Abstract:
A dilution refrigerator includes a still; a mixing chamber; a pump to pump coolant from the still through a still outlet port and a heat exchanger connected between the still and mixing chamber whereby coolant flows under the assistance of the pump from the still to the mixing chamber and from the mixing chamber to the still through respective first and second adjacent paths in the heat exchanger. An access path extends to the mixing chamber. A probe is provided for insertion along the access path, the probe having a displacer which substantially fills the cross-section of the access path in use. Any coolant from the mixing chamber which flows along the access path past the displacer can flow from the access path into the still. The still outlet port is separate from the access path.

Description:
FIELD OF THE INVENTION 
     The invention relates to a dilution refrigerator. 
     DESCRIPTION OF THE PRIOR ART 
     Dilution refrigerators are used for achieving ultra low temperatures for experiments in the millikelvin temperature range. A typical dilution refrigerator includes a still, a mixing chamber, and a heat exchanger connected between the still and mixing chamber whereby coolant flows from the still to the mixing chamber and from the mixing chamber to the still through respective first and second adjacent paths in the heat exchanger. Examples of known dilution refrigerators are described in U.S. Pat. No. 5,189,880, “A Simple Dilution Refrigerator” by J. L. Levine, The Review of Scientific Instruments, Vol. 43, Number 2, February 1972, pages 274-277, “Fully portable, highly flexible dilution refrigerator systems for neutron scattering”, Hilton et al, Revue de Physique Appliquee, Vol. 19, No. 9, pages 775-777, and GB-A-2166535. 
     Typically, such a dilution refrigerator uses  3 He/ 4 He and makes use of the fact that when a mixture of these two stable isotopes of helium is cooled below its tri-critical temperature, it separates into two phases. The lighter “concentrated phase” is rich in  3 He and the heavier “dilute phase” is rich in  4 He. Since the enthalpy of the  3 He in the two phases is different, it is possible to obtain cooling by “evaporating” the  3 He from the concentrated phase into the dilute phase. 
     The properties of the liquids in the dilution refrigerator are described by quantum mechanics. However, it is useful to regard the concentrated phase of the mixture as liquid  3 He, and the dilute phase as  3 He gas. The  4 He which makes up the majority of the dilute phase is inert, and the  3 He “gas” moves through the liquid  4 He without interaction. This gas is formed in the mixing chamber at the phase boundary, in a process analogous to evaporation at a liquid surface. This process continues to work even at the lowest temperatures because the equilibrium concentration of  3 He in the dilute phase is still finite, even as the temperature approaches absolute zero. 
     In a continuously operating system, the  3 He must be extracted from the dilute phase (to prevent it from saturating) and returned into the concentrated phase, keeping the system in a dynamic equilibrium. The  3 He is pumped away from the liquid surface in the still, which is typically maintained at a temperature of 0.6 to 0.7 K by a small heater. At this temperature the vapour pressure of the  3 He is about 1000 times higher than that of  4 He, so  3 He evaporates preferentially. 
     The concentration of  3 He in the dilute phase in the still therefore becomes lower than it is in the mixing chamber, and the osmotic pressure difference drives  3 He to the still. The  3 He leaving the mixing chamber is used to cool the returning flow of concentrated  3 He in the heat exchanger. A room temperature vacuum pumping system draws the  3 He gas from the still, and compresses it to a pressure of a few hundred millibar. The gas is then returned to the refrigerator. 
     In 1987, a modified dilution refrigerator was described which allowed the investigation of samples in high magnetic fields. See “Novel Top-Loading 20 mK/15T Cryomagnetic System” by P. H. P. Reinders et al, Cryogenics 1987 Vol. 27 December, pages 689-692. This type of dilution refrigerator is now known as a top loading dilution refrigerator. 
     Top loading dilution refrigerators have been developed for simple and rapid sample changing for millikelvin experiments without the need to warm up the main cryostat. A common approach is to have a top loading probe which is loaded into the cryostat through a room temperature vacuum lock. The cryostat is then kept at a temperature of 4.2K (or below) during this loading procedure, and the experiment or sample is mounted on the end of the probe. Using this technique, the experiment or sample can be loaded directly into the  3 He/ 4 He mixture inside the mixing chamber. Quite often, the mixing chamber has a tubular extension into the bore of a magnet, allowing samples to be run at millikelvin temperatures in high magnetic fields as described in the Reinders et al paper. Another example of a top loading dilution refrigerator is described in EP-A0675330. 
