Abstract:
Retractable cryogenic leads for superconducting magnets are provide for making low resistance contacts at liquid helium temperature reliably in the presence of heavy frost. A tube defining a bayonet socket on one end extends through a vertical stack cover of a cryostat insert to engage and disengage a vertical terminal rod having a transverse pin. A copper rod is slidably mounted inside the tube and is vertically displaced relative to the tube by a nut rotatably mounted on the other end of the tube and threadingly engaged with the copper rod. The end of the copper rod can be forced against the top of the vertical terminal rod at high pressure after the bayonet socket engages the pin of the vertical terminal rod.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to copending applications, Ser. No. 826,058, entitled &#34;A Horizontal Cryostat Insert With a Vertical Service Stack&#34; and Ser. No. 826,051 entitled &#34;Spring Loaded Valve for Adding Cryogenic Liquid To a Cryostat&#34;, filed on even date herewith and assigned to the instant assignee. 
     BACKGROUND OF THE INVENTION 
     The present invention is generally directed to mechanical devices for making secure electrical connections to superconductive coils disposed within cryostats. 
     In the generation of images in medical magnetic resonance diagnostic systems, it is necessary to provide a temporally stable and spatially homogeneous magnetic field. The use of superconductive electrical materials maintained at a temperature below their critical transition temperatures provides an advantageous means to produce such a field. Main superconductive electrical coil windings are disposed in a vessel containing a cryogenic fluid such as liquid helium. Main coils and correction coils which provide the desired magnetic field uniformity are disposed in a cryostat which is essentially a thermal insulating device. The superconductive windings exhibit the particular advantage that electrical energy need not be supplied to the circuit once the desired current level is achieved in the main coils and the correction coils. However, electrical connections must be made to these interior coils at various intervals such as in the case of a quench condition in which the superconductive windings undergo a transition to the normal, resistive state and the current flow is dissipated. Additionally, it is desirable to be able to adjust the currents in the correction coils from time to time to compensate for changes in the uniformity of the magnetic field as a result of changes in the position of external ferromagnetic objects. In superconducting magnets, the main magnet coils can carry a current of 2,000 amperes while the correction coil currents are typically no more than 50 amperes. 
     While the correction coils and the main magnet coils typically comprise superconductive material, circuit energization is generally accomplished by means of normal (that is, resistive) conductors, which penetrate the nested set of vessels of the cryostat and significantly impair their insulating function or increase the rate of helium evaporation. It is desirable therefore to make and break electrical connections at superconducting temperatures, to minimize helium boiloff in steady state operation when the leads are retracted. Because of the extremely low temperatures (4° K.) at which the connections are made and broken, there is a very strong tendency for frost to form on the electrical contacts. This frost typically includes both ice and solidified air. This frost, whether ice, air or both, can significantly impede the formation of a good electrical connection between the interior magnet circuit and an exterior energizing source. Electrical and mechanical properties of the contact surfaces must be adequate at the cryogenic temperatures since a high resistivity contact junction will generate heat and unnecessarily boil off cryogenic coolants. 
     A cam lever mechanism for a retractable lead system is described in my copending application Ser. No. 621,330 filed June 15, 1984 and assigned to the instant assignee. The cam lever mechanism tightens a split-ring terminal around the leads to exert high pressure at the contacts. One of the mating surfaces is serrated and preferably silver plated, and the other is coated with a thick layer of soft metal such as indium. Wear of the cam and linkages in the low temperature environment is a concern with regard to the life and reliability of the device especially when it is inaccessible for maintenance or repair. 
     It is an object of the present invention to provide a compact arrangement for high contact force electrical connections that can be operated by means of accessible, ambient-temperature, mechanical components. 
     It is another object of the present invention to provide high current leads that can make and break contact repeatedly and still attain a low resistivity connection. 
