Pulse Tube Cryocooler with Compact Size and Decreased Dead Volume

The present invention generally relates to cryogenic refrigerators and, more specifically, to pulse tube cryocoolers (PTC).

In cryocooling technology, generally two types of processes exist, recuperative cryocoolers and regenerative cryocoolers. In recuperative cryocoolers, the cooling fluid is cycled in a continuous flow. Typical recuperative flow cycles are the Brayton or the Joule-Thomson-process.

In contrast, in regenerative cryocoolers, the cooling fluid does not flow in a continuous cycle, but oscillates instead. An example for a regenerative cryocooler is a Stirling-machine operated in a cooling cycle mode. As is known to a person skilled in the art, the traditional Stirling cooler has a moving displacer at each of its hot and cold ends. Instead of moving displacers, valved compressors can be used as suggested by Gifford and McMahon. These latter types of refrigerators are therefore called Gifford-McMahon-refrigerators (G-M-refrigerators).

An important improvement for regenerative cryocoolers is the pulse tube cryocooler (PTC) first invented in the mid 1960s by Gifford and Longsworth (W. E. Gifford and R. C. Longsworth, Pulse tube refrigeration, Trans, of the ASME, Journal of Engineering for Industry, paper No. 63-WA-290, (1964)). Compared with conventional regenerative cryocoolers such as G-M- and Stirling refrigerators, the PTC eliminates the moving displacer at the cold end. This feature results in easy fabrication, much lower vibration and electromagnetic interference, smaller coaxial heat loss, higher reliability and in many cases a longer life time . These advantages are a strong appeal to researchers, and many important structural improvements have been made since the early 1980s. Nowadays, the PTC has become one of the most efficient regenerative cryocoolers for a given size. As promising next generation cryocoolers, PTCs have already been developed for a wide variety of important applications in military, civil, medical and scientific fields.

PCTs can be divided into two types based on their drivers. The first type is usually referred to as "Stirling-type", because this type employs a linear compressor with a piston or a plunger to

linearly move the working gas, just as conventional Stirling cryocoolers usually do. In these Stirling-type PTCs, the frequency of the compressors is identical with the oscillation frequency of the working fluid in the tube. Stirling PTCs are usually operated at frequencies above 30 Hz.

At temperatures below 60 K, PTCs typically work with frequencies as low as 1 to 2 Hz. In order to keep the volume of the compressor small, it is advantageous to decouple the compressor from the pulse tube such that both systems can be optimized independently of each other. The compressor can then be operated at a higher frequency of e.g. 50 Hz to provide a constant high- and low pressure region. The compressor then utilizes a valve system that alternately connects the hot side of the regenerator with low and high pressure. The frequency of valve switching can be adjusted to the desired operation frequency of the PTC and can be chosen to be much smaller than the frequency of the compressor. Since this valve switching is similar to the construction of the above mentioned Gifford-McMahon-refrigerater (G-M- refrigerator), PTCs with such valve compressors are usually called G-M-PTCs.

Stirling-type PTCs which mainly aim at miniaturization, reliability, long life and high efficiency, are gradually replacing Stirling cryocoolers, especially in military and space fields (such as infrared sensors for missile guidance, satellite based surveillance, atmospheric studies of ozone hole and greenhouse effects).

G-M-type PTCs are ideal substitutes for conventional cryocoolers in supplying low-noise cooling for cold electronics such as semiconductor or superconducting detectors, sensors or superconducting magnets, cryomedial instrumentation such as MRI systems or SQUID instruments, cryobiology such as cryosurgery and organ preservation, industrial and commercial applications such as liquification or separation of gases, cryopumping or sensors for nondestructive evaluation and process monitoring, and advanced scientific research.

In Fig. 1, the basic PTC 10 as originally invented by Gifford and Longsworth is shown. The basic PTC 10 comprises a G-M-type compressor 12 which is in fluid connection with the hot end of a regenerator 14. At the hot end of the regenerator 14, a heat exchanger 16 is disposed, which is usually called "aftercooler". The aftercooler 16 is at ambient temperature.