     The problem with top loading into the mixing chamber is that it is necessary to provide a clear access tube into the mixing chamber. This access tube fills up with liquid  3 He/ 4 He. It is therefore necessary to include a displacer on the probe to minimise the cross-sectional area of the liquid column in the central access tube. However, even with a displacer, there is a significant heat leak through the liquid around the displacer and this limits the base temperature. 
     In “A combined  3 He- 4 He dilution refrigerator” by V. N. Pavlov et al, Cryogenics, February 1978, pages 115-119, a route is provided to allow any coolant which flows up the access path to flow into the still. Thus when the displacer is removed, the system of heat exchangers is shunted by the access path and the refrigerator becomes a conventional  3 He circulating refrigerator. 
     In the system of Pavlov, the probe passes down the pumping line into the still. A problem with the system of Pavlov is that a film of superfluid  4 He will flow up the pumping line due to the temperature gradient (since superfluid  4 He flows from low temperature regions to high temperature regions). The film will then progress up the pumping line until it evaporates. The evaporation of  4 He impairs the cooling efficiency of the refrigerator and as a result a very powerful pump must be used. 
     Superfluid  4 He films can only have a thickness up to a fundamental limit of approximately 200 Angstroms. Therefore one approach to the problem of film flow in Pavlov would be to reduce the diameter of the pumping line. However this would then limit the diameter of the probe (since the probe must be passed down the pumping line into the still). 
     SUMMARY OF THE INVENTION 
     In accordance with the a first aspect of the present invention there is provided a top loading dilution refrigerator comprising a still; a mixing chamber; a pump for pumping coolant from the still through a still outlet port; a heat exchanger connected between the still and mixing chamber whereby coolant flows under the assistance of the pump from the still to the mixing chamber and from the mixing chamber to the still through respective first and second adjacent paths in the heat exchanger; means defining an access path extending to the mixing chamber; a probe for insertion along the access path, the probe having a displacer which substantially fills the cross-section of the access path in use; and means to allow any coolant from the mixing chamber which flows along the access path past the displacer to flow from the access path into the still, characterised in that the still outlet port is separate from the access path. 
     In accordance with a second aspect of the present invention there is provided a dilution refrigerator comprising a still; a mixing chamber; a pump for pumping coolant from the still through a still outlet port; a heat exchanger connected between the still and mixing chamber whereby coolant flows under the assistance of the pump from the still to the mixing chamber and from the mixing chamber to the still through respective first and second adjacent paths in the heat exchanger; means defining an access path extending to the mixing chamber; a probe mounted in the access path, the probe having a displacer which substantially fills the cross-section of the access path; and means to allow any coolant from the mixing chamber which flows along the access path past the displacer to flow from the access path into the still, characterised in that the still outlet port is separate from the access path. 
     We have recognised that by physically separating the still outlet port from the access path, film flow through the still outlet port can be controlled without affecting the diameter of the access path. 
     Furthermore, we have also recognised the advantages inherent in providing a route for coolant to flow from access path into the still. We accept that we cannot displace all the coolant in the access path and there will always be at least a thin film around the displacer which will transmit heat from the still to the mixing chamber. We generate a flow of  3 He atoms from the mixing chamber to the still flowing along the access path around the displacer. The heat load mechanism is complex but heat is primarily transported by the gas atoms, typically  3 He dissolved in  4 He, and convection instabilities in the liquid column. The heat flow from the still to the mixing chamber is greatly reduced (compared to a conventional static column) by this small flow from the mixing chamber to the still past the displacer. This advantage was not recognised by Pavlov et. al, who merely provided the flow route from the access path to the still to enable the refrigerator to work as a normal  3 He circulating refrigerator when the probe is removed. The flow is induced by having a connection from the still into the access path. The relative flow through the conventional flow path, compared to the access path route, depends on the relative impedance of the two routes. It is important that the bulk of the flow passes through the conventional dilution refrigerator route as this provides the cooling power, while a small flow is generated up the access path to minimise the heat leak. To control the flow through the access path, the displacer is preferably a tight fit in the access path. 