     SUMMARY OF THE INVENTION 
     A retractable lead assembly is provided for a cryostat having an aperture. The retractable lead assembly comprises a tube defining a bayonet socket at one end and means for slidably and rotatably disposing the tube through the cryostat aperture. The tube extends inside and outside the cryostat with the bayonet end of the tube extending inside the cryostat. An electrically conductive rod is slidably disposed inside the tube. An electrically conductive terminal post with a transverse pin adapted to be engaged by the bayonet socket is mounted in the cryostat. Means external to the cryostat for rotating and slidably moving the tube to engage and disengage the socket with the terminal post are affixed to the tube. Means external to the cryostat are coupled to the rod and the tube for adjusting the relative displacement between the rod and the tube, so that the rod can be urged against the post after the socket engages the pin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
     FIG. 1 is an exploded isometric view of a cryostat and a cryostat insert in accordance with the present invention; 
     FIG. 2 is a partial cross sectional view of the insert positioned in the cryostat penetration; 
     FIG. 3 shows the relationship of FIGS. 4A, B, C and D. 
     FIGS. 4A, B, C and D together show a cross-sectional view of the cryostat insert with selected parts rotated to more easily show features in cross section; 
     FIG. 5 is a partial cutaway view showing in a more detail a portion of FIG. 4C; 
     FIGS. 6A and 6B show a cross sectional view of a spring loaded valve on the end of a liquid helium fill tube situated in the cryostat, with the valve in open and closed position, respectively, FIG. 6C shows a cross-sectional view of the spring loaded valve of FIGS. 6A and B; 
     FIG. 7 is a partial cutaway cross sectional view of the horizontal portion of the 4° K. isothermal envelope located in the exterior portion of the insert, when the insert is situated in the cryostat; 
     FIG. 8A and B are the lower and upper portions, respectively, of a partially cutaway isometric view of the exterior portion of the horizontal 4° K. envelope, part of the vertical 4° K. envelope extension and the vertical stack cover including the retractable lead assembly, when the insert is situated in the cryostat; and 
     FIG. 9 is a partial cross sectional view of the retractable lead assembly in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing wherein like elements are indicated by like numerals throughout and particularly FIG. 1 thereof a cryostat 11 defining a cryostat penetration 13 together with a cryostat insert 15 are shown. The cryostat 11 comprises a cylindrical shaped vessel 16 defining a horizontal bore 17 therethrough, suitable for use in magnetic resonance imaging systems. Extending from the cryostat 11 through the cryostat penetration 13 is an electrical lead assembly 21 providing connections to a superconducting magnet and shimming coils inside the cryostat 11. 
     The cryostat insert 15 comprises a horizontal tubular outer housing having a first portion 23a which is adapted to be inserted in the cryostat penetration 13 and a second portion 23b which remains outside the cryostat when the first portion is inserted. Extending radially in the vertical direction from the second portion of the external housing is a vertical stack 25, having a burst disk 27 extending perpendicularly from the top end of the stack. A slanted liquid helium transfer tube housing 31 also extends radially from the second portion 23b of the tubular housing, longitudinally separated from the vertical stack 25 and extending at a 45° angle, measured about the longitudinal axis of the horizontal tube from the vertical stack. 
     Referring how to FIG. 2 a portion of the cryostat 11 with the insert in position inside the cryostat penetration 13 is shown. The cryostat comprises an inner annular helium vessel 33 which contains magnet windings 35 of superconductor wire for generating a magnetic field as well as shimming coils 37 for field correction. The cryostat penetration extend into the upper portion of the helium vessel. 
     The helium vessel 33 is surrounded by an intermediate shield 41 cooled by helium vapor boiloff and maintained at a temperature below 40° K. The intermediate shield is surrounded by a nitrogen vessel 43 which also serves as a shield and maintains a temperature of approximately 80° K. due to nitrogen cooling. 
