooling element ( s) can be arranged to be provided from outside the still chamber and from outside the dilution circulation of the dilution refrigerator, in particular .

[0025] The present solutions allow higher dilution circulation of helium-3 without need to increase the size of the pump (s) for maintaining the dilution circulation excessively.

[0026] It is to be understood that the aspects and embodiments described above may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment of the invention.

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings, which are included to provide a further understanding and constitute a part of this specification, illustrate examples and together with the description help to explain the principles of the disclosure . In the drawings :

[0029] Fig. 1 schematically illustrates a still chamber according to an example,

[0030] Fig. 2 schematically illustrates an arrangement with a still chamber according to another example,

[0031] Fig. 3 schematically illustrates a system according to an example, and

[0032] Fig. 4 illustrates a method according to an example .

[0033] Like references are used to designate equivalent or at least functionally equivalent parts in the accompanying drawings . DETAILED DESCRIPTION

[0034] The detailed description provided below in connection with the appended drawings is intended as a description of examples and is not intended to represent the only forms in which the example may be constructed or utilized. However, the same or equivalent functions and structures may be accomplished by different examples .

[0035] Fig. 1 shows an example of a still chamber 100. The still chamber is for a dilution refrigerator and is arranged to be coupled to a mixing chamber of the dilution refrigerator for facilitating cooling of the mixing chamber . The dilution refrigerator follows a cooling cycle, which may herein be also referred as the dilution circulation, where helium-3 is pumped through the dilution refrigerator, typically together with some amount of helium-4. In the dilution circulation, gaseous helium-3, and typically some amount of helium-4, is pumped into the dilution refrigerator, where it is first cooled and condensed into liquid form. In the mixing chamber of the dilution refrigerator, helium mixture forms two phases : a concentrated phase of helium-3 and a dilute phase of a mixture of helium-3 and helium-4, helium-4 dominating in the latter . As helium-3 is pumped into the dilute phase, the mixing chamber is cooled. Due to finite solubility of helium-3 into helium-4, helium-3 needs to be removed from the dilute phase to allow more helium-3 to be pumped into the mixing chamber for cooling the mixing chamber . Helium-3 may be removed from the dilute phase by providing a liquid mixture 140 of helium-3 and helium-4 from the dilute phase via a fluid connection to the still chamber and evaporating helium vapor from the liquid mixture in the still chamber . This can be done by heating the liquid mixture . Since the vapor pressure of helium-3 is larger than the helium pressure of helium-4, evaporation of helium-3 dominates over evaporation of helium-4. The helium vapor may be directed from the still chamber into a still pumping line, thereby restarting the dilution circulation as helium-3 (along with some helium-4 ) is again pumped in gaseous form back to the dilution refrigerator .

[0036] The still chamber 100 has a base region 110 for accommodating the liquid mixture 140 of helium-3 and helium-4. The base region may simply be provided as a bottom part of the still chamber, for example as a bottom part of a housing of the still chamber. The base-region may be bowl-shaped. The still chamber, or the housing thereof, may have an elongated shape with height 102 larger than width 104, e . g. the maximum width, for example two or more times as large . This allows increasing the relative amount of helium-3 in the helium vapor at the outlet with respect to helium-4. Moreover, with such a construction the still chamber can be provided without excessively impeding pumping.

[0037] The still chamber 100 may have an inlet 112 for receiving helium-3 from the mixing chamber of the dilution refrigerator, e . g. as a part of a dilute mixture of helium-3 and helium-4 from the mixing chamber . Along helium-3, some helium-4 may thus also flow from the mixing chamber during the dilution circulation. The inlet may be arranged at the base region.

[0038] The still chamber 100 may also have an outlet 120 for directing helium vapor evaporated from the liquid mixture into a still pumping line, thereby continuing the dilution circulation with a new cycle . The still chamber may be arranged for coupling to a gas handling system (GHS) for the dilution refrigerator through the still pumping line, wherein the GHS may comprise one or more pumps for pumping helium-3 in gaseous form from the still chamber back to the dilution refrigerator . The still chamber may thus be pumped with one or more pumps of the GHS arranged downstream from the outlet .