The regenerator comprises a porous or fibrous material having a high heat capacity. Further, the PTC comprises a pulse tube 18. A cold heat exchanger 20 is placed between the regenerator 14 and the pulse tube 18. This cold heat exchanger 20 absorbs heat from an object to be cooled (not shown) by the PTC 10. Finally, a hot heat exchanger 22 is provided at the hot end of the pulse tube 18 and is at ambient temperature. In the present documents, terms like "hot" or "cold" of course always refer to the PTC when it is in operation.

The following description of the functionality is based on the heat pump theory. The basic PTC of Fig. 1 is operated in a two-step cycle. In a first step, the compressor 12 generates high pressure. Accordingly, the working gas is compressed and moved slightly to the right in Fig. 1. Thereby, the temperature rises above the ambient temperature. Heat is transferred to the walls of the pulse tube 18 and the gas acquires ambient temperature again.

In the second step, the compressor 12 generates low pressure. The working gas expands and moves slightly to the left. Thereby, the temperature will decrease. Heat from the wall of the pulse tube 18 will be transferred to the gas and the gas warms up again.

Essentially, heat is transferred from a low pressure position to the high pressure position of the gas within the pulse tube 18. The regenerator 14 prevents heat from flowing from the left into the pulse tube 18. This is achieved by the fast heat exchange between the gas and the regenerator 14. The cold heat exchanger 20 is thus cooled, and heat is accumulated at the right hand side of pulse tube 18 and is dissipated to the environment via the hot heat exchanger 22.

The basic PTC 10 as shown in Fig. 1 has only a comparatively small efficiency. Many structural improvements have been made since the early 1980s, and a so called "double inlet PTC" with increased efficiency has been developed since the early 1990s. A schematic illustration of a double inlet PTC 24 is shown in Fig. 2. In Fig. 2 and all of the following figures, similar or like parts may be denoted by identical reference signs. Particularly, Fig. 2 shows an in-line type double inlet PTC, i.e. a construction in which the regenerator 14 and the pulse tube 18 are arranged one after another along a straight line. Compared with the basic PTC 10 of Fig. 1, the in-line double inlet PTC 24 of Fig. 2 additionally comprises a reservoir 26 which is connected with the hot end 22 of the pulse tube 18 via a reservoir- flowpath 28, and a bypass- flowpath 30 connecting the hot ends 16, 22 of the regenerator 14 and the pulse tube 18, respectively. A first flow restriction means 32 is disposed in the reservoir- flowpath 28. This

first flow restriction means is usually called "orifice" in the art and may be a needle valve. A second flow restriction means 34 is provided in the bypass-flowpath 30 and is usually called "double-inlet valve" in the art. The double-inlet valve 34 may be a needle valve as well. Also, a Stirling type compressor is also shown in Fig. 2 to indicate that the double-inlet PTC 24 can be driven by either one of a G-M-type compressor 12 or a Stirling-type compressor 35. The double-inlet PTC 24 of Fig. 2 allows for a much higher efficiency than the basic PTC 10 of Fig. 1.

With the orifice 32 and the reservoir 26, the operation is quite different from the operation of the basic PTC of Fig. 1. When gas flows from the compressor 12, 35 to the right in Fig. 2, heat is transferred to the regenerator 14 and stored therein. When the gas returns to the compressor 12, 35, the regenerator 14 transmits heat to the gas. The cold heat exchanger 20 absorbs heat from the object to be cooled and transmits this heat to the gas oscillating within the pulse tube 18. The hot heat exchanger 22 transmits heat transported by the oscillating gas to the environment. The reservoir 26 provides a buffer volume which could be for example 10 times the volume of the pulse tube 18, such that the pressure in the reservoir 26 is nearly constant over time.

The combination of orifice 32 and the buffer volume of the reservoir 26 is used to establish a phase difference between the mass flow of the gas and the pressure in the system. Without such phase difference, the temperature would only oscillate within the system, but no cooling effect would be obtained. The orifice 32 in combination with reservoir 26 alone is responsible for a great increase in efficiency as compared to the basic PTC 10 of Fig. 1. A further increase in efficiency is obtained by the bypass-flowpath 30, which was first suggested from Zhu Wu and Chen in 1990. This bypass-flowpath 30 allows a small portion of the gas in the PTC 24 to directly flow between the compressor 12, 35 and the hot end of the pulse tube 18. The amount of flow is adjusted by the valve 34.

