Process and apparatus for obtaining very low temperatures

Process and apparatus for obtaining very low temperatures by making use of the endothermic dissolution of 3He in 4He. A mixing box placed in a vacuum enclosure cooled to approximately 2.degree. K. or less receives continuously liquid 3He and 4He via separate conduits. The solution produced is extracted therefrom via a third conduit at a rate such that, bearing in mind the diameter of the conduit, 3He cannot diffuse countercurrentwise in the solution sufficiently to raise appreciably the content of 3He in the liquid 4He introduced and to reduce the dissolution in this 4He of the liquid 3He introduced simultaneously.

The present invention relates to a process and an apparatus for obtaining 
very low temperatures. 
Among the methods for obtaining very low temperatures, one of the most 
advantageous makes use of the dilution of the isotope 3He in ordinary 
helium 4He. Below approximately 2.17.degree. K., a 4He-3He mixture 
exhibits two phases, the limiting content of 3He in 4He being 
approximately 6%. 
To describe the principle of known processes in a somewhat simplistic 
manner, let us assume that, in a "mixing box" placed in a vacuum enclosure 
cooled to a temperature of the order of 2.degree. K. or less, there are 
3He and 4He in such quantities that there are two phases, a "solution" 
phase and a pure 3He phase, if we decrease the concentration of 3He in the 
solution phase, pure 3He will dissolve therein to reestablish the 
equilibrium concentration, and the energy needed for this dissolution will 
be taken from the mixing box, which will consequently be cooled. 
The word "solution" is employed to simplify matters, it is known that the 
usual definitions do not apply well at very low temperatures. 
In practice, the mixing box is used in combination with a column filled 
with solution at equilibrium, and at the top of which is placed an 
evaporator which allows 3He to be extracted from the solution. A 
concentration gradient is created in the column, and 3He migrates from the 
mixing box towards the evaporator, which results in the dissolution of 
pure 3He and the cooling of the mixing box. 
For such a device, or cryostat, to operate continuously, it suffices to 
introduce liquid 3He into the mixing box, optionally mixed with a little 
4He, to compensate for escapes. 
It would be possible to, in a similar manner, operate a cryostat in which 
the mixing box contained a solution phase and a pure 4He phase. 
With such cryostats, the presence of a distillation unit requires a pumping 
unit incorporating large-diameter pipes, and makes operation in all 
orientations or in the absence of gravity (for example use in space) 
difficult, or even impossible. 
The purpose of the invention is to make it possible to get rid of these 
constraints by the construction of a cryostat comprising neither a 
distiller nor a low-pressure pumping line. 
To obtain this result, the invention provides a process for obtaining very 
low temperatures, according to which there is created, in a mixing box 
placed in a vacuum enclosure cooled to a temperature of the order of 
2.degree. K. or lower, a two-phase system comprising a phase of solution 
of 3He in liquid 4He, and a liquid phase consisting of the pure isotope 
3He, this isotope is transferred into the solution phase, the energy of 
dissolution being taken from the mixing box in order to cool it, 3He is 
extracted from the mixing box in the state of solution and the 3He in the 
pure liquid state is introduced into the mixing box in a quantity equal to 
the quantity of this isotope which leaves the mixing box through the 
solution, a feature of this process being that the two-phase system is 
created by continuously feeding the mixing box with liquid 4He and 3He 
which are introduced separately, in that the solution phase is extracted 
in such velocity conditions that the 3He which it contains cannot diffuse 
countercurrentwise sufficiently in the solution to raise appreciably the 
content of 3He in the liquid 4He introduced and reduce the dissolution, in 
this 4He, of the liquid 3He introduced simultaneously. 
The process of the invention introduces appreciable differences from the 
prior art. In both cases, an attempt is made to have a 4He with a low 
content of 3He, and in which the liquid 3He introduced will be capable of 
dissolving easily, with production of cold. 
In the prior art, the content of 3He is lowered by making the latter 
diffuse through the solution in the direction of an evaporator. According 
to the invention, on the other hand, the solution is extracted at a 
velocity such that 3He cannot return backwards to raise the content of 3He 
in 4He and consequently to make it less capable of dissolving the liquid 
3He. In the system of the prior art, it is instinctively guessed that it 
would be necessary to prefer a wide discharge conduit, where the velocity 
of travel will be low, in fact, the 4He remains practically stagnant. On 
the other hand, the discharge conduit of a cryostat according to the 
present invention will be, preferably, narrow, and the velocity of travel 
relatively high.