[0039] The still chamber 100 may comprise one or more cooling elements 130 (herein also "the cooling elements") between the base region 110 and the outlet 120 for recondensing helium-4 from the helium vapor . The cooling elements can thus act as condensing surfaces . Fig. 1 illustrates the principle of recondensation as the liquid mixture is evaporated to introduce both He-3 vapor 142 (dotted lines) and He-4 vapor 144 (dash-dotted lines) into the mixing chamber . As the boiling point of He-3 is lower and vapor pressure higher than the corresponding values for He-4, generation of He-4 vapor 144 is relatively weaker and the He-4 vapor is also more susceptible to recondensing to liquid form than the He-3 vapor 142 when subj ected to cooling from the one or more cooling elements 130. Accordingly, the one or more cooling elements situated between the base region and the outlet and arranged to cool the still chamber from within allow the share of helium-3 in the helium vapor directed into the still pumping line from the still chamber to be increased relative to the share of helium-4. The solution allows the helium vapor to be brought into direct contact with the cooling elements within the still chamber .

[0040] Helium-4 may recondense into helium-4 droplets 146. Accordingly, the cooling elements 130 may comprise one or more protrusions for inducing formation of droplets of helium-4 from the helium vapor. The surface of the liquid mixture 140 is indicated in Fig. 1 as a liquid-gas interface 148.

[0041] The cooling elements 130 may comprise two or more cooling elements in cascade (along the height dimension of the still chamber) , where subsequent cooling elements may be at same or at different temperatures . The helium-4 from the helium vapor may be recondensed back into the liquid mixture 140 and the still chamber may be arranged accordingly. As such, the helium-4 is removed from the dilution circulation and the still chamber may be arranged for maximizing helium-3 circulation from the still chamber . While not necessary for all embodiments, the still chamber may comprise one or more guiding surfaces 150 for directing recondensed helium-4 to the liquid mixture 140. The one or more cooling elements 130 may also be shaped for directing He-3 towards the outlet 120 and / or He-4 towards the liquid mixture 140. The still chamber may be arranged for guiding recondensed helium-4 to a periphery of the liquid mixture, i . e . away from the centre of the liquid mixture, for example as illustrated in Fig. 1. This allows one or more regions of evaporation and one or more regions of recondensation to be formed within the still chamber, where the ratio of recondensation per evaporation of helium-4 in the one or more regions of recondensation is higher than the ratio of recondensation per evaporation of helium-4 in the one or more regions of evaporation, e . g. so that evaporation of helium-4 dominates over recondensation of helium-4 at the centre of the still chamber and recondensation of helium-4 dominates over evaporation of helium-4 along a periphery of the still chamber . For this purpose, the still chamber may comprise one or more dividing surfaces 152 for separating the liquid mixture, e . g. arranged to separate the centre of the liquid mixture from the periphery of the liquid mixture at the surface of the liquid mixture while allowing fluid connection between the centre of the liquid mixture and the periphery of the liquid mixture below the surface of the liquid mixture . Any or all of the one or more guiding surfaces 150 may also be or arranged as a part of the one or more dividing surfaces 152. For example, the one or more guiding surfaces may comprise one or more first guiding surfaces joined to or arranged as a part of the cooling elements 130 and / or one or more second guiding surfaces joined to or arranged as a part of the one or more dividing surfaces 152. The one or more first guiding surfaces may thus be arranged above the one or more second guiding surfaces within the still chamber, for example directly above .

[0042] In general, the base region 110 may have a collection area for collecting recondensed helium-4, e . g. as helium-4 droplets . The collection area may be partially separated from an evaporation area of the base region for evaporating helium-3 from the liquid mixture in particular, e . g. by the one or more dividing surfaces 152 as described above . The collection area and the evaporation area may be separated by one or more fluid flow constrictions 160, such as superleaks and / or other constrictions, to limit flow of helium-3 to the collection area . In the examples above, the one or more dividing surfaces 152 may thus form one or more superleaks to limit flow of helium-3 from the centre of the liquid mixture to the periphery of the liquid mixture . These solutions can be used for enhancing distillation of helium-4 .

[0043] The one or more cooling elements 130 may comprise one or more openings 132, which can be provided as through-holes . This allows recondensing helium-4 from the helium vapor passing through the one or more openings (herein also "the openings") . Any or all of the openings may be arranged to allow direct passage for the helium vapor from the liquid mixture 140 to the outlet 120. The cooling elements may comprise one or more tubular conduits defining the openings so that the openings can be provided with an elongated path through, thereby increasing the time each of the openings cools the helium vapor, thus allowing improved recondensation of helium-4. Any or all of the one or more tubular conduits may have a height that is longer than a width of the conduit, e . g. a maximum width of the conduit, for example two or more times the width.