This bypassing of the regenerator 14 and the pulse tube 18 may at first sight appear to reduce the cooling efficiency of the PTC to some extent, since less gas passes the cold heat exchanger 20 to absorb heat. Also, the pressure drop at valve 34 leads to irreversibilities. However, by adding the bypass-flowpath 30, the amount of entropy generated in the regenerator 14 is decreased so significantly that this advantage outweighs the aforementioned disadvantage and leads to an improved overall cooling efficiency compared to a situation having ori-

fice 32 and reservoir 26, but no bypass-flowpath 30. For a more detailed explanation, reference is made to P. J. Storch, R. Radebaugh, and J. E. Zimmerman, "Analytical Model for the Refrigeration Power of the Orifice Pulse Tube Refrigerator", NIST Technical Note 1343 (1990); S. Zhu et al., "Double inlet pulse tube refrigerator-an important improvement", Cryogenics 30, (1990), 514; and R. Radebaugh, "Development of the Pulse Tube Refrigerator as an efficient and reliable cryocooler", Proc. Institute of Refrigeration, Vol. 1999-2000, London, (2000), 1.

The in-line double-inlet PTC 24 of Fig. 2 has the practical disadvantage that the cold heat exchanger 20 is located in the middle of the PTC, which is often difficult to access by the to be cooled object, in particular since the object to be cooled must be isolated from the after- cooler 16 and the hot heat-exchanger 22. For this reason, two variants of the double-inlet PTC 24 are common. The first is the so called U-type double-inlet PTC 36 shown in Fig. 3 and the second is the coaxial double-inlet PTC 38 shown in Fig. 4. The U-type double-inlet PTC 36 of Fig. 3 is very similar to the in-line double-inlet PTC 24 of Fig. 2 except that, as the name suggests, the regenerator 14 and the pulse tube 18 are arranged in parallel to each other, and their respective cold ends are connected with a pipe 40. An object to be cooled can then easily be brought in contact with cold heat-exchangers 20a and 20b and at the same time be sufficiently remote from the aftercooler 16 and the hot heat-exchanger 22.

The coaxial double-inlet PTC 38 of Fig. 4 differs in that the regenerator 14 and the pulse tube 16 are not arranged next to each other (such as to form the legs of a U), but the pulse tube 18 is coaxially disposed inside the regenerator 14.

While the U-type and coaxial double-inlet PTCs 36, 38 already have attractive performance in comparison to many traditional cryo-refrigerators, there is still room for improvement as to efficiency and practicability. Accordingly, it is an object of the present invention to improve the known double-inlet PTCs of Figs. 3 and 4 with regard to practicability and efficiency.

This object is achieved by a PTC with the features of claim 1. Preferable embodiments are defined in the dependent claims.

According to the invention, the PTC comprises a holding structure to which the hot ends of the regenerator and the pulse tube are mounted, and at least the bypass-flowpath is formed

inside the bulk of the holding structure, hi addition, the reservoir flowpath may also be formed inside the bulk of the holding structure. The bypass-flowpath and/or the reservoir- flowpath may for example be formed by bores within the holding structure.

By integrating the bypass-flowpath in the holding structure, this flowpath can be designed as short as possible. With reference again to Fig. 3, it can be seen that the bypass-flowpath 30 has a considerable length and therefore includes a considerable volume of working gas. The gas contained in the pipes forming the bypass-flowpath 30 do not contribute to the work of the cycle. Accordingly, the volume contained in the bypass-flowpath 30 can be viewed as dead volume. The dead volume is harmful for the efficiency of the cryocooler. In general, for a fixed swept volume compressor, the larger the dead volume, the smaller is the efficiency thereof. Especially for miniature PTCs, sometimes the dead volume even accounts for the larger fraction of the overall volume, which may lead to a poor refrigeration performance.

