BACKGROUND OF THE INVENTION Superconductive phenomenon where an electrical resistance disappears in the ultra low temperature around 4K, has been widely applied to devices using a high magnetic field superconductive magnet. Such devices use, as a refrigerant, a liquid helium which is expensive, to create an ultra low temperature condition. Such devices also use a cryovessel which is specially designed and manufactured in consideration of thermal condition so as to suppress evaporation of helium. Recently, since a GM cryocooler which can be cooled to the temperature lower than 4K has been developed, a superconductive magnet that adopts a conductive cooling system, in which superconductive state is maintained by directly cooling the bobbin of electromagnet without using a refrigerant, has been developed and applied to a small-sized high magnetic field generating electromagnet.

The above-described superconductive magnetic has advantages in that the configuration of the magnet and operating method became more simple than the conventional method of using the liquid helium as a refrigerant, while achieving a reduced maintenance cost. However, the superconductive magnet still has drawbacks in that the magnetic is weak to the thermal disturbance since it is positioned in the vacuum state and has no refrigerant around.

In an ultra low temperature state, the portion for leading current into the superconductive magnet has mechanical stresses concentrated thereon during the thermal

contraction of bobbin, which may cause a mechanical vibration. Moreover, vibrations may be caused due to the electromagnetic force during the excitation of electromagnet when the support structure for the superconductive coil is not sufficiently strong. Thus-caused mechanical vibrations may generate heat which quenches superconductive characteristics of the current lead-in portion, resulting in the quenching of overall superconductive state of the magnet. The current lead-in portion and the bobbin are electrically insulated from each other, and thus form a thermal insulation relationship. Therefore, in the superconductive magnet adopting a conductive cooling system where no refrigerant is employed, heat generated by the mechanical vibration may prevent the current lead-in portion from being cooled. This may result in a significant damage to the superconductive state.

Furthermore, in cases where superconductive characteristic is quenched while the electromagnet is in operation, the electromagnet may be changed into a phase conductor, which causes emission of remarkable amount of heat. A superconductive magnet adopting the conventional conductive cooling system uses a thermally isolated diode circuit to emit thermal energy. However, problems still exist in that the thermal energy is concentrated to the part of magnet because such a superconductive magnet adopting the conventional conductive cooling system has no refrigerant around, resulting in the damage of the superconductive coil.

SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to overcome the above-described problems of conventional superconductive magnet, and provide a bobbin for superconductive magnet using a GM cryocooler in that advantages are achieved as follows; First, bobbin has a configuration capable of overcoming the difficulty of electrical insulation between the current lead-in portion and bobbin.

Second, means for improving cooling efficiency through the enhancement of thermal conductivity of bobbin is provided.

Third, a current lead-in portion is directly coupled to the bobbin so as to improve mechanical strength and prevent unnecessary thermal energy.

Fourth, means for reliable attachment of superconductive coil is provided.

Fifth, conducting means for achieving an electrical insulation between the GM

cryocooler and bobbin while improving conductive cooling from the GM cryocooler toward the bobbin is provided.

Sixth, a superconductive magnet adopting a conductive cooling system and having an improved performance is provided, while ensuring a stable operation of magnet through the bobbin of the present invention.

To achieve the above-described objects of the present invention, there is provided a bobbin for superconductive magnet using a GM cryocooler, the bobbin including a bobbin constituted by a central insulating portion and upper and lower conduction portions each of which has an end coupled to both ends of the central insulating portion and the other end with edges, the bobbin having a superconductive coil being wound along the outer periphery thereof and fixed to the edges of upper and lower conduction portions of the bobbin; a rectangular thermal capacitor having a side an upper end of which being perpendicularly coupled with a lower end of a head portion of the GM cryocooler; a conductive cooling rod having an end coupled with upper and lower ends of the other side of the thermal capacitor, with an insulation member interposed between the conductive cooling rod and the thermal capacitor, the conductive cooling rod having the other end coupled to sides of edges of the bobbin; a rod cable serving as a current lead-in portion, and which is coupled to sides of the edges of the bobbin so as to apply an electric power from a power source to the bobbin; and a pair of diodes connected in parallel to a protrusion formed at the other sides of the edges.

The thermal capacitor is constituted by outer PI (polyimide) films for insulation and a plurality of copper foils stacked and fixed between flanges and press plates which are spaced apart from each other. Preferably, the thermal capacitor is fixed to the conductive cooling rod via a bolt through a flange portion.

The rod cable coupled to the lower edge of the bobbin is fixed to the upper edge of the bobbin by a thermal anchor so as to improve cooling and fixing effects, and an insulating plate is interposed between the anchor and the upper edge of the bobbin, to thereby achieving improved mechanical durability and insulation properties.

The coupled portion between the edges of the bobbin and both ends of the superconductive coil is soldered to a coil insertion hole of the groove formed at the edges of the bobbin so as to ensure a reliable fixation.

DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view illustrating a coupling relation between the bobbin for superconductive magnet and GM cryocooler according to the present invention; Fig. 2 is an exploded perspective view illustrating the bobbin of the present invention; Fig. 3 is a perspective view illustrating the bobbin with a groove for fixation of superconductive coil; Fig. 4 is a perspective view illustrating a coupling relation among the GM cryocooler, thermal capacitor and conductive cooling rod; Fig. 5 is a schematic view illustrating a coupling relation between diodes and superconductive coil; and Fig. 6 is a perspective view illustrating flow of heat and current at the bobbin upon damage of superconductive magnet.

DETAILED DESCRIPTION OF THE PRESENT INVENTION A bobbin for superconductive magnet using GM cryocooler according to the present invention will be explained in more detail with reference to the attached drawings.

Referring to Figs. 1 and 2, a bobbin 1 of the present invention is constructed in three stages constituted by a central insulating portion 7 which is formed of a material like an FRP so as to achieve improvements in mechanical strength and electrical insulation, and an upper conduction portion 3 and a lower conduction portion 5 each end of which is coupled to both ends of the central insulating portion and other ends with edges 3a and 5a. The upper and lower conduction portions are formed of a material like a coalesced copper, tough pitch copper and an aluminum which have a superior thermal conductivity around the temperature of 4K. As shown in Fig. 2, it is preferable that the central insulating portion 7 and conduction portions 3 and 5 are screw-coupled with each other through female screws 3f and 5f and a male screw 7a. It is more preferable that rings 9a and 9b formed of a metal (for example, indium (In), lead and aluminum) having a high thermal conductivity and flexibility are interposed between the central insulating portion 7 and conduction portions 3 and 5 so as to achieve an improved thermal contact.

The bobbin 1 which is coupled in three stages, has a superconductive coil (designated

as C in Fig. 5) wound along the outer periphery of the bobbin, and both ends of the coil are fixed to the edges 3a and Sa of the bobbin 1. As shown in Fig. 3, it is preferable that the superconductive coil C and the bobbin 1 are fixed in such a manner that both ends of the coil are inserted and soldered to insertion holes 3e and 5e of grooves 3d and 5d formed at the edges 3a and 5a of the bobbin.

A thermal capacitor 20 and conductive cooling rods 31 and 33 are arranged to improve a conductive cooling performance for the bobbin 1 through a GM cryocooler 10.

The thermal capacitor 20 is shaped as a rectangle and has an upper end of a side thereof being perpendicularly coupled to a lower end of a head portion 11 of the GM cryocooler 10.

As shown in Fig. 4, it is preferable that the thermal capacitor 20 is configured in such a manner that outer polyimide (PI) Elms 23a and 23b (for example, Kapton of DuPont Company) and copper foils 21a and 21b are stacked between flanges 31a and 33a of conductive cooling rods 31 and 33 and press plates 25a and 25b formed of a material like an FRP, so as to thereby accomplish both thermal connection and electrical insulation. The press plates are spaced apart from each other at a predetermined distance. It is preferable that the thermal capacitor 20 is firmly fixed to conductive cooling rods 31 and 33 via a bolt B through flanges 31a and 33a. The thermal capacitor 20 having copper foils 21a and 21b is firmly pressed to the conductive cooling rods 31 and 33 through the bolt B so as to improve a thermal conductivity. In cases where the thickness of the thermal capacitor 20 and the spacing between flanges 31 and 33a of conductive cooling rods 31 and 33 are not matched with each other, it is difficult to compress the thermal capacitor 20. Therefore, the thermal capacitor 20 is divided into two parts, and compressed and fixed toward flanges 31a and 33a of conductive cooling rods 31 and 33through press plates 25a and 25b and the bolt B. It is understood by those skilled in the art that the thermal capacitor can be constituted by a single layer. It is also possible that the thermal capacitor 20 is fixed to the head portion 11 of the GM cryocooler 10 through the bolt B.

Referring back to Fig. 1, conductive cooling rods 31 and 33 for conducting the cooling effect generated from the GM cryocooler 10 from the thermal capacitor 20 to the bobbin 1 have ends coupled to upper and lower ends of the other side of the thermal capacitor 20 while contacting members, and the other ends coupled to edges 3a and 5a of the bobbin 1. Conductive cooling rods 31 and 33 are formed of a coalesced copper, tough

pitch copper and an aluminum which have a superior thermal conductivity.

The power lead-in portion for applying a power to the bobbinl from a power source is constituted by rod cables 35 and 37 which are coupled to edges 3a and 5a of the bobbin 1.

The rod cables 35 and 37 are connected to grooves 3d and 5d where the superconductive coil C is soldered. Preferably, the rod cable 37 coupled to the lower edge 5a of the bobbin 1 is fixed to the upper edge 3a by a thermal anchor 37a so as to improve cooling and fixing effects. In case where the anchor 37a is an electrical conductor metal material having a superior thermal conductivity, like coalesced copper, it is preferable that an insulating plate 37b is interposed between the anchor 37a and the upper edge 3a so as to prevent short circuit at the upper edge 3a.

As shown in Figs. 1 and 5, a pair of diodes 39a and 39b are connected in parallel to protrusions 3b and 5b which are formed at edges 3a and 5a and electrified so as to by-pass the current and discharge electrical energy when the magnet is broken down. As shown in Fig. 3, protrusions 3b and 5b of edges 3a and 5a have coupling holes 3c and 5c for diodes.

As described above, the bobbin 1 of the present invention is constituted by the central insulating portion 7 and conduction portions 3 and 5 arranged at both ends of the bobbin 1.

Conduction portions 3 and 5 are electrically connected to a power source through rod cables 35 and 37 serving as a current lead-in portion, and divided through the central insulating portion 7 as a boundary. The superconductive coil C taken up at the bobbin 1 has both ends electrically connected at grooves 3d and 5d. Conduction portions 3 and 5 of the bobbin 1 serve as a current lead-in portion, and mechanically fix both ends of the superconductive coil C through insertion holes 3e and 5e communicated to grooves 3d and 5d, respectively.

The bobbin 1 has a thermally strong coupling force through the GM cryocooler 10, thermal capacitor 20 and conduction cooling portions 31 and 33, thus the thermal coupling force between the superconductive coil C and rod cables 35 and 37 is reinforced. Since rod cables are reinforced through the thermal anchor 37a in terms of a mechanical and thermal view, and electrically insulated from the upper edge 3a through the insulating plate 37b, the bobbin 1 has a significantly enhanced thermal conductivity, resulting in a superior conductive cooling efficiency.

A superconductive magnet enters a superconductive state by being cooled to 4K or lower by the GM cryocooler 10, and the bobbin 1, rod cables 35 and 37 coupled to the

bobbin 1, and diodes 39a and 39b are cooled to the ultra low temperature state through a conductive cooling upon starting of operation of the GM cryocooler 10. Heat generated from each component is emitted to outside through the thermal capacitor 20 and conductive cooling portions 31 and 33. The thermal capacitor 20 of the present invention is formed by stacking thin copper foils 21a and 21b for ease of coupling and protecting the head portion 11 of the GM cryocooler 10 from stresses during thermal contraction.

When the superconductive magnet enters a superconductive state after elapse of cooling time, a current is applied through rod cables 35 and 37. Here, the superconductive magnet is operated at the voltage lower than the forward direction voltage of diodes 39a and 39b so as to prevent diodes 39a and 39b from being electrified with the current. As shown in Fig. 5, diodes 39a and 39b are coupled in parallel to the superconductive coil C and may not electrified at the voltage lower than the forward direction voltage.

In case where the superconductive magnet has escaped from the superconductive state due to the problems including a mechanical vibration or an abnormal operation of the GM cryocooler 10, the superconductive magnet is partially turned into a phase conducting state, and a voltage exceeding the forward direction voltage of diode is generated at the magnet. Here, diodes 39a and 39b are electrified and by-pass the current flowing at the magnet, and an electrical energy is converted into a thermal energy and consumed. As shown in a dotted arrow mark I in Fig. 6, since diodes 39a and 39b are positioned in the direction opposite to the rod cables 35 and 37 serving as a current lead-in portion, the current flows across conduction portions 3 and 5 of the bobbin 1 through diodes. Since conduction portions 3 and 5 of the bobbin 1 have electrical resistances, heat is generated by the current.

Thus-generated heat is diffused all over the bobbin 1 as shown in a solid line arrow mark T in Fig. 6, thus avoiding thermal energy from being locally concentrated to a specific part of the magnet As described above, the bobbin for superconductive magnet using GM cryocooler according to the present invention has advantages as follows; First, difficulty of electrical insulation between the current lead-in portion and bobbin is eliminated through the bobbin which is coupled in three stages by upper and lower conduction portions and central insulating portion.

Second, cooling efficiency is improved through the enhancement of thermal

conductivity between the bobbin and GM cryocooler as well as the thermal conductivity of the bobbin itself through the thermal capacitor, conduction cooling portion and thermal anchor.

Third, mechanical strength of the bobbin is improved through the press plate, grooves for soldering and thermal anchor, to thereby prevent generation of unnecessary thermal energy.

Fourth, thermal conductivity among components of the bobbin and thermal conductivity between the bobbin and GM cryocooler are enhanced through the press plate, PI film, insulating plate, central insulating portion and ring, while achieving an electrical insulation.

Fifth, since diodes are electrically and thermally connected in parallel to the bobbin, the current is by-passed upon breakdown of the magnet. Thus, electrical energy is emitted in a thermal energy, to thereby prevent local concentration of heat.

Sixth, since heat is compulsorily conducted through the bobbin of the present invention such that the heat is prevented from concentrating to the initial quenching point upon occurrence of quenching of superconductive characteristics caused due to a variety of sources, the magnet is protected from being damaged or burned. As a consequence, it is possible to obtain a high performance superconductive magnet adopting a conductive cooling system where a stable operation of the magnet is secured.