US20130133341A1

US20130133341A1 - Cryorefrigeration Device and Method of Implementation

Info

Publication number: US20130133341A1
Application number: US13/813,576
Authority: US
Inventors: Francois Viargues; Daniel Communal; Marc Chichoux; Gerard Marzo
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2010-08-03
Filing date: 2011-07-27
Publication date: 2013-05-30
Also published as: FR2963667A1; WO2012017357A1; FR2963667B1; EP2601457A1

Abstract

The invention proposes a reliable high-performance cryorefrigeration device, the level of thermal oscillations of which may be minimized so as to be below a threshold value. For this purpose, the subject of the invention is a cryorefrigeration device comprising N periodically operating cryorefrigerators (100), N being an integer equal to or greater than 2, each cryorefrigerator being provided with a cold end coupled to a common cold end (300), the cryorefrigerators being linked to a control device provided with a phase-shift means capable of making the cryorefrigerators operate in phase-shifted relationship one with respect to another.

Description

The present invention relates to a cryorefrigeration device and a method of use thereof.
A cryorefrigeration device is a cyclically operating apparatus producing cooling capacity at a temperature below 120 K, without material removal outside the cycle.
Among known cryorefrigerators, pulse tube refrigerators are particularly advantageous, owing to the absence of moving parts at low temperature, which results in a low level of vibrations generated high reliability and long service life. Pulse tube refrigerators are therefore advantageously used in the space field or for cooling sensitive detectors.
The operation of cryorefrigerators (pulse tube refrigerators, Gifford-MacMahon refrigerators, etc.) is based on a reciprocating cycle using a gas (advantageously helium). The temperature behavior of a cryorefrigerator is not perfectly sinusoidal, but it is periodic. The cooling capacity is obtained by successive expansions of the cycle helium, cadenced by a rotary valve.
Cryorefrigerators comprise a drive portion (compression) which circulates the gas in a second “cold head” portion, in compression/expansion cycles, to generate a useful temperature for the user.
The cold head has a generally elongated shape of which the free end (that is to say opposite the drive portion) represents the useful interface for the user, and is called the “cold end”. The cold end is enclosed in an insulating chamber which contains the object to be cooled. The interior of the chamber is placed under vacuum to limit entries of heat.
The reciprocating operation naturally generates high thermal oscillations at the cold end of the cryorefrigerator. This cold end, generally made from copper, has a much higher mass than the cycle gas undergoing the compressions-expansions. It is, however, incapable of effectively smoothing the oscillations by thermal inertia, because at the temperatures concerned (typically 3 to 20 K), solid materials have a very low volumetric heat capacity compared to that of helium.
Thermal oscillation measurements were taken on various cryorefrigerators (Gifford-MacMahon and pulse tube refrigerators). The peak-to-peak amplitudes temperature taken at the cold end were measured between 300 mK and 2200 mK. These thermal oscillations are therefore considerable and are disturbing for certain applications.
Systems intended to reduce the mechanical vibrations of cryorefrigerators are proposed by manufacturers: a “sock” surrounds the cold end, with a clearance filled with heat exchange gas (helium). This system procures a (moderate) damping of thermal oscillations and hence of the mechanical vibrations, but at the cost of a wide temperature difference between the useful temperature for the user and the cold end temperature, which is detrimental to performance. In other words, the temperature usable by the user will not be as low as the temperature generated by the pulse tube refrigerator.
Similarly, it has been proposed to reduce the thermal oscillations by filtering, by associating a thermal resistance and a heat capacity with a pulse tube refrigerator, as for an RC circuit in electricity. However, while the presence of an artificially introduced thermal resistance is effective for filtering, it has the drawback of also generating a wide temperature difference between the useful temperature for the user and the cold end temperature.
Document US 2005/0028534 describes a cryorefrigeration device incorporating a plurality of periodically operating cryorefrigerators. To reduce the level of vibrations generated at the cold end by the cyclic operation of the cryorefrigerators, this document proposes a phase-shifted operation of the cryorefrigerators on the one hand, and the merging of the cold ends of the individual cryorefrigerators into a single common cold end, on the other hand. This particular arrangement consists in connecting the tubes of each cryorefrigerator to the common cold end in order to obtain a symmetrical arrangement about the center of the common cold end. Thus, during the gas compression/expansion cycles, the deformations on the tubes are partially offset and produce a lower level of vibrations at the common cold end than for individual cryorefrigerators.
However, such a solution necessarily implies the fabrication of a specially designed multiple cryorefrigerator, in which the cold end connects the various tubes of the individual cryorefrigerators.
It is the object of the invention to overcome these drawbacks by proposing an economical cryorefrigeration device, that is to say, which can be made with commercial cryorefrigerators, which is reliable, efficient, that is to say, without a significant difference between the temperature generated and the useful temperature, and in which the level of thermal oscillations may be reduced below a threshold value, preferably less than 2% of the mean temperature generated.
For this purpose, the invention relates to a cryorefrigeration device comprising N periodically operating cryorefrigerators, where N is an integer equal to or greater than 2, each provided with a cold end connected to a common cold end, the cryorefrigerators being associated with a control device provided with a phase-shift means suitable for actuating phase-shifted operation of the cryorefrigerators with regard to one another, the cold end of each cryorefrigerator being connected to the common cold end via a heat conducting mechanical uncoupling means.
According to other features of the invention:
the phase-shift means may be suitable for actuating a phase-shifted operation with a phase difference of 2π/N, to within 5 degrees, between each cryorefrigerator;
the cryorefrigerators may be identical;
the cryorefrigerators may be pulse tube refrigerators or Gifford-MacMahon cryorefrigerators;
the heat conducting mechanical uncoupling means may comprise a plurality of heat conducting wire braids fixed between two mounting plates in thermal contact with the cold end of a cryorefrigerator and the common cold end, respectively;
the heat conducting wires of the braids may be made from Cu/a1 copper and have a diameter between 0.03 mm and 0.1 mm, preferably equal to 0.05 mm, and the mounting plates have a residual resistivity ratio of at least 50;
the mounting plates and/or the common cold end may be made from Cu/a1 or Cu/c1 copper;
the heat conducting wires of the braids may be welded to the mounting plates by electron beam welding so as to ensure material continuity; and/or
the common cold end may comprise a temperature sensor connected to the control device.
The invention also relates to a method for using the above cryorefrigeration device, comprising a step of actuating the N cryorefrigerators in a phase-shifted manner with regard to one another.
According to a first embodiment of the invention, the method comprises a step of adjusting the phase difference between each of the N cryorefrigerators, this adjustment step comprising the following steps:

- a1) operating the N cryorefrigerators simultaneously, in any phase-shifted manner;
- b1) measuring the temperature variations of the common cold end with the temperature sensor and calculating the mean temperature of the common cold end;
- c1) actuating a phase-shifted operation between each cryorefrigerator until the temperature variations of the common cold end during the operation of the cryorefrigeration device are lower than a threshold value, preferably less than 2% in absolute value of the mean temperature of the common cold end, advantageously less than 1% in absolute value of the mean temperature of the common cold end; and/or
- d1) setting the operating phase of each of the N cryorefrigerators.

According to a second embodiment of the invention, the method comprises a step of adjusting the phase difference between each of the N cryorefrigerators, this adjustment step comprising the following steps:

- a2) operating the N cryorefrigerators simultaneously, with a phase difference of 2π/N between each cryorefrigerator;
- b2) measuring the temperature variations of the common cold end with the temperature sensor and calculating the mean temperature of the common cold end;
- c2) varying the phase of N-1 cryorefrigerators about their initial phase in step a) until the temperature variations of the common cold end during the operation of the cryorefrigeration device are lower than a threshold value, preferably less than 2% in absolute value of the mean temperature of the common cold end, advantageously less than 1% in absolute value of the mean temperature of the common cold end;
- d2) setting the operating phase of each of the N cryorefrigerators.

By convention, the steps of this method are carried out in alphabetical order.

Other features of the invention are described in the detailed description below with reference to the appended figures which show, respectively:

in FIG. 1, a schematic perspective view of a cryorefrigeration device according to the invention;

in FIG. 2, a schematic perspective view of a thermal coupler (common cold end) provided with two mechanical uncoupling means according to the invention, intended to be fixed in thermal contact with the cold ends of the two cryorefrigerators;

in FIG. 3, a graph showing the cold end temperature of two phase-shifted cryorefrigerators and the temperature of the thermal coupler;

in FIG. 4, a diagram showing the phase-shift regulation of a system for implementing the device according to the invention;

in FIG. 5, a graph showing the thermal oscillations of the common cold end when the phase difference between two pulse tube refrigerators is varied continuously, for two mean temperature levels of the common cold end; and

in FIG. 6, a schematic plan view of a second embodiment of a cryorefrigeration device according to the invention.