Further dead volume is contained in the part of the reservoir-flowpath 28 between the hot end 22 of pulse tube 18 and orifice 32 of prior art PTC 36 of Fig. 3. Accordingly, it is advantageous to integrate at least this part of the reservoir-flowpath within the holding structure as well. Finally, the volume of a pipe connecting compressor 12, 35 with the cold end 16 of the regenerator 14 resembles further dead volume. Accordingly, at least a part of the flowpath connecting the compressor 12, 35 and the regenerator may further be formed inside the bulk of the holding structure.

Accordingly, using the holding structure of the invention, the dead volume can be significantly reduced and the efficiency of the PTC can correspondingly be increased.

Integrating the respective flowpaths in the holding structure has the further advantage that it allows to dispense with external pipes that are used for conventional double-inlet PTCs. Due to the external pipes, conventional double-inlet PTCs have a loose and often clumsy structure. In many practicable applications, especially in space, underground or other special scientific experiment fields, room is strictly limited, and a compact cooling system is highly desirable. So a second advantage of the invention is that it provides for a more compact PTC.

In a preferred embodiment, the holding structure comprises a first bore connecting the hot end of the pulse tube and the first flow restriction means, a second bore connecting the hot end of

the regenerator and the second flow restriction means and a third bore connecting the second flow restriction means and the first bore. In addition, the holding structure preferably further comprises a fourth bore connecting the hot end of the regenerator with the compressor.

In a preferred embodiment, the holding structure is a plate like structure, and in particular, a flange structure. The holding structure preferably also serves as a hot-end heat-exchanger. In such an embodiment, the holding structure simultaneously provides mounting of the pulse tube and the regenerator, heat exchange and the conduits necessary for a double-inlet PTC in a simple, compact structure and with minimal dead volume.

Preferably, the holding structure has a first and a second side, wherein the regenerator and the pulse tube are attached to the first side thereof and the reservoir is attached to the second side thereof. The reservoir may contain a reservoir casing attached to or attachable with the second side of the holding structure. Herein, the term "casing" shall comprise any type of enclosure having any shape. Preferably, however, the reservoir casing is dome- or bell-shaped.

In a preferred embodiment, the first and second flow restriction means are accessible from the second side of the holding structure. The flow restriction means will typically be adjustable such that their flow resistance is set such as to allow for maximum cooling efficiency. This adjustment of the flow restriction means is preferably performed in test cycles under operation of the PTC. hi order to allow this adjustment during operation, the PTC preferably comprises an auxiliary reservoir connectable with the reservoir flowpath when the reservoir casing is detached from the holding structure for providing access to the flow restriction means. Preferably, this auxiliary reservoir is comprised of the reservoir casing and a cover board attachable to the reservoir casing. That is, one does not need to provide a full additional reservoir to serve as the auxiliary reservoir, but it is sufficient to provide a cover board which closes the reservoir case when the reservoir case is detached from the holding structure to provide access to the flow restriction means.

The holding structure may comprise a first part facing the first side and a second part facing the second side of the holding structure. Then, the first, second and third bore may be formed in the second part of the holding structure, and the fourth bore may be formed in the first part of the holding structure.

In a preferred embodiment, the first and/or second flow restriction means may comprise a threaded element projecting into the reservoir-flowpath and/or the bypass-flowpath, respectively, wherein the extent of projection into the respective flowpath is adjustable by turning said threaded element. Preferably, said threaded element is a screw-like member having a conical front end, a threaded intermediate portion and a screw head provided at its rear end.

Preferably, the threaded element is located at a corner portion of the respective flowpath. In particular, the threaded element is preferably coaxial with one of the two portions of the respective flowpaths forming said corner portion. Such structure of the flow restriction means can be very easily integrated into the holding structure and at the same time allows for very precise adjustment of the flow resistance caused by the flow restriction means.

The regenerator and the pulse tube may be arranged in parallel adjacent to each other. That is, the PTC may be a U-type double-inlet PTC as generally shown in Fig. 3.