[0044] The cooling elements 130 may also comprise or be arranged as one or more cooling grids (herein also "the cooling grids") , which comprise the openings . The openings can thus be arranged in grid-form, in two-dimension . The openings may be arranged in the cooling grids regularly or irregularly, for example in rectangular pattern or in honeycomb pattern. The pattern may be in two-dimensions, for example on a horizontal level . The one or more cooling grids may also comprise the one or more openings, as described above, thereby defining a grid or grids of tubular conduits . The cooling grids may also have a grid pattern in the height dimension of the still chamber .

[0045] The one or more cooling elements 130 may be arranged for recondensing helium-4 at various temperatures but in particular at a temperature of 0.7-1. 2 Kelvin, preferably 0.7-0. 9 Kelvin. The one or more cooling elements may thus be arranged to have this as a surface temperature during the dilution circulation. Correspondingly, the still chamber may be arranged to accommodate the liquid mixture at the base region at a temperature of 0. 6-1.5 Kelvin and the dilution refrigerator may be arranged accordingly for performing the dilution circulation. The dilution refrigerator, or the still chamber, may comprise one or more heaters (not illustrated) for heating the liquid mixture at the still chamber for evaporating the helium vapor . The dilution refrigerator, or the still chamber, may also comprise one or more heaters (not illustrated) coupled to the one or more cooling elements for directly heating the one or more cooling elements . This allows the temperature of the cooling elements to be raised above the ambient temperature at the location of the cooling elements, which in turn can be used in a situation where the condensation of the helium vapor is too strong, for example leading to increased recondensation of helium-3 as well as helium-4 . The still chamber may comprise one or more temperature sensors (not illustrated) for measuring temperature, in particular at the one or more cooling elements and / or at the liquid mixture, thereby allowing improved control of the evaporation of the helium vapor . A still arrangement for the dilution refrigerator may comprise the still chamber 100 together with the one or more heaters for heating the liquid mixture at the still chamber for evaporating the helium vapor, which one or more heaters can also be arranged outside the still chamber 100.

[0046] Fig. 2 schematically illustrates an arrangement 200 with a still chamber 100 according to another example, which still chamber may involve any of the features described above . The arrangement is arranged for a first helium flow 212 comprising helium-3 to be provided to the mixing chamber 210 for cooling the mixing chamber . The arrangement may further be arranged for a second helium flow 214 comprising helium-3 to be provided from the mixing chamber 210 to the inlet 112 of the still chamber 100 and for directing helium vapor evaporated from the liquid mixture into a still pumping line through the outlet 120. The arrangement may be configured for thermalizing the first helium flow 212 with the still chamber 100. The arrangement may comprise one or more heat exchangers for this purpose .

[0047] In addition, the one or more cooling elements 130 here comprise or consist of one or more electric coolers (herein also "the electric coolers") . Electric coolers are solid-state coolers operating electrically and therefore do not provide their cooling effect via cryofluids . However, they still may be cooled to their operating temperature, for example with a cryofluid. The electric coolers may comprise one or more tunnel junctions such as normal (N) -superconducting (S) tunnel junctions, which may involve an insulating interface and / or a constriction ( I ) in between, thereby being referred to also as NIS-j unctions . The electric coolers can thereby comprise or consist of NIS-coolers .

[0048] Importantly, the one or more cooling elements may have a first surface 232 facing the base region 110, or the liquid mixture 140 in particular, and / or a second surface 234 facing the outlet 120. The one or more cooling elements, in particular the electric coolers, can be then arranged to transfer heat from the second surface to the first surface . For example, a normal-conducting side of a normal-superconducting tunnel junction may be facing the outlet, thereby corresponding to the second surface 234, and a superconducting side of the normal-superconducting tunnel junction may be facing the base region, or the liquid mixture 140 in particular, thereby corresponding to the first surface 232. The cooling elements 130 may thus have a cooling effect 244 between said cooling elements and the outlet 120 and a heating effect 242 between said cooling elements and the base region 110, or the liquid mixture 140 in particular . The still chamber may be arranged for the heating effect to extend all the way to the liquid mixture 140. In these and in other cases, the one or more cooling elements may comprise or consist of one or more cooling grids and / or one or more cooling tubes .

[0049] Fig. 2 also illustrates how the helium vapor may in general evaporate from the liquid mixture 140. In the liquid mixture, helium-4 proportionally dominates helium-3 and the relative proportion of helium-3 may be well below 10% in the liquid mixture, for example 1 percent, as illustrated. In the helium vapor, on the other hand, helium-3 proportionally dominates helium-4 and the relative proportion of helium-3 may be 90% or more in the helium vapor at the outlet 120.