     The first portion of the horizontal tubular housing 23a extends into the cryostat through a cryostat penetration tube 45 which is part of cryostat. Penetration tube 45 defines the cryostat penetration 13 as well as extending outside the cryostat. A clearance gap is created between the outside of the first portion of the tubular housing 23a and the inside of the cryostat penetration tube 45, where helium vapor form the helium vessel 33 flows and intercepts heat conducted through the tube 45 and the insert portion 23a from the ambient temperature outside the cryostat. The gap extends from the ambient to the inside of the helium vessel 33. The flow of helium vapor through the gap is guided by means of two or more variable pitch spiral Teflon® insulated wires 47 that are wrapped around the external housing of the insert 23a. When two wires are used, they start on opposite sides on the housing and spiral around spaced apart from one another. The spiral flow of helium vapor suppresses the secondary flow that could be induced in the gap as a result of the helium density gradient from 4° K. to 300° K. Vapor cooling of the penetration tube 45 and the insert housing tube 23a is the most effective way to reduce the heat transferred to the liquid helium in the helium vessel 33. A portion of the helium vapor flowing in the gap is diverted to flow through a spiral of aluminum tubes 51 adjacent to and in contact with the flat circular end of the cylindrically shaped intermediate shield 41. The remaining helium vapor continues along the gap between the outer housing 23a and the inside of the cryostat penetration tube 45 and is vented through a hole in a flange 53 welded to the outside of penetration tube 45 that extends beyond the cryostat. Flange 53 is supported by bolts 55 from the exterior of the cryostat. A flange welded around the outside of the outer housing 23a of the insert is bolted to the cryostat flange 53. 
     A 4° K. isothermal envelope 59 surrounds the electrical lead assembly 21 extending from the cryostat. The 4° K. envelope is in flow communication with the interior of helium vessel 33. To reduce radiation heat transfer from the ambient temperature of the outside of the tubular housing 23a to a 4° K. isothermal envelope 59, two copper thermal radiation shields 61 and 63 are employed each of which surround the 4° K. envelope. In addition, a vacuum is maintained between the outer tubular housing 23 and the 4° K. envelope 59. Getter material 64 is situated in the end of the first portion of the housing where the outer housing is welded to the 4° K. envelope. The getter material serves as a molecular sieve to absorb gases that would degrade the vacuum. Gases including helium and air might enter the vacuum due to metal porosity or small leaks at welded joints. The inner radiation shield 61 of the insert is heat stationed to the intermediate shield 41 of the cryostat by means of a copper ring 71 which surrounds the penetration tube 45. A copper ring 73 located inside the insert 15 is situated concentric with copper ring 71. Copper ring 73 conducts heat from the inner shield 61 of the insert to ring 71 and to the intermediate shield 41. Conduction cooling occurs between the inner shield 61 of the insert and the cryostat intermediate shield 41 to maintain a temperature below 40° K. at the inner shield 61. Similarly, outer radiation shield 63 is heat stationed to the nitrogen vessel 43 by means of a copper ring 65 surrounding the penetration tube 45. Tube 45 is concentric with a copper ring 67 situated inside the horizontal housing 23a. Ring 67 conducts heat from the outer shield 63 to ring 65 and then to the nitrogen shield 43. Conduction cooling occurs between the outer radiation shield 63 and the cryostat nitrogen vessel, maintaining an 80° K. outer radiation shield temperature. The heat stations which comprise copper rings 65 and 71 are also cooled by convection heat transfer to the helium vapor through spiral flow passages defined by wires 47 surrounding the first portion of the horizontal tubular housing 23a. 
     Referring now to FIGS. 4A, B, C and D, the insert 15 is shown in cross section prior to insertion in the cryostat with FIGS. 4A, B, C, and D arranged as shown in FIG. 3. The electrical lead assembly 21 is shown in phantom. For ease of illustration in cross section, the burst disk 27 has been rotated 90° about the vertical axis of the vertical stack 25 and the slanted liquid helium fill tube housing 31 has been rotated about the longitudinal axis of the horizontal tubular housing 23a and b, as compared to their actual position shown in the isometric view of FIG. 1. 