Alternatively, the pulse tube may be arranged coaxially inside the regenerator. That is, the PTC may be a coaxial double-inlet PTC as generally shown in Fig. 4. In this case, a flow straightener is preferably provided at the cold end of the pulse tube for straightening the flow entering the pulse tube at its cold end. Namely, a difficulty with coaxial double-inlet PTCs is that the flow direction is turned by 180° when the gas exits the regenerator and enters the pulse tube or vice versa. The purpose of the flow straightener is to cause the flow of the gas to enter the pulse tube or regenerator in a laminar flow manner directed along the axis thereof without causing turbulence, such as to preferably generate a one-dimensional standing wave within the pulse tube. This "straightening" of the flow is what the flow straightener is for.

In a preferred embodiment, the flow straightener has a cap-like structure covering the end of the pulse tube and having through holes provided therein, said through holes communicating with the pulse tube and the axis of said through holes being parallel to the axial direction of the pulse tube. In addition, the cap-like flow straightener preferably has a dome-like surface facing away from the pulse tube. The dome-like surface of the flow straightener provides guiding of the gas upon turning its flow direction by 180° when it moves between the pulse tube and the regenerator and vice versa.

Preferably, the flow straightener has channels on its outer circumference, said channels communicating with the regenerator and being parallel with the axis of the pulse tube.

Brief Description of the Drawings

Fig. 1 shows a schematic view of a basic pulse tube cryocooler (PTC).

Fig. 2 shows a schematic view of an in-line type double-inlet PTC.

Fig. 3 shows a schematic view of a U-type double-inlet PTC.

Fig. 4 shows a schematic view of a coaxial double-inlet PTC.

Fig. 5 shows a sectional view of a U-type PTC according to an embodiment of the invention.

Fig. 6 shows a sectional view of a coaxial PTC according to an embodiment of the invention.

Fig. 7 shows an enlarged view of a part of Fig. 6.

Fig. 8 shows a cross-section of a flowpath-corner portion in which the flow-restriction means is provided.

Fig. 9 is a side view of a threaded element of the flow restriction means.

Fig. 10 is a front view onto the head portion of the threaded element.

Fig. 11 is a sectional view of the U-type PTC of Fig. 5, wherein the reservoir casing is removed from the holding structure and closed by a cover board.

Fig. 12 is a sectional view of a flow straightener.

Fig. 13 is a top view onto the flow straightener of Fig. 12 in axial direction.

Fig. 14 is a sectional view of the coaxial PTC of Fig. 6, wherein the reservoir casing is removed from the holding structure and closed by a cover board.

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of invention is hereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates.

In Fig. 5, a cross-sectional view of a U-type PTC 42 according to an embodiment of the invention is shown. PTC 42 comprises a holding structure 44 which is a plate-like structure or hot-end flange of the PTC. The holding structure 44 comprises a first part 44a facing toward a first direction (downward in Fig. 5) and a second part 44b facing in a second direction (upward in Fig. 5). On the first part 44a of the holding structure 44, a regenerator 14 and a pulse tube 18 are mounted with their hot ends, while their cold ends are connected with a U-shaped pipe 46. Cold heat exchanges 20a, 20b are provided at the cold ends of the regenerator 14 and the pulse tube 18, respectively.

Since the hot ends of the regenerator 14 and the pulse tube 18 are each mounted to the first part 44a of the holding structure 44, the holding structure 44 acts as a hot-end heat-exchanger and thus serves the same function as the aftercooler 16 and hot heat exchanger 22 of the conventional U-type double-inlet PTC shown in Fig. 3.

In the second part 44b of the holding structure 44, a first bore 48 is drilled which extends vertically from the hot end of the pulse tube 18 toward a first flow restriction means 50. Also in the second part 44b, a second bore 52 is drilled extending vertically from the hot end of regenerator 14 to a second flow restriction means 54. hi addition, a third bore 56 is provided to connect the second flow restriction means 54 with the first bore 48. In Fig. 5, the left end of the third bore 56 and the upper end of the second bore 52 form a corner portion, in which the second flow restriction means 54 is disposed. Similarly, a fifth bore 58 is formed connecting the first flow restriction means 50 with the outside of the first part 44a of holding structure 44 and forming a corner portion with the first bore 48. Finally, a fourth bore 60 is formed in the first part 44a of holding structure 44 communicating with the hot end of the regenerator 14 and a port 62 for mounting a compressor (not shown).