With reference to FIG. 1, a cryorefrigeration device according to the invention comprises two identical periodically operating cryorefrigerators 100. These cryorefrigerators are pulse tube refrigerators of a known type (for example CRYOMECH PT415 pulse tube refrigerators) the structure of which is shown in FIG. 1, but is not completely described in detail, as those skilled in the art know this structure.
Chiefly, each cryorefrigerator 100 comprises a drive portion 110 which circulates the gas in a second “cold head” portion 120, in compression/expansion cycles in order to generate a useful temperature for the user.
The drive portion 110 is connected, in the embodiment shown, to a compressor (not shown).
The cold head 120 has a generally elongated shape of which the end opposite the drive portion is the cold end 121.
The two cryorefrigerators 100 are mounted side by side on a common mounting plate 200 of a vacuum chamber and supported mechanically by a mounting flange 101.
During operation, the drive portion of the plate 200 supporting the two cryorefrigerators remains at ambient temperature (300 K). Thus, the centerline distance between the two machines does not vary. On the contrary, the cold head 120, located under the plate 200 is enclosed in a vacuum chamber and undergoes thermal contractions.
A coupling rod 300 of heat conducting material (preferably copper, advantageously Cu/a1 or Cu/C1 copper) is thermally connected to the cold ends of the two cryorefrigerators, which it joins. This rod 300, or “thermal coupler”, constitutes the common cold end. The object to be cooled is, as it would be mounted mounted on the coupling rod 300 under the cold end in the case of a single pulse tube refrigerator.
The denominations of Cu a1 and Cu c1 according to standard NF A 51-050 correspond to the following ISO 431 denominations:

- Cu-a1 becomes Cu-ETP
- Cu-c1 becomes Cu-OF

Cu-A1 (or Cu-ETP) copper has a copper content above 99.90%. This is an electrolytically refined copper, not deoxidized, with guaranteed conductivity. Cu-C1 (or Cu-OF) copper has a copper content above 99.95%. This type of copper is an oxygen-free or deoxidized copper with trace residual deoxidant. In any case, the residual deoxidant content is too low to affect the conductivity.
The geometry of the coupling rod 300 is selected to optimize the thermal behavior. For a number of pulse tubes higher than 2, the rod advantageously has the shape of a triangle, a square, a disc, etc.
All of the coupling rod 300 is cooled to low temperature and contracts (4 mm/m for copper). For a cryorefrigeration device according to the invention, with two pulse tubes, the rod may reach a length of about fifty centimeters, so that the contraction may reach about 2 mm. If the number of tubes increases, the length of the rod and hence the contraction increase.
The invention provides for a heat conducting mechanical uncoupling means 400 between the cryorefrigerators 100 and the coupling rod 300, so that the stress generated by the contraction of the rod is not transmitted to the pulse tubes.
A first embodiment, not shown, consists in providing a bellows under the mounting flanges of the cryorefrigerators at the plate of the vacuum chamber. The centerline distance of the cryorefrigerators is then free to reduce by 1 to 2 mm during the contraction of the cold rod. However, very flexible bellows must be provided to limit the stresses generated in the tubes of the refrigerators, while the external atmospheric pressure bears considerably on the bellows which communicate internally with the vacuum of the chamber.
A second preferred embodiment is shown in FIGS. 1 and 2.
In this embodiment, the cold end 121 of each cryorefrigerator 100 is connected to the common cold end 300 via a heat conducting mechanical uncoupling means 400.
In an advantageous exemplary embodiment, the heat conducting mechanical uncoupling means 400 comprises a plurality of braids 410 of heat conducting wires fixed between two mounting plates 420-430 in thermal contact with the cold end 121 of a cryorefrigerator 100 and the common cold end 300, respectively.
This exemplary embodiment must be designed to ensure the least possible deterioration of the thermal connection between the common cold end and the cold source consisting, here, of the pulse tubes. The heat conducting mechanical uncoupling means 400 must transmit a heat flux with the lowest possible temperature drop (ΔT).
Preferably, the heat conducting mechanical uncoupling means 400 is made using the shortest and largest possible number of copper braids. The braids are selected for their high thermal conductivity properties at low temperature, and their high flexibility (very thin wires).
Preferably, the heat conducting wires of the braids are made from Cu/a1 or Cu/c1 copper, and have a diameter between 0.03 mm and 0.1 mm, preferably 0.05 mm. The flexibility between the plates 420-430 and the rod 300 depends considerably on the diameter of these wires.
The dimensions of the braids are the result of a mechanical-thermal compromise. The heat transfer calculations encourage the use of braids having a high total cross section and short length (heat conduction) to limit the temperature drop (ΔT) lost by conduction. The mechanical calculations encourage the use of the most flexible possible braids, hence the use of thin wires (typically having a diameter of 0.05 mm) and which are not too short (typically about 30 mm for braids with a cross section of about 25 mm²). For example, a braid may consist of 12 strands of 1062 wires having a diameter of 0.05 mm.
Also preferably, the mounting plates 420-430 have a residual resistivity ratio (RRR) of at least 50. For this purpose, the mounting plates 420-430 are selected to be made from copper having high thermal conductivity at low temperature (Cu/a1 or Cu/c1). The same applies to the common cold end.
The residual resistivity ratio provides a good image of the low temperature conductivity of the copper. It can be measured according to international standard (IEC 61788-11) of 2003.
Alternatively, the following method can be used to calculate the residual resistivity ratio of the mounting plates according to the present invention:

- A slender test specimen having a cross section of about 4 mm²and a length of about 150 mm is taken from the block of material to be qualified.
- The test specimen is mounted on a cryogenic rod intended to be immersed in a Dewar flask of liquid helium.
- The ends of two power input cables (diameter 1 mm) are soldered with tin at the two ends of the test specimen.
- Two other electrical cables (diameter 0.2 mm) serving as power connectors are placed in contact (but not soldered) with the ends of the test specimen, in the area not polluted by the tin solder of the test specimen. This is because any tin that has migrated into the copper can affect the value of the RRR. The power connections are separated by a distance of about 100 mm.
- With the rod and specimen at ambient temperature, a current of 10 A is sent through the specimen. The voltage generated between the two power connections is measured using a microvoltmeter.
- The rod is then immersed in the liquid helium, so that the specimen is cooled completely to the helium saturation vapor temperature (4.2 K).
- With the 10 A current maintained across the specimen, the new value of the potential difference developed along the specimen is measured using the microvoltmeter.
- The ratio of the voltage developed at ambient temperature to the voltage at 4.2 K directly gives the RRR of the test specimen, without the need to determine the real electrical resistance of the specimen. RRR values above 50 are considered to be the most effective for implementing the invention. Low-RRR coppers, such as Cu-b, have an RRR lower than 8 and are avoided for applications in which thermal conductivity is important.