     The coolant may flow from the access path into the still via the second path in the heat exchanger. However preferably the coolant flows from the access path directly into the still. 
     The invention is applicable to several different types of top loading dilution refrigerator. For example, the Reinders et al paper discloses a dilution refrigerator with a metallic dilution unit in which the still is laterally offset from the access tube. In this case, the means to allow coolant to flow into the still will comprise a conduit extending from the access path to the still. 
     In other applications, the still and heat exchanger are mounted coaxially with the access path as, for example, in EP-A-0675330, and the means can comprise a simple aperture in the wall of the access tube (which defines the access path). 
     The aperture or conduit can communicate with the still or the second path in the heat exchanger at a point below the coolant level in the still. However preferably the coolant flows from the access path into the still at a point above the level of coolant in the still. 
     In the first aspect of the invention, the probe is inserted, in use, along the access path (typically after the refrigerator has been pre-cooled). The probe may provide experimental services to a sample which has been previously mounted (either via the access path or be some other route) in the mixing chamber. For instance the probe may comprise a drive rod which is inserted along the access path, attached to the sample in the mixing chamber, and rotated to rotate the sample in the mixing chamber. Alternatively the probe may comprise a waveguide which transmits radiation to the sample. However preferably the probe comprises a sample holding device which is inserted along the access path to introduce the sample into the mixing chamber. In this case electrical wiring for connection to the sample may extend along the sample holding device. 
     Preferably, the probe is removable from the dilution refrigerator without purging coolant and in that case, the probe further comprises a seal for sealing the probe to the refrigerator when inserted. Preferably the seal is defined by a cone shaped member, located in the dilute or concentrated mixture, which mates with a corresponding cone shaped portion on the refrigerator. 
     In the second aspect of the invention, the probe is permanently mounted in the access path and the sample is introduced to the mixing chamber via some other route. Again, the probe may be used to rotate the sample or to transmit radiation to the sample. 
     In the preferred example, the access path extends through the centre of the heat exchanger. 
     In the case of pulsed magnetic fields, it is preferable if all the components making up the still, heat exchanger and mixing chamber are made of non-metallic materials such as plastics, preferably PEEK. PEEK (polyetheretherketone) is particularly suitable because it has low diffusibility to helium gas, even at room temperature (300K) for the time periods required for conventional dilution unit leak testing. This simplifies leak testing procedures. 
     Preferably, the probe is sealed to the heat exchanger, for example by a seal comprising cooperating cone shaped members on the probe and heat exchanger. Other seals could be used such as cooperating screw shaped members. 
     Film flow may simply be restricted by providing a pumping path (which terminates at the still outlet port) with a small diameter. However this increases the fluid impedance of the pumping path which can result in a more powerful pump being required. Therefore in a preferred example a film flow restrictor is provided to restrict the flow of coolant film through the still outlet port without significantly increasing the fluid impedance presented to the pump. For example the walls defining the still outlet port may be coated with a material (such as pure Caesium) which repels the liquid coolant film. Alternatively the cross-sectional area of the pumping path may reduce to an orifice at the still outlet port. The relatively small diameter of the pumping path at the orifice restricts the film flow, but does not significantly increase the impedance of the pumping path. Preferably the length of pumping path with relatively small cross-sectional area is minimised by tapering the walls defining the orifice to a knife-edge. In a further alternative, a film burner may be provided at the still outlet port. An example of a suitable film burner is described by G. Frossati in J. de Physique  39  (C 6 ), 1578 (1978); and J. Low Temp. Phys. 87, 595 (1992). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of a dilution refrigerator incorporating a probe according to the invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic, partially cut away view of the dilution refrigerator situated within a cryostat containing a magnet; 
     FIG. 2 illustrates the components of the dilution refrigerator in more detail; 
     FIG. 3 illustrates the dilution refrigerator shown in FIG. 2 with a probe inserted; and 
     FIG. 4 is a schematic view of an alternative dilution refrigerator with a probe inserted. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The apparatus shown in FIG. 1 comprises a cryostat  1  having a cylindrical outer wall  2 , radially inwardly of which is mounted a cylindrical wall  3  with a vacuum defined in the space between the walls  2 , 3 . The wall  3  defines a chamber filled with liquid nitrogen and containing a magnet  4  having a bore  5 . Axially positioned above the magnet  4  within the liquid nitrogen reservoir is a cylindrical liquid helium reservoir  6  separated from the liquid nitrogen reservoir by an evacuated region  7 ′ defined between the reservoir  6  and a wall  7 . An inner vacuum vessel  45  is positioned within the reservoir  6 . Conventional ports  8 A, 8 B are coupled with the liquid nitrogen reservoir for supplying and exhausting nitrogen respectively and similar ports  9  (only one shown) are provided for the helium reservoir  6 . Each port  8 B and  9  has an associated pressure relief valve  8 ′,  9 ′ respectively. 