Accordingly, the second bore 52, the third bore 56 and the lower half of the first bore 48 form the bypass-flowpath 30 described with reference to the Fig. 2 to 4 above. The first bore 48 and the fifth bore 58 form the reservoir flowpath 28 described with reference to Fig. 2 to 4 above. Finally, the fourth bore 60 forms a compressor flowpath. The volume contained in the first to fourth flowpaths is a dead volume which does not contribute to the cooling. However, as can be seen from Fig. 5, this dead volume is extremely minimized by integrating the respective flowpaths into the holding structure 44. hi addition, using the first to fifth bore, ex-

temal pipes which tend to be clumsy, loose and space consuming are dispensed with, leading to a very compact structure.

On the first part 44a of holding structure 44, a vacuum chamber dome 64 is mounted which in combination with the holding structure 44 forms a vacuum chamber 66. Vacuum chamber 66 can be evacuated via a vacuum port 68.

On the second part 44b of holding structure 44 a reservoir dome 70 is mounted. That is, holding structure 44 and reservoir dome 70 in combination form the reservoir 26 of the PTC.

hi Fig. 6, a coaxial PTC 72 of the invention is shown. The structure of coaxial PTC 72 is very similar to the structure of U-type PTC 42 of Fig. 5, and the description of like structures and elements is not repeated. The main difference is that in the coaxial PTC 72 of Fig. 6, the pulse tube 18 is coaxially disposed in a regenerator tube 14. At the cold end of pulse tube 18, a cap- like flow straightener 74 is shown, which is described in more detail below. Further flow straighteners 76 and 78 are provided at the hot ends of regenerator 14 and pulse tube 18, respectively.

The structure of holding structure 44 of coaxial PTC 72 is very similar to that of holding structure 44 of the U-type PTC shown in Fig. 5. In particular, holding structure 44 of coaxial PTC 72 also comprises first to fifth bores having the same function and similar geometries as the first to fifth bores described with reference to U-type PTC 42 of Fig. 5. Note, however, that the structure is even more compact in the case of the coaxial PTC 72 and that the dead volume is even further decreased.

hi Fig. 7, a section of coaxial PTC 72 of Fig. 6 is shown in an enlarged view, hi this section, the first to fifth bores 48, 52, 56, 60, 58 provided in holding structure 44 are again shown. As can be seen in Fig. 7, the first flow restriction means 50 is formed at a corner portion of the reservoir flowpath 28, where the first and fifth bores 48, 58, meet. A threaded element 80 is coaxially provided within the first bore 48 and is projecting into the reservoir-fiowpath 28 at the corner portion thereof. The threaded element 80 is shown in an enlarged side view and in a front view in Fig. 9 and 10, respectively, and an enlarged view of the corner portion at which the first and fifth bores 48, 58 meet is shown in Fig. 8.

As can be seen in Fig. 9, the threaded element 80 comprises a conical tip 82, a cylindrical portion 84, a threaded portion 86 and a screw head 88. A view onto screw head 88 along the axial direction of threaded element 80 is shown in Fig. 10. An O-ring 90 is disposed in an associated groove of threaded element 80.

With reference to Fig. 8, a female thread 92 is provided in the holding structure 44 close to the corner at which the first and fifth bores 48, 58 meet. As can be seen from Fig. 7, the threaded portion 86 of threaded element 80 can be screwed into the female thread 92 provided in the holding structure 44, such that the conical tip 82 of threaded element 80 projects into the upper end of first bore 48. By turning the threaded element 80 using an ordinary screwdriver, the flow resistance of first flow restriction means 50 can be adjusted. The second flow restriction means 54 has the same structure as the first flow restriction means 50.