The invention will now be described in greater detail with the aid of a 
practical example, illustrated with the aid of the single FIGURE which is 
in a diagrammatic cross-section, of an experimental cryostat in accordance 
with the invention. 
The cold part of the cryostat alone is shown in the FIGURE. The unit is 
included in an enclosure 1 cooled to the temperature of 1.8.degree. K. The 
two isotopes 3He and 4He arrive into the enclosure via capillaries 2 and 3 
respectively, in liquid phase and under a pressure in the region of 1/2 
atmosphere. The mixture leaves the enclosure again via a capillary 4. In 
the cryostat described here, these three capillaries have an internal 
diameter of 0.3 mm. A heat exchanger 5 consists of three capillaries 2A, 
3A, and 4A made of CuNi with an internal diameter of 0.1 mm and an 
external diameter of 0.5 mm, which are tin-soldered together over their 
entire length and connected, on the one hand, to the capillaries 2 to 4 
and, on the other hand, to the mixing box. Mixing of the two components 
takes place in the copper mixing box 6 of a very small capacity (a few 
cubic millimeters) whose wall is coated with sintered silver powder in 
order to increase its exchange surface area. A thermometer 7, consisting 
of a germanium resistor is screwed into the outside of this box. 
Two different assemblies have been employed to test this cryostat. The 
first consists of a conventional cryostat which enables a temperature 
close to 1.8.degree. K. to be obtained by pumping a bath of 4He. In this 
helium bath is immersed a vacuum enclosure 1 containing the cold part of 
the cryostat described above. The two pure isotopes of helium are injected 
into the cryostat in gaseous form at a rate which is controlled by two 
flow regulators. These gases are liquefied with the aid of exchangers in 
the main helium bath before being directed into the cold enclosure 1. 
The second device is a rotating cryostat operating by continuous 
circulation of 4He from a stationary reservoir. Rotation is obtained with 
the aid of a horizontal rotating coupling in the liquid 4He feed line. 
This cryostat enables an exchanger to be cooled to a temperature close to 
1.6.degree. K. by pumping 4He and by using a cold decompression valve. The 
enclosure 1 containing the mixing chamber is fixed on to this exchanger. 
The two isotopes of helium are injected in gaseous form and are then 
cooled and liquefied in the helium vapours of the main circulation. 
The operation of a system of this kind requires a continuous supply of 3He. 
In view of the price of this isotope it is preferable to recover the 
dilute mixture leaving the cryostat and to separate its two constituents. 
To do this, an adjoining distillation unit has been employed, operating in 
a separate cryostat and enabling the two isotopes to be obtained in a 
purity better than 99% in the case of 3He and 99.99% in the case of 4He. 
This unit, which employs only well-known techniques, has not been 
described. 
Using the cryostat in the form described, a stable temperature of less than 
180 mK and completely insensitive to the orientation of the system 
relative to the vertical has been obtained. 
It is advantageous to employ, in the exchanger, capillaries of smaller 
diameter in the warm part than in the cold part. In another embodiment, 
the use of a two-part exchanger, the warmer part being made with 0.05 mm 
tubing and the other with 0.2 mm tubing, has made it possible to obtain a 
temperature of 125 mK with flow rates of 9 ml/mm in the case of 3He and of 
90 ml/min in the case of 4He (NTP gas flow rate). This corresponds to 
linear velocities of approximately 10 cm/s. It has been observed that, if 
the diameter increases, the critical velocity decreases, at least within 
certain limits. In principle, nothing prevents lower temperatures, below 
100 mK, being obtained. The flow rates employed are sufficiently low to 
envisage the use of the system in a satellite. In fact, under these 
operating conditions, the consumption over one year is 5 liters of 3He and 
50 liters of 4He (liquid). 
The system actually looks very simple but, as far as the inventor is aware, 
such a cryostat has never been proposed. The reason is that this system 
operates only out of equilibrium, at well-defined flow rates, 
corresponding to the tubing diameters. Understanding of the operation of 
this system makes it necessary, furthermore, to take into account the 
mutual friction between the superfluid 4He and the 3He, a phenomenon which 
has only recently become known. 
Two very different cases of use may be envisaged: 
the use of a system of this kind in space appears highly promising because 
the reliability of the system can be extremely good owing to its 
simplicity. In fact, in an application of this kind, the two isotopes are 
initially taken on board in liquid form and the mixture is discharged into 
space (it could also be stored as such, in the case of a recoverable 
device). In principle, therefore, a system of this kind requires no 
pumping system (only a flow control system is needed). The absence of cold 
decompression (in contrast to traditional systems) makes it possible, 
moreover, to get rid of blockage problems due to impurities in the helium, 
in normal conditions (on Earth) this system applies in all cases where a 
high mobility and a small bulk are sought after. In fact, on the one hand, 
the cryostat is insensitive to gravity (no liquid-gas phase separation 
surface) and, on the other hand, the only external connections are 
effected by three capillaries less than one millimeter in diameter. In 
particular, a system of this kind can be mounted on a carrier which can be 
pointed in all directions. However, in view of the price of 3He, it is 
necessary in this case to employ an appended distillation unit, as 
described above.