[0050] The dilution refrigerator, or the arrangement 200 in particular, may optionally comprise a Joule-Thom-son stage, where Joule-Thomson (JT) expansion may take place facilitating condensation of helium-3 before it enters the mixing chamber 210 of the dilution refrigerator . This stage may involve heat exchange with the liquid mixture and can therefore be arranged in conjunction with the still chamber 100. The Joule-Thompson stage (and / or one or more heat exchangers) may be provided at the level of the still chamber 100 and / or between the still chamber 100 and the mixing chamber .

[0051] Fig. 3 illustrates a system 300 according to an example . The system may include the still chamber 100 and any combination of features according to any of the examples above . However, the still chamber 100 may be arranged for the cooling elements to be cooled by an external cooling system 320. The external cooling system may be arranged to provide cooling, for example by cooling circulation, in particular helium-3 circulation. Such cooling circulation for the external cooling system would be arranged as a separate circulation from the dilution circulation 310, although they may share one or more pre-coolers 330 such as pulse tubes (PT) for cooling both the cooling circulation and the dilution circulation. Alternatively or additionally, the external cooling system may have one or more coolers, e . g. pulse tubes, of its own for cooling the cooling circulation instead of the dilution circulation. Cooling the cooling circulation and / or the dilution circulation may be read as cooling a cryofluid, such as helium-3, circulated in the corresponding circulation. Various cooling phases with heat exchangers 340 together with temperature values (in Kelvins) and pressure values (in millibars) in the corresponding phases are illustrated (the directions of the arrow illustrating the direction of cryofluid flow, e . g. helium-3 flow, within the corresponding circulation) but it should be understood that details for both the dilution circulation and the external cooling system may be arranged in various ways known to a person skilled in the art of dilution refrigeration. As the dilution circulation can typically involve a JT stage, also the external cooling system and the helium-3 circulation thereof may include a JT stage 350 before the external cooling system, and the helium-3 circulation thereof, is coupled to the cooling elements 130 for cooling the cooling elements .

[0052] The still chamber 100 may be arranged to be coupled to an external cooling system, in particular to the helium-3 circulation, for cooling the one or more cooling elements 130. For this purpose, the still chamber 100 may comprise one or more cryofluid circulation channels, such as cooling tubes, for facilitating transfer of cryofluid for cooling the cooling elements . The still chamber may have one or more feedthrough regions for input and output of the cryofluid circulation channels . The cooling elements 130 can be maintained at a temperature lower, e . g. 0 . 8K, than ambient temperature above, e . g. 1 . OK, and / or below the cooling elements, e . g. 1.2K at the liquid mixture .

[0053] As an example, the dilution circulation 310 may be arranged with a flow of at least 5 mmol / s . A single pump for pumping the still chamber may be used for this purpose . The external helium-3 circulation may be arranged with a flow of over 5 mmol / s, for example at 9 mmol / s or more .

[0054] Fig. 3 also illustrates the cooling elements 130 with the cooling grids, which may have a grid pattern in the height dimension of the still chamber .

[0055] In general, the dilution refrigerator as disclosed herein, such as a He-3 / He-4 dilution refrigerator, may be a so-called dry dilution refrigerator, where helium-3 may be pre-cooled with a cryocooler, or a so-called wet dilution refrigerator, where helium-3 is pre-cooled with a cryofluid, for example helium-4, which may be in liquid form, instead. The dilution refrigerator can be arranged to be operated as a continuous dilution refrigerator .

[0056] The dilution refrigerator comprises the still chamber 100 and, in addition, may comprise any components of a dilution refrigerator as known to a person skilled in the art of dilution refrigerator. In particular, the dilution refrigerator may comprise the mixing chamber, which may be coupled to the still chamber with a fluid connection for helium-3 circulation for the dilution circulation. The dilution refrigerator may comprise an operation space, such as an experimental space, for receiving one or more devices to be cooled. The mixing chamber may be coupled to the operation space for cooling the operation space . The dilution refrigerator may also comprise the still arrangement as mentioned above . In particular, the dilution refrigerator may comprise the one or more heaters for heating the liquid mixture at the still chamber for evaporating the helium vapor . The dilution refrigerator may comprise various other units, such as heat exchangers, but these are not described here further as the present solution may be used with any range of different types of dilution refrigerators available to a person skilled in the art of dilution refrigeration.

[0057] Fig. 4 illustrates a method 400 for a dilution refrigerator, such as the dilution refrigerator described above, according to an example . The method may involve any features, actions or solutions described above .

