Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2001-12-21
2003-12-02
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C505S220000, C505S230000, C505S706000, C505S879000
Reexamination Certificate
active
06657526
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a power lead for a superconductive magnet to supply a current to superconductive coils of the superconductive magnet to be mounted on a vehicle of a levitated train, and for use in, e.g. an energy storage unit, a power generator, a medical equipment.
2. Description of the Related Art
The most widely used means at present for supplying a current to superconductive coils of a superconductive magneto unit comprises disposing a power source in a room temperature section of the superconductive magneto unit and supplying a current from the power source to the superconductive coils which are placed on a cryogenic section using a lead wire (hereinafter referred to as a power lead) made of copper.
However, according to the method of supplying a current using the power lead made of copper, heat loss (deterioration of cooling efficiency) caused by the employment of the power lead cannot be ignored.
The heat loss caused by the employment of the power lead comprises a loss caused by Joule heat produced by electric resistance of the power lead and another loss caused by heat which inflows through the power lead due to thermal conduction, and it occupies most of the total heat loss in a superconductive magneto unit.
If a sectional area of the power lead is rendered large, Joule heat becomes small. However, in that case, heat inflowing from the outside becomes large. On the contrary, if the sectional area of the power lead is rendered small, the heat inflowing from the outside becomes small, but Joule heat becomes large.
Accordingly, the sectional area of the power lead is set at a proper value by which the foregoing two heat losses are balanced.
Meanwhile, in a normal superconductive magneto unit, the cryogenic section in which superconductive coils are placed is cooled by liquid helium supplied thereto wherein the unit is operated as liquid helium is supplied to the cryogenic section and a helium gas which is evaporated at the cryogenic section is collected. However, since the helium gas immediately after evaporated is at a cryogenic temperature of 4.2 K, the power lead is cooled by the collected evaporated helium gas, thereby reducing the heat loss at the cryogenic section.
There is employed a power supply member (it is also referred to as a gas cooling lead wire) having a structure to seal a power lead inside a gas transfer tube through which evaporated helium gas is collected so as to cool the power lead by the foregoing evaporated helium gas.
For example,
FIG. 2
is a schematic view showing a main portion of a superconductive magneto unit for a vehicle which is used by a testing vehicle of a levitated train, and comprises baths
3
,
3
that are cooled at a cryogenic temperature i.e. at the temperature of liquid helium (4.2 K) (hereinafter referred to as liquid helium temperature) and disposed inside a radiant heat shielded section (radiant heat shielded system), which is kept at a liquid nitrogen temperature (77.3 K) while shielded by a radiant heat shielded plate
2
provided for preventing invasion of heat from the outside of an outer bath
1
, superconductive coils
4
,
4
which are sealed and installed inside the inner baths
3
,
3
, and an external power source by a gas cooling lead wire (a power lead
5
made of copper is sealed by a coolant gas transfer tube
6
) for connecting between an external power source and the superconductive coils
4
,
4
.
In
FIG. 2
, depicted by
7
is permanent current switches (which are cooled at a liquid helium temperature) wherein terminals of the superconductive coils are short circuited in a superconducting state or manner when a current supplied thereto reaches a given value so that a permanent current flows inside the superconductive coils
4
,
4
.
Depicted by
8
is a thermal anchor (which is formed of a metal block having high thermal conductivity such as pure copper and is cooled at a liquid nitrogen temperature) serves to prevent a heat from invading from the outside to a cryogenic section through the power lead
5
.
However, even if the power lead has the foregoing gas cooled structure, the length of the power lead has to be rendered as long as possible so that the invasion of heat can be sufficiently prevented and heat can be escaped while contacting the coolant gas because the power lead member made of copper is very excellent in thermal conductivity as well as electric conductivity (in the unit shown in
FIG. 2
, the power lead is rendered long to the extent of 1 m and is accommodated in a U shape). As a result, it is difficult to render the unit small sized, and the generation of Joule heat increases when the power lead is rendered long so that the heat loss reduction effect fell short of an intended value.
Further, the power lead having the gas cooled structure has to be used in the manner that it is cooled by evaporated helium gas when turning on an electricity so that the generation of Joule heat increases, causing a problem that an electricity cannot be turned on for a long period of time.
SUMMARY OF THE INVENTION
Therefore, it is an object of the invention to provide a power lead for use in superconductive magnet capable of effectively controlling the invasion of heat to a liquid helium system (inner baths and permanent current switches) without influencing upon the miniaturization or simplification of a superconductive magneto unit, but supplying a current for a long period of time while securing current-carrying capacity.
Accordingly, inventors of this application endeavored themselves to achieve the foregoing object and obtained the following finding.
That is, although high temperature oxide superconductors each having a critical temperature which exceeds a liquid nitrogen temperature have been sequentially discovered and techniques for make up a bulk body thereof having a high critical current density have been remarkably developed, the high temperature oxide superconductor bulk body is very low in thermal conductivity compared with a metal material such as copper which has been so far applied to the power lead member for a superconductive magnet. Accordingly, if at least a part of “a section positioned inside a radiant heat shielded section” of a power lead member of a superconductive magnet for supplying a current to “superconductive coils which are cooled at a liquid helium temperature” through “the radiant heat shielded section which is cooled to the liquid nitrogen temperature” is replaced by such a high temperature oxide superconductor bulk body having a low thermal conductivity but high critical current density, the high temperature oxide superconductor bulk body having a low thermal conductivity becomes the obstruction to the heat transfer so that invasion of heat from the radiant heat shielded section to the permanent current switches or inner baths (inner bath system) through the power lead member can be remarkably reduced.
That is, since the high temperature oxide superconductor bulk body section becomes an obstacle for preventing invasion of heat from the radiant heat shielded section to the permanent current switches or inner baths, even if the lead member extending to the high temperature oxide superconductor bulk body is formed of a copper wire having a large sectional area through which heat inflows becomes large, the heat inflowing to the permanent current switches and the inner baths can be effectively prevented by the high temperature oxide superconductor bulk body section, and also a large current can be supplied to the power lead member under the condition where Joule heat generates less by the use of the power lead member having a large sectional area.
It is a matter of course that since the high temperature oxide superconductor bulk body section per se is kept up at a temperature under a transition temperature (liquid nitrogen temperature), electric resistance becomes 0, so that a large current can be supplied to the power lead member without loss, i.e. without generation of heat.
Further, when the high temperature oxi
Herai Toshiki
Murakami Masato
Nagashima Ken
Nemoto Kaoru
Tomita Masaru
Barrera Ramon M.
Flynn ,Thiel, Boutell & Tanis, P.C.
International Superconductivity Technology Center
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