US 3317651 A
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Description (OCR text may contain errors)
s. osmass 3,317,651
LOW TEMPERATURE DEVICE WITH A CURRENT SUPPLY MEMBER May Z, 1967 Filed Nov. 30, 1965 INVENTOR. SEWCE'E DEIMESfi AGE@? United States Patent 6 Claims. (01. 174-15 The present invention relates to a device for supplying current, in particular to a device operating at low temperature, for example, a device which is arranged in a cryostat in which the cold liquid is formed, for example, by liquid helium and which has to be fed by a strong current which may reach, for example, several hundreds of amperes.
.' .The insertion of current supply members having a large cross-section in the cryostat causes loss of cold (liquid helium) as a result of the conductivity and the Joule effect, which loss must be kept as low as possible by a correct choice of the various characteristic parameters of the material, which are: the electric resistance the coefficient of thermal conductivity (x) the cross section (S), the surface (E) available for heat exchange. Such a choice is determined by the following requirements: On the one hand the metal must be readily electrically conducting (a small without on the other hand being too readily heat-conducting (A must be small); on the one hand the cross-section must be so large that the Joule heat produced is moderate but on the other hand it must be so small that the thermal conductivity is kept within bounds; the surface (E) must be so large that dissipation of the Joule heat is possible so that a considerable heat supply to the regions with the lowest temperature is avoided.
In the most commonly used devices the current supply members are constituted by copper wires. In general, wires are used of a small cross-section, so with a low thermal conductivity. However, this involves that the Joule heat produced will be considerable while the produced heat can be dissipated with difiiculty only since the surface for heat-exchange is small (even insufficient) when rib bons are used. As a result of this a considerable amount of heat will penetrate to the helium level.
To avoid this drawback it is known to provide the current supply wires with ribs which facilitate the dissipation of the heat, but do not cooperate in the current passage. Such a system is found to be very bulky and on the one hand impedes the visibility for the operators, while on the other hand it is impossible to provide therein further devices for which no entrance apertures would be provided. In addition it is necessary to provide supporting members for the copper wires to give them the required rigidity (for example, stainless steel pipe) which increases the thermal losses and the bulkiness considerably.
The present invention mitigates the various drawbacks. The device according to the invention which is of the type which comprises at least one current supply member of readily electrically conducting and poorly heat-conducting material is characterized in that the cross-section of each of the current supply members decreases from the part of the device which is in contact with the warmer surroundings to that which is in contact with the colder surroundings.
In order that the invention may readily be carried into effect, it will now be described in greater detail, by way of example, with reference to the accompanying drawing, in which:
FIGURES 1 and 2 show two preferred embodiments of a current supply member with decreasing cross-section, and
3,317,651 Patented May 2, 1967 FIGURES 3 shows a device comprising two current supply members.
In the embodiment shown in FIGURE 1, the current supply device is constituted by a bundle of pipes 1 of a conducting material and having a constant thickness and cross-section, the upper ends of which are secured to a yoke 2 which likewise consists of conducting material and has the shape of a segment in such manner that the pipes extend in parallel with one another over some distance. The length of the pipes differs (for example, the pipes 1b and 1c have a length of two and three times respectively the length of the pipe 1a). Each level of the bundle is bounded by a yoke which connects the lower ends of the pipes of a given length to the pipes of a longer length.
In the embodiment chosen the number of levels is restricted to 3, the last level being constituted by one pipe, the lower end of which is to be connected to the apparatus to be supplied. In these circumstances the total crosssection of the bundle of pipes decreases from the upper part which is in contact with the warmer surroundings to the lower part which is in contact with the colder surroundings.
As a result of this the current supply member has a large surface for heat exchange in its upper part and a small cross-section in its lower part so that the heat which is produced as a result of the Joule effect is laterally dissipated without propagating too much along the conductor in the direction of the regions of low temperature, which would cause the evaporation of helium to increase. To facilitate the cooling of the inner Walls of the pipes, said pipes are provided with small apertures by which a flow of the cold gas through the pipe is possible, and a ready cooling is obtained, also of the inner wall.
