Low-temperature refrigerator with cold head and a process for op

Details Low-temperature refrigerator with cold head and a process for op Low-temperature refrigerator with cold head and a process for op

1998-05-20

2000-05-23

Capossela, Ronald

Refrigeration

Gas compression, heat regeneration and expansion, e.g.,...

60520, 165 4, 165 10, F25B 900

Patent

active

060652950

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a low-temperature refrigerator for producing temperatures in a defined low-temperature range, with a cold head which is fitted with a regenerator and in which a thermodynamic circulation process (regenerative cold generation process) is carried out to generate cold. Moreover, the present invention relates to a process for optimising said cold head for a desired temperature range as well as applications.
It is known to employ low-temperature refrigerators in connection with cryogenic pumps or the cooling of magnets in nuclear spin tomographs. In these applications the cold head shall generate temperatures in the range of 20 K or below. Commonly therefore, refrigerators designed to have two or three stages operating according to the Gifford-McMahon.sup.1) principle, for example, are employed in these applications. However, at these low temperatures the refrigerating capacity of these machines is limited. In commercially available two-stage cold heads, the refrigerating capacity amounts to about 5 to 10 W at a temperature of 20 K. If the refrigerating capacity "Gifford-McMahon" would be appropriate and in line with the remaining text as well as other publications on this topic. Thus the word "Gifford-McMahon" has been assumed for the translation. requirement is higher, several two-stage cold heads must be employed in order to attain the corresponding refrigerating capacity through the total.


SUMMARY OF THE INVENTION

It is the task of the present invention to reduce the technical complexity of generating low temperatures, and moreover, to apply a relatively simple means in order to make the cold head for a refrigerator of the aforementioned kind particularly suitable for a certain temperature range.
This task is solved by the present invention in that the cold head is designed to be of the single-stage type, and that the physical characteristics of the refrigerating machine's regenerator are so selected that the cold head delivers an optimal refrigerating capacity within the desired temperature range. Alone by employing a single-stage refrigerating machine, the technical complexity is already reduced significantly. Moreover, it has been determined that also single-stage refrigerating machines are capable of generating temperatures below 20 K. In this it has been found, surprisingly, that with cold heads which are equipped with suitably designed regenerators, much higher refrigerating capacities can be attained also in the temperature range around 20 K, as would be possible with the two-stage cold heads mentioned above. The root cause for this improvement in refrigerating capacity is, on the one hand, the special design of the regenerator. On the other hand, in two-stage cold heads a part of the refrigerating capacity is consumed for the first stage and it is thus not optimally utilised at low temperatures, so that the attained efficiency is in total poor. Therefore the realisation of the present invention thus permits--instead of the previously commonly used two-stage refrigerating machine for temperatures around 20 K--the deployment of a simple and thus reliable single-stage machine with the additional benefit of an increased refrigerating capacity.
Cold heads of the type designed according to the present invention are especially suited in applications, in which the target temperatures are slightly above 20 K. Through the selection of the properties of the regenerator material and corresponding layering of different materials, the cold head or the refrigerating machine may be optimised with respect to the desired temperature range.
An example for an application in which the employment of a cold head designed according to the present invention is particularly advantageous, is the cooling of high-temperature superconductors (HT.sub.c material) with transition temperatures T.sub.c above the temperature of liquid nitrogen, in particular in applications involving high currents or strong magnetic fields (magnets for the Tesla range, energy storage, MRI, curre

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