Dilution refrigerator

Details Dilution refrigerator Dilution refrigerator

1999-06-30

2001-03-20

Doerrler, William (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Liquefaction

C062S051100

Reexamination Certificate

active

06202439

ABSTRACT:

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 f

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