The material of the current conductor, for example, pure nickel, has the following characteristic features.
The term (hp) which occurs in the known theoretical calculations of the optimum conditions and in which a is the heat conductivity and p the electrical resistance is small. It has been found that said term for nickel is smaller than for any other metal in the temperature range to be considered between 4 and 300 K.
It makes it possible to obtain optimal cross-sections both from a point of view of thermal losses and of mechanical properties.
In the embodiment shown in FIGURE 2 the pipes of the same length are provided between two subsequent yokes 2, the number of which pipes decreases from one level to the next in the direction of the regions of low temperature; the pipes of two successive levels are not in alignment so that the cold gas easily penetrates into the pipe and traverses a shorter distance which results in a better cooling. In this case the apertures 3 may be omitted.
FIGURE 3 shows a combination of three coaxial current supply members. The pipes 1 have small diameters. As far as the outer current supply member is concerned, the pipes engage one another and are soldered or welded together throughout their length; the pipes of the inner current supply member are welded or soldered to the outer surface of a guiding pipe 4 which has a larger diameter than the pipe 1 to increase the surface of heat exchange. The pipes have a length which is different so that an optimum cross-section of the current supply member is obtained for the temperature prevailing at a given level as a function of the current to be conducted. The diameter of the pipes may be very small (for example, a diameter of 2 mm.) and the number of pipes may be large. The variation of the total cross-section is substantially continuous.
In addition it may be noted that because nickel can so easily be soldered or welded (to tin) a member may be altered arbitrarily to perfect the optimum properties thereof while the dismantled elements can be used again for a new device.
Such devices can withstand the current passage of 200 ampere and over without causing too great evaporation of the helium.
The invention is not restricted to the embodiments described and variations may be made, in particular in as far as the nature of the conductors used (pipes or other profiled conductors with ready rigidity), their number (if required only one), the material used (nickel may be replaced, for example, wholly or partially by iron, aluminum and so on) are concerned without leaving the scope ofthis invention.
In addition it may be noted that in the embodiment chosen the decrease of the cross-section of the assembly is obtained by decreasing the number of pipes but this can also be obtained by any other means in particular by using pipes of decreasing cross-sections.
What is claimed is:
I. In a low temperature device, a current supply member for said low temperature device of good electrical conduction and poor heat conduction, said current supply member being constituted of a plurality of conductors of approximately the same cross-section, said conductors being located in interconnected levels, each of said conductors in each of said levels being parallel to one another, the number of conductors in each level decreasing from that part of said current supply member which is in contact with a warmer part of said low temperature device to that part of said current supply member which is in contact with a colder part of said low temperature device.
2. In a low temperature device as claimed in claim 1 further comprising yokes of conducting material connecting the ends of each of the conductors on one level with adjacent conductors on the next level, and each of the groups of the pipes on one level having a different length than adjacent groups of pipes on a different level.
3. In a low temperature device, a current supply member for said low temperature device of good electrical conduction and poor heat conduction, said current supply member being constituted of a plurality of conductors of approximately the same cross-section, said conductors being in groups spaced from each other, and each of the conductors in one of said groups having a length which is different from each of the other conductors in another of said groups.
4. In a low temperature device as claimed in claim 3 wherein said groups of pipes having different lengths engage one another throughout their lengths and are secured together.
5. In a low temperature device as claimed in claim 1 wherein said conductors are in the form of pipes having relatively small apertures therein.
6. In a low temperature device as claimed in claim 2 wherein said conductors and said yokes are constituted of nickel.
References Cited by the Examiner UNITED STATES PATENTS 485,315 11/1892 Yaryan 165-146 2,538,957 1/ 1951 Askevold 165185 2,992,538 7/1961 Poganski 165--185 LEWIS H. MYERS, Primary Examiner.
L. E. ASKIN, Examiner.
E. GOLDBERG, Assistant Examiner.