     A dilution refrigerator is inserted along a central axis of the cryostat  1 . The dilution refrigerator is generally of the form described in EP-A-0675330 and is shown in more detail in FIG.  2 . The refrigerator includes a plastics machined cylinder  10  defining a central cylindrical bore or access tube  11  which defines a probe access path. The cylinder  10  is connected to a  1 K pot of conventional form  12  (FIG. 1) via a metal tube  13  located on a tubular extension  14  of the cylinder  10 . The tube  13  is bonded to the  1 K pot  12  by an indium seal flange  15 . A tube  60  extends from the top of the  1 K pot  12  in alignment with the tube  13  to a gate valve  61  above which is positioned a vacuum lock  62  for connection to a vacuum pump (not shown). 
     The  1 K pot  12  is filled with helium from the reservoir  6  via a needle valve  63  which is connected via a tube (not shown) with the reservoir  6  on one side and to the  1 K pot  12  on the other side. The needle valve  63  is controlled from a control position  64  external to the refrigerator. 
     The upper end of the cylinder  10  defines an upwardly opening, cylindrical bore  16  forming the still which is closed by a plug  17  into which extends a tube  18  defining a still pumping path which terminates at a still outlet port  113 , and electrical wiring contained in a tube  19 . A 5-6 mm diameter aperture  100  extends through the inner wall of the still  16  into the bore  11  below the still outlet port  113 . The aperture  100  is shown above the liquid level in the still but it can also be below the liquid level. 
     The tube  18 , tube  60 , and control  64  extend through a neck  65  of the reservoir  6  and four radiation baffles  66  are positioned within the neck  65 . Each baffle has a small clearance (4-5 mm) between its circumference and the facing surface of the neck  65 . 
     As will be explained below,  3 He is pumped along the pumping path  18  (having a pressure relief valve  18 ′) out of the still  16  by a pump (indicated schematically at  210 ) and is returned to a conduit  20  which extends into a helical groove  21  extending around the plastics cylinder  10 . The conduit  20  terminates in a mixing chamber  22  in another plastics cylinder  23  having a socket  24  into which the end of the cylinder  10  is received. A tube extension  46  is provided in the mixing chamber  22 . A non-metallic tube  25  extends around the groove  21  and part of the cylinder  23 . The groove  21  and conduit  20  cooperate together to define a heat exchanger  26 . 
     A member  27  defines an elongate extension tail of the mixing chamber  22  and is situated in use in the bore  5  of the magnet  4  as shown in FIG.  1 . Typically, the clear diameter of the bore  5  would be about 15 mm although the diameter of the access tube can be as high as 34 mm. 
     FIG. 3 illustrates the dilution refrigerator of FIG. 2 but with a probe inserted. The probe is indicated at  30  and comprises a plastics cylinder forming a displacer  101  which extends as a tight fit through the bore  11  of the plastics cylinder  10 . The end of the probe  30  has towards its lower end a cone shaped cold seal  31  which sits in a correspondingly shaped seat  32  defined by the plastics cylinder  23 . A narrower section  33  of the probe  30  extends through the mixing chamber  22  and terminates near the bottom of the extension tail  27 . A sample  35  is secured to the lower end of the section  33  as described in EP-A0675330. 
     The lower section  33  of the probe  30  also includes a number of orifices  36  circumferentially spaced around the section  33  to allow  3 He to pass into the section  33 . The passage in the section  33  terminates in a radially opening orifice  37  which communicates in use with the groove  21  in the heat exchanger (See FIG.  3 ). 
     Typically, the inside diameter of the tubular section  33  is about 2 mm. Electrical wiring (not shown) may extend through this section  33  for connection to the sample. 