     The space between horizontal tubular housing 23a and b and the 4° K. envelope 59 forms an evacuable jacket around the 4° K. envelope 59 which can be evacuated at pump out port 75, located at the exterior end of horizontal housing 23b. The radiation shields 61 and 63 are spaced apart from one another, the 4° K. envelope 59 and the outer housing 23 to avoid conduction thereamong. The copper radiation shields 61 and 63 each comprise a cylindrical tube located in housing portion 23a which transitions to a larger cylindrical tube in housing portion 23b. The two tubes of each of the shields are joined by transition pieces all of which are joined together by soldering. The 4° K. envelope extends vertically inside the vertical service stack where it is affixed to the outer wall by a cylindrical bellows which surrounds the upper portion of the 4° K. tube. The envelope is joined to the upper end of the bellows and the lower end of the bellows is affixed to the outer wall of the service stack. The connection using bellows 77 provides flexible support of the 4° K. vapor cooled extension in the vertical stack that will maintain a sealed vacuum jacket. 
     Referring now to FIGS. 4A, B, C and D, and 5, vapor cooling of the extension of the 4° K. envelope in the vertical stack is accomplished by a fiberglass tube 81 having a flange 83 on its upper end and a spiral groove 85 on its outer surface. The fiberglass tube 81 is bonded to the inside of the extension of the 4° K. envelope 59, forming a spiral passage extending up the inside of the stack, providing vapor cooling of the 4° K. envelope extension. Inner shield 61 extends partway up the vertical stack and is secured around the 4° K. envelope extension by a spring 87 of the type shown in U.S. Pat. No. 4,562,703, heat stationing that portion of the vertical stack. U.S. Pat. No. 4,562,703, entitled &#34;Plug Tube for NMR Magnet Cryostat&#34;, and assigned to the instant assignee is hereby incorporated by reference. Further up in the tube the outer shield 63 is secured around the vapor cooled 4° K. envelope extension by a spring 91. The shields are conduction cooled by convection heat transfer to the helium vapor through the spiral flow passages. The helium vapor from the spiral flow passages is vented at the top of the stack via tube 93. 
     The copper radiation shields 61 and 63 each comprise cylindrical tube sections located in housing 23a which transition to larger cylindrical tubes in housing 23b, ending with end plates closing the end of the tubes. The cylindrical tubes open into and extend the shields up into the vertical stack. Shield 63 has an additional cylindrical tube section extending partway into the slanted fill tube 31. All the sections of each of the shields are joined together, preferably by soldering. 
     A cylindrical bellows 77 is affixed at its lower end to the housing wall 25 and at its upper end to the top edge of the 4° K. envelope creating a vacuum tight enclosure between the terminator of the 4° K. envelope in the vertical stack and the housing wall. 
     The bellows 77 accommodates the axial thermal contraction of the 4° K. envelope in the horizontal portion of the insert relative to the housing of the vertical stack. The vacuum insulation between the exterior of the 4° K. envelope and the interior of the housing results in a vertical unbalance force on the vertical extension of the 4° K. envelope in the vertical stack which could overstress the welds of the horizontal portion of the 4° K. envelope. To reduce stress on the horizontal portion of the 4° K. envelope, a retainer ring 95 is welded to the inside of the vertical portion of the housing above the bellows 77 to support the flanged end 83 of the fiberglass tube 81 which in turn supports the vertical extension of the 4° K. envelope. 
     To reduce the radiation heat transfer to the liquid helium inside the 4° K. envelope, the inside of the outer vertical housing above the bellows 77 and a cover 97 situated over the top of the stack are lined with foam insulation 101. Aluminum foil 103 is bonded on the inside surface of the foam insulation to serve as a reflective surface. The inside surface of the fiberglass tube is painted black to absorb all the thermal radiation emitted from the reflective surface. Burst disk 27 is located near the top of the vertical stack to serve as a pressure relief device. The top cover 97 has apertures which permit retractable leads 105, 107 and 109 to pass therethrough, which power and shim the magnet 35 in the cryostat 11. 
     Multilayer insulation 111 is wrapped around the 80° K. shield in the exterior portion 23b of the insert 15 to reduce radiation heat transfer. Also, the vacuum side of the 4° K. envelope and the housing 23a and 23b as well as both radiation shields 61 and 63 are silverplated to reduce their emissivity. 