[0058] The method comprises, within a still chamber of the dilution refrigerator, e . g. the still chamber 100 as described above, cooling 410 helium vapor evaporating from a liquid mixture of helium-3 and helium-4. This is done so as to cause recondensing 420 of helium-4 from the helium vapor . As described above, the helium-4 may be recondensed back into the liquid mixture . The helium vapor may be cooled, as described above, with one or more cooling elements within still chamber, in particular situated between the liquid mixture and an outlet of the still chamber . The one or more cooling elements may correspond to any of the cooling elements described above . The one or more cooling elements may be cooled externally from the still chamber, or externally from the dilution refrigerator altogether. For this purpose, the external cooling system as described above may be used .

[0059] In all the indicated solutions, one or more controllers (herein also "the controllers") , such as PID-controllers , may be used for adjusting the temperature of the cooling elements . The controllers may be coupled to the one or more temperature sensors allowing the controllers to utilize measurements for temperature at the one or more cooling elements and / or at the liquid mixture for adjusting the temperature of the cooling elements . The controllers can be arranged to adjust the temperature of the cooling elements, e . g. with the one or more heaters coupled to the one or more cooling elements for directly heating the one or more cooling elements, and / or the temperature of the liquid mixture, e . g. with the one or more heaters for heating the liquid mixture at the still chamber for evaporating the helium vapor . Accordingly, the controllers may be arranged to optimize the temperature of the cooling elements and / or the temperature at the liquid mixture .

[0060] The different functions discussed herein may be performed in a different order and / or concurrently with each other . Although the subj ect matter has been de-scribed in language specific to structural features and / or acts, it is to be understood that the subj ect matter for which protection is sought is defined in the appended claims and not necessarily limited to the specific features or acts described above . Any example disclosed herein may be combined with another example unless explicitly disallowed .

[0061] The term ' comprising' is used herein to mean including the method, blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements .

[0062] Numerical descriptors such as 'first' , 'second' , and the like are used in this text simply as a way of differentiating between parts that otherwise have similar names . The numerical descriptors are not to be construed as indicating any particular order, such as an order of preference, manufacture, or occurrence in any particular structure .

[0063] Expressions such as 'plurality' are in this text to indicate that the entities referred thereby are in plural, i . e . the number of the entities is two or more .

Claims

CLAIMS1. A still chamber for a dilution refrigerator, comprising :- a base region for accommodating a liquid mixture of helium- 3 and helium-4 ;- an outlet for directing helium vapor evaporated from the liquid mixture into a still pumping line; and- one or more cooling elements between the base region and the outlet for recondensing helium-4 from the helium vapor .

2. The still chamber of claim 1, wherein the one or more cooling elements comprises one or more openings for recondensing helium-4 from the helium vapor passing through the one or more openings .

3. The still chamber of claim 2, wherein the one or more cooling elements comprises one or more cooling grids comprising the one or more openings .

4. The still chamber of claim 2 or 3, wherein the one or more cooling elements comprises one or more tubular conduits defining the one or more openings .

5. The still chamber of any preceding claim, wherein the one or more cooling elements are arranged for recondensing helium-4 at a temperature of 0.7-1. 2 Kelvin, preferably 0.7-0. 9 Kelvin.

6. The still chamber of any preceding claim, arranged to accommodate the liquid mixture at the base region at a temperature of 0. 6-1.5 Kelvin.

7. The still chamber of any preceding claim, wherein the one or more cooling elements comprise one or more electric coolers .

8. The still chamber of claim 7 , wherein the one or more electric coolers comprise one or more normal- superconducting tunnel junctions .

9. The still chamber of any preceding claim, comprising a helium-3 circulation channel for facilitatingtransfer of helium-3 for cooling the one or more cooling elements .

10. The still chamber of any preceding claim, wherein the one or more cooling elements have a first surface facing the base region and a second surface facing the outlet and the one or more cooling elements are arranged to transfer heat from the second surface to the first surface .

11. The still chamber of any preceding claim, comprising one or more temperature sensors for measuring temperature at the one or more cooling elements and / or at the liquid mixture .

12. The still chamber of any preceding claim, comprising one or more heaters coupled to the one or more cooling elements for directly heating the one or more cooling elements .

13. The still chamber of any preceding claim, arranged to recondense the helium-4 from the helium vapor back into the liquid mixture .

14. A dilution refrigerator comprising the still chamber of any preceding claim.

15. A system comprising the dilution refrigerator of claim 14 and an external cooling system arranged to be coupled to the dilution refrigerator for cooling the one or more cooling elements .

16. Method for a dilution refrigerator, the method comprising, within a still chamber of the dilution refrigerator, cooling helium vapor evaporating from a liquid mixture of helium-3 and helium-4 for recondensing helium-4 from the helium vapor .

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