     The operation of the dilution refrigerator can be briefly explained as follows. The mixing chamber  22  includes a mixture  110  of  3 He and  4 He. There exists a phase boundary  111  within the mixing chamber and  3 He gas is “evaporated” from a “concentrated phase”  112  into the dilute phase  110  defined principally by  4 He. The  3 He “gas” then moves through the liquid  4 He down into the tail  27 , through the apertures  36  and up through the tubular section  33  of the probe  30 . The primary flow of  3 He/ 4 He is then into the groove  21  of the heat exchanger  26 . This  3 He/ 4 He then moves up through the helical groove  21  into the still  16  from where the  3 He is pumped through the tube  18  and back in concentrated form to the return line  20 . The relatively small diameter of the tube  18  ensures that only a small amount of superfluid  4 He flows up the sides of the tube. This reduces the concentration of  4 He in the vapour passing up the tube  18 . Furthermore, the diameter of the access tube  11  can be increased without increasing the concentration of  4 He in the vapour passing up the tube  18 . The  3 He is maintained at a temperature of 0.6 to 0.7K in the still  16  by a heater  40 . The returned  3 He passes through the conduit  20  within the groove  21  where it is cooled by the  3 He leaving the mixing chamber  22  until it is fed into the mixing chamber  22  and the cycle continues. 
     Some  3 He/ 4 He will leak past the cold seal  31  into the bore  11  of the moulding  10 . Traditionally, this has been ignored on the basis that the impedance of this path is much greater than that of the flow from still through heat exchanger to mixing chamber and so this leak path will not adversely affect the refrigerators performance. The wall of the heat exchanger  26  adjacent the helical groove  21 , for example at  41 , is made sufficiently thin so that heat exchange can take place between the liquid and probe in the central bore  11  and liquid within the groove  21 . 
     In the present invention, however, this path is promoted by use of the aperture  100 . The presence of this aperture generates an osmotic pressure as a result of the concentration gradient in the  3 He/ 4 He so producing a positive flow through the bore  11  past the displacer  101 . In view of the tight fit of the displacer  101  in the bore  11 , this flow is small compared to the primary flow along the tube  21  but we have found that it can be made sufficient to reduce significantly the heat leak from the still  16  to the mixing chamber  22 . The  3 He atoms dissolved in  4 He flowing away from the mixing chamber greatly reduce the heat flow from the still to the mixing chamber. 
     The reason for the tube extension  46  is that if the phase boundary between the dilute and concentrated phases is set up correctly, any “crossover” leak occurring at the cone seal would still cause  3 He to cross the phase boundary thereby creating cooling. Without the extension tube a crossover leak would cause the  3 He just to be taken from the concentrated phase without forcing it to cross the phase boundary. 
     The embodiment described in FIGS. 1-3 is a special non-metallic top loading system as described in EP-A0675330. However the invention can also be employed in a conventional metal top loading dilution refrigerator. 
     Furthermore, although a top loading refrigerator is described, it will be appreciated that the invention is also applicable to a system in which the probe is permanently mounted in the access path. 
     In an alternative embodiment, instead of providing an aperture  100  which allows the coolant to flow directly from the access path into the still  16 , an aperture may be provided in the wall of the heat exchanger  26  adjacent the helical groove  21  (for example at  41 ) so that the coolant flows from the access path to the still via the helical groove  21 . 
     A further alternative embodiment of a dilution refrigerator according to the present invention is illustrated schematically in FIG.  4 . The heat exchanger and return flow path from the pump to the still are omitted for clarity. An access tube  200  extends into a mixing chamber  201 . A displacer  202  is inserted into the access tube  200 .  3 He flows up the access tube  200  outside the displacer  202  and into a still  204  through a 5-6 mm diameter hole  203  in the side of the access tube  200 , the hole  203  being located in alignment with the liquid level  206  in the still  204 . A groove  205  is provided around the circumference of the displacer  202  at the level of the hole  203  to ensure that all of the fluid flowing up the access tube  200  flows through the hole  203 . A pumping path  207  to a pump (not shown) narrows to an orifice  208  which forms the still outlet port. The wall  209  defining the orifice  208  is tapered to a knife-edge as shown, to minimise the fluid impedance of the orifice  208  and maximise its film restricting effect.