     The slanted liquid helium transfer tube housing 31 (shown rotated in FIG. 4 from its actual position as shown in the isometric view of FIG. 1) has a tube 112 coaxial with the outer housing 31. The end of the tube 112 extends inside the insert and terminates in a housing 113 located inside the 4° K. envelope 59. Tube 112 penetrates the 4° K. envelope, with the 4° K. envelope soldered around tube 112. Tube 112 is in flow communication with two concentric horizontal tubes 115 and 117 which extend the entire horizontal length of the insert and extend from the end of the tubular housing that is inserted in the cryostat, terminating in a spring loaded valve 121. The inner horizontal tube 115 is a liquid helium fill tube and the outer concentric tube 117 is a helium vapor return tube. The tube 112 is coupled to a bleed off nozzle 123 at the exterior portion of the insert. Also coupled to tube 112 is a quick-seal connector 125 which allows a helium filler pipe (not shown) to be inserted through the connector 125 and extend down the inside of tube 112 to the housing 113 allowing liquid helium to flow to the inner tube 115 of two concentric tubes 115 and 117, but blocking the passage of liquid helium from reaching the outer tube 117 through housing 113. The two concentric tubes extend through a horizontal aperture at the center of electrical leads assembly 21 extending from the cryostat, shown in phantom in FIG. 4. The inner radiation shield 61 does not extend into the slanted liquid helium transfer tube housing 31. The outer radiation shield 63 extends only partway into the housing 31. Multilayer insulation surrounds tube 112 and extends into the housing 31. To maintain the vacuum surrounding tube 112, a collar 127 is welded to the inside of the housing vertical wall, partway up the slant tube. The tube 112 extends through the collar 127 and through a bellows 131 one end of which is brazed to the inside diameter of the collar. The other end of bellows 131 is brazed to tube 112 forming a vacuum seal, while allowing relative movement between the housing 31 and the tube 112. The portion of tube 112 extending beyond the collar 127 and surrounded by the bellows 131 is enclosed by a cover 133. 
     Referring now to FIGS. 6A, B which show the spring loaded valve 121 situated in the cryostat fill tube termination block 135 in the open and closed positions, respectively, and FIG. 6C which shows the spring loaded valve, the spring loaded valve 121 comprises a sleeve 137 having a flange 141 at one end. The other end of sleeve extends partway inside the end of liquid helium fill tube 115. A spring 143 surrounds sleeve 137 and is captured between the flange 141 and the welded end of tubes 115 and 117. A valve 145, comprising a valve head 147 and valve stem portion 149, has its valve stem portion extending partway down the center of inner tube 115 and held in the tube by pins 151 extending through the walls of the tube 115 and through the stem 149. The head 147 of the valve has a diameter larger than tube 137, retaining the sleeve 137 in the end of tube 115 when the insert is not situated in the cryostat. The flange 141 however has a larger diameter than head 147, permitting the head to pass through an aperture 153 in block 135, but not the flange 141. 
     An epoxy glass tube 155 extends from the cryostat fill tube termination block 135 through the cryostat electrical lead assembly 21, forming the inner diameter of the electrical lead assembly (see FIG. 2). The epoxy glass tube 155 surrounds the horizontally extending concentric liquid helium fill tube 115 and the vapor return tube 117 when the insert is positioned in the cryostat. 
     Referring now particularly to FIG. 6A, the spring loaded valve 121 is shown in the opening position. Valve head 147 extends through the aperture 153 in the termination block 135. The spring 143 is compressed when progress of sleeve 137 through the block is stopped during insert installation by flange 141 which contacts the wall surrounding aperture 153. With the valve open liquid helium flowing in the inner concentric tube 115 can be supplied to the top of the magnet through aperture 153. 
     Referring now to FIG. 6B, the spring loaded valve 121 is shown in the closed position, which can be used during initial magnet cooldown. The spring loaded valve 121 is closed by retracting the insert 1/4&#34; by adjusting bolts 55 moving flange 53 further from the cryostat. Retracting the insert causes concentric tubes 115 and 117 to be retracted, which in turn retracts the valve head 147 inside the termination block aperture 153, closing aperture 153. Sleeve 137 does not retract since it is still held against the termination block by spring 143. Holes 159 in tube 137 are exposed allowing liquid helium to flow from tube 115 to holes 159 in tube 137 to holes 160 in the epoxy tube 155 to a chamber in the termination block which is in flow communication with a tube 157 leading to the bottom of the helium vessel 33. 