As can be seen from Fig. 5 and 6, the threaded elements 80 of first and second flow restriction means 50, 54 are accessible from the second side of holding structure 44. However, during normal operation, the second side of holding structure 44 is covered by reservoir dome 70, such that access to flow restriction means 50, 54 is not possible. For optimizing the performance of the PTC, the flow resistance of flow restriction means 50, 54, i.e. the position of threaded element 80 must be delicately adjusted, and this adjustment can most conveniently be done during operation of the PTC.

Fig. 11 shows how an adjustment of flow restriction means 50, 54 can be performed during operation of the parallel type PTC 42 of Fig. 5. As is seen in Fig. 11, during adjustment the reservoir dome 70 is removed from holding structure 44, and it is closed by a cover board 94 instead. Dome 70 and cover board 94 thus form an "auxiliary reservoir" to be used during the adjustment procedure. A port 96 provided in cover board 94 is connected with the outlet of fifth bore 58 via a connecting tube 98. With this arrangement, the threaded element 80 of the first and second flow restriction means 50, 54 are easily accessible while PTC 42 is in operation. Note that although an additional connecting tube 98 is provided, this connecting tube can be regarded as a part of the auxiliary reservoir and does not have any effect on the performance of the PTC. In other words, an adjustment of the first and second flow restriction means 50, 54 established in the state of Fig. 11, where the reservoir dome is removed from the holding structure 44, will lead to the same behaviour of the PTC when reservoir dome 70 is mounted to holding structure 44 during ordinary operation.

Fig. 14 shows how an adjustment of flow restriction means can be performed during operation of the coaxial type PTC 72 of Fig. 6. Again, during adjustment, the reservoir dome 70 is removed from holding structure 44, and it is closed by a suitable cover board 94 instead. The functionality and the process of adjusting the flow restriction means is identical to the situation of Fig. 11 and will not be repeated here.

As can be discerned from the forgoing description, holding structure 44 has multiple purposes. First of all, it serves as a mounting flange for mounting the regenerator 14, the pulse tube 18, the vacuum chamber dome 64 and the reservoir dome 70. Secondly, holding structure serves as the hot- end heat-exchanger. And third, holding structure 44 provides the conduits for the reservoir-flowpath 28 and the bypass-flowpath 30 without any need for external piping.

hi Fig. 12, the cross section of flow straightener 74 of Fig. 6 is shown. Flow straightener 74 has a cap-like structure with a dome- shaped outer surface 100 and a cylindrical inner volume 102, in which pulse tube 18 is inserted.

Fig. 13 is a top view onto flow straightener 74 from above. As can be seen in Fig. 12 and 13, a number of parallel holes 104 are provided in an inner portion of flow straightener 74, which are parallel to the center axis of pulse tube 18. In addition, a number of outer channels 106 are provided at the circumference of flow straightener 74.

When working gas moves from regenerator 14 into pulse tube 18, it flows along the dome- shaped surface 100 and passes through the set of parallel holes 104. Accordingly, the flow of gas is straightened such as to provide a nearly one-dimensional standing wave in pulse tube 18. Conversely, if gas moves from pulse tube 18 into regenerator 14, the flow of the gas will be straightened by the gas passing through said outer channels 106. With flow straightener 74, turbulences in the working gas can be suppressed.

Although a preferred exemplary embodiment is shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiments are

shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of the appending claims.

List of Reference Signs

basic pulse tube cryocooler (PTC) G-M-type compressor regenerator aftercooler pulse tube cold heat-exchanger hot heat-exchanger in-line type double-inlet PTC reservoir reservoir-flow path bypass-flow path orifice double-inlet- valve Stirling-type compressor U-type double-inlet PTC coaxial double-inlet PTC pipe U-type PTC according to an embodiment of the invention holding structurea first part of holding structureb second part of holding structure connecting pipe first bore first flow restriction means second bore second flow restriction means third bore fifth bore fourth bore

compressor port vacuum chamber dome vacuum chamber vacuum chamber port reservoir dome coaxial PTC according to an embodiment of the invention flow straightener flow straightener flow straightener threaded elemenrt conical tip cylindrical portion threaded intermediate portion screw head

O-ring female thread cover board cover board port connecting tube dome shaped surface of flow straightener 74 cylindrical volume inside flow straightener 74 through holes outer channels