Furthermore, the heat conducting wires of the braids are advantageously electron beam welded to the mounting plates so that material continuity is guaranteed and the conduction of heat is feasible with the minimum temperature drop (ΔT) between the cold end 121 of each cryorefrigerator and the common cold end.
The above cryorefrigeration device has been described with N existing individual cryorefrigerators. However, the invention also relates to a cryorefrigeration device (not shown) comprising a single or “integrated” cryorefrigerator, incorporating all the preceding functions, for example using N independent refrigeration circuits each comprising a cold end connected to a common cold end.
The cryorefrigerators are very advantageously identical, that is to say, having the same thermal response in their operating cycles, this response being more or less symmetrical about a mean temperature. Typically, two identical cryorefrigerators are of the same brand and the same model.
The principle according to the invention of damping the thermal oscillations by phase-shift operation of the cryorefrigerators is very easily implemented when the cryorefrigerators have the same thermal response in their operating cycles, and when this response is more or less symmetrical about a mean temperature. The temperature generated at the cold end of each of these machines is alternatively higher and lower than the mean temperature (thermal oscillations). If two identical machines are synchronized in phase opposition, one will have a higher temperature than the mean at the same time that the other has a lower temperature.
For theoretical cyclic temperature response curves that are perfectly symmetrical about the constant mean temperature (examples: sinusoidal response or rectangular or triangular shapes, etc.) the compensation would be perfect and the resulting temperature would be constant and equal to the mean temperature. In practice, real response curves are neither sinusoidal nor even perfectly symmetrical about the mean temperature. The compensation is therefore not perfect. By using identical cryorefrigerators, the compensation tends more easily toward perfection. The settings of the control electronics serve to synchronize the cryorefrigerators as well as possible, but do not serve to eliminate the asymmetries. Hence the compensation is much more effective when using identical cryorefrigerators.
In practice, an attempt is made to approach symmetry most closely (allowing good compensation), by preferably selecting identical cryorefrigerators, by controlling them to operate with exactly the same frequency and with a phase difference (the phase being calculated according to the number of cryorefrigerators used). If identical pulse tube refrigerators with two heat exchange stages are used (as shown in FIG. 6), the first stages of the cryorefrigerators are regulated at exactly the same temperature in order to make their operation, and hence their temperature response, as symmetrical as possible.
The implementation of the cryorefrigeration device according to the device is described below, with reference to FIGS. 3 to 5.
A cryorefrigeration device according to the invention, and described above, comprises N periodically operating cryorefrigerators 100 (or one integrated cryorefrigerator), where N is an integer equal to or greater than 2, each provided with a cold end 121 connected to a common cold end 300. These N cryorefrigerators 100, or the integrated cryorefrigerator, are associated, according to the invention, with a control device provided with a phase-shift means suitable for actuating a phase-shifted operation of the cryorefrigerators 100, or of the N refrigeration circuits of the integrated cryorefrigerator, with regard to one another.
The result of an actuation of two cryorefrigerators in a phase-shifted manner of A with regard to B is shown in FIG. 3.
Tests were performed with two PT415 pulse tube refrigerators A and B (manufactured by CRYOMECH) connected by a mechanical uncoupling means according to the invention (FIG. 2). A CERNOX type AA temperature sensor (manufacture by LAKESHORE) mounted at the center of the common cold end and connected to a rapid data acquisition system serves to observe the temperature fluctuations.
Each cryorefrigerator is normally associated with a compressor and an electronic module for control of the drive motor and its rotary valve. Each of these modules incorporates an oscillator and a power circuit which controls the motor.
For a preferred implementation of the invention, these modules were disconnected and replaced by simple power modules without oscillator (MDP model MFM1CSZ34N7). The oscillators were replaced by a double-output signal generator (TEKTRONIX model AGF3102). The generator outputs can be synchronized in normal operation or separated during the adjustment phase.
As shown in FIG. 3, the temperature behavior of each of the two cryorefrigerators A (dotted line) and B (dashed line) is not perfectly sinusoidal, but it is periodic. The mean temperature obtained in this case is 10.6 K. The temperature of the cold end of each of the cryorefrigerators A and B varies periodically between −800 mK and +800 mK. When the two machines operate substantially in phase opposition, the temperature variations are partly offset: the temperature variations of the common cold end (solid line) are lower than 100 mK in absolute value. In FIG. 3, the phase difference Δφ between the two cryorefrigerators is 177°, or about 2π/2 (N=2 in this embodiment). If the cryorefrigeration device had comprised 3, 4 or N pulse tubes, the phase difference between each of the tubes would have been about 2π/3, 2π/4, or 2π/N.
During normal operation of the device according to the invention substantially in phase opposition (N=2) or in phase-shifted mode (N greater than 2), a misadjustment of the device may occur, so that the phase difference no longer serves to minimize the temperature variations of the common cold end, or to maintain these temperature variations below a threshold value. A step of adjusting the phase difference between each of the N cryorefrigerators is then necessary. This step is described below, with reference to FIGS. 4 and 5.
Cryorefrigerators of the pulse tube type or Gifford-MacMahon type are periodic machines in which the cycle is cadenced by a distributor driven by a motor.
The synchronization of several machines, in order to smooth the temperature fluctuations, requires identical speeds for the distributors, but with an appropriate phase difference.
In an integrated system which comprises a plurality of refrigeration circuits in the same device, it is possible to couple the distributors together mechanically, or even to produce a single distributor which, by construction, provides the necessary phase difference between the refrigeration circuits.
The implementation method according to the invention is described below in relation to a device comprising N individual cryorefrigerators. However, this method is perfectly applicable to an integrated cryorefrigerator, comprising N refrigeration circuits.
To synchronize N identical independent cryorefrigerators, the invention proposes making an “electrical tree” between the drive motors of the distributors to obtain identical speeds (see FIG. 4). The invention also proposes allowing a temporary phase difference between the movements of these distributors during the adjustment phase, which consists in seeking the minimum of the temperature fluctuations or in reducing the temperature fluctuations below a threshold value.
In pulse tube or Gifford-MacMahon cryorefrigerators, the distributor is driven by a synchronous stepping motor. FIG. 4 shows the principle of a control system incorporating a system of automatic adjustment in phase opposition for two cryorefrigerators. This system can obviously be generalized to N cryorefrigerators.
The device measures the central temperature of the common end (“thermal coupler”) 300 using a thermometer 310 and extracts the fluctuations of this temperature about the operating frequency by passband filtering and amplitude detection.
In the automatic position, the device makes a phase adjustment to adjust to the minimum amplitude of the fluctuations by adjusting the phase-shift means by increasing or decreasing increments according to the sign of the derivative of the amplitude signal. When the adjustment is completed, the control loop can be inhibited by switching to “locked” position. In the “manual control” position, the phase shift can be adjusted gradually and virtually continuously by an operator.
Thus, in general, the adjusting step comprises the following steps:

a1) operating the N cryorefrigerators simultaneously, in any phase-shift manner;
- b1) measuring the temperature variations of the common cold end with the temperature sensor and calculating the mean temperature of the common cold end;
- c1) actuating a phase-shift operation between each cryorefrigerator until the temperature variations of the common cold end during the operation of the cryorefrigeration device are lower than a threshold value in absolute value (preferably less than 2% in absolute value of the mean temperature of the common cold end, advantageously less than 1% in absolute value of the mean temperature of the common cold end);
- d1) setting the operating phase of each of the N cryorefrigerators.

Obviously, the phase of all the N cryorefrigerators is not varied. A reference cryorefrigerator is arbitrarily selected among the N cryorefrigerators, and the phase of the other N-1 cryorefrigerators is varied so that all the N cryorefrigerators are offset with regard to one another, substantially by 2π/N.
According to a preferred embodiment, the operation of each of the N cryorefrigerators is initially phase-shifted by 2π/N between them (step a2) and not randomly phase-shifted. During a step c2), the phase of N-1 cryorefrigerators is adjusted about their initial phase of step a2) until the temperature variations of the common cold end during the operation of the cryorefrigerators device are lower than the threshold value in absolute value.
FIG. 5 shows a recording of the thermal oscillations recorded in the central portion of the common cold end when the phase-shift between two pulse tube refrigerators is varied continuously, and for two mean temperature levels of the common cold end: 5 K (solid line) and 11 K (dotted line).
These measurements were taken with two PT415 pulse tube refrigerators A and B (CRYOMECH) connected by a mechanical uncoupling means according to the invention (FIG. 2). A temperature sensor mounted at the center of the common cold end is connected to a data acquisition system which serves to observe the temperature fluctuations on a recorder.
FIG. 5 shows that the higher the mean temperature of the cold end, the wider the temperature variations may be. Thus, for a mean temperature of 11 K, the temperature variations may reach almost 500 mK. For a mean temperature of 5 K, the temperature variations may reach nearly 200 mK.
By changing the frequency of one of the tubes with regard to the other (or more generally of N-1 tubes with regard to the final tube), the phase of the first tube is caused to slide continuously with regard to the other: for a frequency difference of 0.6% between the two tubes, the phase difference reaches 180° (2π/2) in about 60 s.
In the graph in FIG. 5, the maximum variation in the mean temperature of the common cold end is obtained when the tubes A and B are in phase (substantially at 3 and a half seconds). The minimum variation in mean temperature of the common cold end is obtained when the tubes A and B are in phase opposition (substantially at half a second and at 7 seconds).
Once the adjustment procedure is completed, the outputs of the generator are synchronized to the set the operating phase of each of the N cryorefrigerators.
A second embodiment (not shown) consists of an electronic circuit board combining the functions described in the diagram in FIG. 4, and a simplified man-machine interface for controlling the adjustments and monitoring the level of fluctuations.
The device described above is based on the use of two commercial pulse tube refrigerators, as they exist in catalogues. As shown in FIG. 6, it is similarly possible to combine 3, 4 or N cryorefrigerators 100, by connecting their cold ends by a common cold end, and by phase-shifting their operation by 2π/N. The larger the number N of cryorefrigerators, the smoother the temperature response of the common cold end, that is to say, having small thermal oscillations.
If pulse tube refrigerators with two heat exchange stages 102-103 are used (as shown in FIG. 6), it may be advantageous to connect the first stage 102 of each tube 100 together thermally by a flexible heat conducting braid 104, thereby serving to create a first “common” stage usable for cooling thermal screens, for example. The problem of the temperature drop ΔT lost is much less important than for the common cold end of the second stage 103.
In this embodiment, the phase difference of the various cryorefrigerators is obtained by a common rotary valve 500, distributing the high pressure and the low pressure to the various cryorefrigerators, through openings made therein. The rotary valve 500 is actuated by a motor 501 controlled by a control device 600 incorporated in the compressor.

Claims

1. A cryorefrigeration device comprising N periodically operating cryorefrigerators, where N is an integer equal to or greater than 2, each provided with a cold end connected to a common cold end, the cryorefrigerators being associated with a control device provided with a phase-shift means suitable for actuating phase-shifted operation of the cryorefrigerators with regard to one another, characterized in that the cold end of each cryorefrigerator is connected to the common cold end via a heat conducting mechanical uncoupling means.

2. The device as claimed in claim 1, in which the phase-shift means is suitable for actuating a phase-shifted operation with a phase difference of 2π/N, to within 5 degrees, between each cryorefrigerator.

3. The device as claimed in claim 1, in which the cryorefrigerators are identical.

4. The device as claimed in claim 1, in which the cryorefrigerators are pulse tube refrigerators or Gifford-MacMahon cryorefrigerators.

5. The device as claimed in claim 1, in which the heat conducting mechanical uncoupling means comprises a plurality of heat conducting wire braids fixed between two mounting plates in thermal contact with the cold end of a cryorefrigerator and the common cold end, respectively.

6. The device as claimed in claim 5, in which the heat conducting wires of the braids are made from Cu/a1 copper and have a diameter between 0.03 mm and 0.1 mm, preferably equal to 0.05 mm, and the mounting plates have a residual resistivity ratio of at least 50.

7. The device as claimed in claim 5, in which the mounting plates and/or the common cold end are made from Cu/a1 or Cu/c1 copper.

8. The device as claimed in claim 5, in which the heat conducting wires of the braids are welded to the mounting plates by electron beam welding so as to ensure material continuity.

9. The device as claimed in claim 1, in which the common cold end comprises a temperature sensor connected to the control device.

10. A method for using a cryorefrigeration device as claimed in claim 1, comprising a step of actuating the N cryorefrigerators in a phase-shifted manner with regard to one another, characterized in that it comprises a step of adjusting the phase difference between each of the N cryorefrigerators, this adjustment step comprising the following steps:

a1) operating the N cryorefrigerators simultaneously, in any phase-shifted manner;

b1) measuring the temperature variations of the common cold end with the temperature sensor and calculating the mean temperature of the common cold end;

c1) actuating a phase-shifted operation between each cryorefrigerator until the temperature variations of the common cold end during the operation of the cryorefrigeration device are lower than a threshold value, preferably less than 2% in absolute value of the mean temperature of the common cold end, advantageously less than 1% in absolute value of the mean temperature of the common cold end;

d1) setting the operating phase of each of the N cryorefrigerators.

11. The method for using a cryorefrigeration device as claimed in claim 1, comprising a step of actuating the N cryorefrigerators in a phase-shifted manner with regard to one another, characterized in that it comprises a step of adjusting the phase difference between each of the N cryorefrigerators, this adjustment step comprising the following steps:

a2) operating the N cryorefrigerators simultaneously, with a phase difference of 2π/N between each cryorefrigerator;

b2) measuring the temperature variations of the common cold end with the temperature sensor and calculating the mean temperature of the common cold end;

c2) varying the phase of N-1 cryorefrigerators about their initial phase in step a) until the temperature variations of the common cold end during the operation of the cryorefrigeration device are lower than a threshold value, preferably less than 2% in absolute value of the mean temperature of the common cold end, advantageously less than 1% in absolute value of the mean temperature of the common cold end;

d2) setting the operating phase of each of the N cryorefrigerators.