     Referring again to FIGS. 2 and 7, the insert is shown in cross section after insertion in the cryostat. FIG. 7 shows only the portion of the 4° K. envelope and its contents which are located in housing portion 23b. The electrical lead assembly 21 includes two main power leads 161 and 163 (see FIG. 2) comprising concentric copper tubes with longitudinally extending slots on the outside surface of the tubes for vapor cooling. The leads 161 and 163 are canvilevered from rectangular shaped copper terminals 165 and 167 surrounding the tubes located on the magnet support structure 171 of the cryostat. An electrical insulating tube 173 is positioned between the two power lead tubes 161 and 163 and another electrical insulating tube 175 surrounds both copper tubes 161 and 163. An electrical insulating tube 177 extends inside the inner power lead tube 161. Correction coil leads 181, which are part of the electrical lead assembly, extend through the annulus formed between the outside of the epoxy tube 155 and the inside of the insulating tube 177. 
     Referring now to FIGS. 7 and 8A where FIG. 7 shows only the portion of the 4° K. envelope and its contents located in housing portion 23b when the insert is inserted in the cryostat and FIG. 8A is an isometric view of FIG. 7 together with the lower portion of the 4° K. envelope in the vertical stack, the correction coil leads 181 are shown connected to a multipin connector 183 situated at right angles to the electrical lead assembly, with the multipin connector pointing upward. In the portion of the electrical lead assembly outside the cryostat, the concentric copper tubes 161 and 163 terminate in staggered rings 185 and 187 which by means of bus bars 189 and 191 are coupled to vertical terminal rods 193 and 195, respectively, situated on either side of the multipin connector, radially opposed from one another. The vertical terminal rods 193 and 195, each have a horizontal pin 197 and 199 (now shown), respectively, extending through the rod and beyond the rod on either side. The copper rings 185 and 187 are enclosed by an electrical insulating sleeve 201 which leaves the multipin connector 183, bus bars 189 and 191 and vertical terminal rods 193 and 195 uncovered. Insulating sleeve is part of the electrical lead assembly 21 of the cryostat. 
     Referring now to FIGS. 2, 7 and 8A, cooling of the main power leads 161 and 163 and correction coil leads 181 is accomplished by helium vapor flow originating in the cryostat helium vessel 33 as indicated by arrows in FIGS. 2 and 7. The helium vapor flows in the annular containing the correction coil wires, and in the longitudinal slots of the main power leads. The vapor is discharged in the chamber created by the insulating sleeve 201. Pipe 203 provides a discharge outlet for vapor traveling in the slots of the outside main power lead 163, pipe 205 provides a discharge outlet for vapor traveling in the slots of the inner main power lead 161 and pipe 207 provides a discharge outlet for vapor cooling the correction coil leads 181. The vapor passes through holes in a pipe 209 which surrounds the two concentric tubes 115 and 117 and then through holes in helium vapor return tube 115 to housing 113 which is in flow communication with tube 112 inside the slanted helium transfer tube housing 31. Tube 209 is part of the cryostat lead assembly 21. 
     The cryostat insert which has the concentric liquid helium tube 115 and helium vapor tube 117 extending through the electrical lead assembly 21 and the 4° K. envelope 59, shields 63 and 61, multilayer insulation 111 and outer housing 23b surrounding it, provides thermal insulation to the leads and access through the vertical stack 25 of the insert to the multipin connector 183 and vertical terminal rods 193, and 195. 
     Referring now to FIGS. 8A and B, and FIG. 9 a retractable lead assembly 211 is shown, which comprises two power leads 105 and 109 each including a copper rod (shown in FIG. 9) with axial cooling slots 214 surrounded by a stainless steel tube 215 and insulated by a fiberglass sleeve 217 bonded on the outside surface of the stainless steel tube 215. The axial cooling slots are in flow communication with a vent 218 in the center of the copper rod portion that extends outside of the vertical stack. The vent 218 is connected to a flow meter and valve (not shown) which screws in the threaded aperture of the vent. The retractable lead assembly also includes a conduit 107 containing a cable of wires 219 connected at the lower end to a multipin connector 221 located inside the insert and at the upper end to another multipin connector outside the insert (not shown). The retractable lead assembly 211 has conduit 107 extending through the center of the vertical stack cover 97 with the power leads 105 and 107 extending through the vertical stack cover 97 on either side of conduit 107. The three leads 105, 107 and 109 are spaced apart inside the vertical stack by a horizontal guide bar 223. The conduit 107 passes through and is secured to, the guide bar 223. The two power leads 105 and 109 slide in tapered notches located on either end of the guide bar 223. The copper rods 213 inside of each of the power leads 105 and 109 slide relative to the surrounding stainless steel tube 215, within the constraints imparted by nuts 225 situated outside the vertical service stack. Nuts 225 are rotatably captured by the stainless steel tube 215 of each of the power leads 105 and 109, respectively, and threadingly engage the copper rods 213 in each of the stainless steel tubes 215. Power leads 105 and 109 are surrounded by a cap nut 227 which has an internal seal 231 which surrounds the power leads to allow the power leads to slide through the cover 97 without admitting air to the insert. The cap nut is threadingly engaged with a nipple 232 which is brazed to the cover. To allow sliding motion of the conduit 107, the conduit 107 extends through a pipe 233 soldered to the top of the cover and passes through a cap nut 235 which has an internal seal 231 which surrounds the conduit. Cap nut 235 is threadingly engaged with a nipple 237 which is brazed to pipe 233. 
     The bottom end of each of the power leads 105 and 109 has a bayonet socket 241 defined by the stainless steel tube 215. An additional stainless steel tube 243 surrounds the stainless steel tube 215 at the bottom of the tube 215 permitting the bayonet socket to have a double wall thickness to prevent bending of the socket as a result of the load transmitted to the socket when the bayonet socket is engaged. A lever 245 is affixed outside the service stack to each of the stainless steel tubes 215 of leads 105 and 109. Lever 245 is used to manipulate the bayonet sockets 241 to engage the pins 197 and 199 of the vertical terminal rods 193 and 195, respectively. When the bayonet sockets are engaged with the pins, nut 225 is rotated using a spanner wrench (not shown), while holding the copper bar from rotating by means of a lever 247 affixed to the copper rod 213 (outside the stack) to force the copper rod end down the stainless steel tube 215, in contact with the serrated top of vertical rod 193 or 195 at high pressure. The bottom end of the copper rod is coated with a soft metal such as indium, typically 0.010-0.020 inches thick. To assure a low resistance connection the high pressure engagement allows the contacts to break through heavy frost and establish low contact resistance. The correction coils are connected to the retractable leads by pushing the conduit 107 into the stack so that the tapered perimeter of the two halves of the multipin connectors 183 and 221 together with the keys and keyways on the multipin connector assure an aligned joining of the male and female multipin connectors. 
     During installation the insert 15 is slid into the cryostat penetration 13 with the concentric tubes 115 and 117 situated inside electrical lead assembly 21 and the spring loaded valve 121 at the end of the concentric tubes situated in the cryostat fill tube termination block 135. The open end of the isothermal envelope 59 surrounds the electrical lead assembly 21 and extends into the helium vessel 33 of the cryostat 11. The insert flange 57 is bolted to the cryostat flange 53. A vacuum is created between the outside of the 4° K. isothermal envelope 59 and the insert housing by evacuating the air by means of pump out port 75. During magnet cooldown it is desirable to supply liquid helium to the bottom of the helium vessel 33 of the cryostat. This is accomplished by retracting the insert 15 approximated 1/4&#34;, by loosening the bolts at the flange connection closing valve 145 and exposing holes 159. A tube (not shown) is inserted inside tube 112 located in the slant fill tube 31 and liquid helium flows down the inserted tube and through the inner of the two concentric tubes 115 to the termination block 135, down tube 157 to the bottom of the helium vessel 33. The valve is then opened by inserting the insert 15 an additional 1/4&#34; causing any additional helium added after initial cooldown to flow into the top of the helium vessel. The tube is removed from the slant tube. 
     With the cryostat helium vessel filled with helium and the nitrogen shield filled by a separate insert (not shown), insert 15 is able to maintain a 4° K. environment within the 4° K. envelope 59. 
     Helium vapor flow spiralling in the clearance gap between the insert housing and the inner portion of cryostat penetration tube 45 intercepts heat conducted into the cryostat penetration 13 from ambient temperature. To reduce heat transfer from the ambient temperature surrounding the housing 23a and 23b of the insert a vacuum surrounds the inner 4° K. envelope 59 and the two copper radiation shields 61 and 63 are heat stationed to the shields 41 and 43 of the cryostat 11 and shields 61 and 63 are heat stationed in the vertical stack 25. The vertical stack has a vapor cooled inside tube 83 with a spiral vapor path to cool the 4° K. envelope extending partway up the vertical stack. The vapor cooling provided by the spiral grooves of tube help maintain shield 61 below 40° K. and shield 63 heat stationed further up the stack below 80° K. by force convection. The vertical arrangement of the stack also benefits from helium stratification, keeping the colder helium at lower levels in the stack. Bellows 77 accommodate the axial thermal contraction which occurs during cooldown between the housing 23a and 23b and the 4° K. envelope 59. 
     To energize the magnet 35, the electrical connections are made by lowering from the upper portion of the vertical stack the multipin connector 221 to engage its mating half 183. Th power leads 105 and 107 are lowered and rotated to engage the horizontal pins 197 and 199 in the vertical terminated rods 193 and 195. The nut 225 is rotated and the copper rods 213 are lowered in the stainless steel tubes 215 with the soft indium metal at the bottom of the copper rods forced against the serrated tops of the vertical terminal rods 193 and 195 forming a good electrical connection. During energization, helium vapor cooling is used to cool the electrical lead assembly of the cryostat 21 and the copper rods 213. Vents 218 at the ends of the copper rods are opened allowing vapor to flow inside the stainless steel tube which are vented outside the insert. 
     When power is supplied to the magnet windings 35 the main power leads 105 and 109 and the leads of the cryostat lead assembly 21 are not superconductive and helium vapor cooling allows large current to be supplied to the windings in the cryostat while minimizing helium vapor loss and not permitting air to enter the cryostat. When the desired superconducting currents have been established in the cryostat the main power leads 105 and 109 can be disconnected from outside the insert and the leads withdrawn to the warmer top portion of the vertical stack 25 to reduce conduction heat losses. Vent valves in the vents 218 of the copper rods 213 are closed. 
     During operation of the magnet in the persistent current mode helium vapor is vented from tube 93 in the vertical stack and the bleed off nozzle 123 on the slant horizontal fill tube, as well from the flange 53. Helium vapor can be recovered from the outer concentric tube 117 through housing 113 and returned through the slant tube by using a liquifier (not shown) which has tube portions which extend down the slant tube and receive vapor from the aperture in housing 113 and return liquid helium down the center of the slant tube to the inner of the two concentric tubes 115. When a liquifier is not used in the persistent current mode a thin wall tube of low heat conductivity material, such as fiberglass, filled with styrofoam is inserted in the slant fill tube closing off the aperture leading to the center concentric tube 115. The space between the outside of the inserted tube and tube 112 of the slant fill tube permits vented helium to flow from the outer concentric tube 117. Shimming coil corrections can be made by lowering the multipin connector without changing inserts or admitting air into the insert. 
     The foregoing has described a compact arrangement for high contact force electrical connections that can be operated by means of accessible, ambient temperature, mechanical components. The cryogenic leads can carry high currents and can make and break contact and still attain a low resistivity connection. 
     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention.