Patent Publication Number: US-5154637-A

Title: High current cable termination for pulsed power applications

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to high current cable terminations for pulsed power applications; and more particularly to terminations for a &#34;High Energy Coaxial Cable for Use in Pulsed High Energy Systems&#34;, covered by our copending patent application Ser. No. (AF Inv. 19975), filed Dec. 11, 1991, which is hereby incorporated by reference. 
     The art of making non-arcing contacts is well documented. In a book by R. Holm, &#34;Electric Contacts&#34;, Springer-Verlay, on electrical contact design, at page 438, it is pointed out that contact resistance primarily depends on contact pressure and that voltage drop, which equals current through the contact multiplied by contact resistance, must be lower than a critical value. The critical value given for a copper-to-copper contact is volts. Thus, as current increases in a contact, pressure must also increase proportionately. 
     Previously, when electrical interconnects were at currents of hundreds of kiloamperes, rigid conductors were bolted in place with sufficient force to avoid arcing at contact points. When conventional cables were used, it was necessary that the current path be broken into many parallel paths having relatively low coulomb rating. Current per path is then small relative to total current, and so high resistance and subsequently low contact pressure can be tolerated. Connections may be made for such contacts using techniques including soldering, brazing, or crimping. 
     SUMMARY OF THE INVENTION 
     An objective of the invention is to provide a means for connecting power cables carrying tens to hundreds of kiloamperes of pulsed current to electrical devices without producing arcing or melting during operation. A further objective is to provide mechanical support at cable ends where otherwise unsupported ends would be damaged due to magnetic force acting on current carrying members. 
     At the high current per contact required in the use of the high energy cable, three important design criteria must be met if contact arcing is to be avoided. First, the contact must supply mechanical support to the cable at the termination, to prevent magnetic forces from moving the conductor in any direction which would loosen the contact. Second, the connector must be installed with sufficient force to meet the Holm resistance criterion, and force must be maintained during the expected life of the cable and termination. Third, a smooth, well-defined surface is needed at the surface of the connector where it interfaces with the mating system contact. 
     During development of the high current flexible cable interface, three different termination assembly techniques were used. Each technique was found to produce satisfactory results and the technique selected for a given installation depended primarily on availability of equipment to accommodate a particular technique. 
     In one embodiment, the mechanical force is provided by use of an 8-jaw hydraulic powered swager. This tool produces a precise but intense pressure which deforms a thick walled connector inward. To provide a precise, smooth surface, the deformed conductor is threaded, and a mating threaded sleeve is screwed in place and torqued to high pressure. The threaded sleeve has the smooth contact surface. 
     In a second embodiment, the connector is manufactured to have two different outside diameters. The connector has a large counter bore at the end having the large outer diameter to provide an insulator support region which fits over the inner insulator. The remainder of the connector has a bore the size of the inner conductor. The smaller thin-walled section is designed such that use of an intense pulsed magnetic force (magnetic swaging) deforms the conductor inward, producing the desired high pressure contact, while the larger and thicker portion of the conductor is unaffected by the swaging process and provides the smooth surface. 
     In another embodiment, connector manufacturing is straightforward and desired contact is provided by separating wire bundles and flaring them between two contact surfaces. Contact force is provided by use of screws to produce the desired clamping force between two contact surfaces. The connector contact surface remains smooth. 
     The techniques described above are also used to provide support and non-arcing contact at the cable outer conductor. 
     ADVANTAGES 
     1. The cable terminations according to the invention utilize large cross-section deformable conductors, along with various techniques for applying force to the conductor and sustain the force, to meet the non-arcing contact criteria described by Holm. It produces a satisfactory termination for a new high energy cable capable of conducting peak currents as great as 200 kiloamperes, with current pulse duration of up to several tens of milliseconds. 
     2. The cable terminations, while providing hundred kiloampere, millisecond pulse current carrying capability through a pressure contact, also provides mechanical support to resist damage to the cable due to intense magnetic forces at the otherwise unsupported cable terminations. 
     3. The cable terminations provide a smooth, uniform outer dimension contact surface, after a non-arcing pressure contact has been formed. The uniform dimension surface permits rapid, interchangeable interconnections to pulsed power equipment operating to hundreds of kiloampere peak currents. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1, 2 and 3 are cross-section views of three embodiments of a cable termination assembly technique, with FIG. 1 showing use of hydraulic force, FIG. 2 showing use of a magnetic swaging force and FIG. 3 showing mechanical force applied with screws; 
     FIG. 4 shows the embodiment of FIG. 1 modified to provide a uniform dimension contact surface; 
     FIG. 5 shows hydraulic swaging terminals for both the center and outer conductors; 
     FIG. 5a shows magnetic swaging for both the center and outer conductors; 
     FIG. 6 shows mechanically (screw) clamped terminals for both center and outer contacts; and 
     FIG. 7 is a cut-away pictorial view of the cable alone. 
    
    
     DETAILED DESCRIPTION 
     The invention is disclosed in a paper titled &#34;High Energy Cable Development for Pulsed Power Applications&#34; by Jamison et al in the IEEE Transactions of Magnetics, Vol. 27, No. 1, January 1991, based on an oral presentation at the 5th Symposium on Electromagnetic Launcher Technology, San Destin, Fla., April 1990. The IEEE paper is hereby incorporated by reference. 
     The three terminal assembly techniques according to the invention, along with detailed design information and test results are described in a technical information memorandum (TIM-1-308) by applicants Ron Stears and Keith Jamison. A technical information memorandum (TIM-1-315) by applicants Ron Stears and Keith Jamison shows improvement in cable termination techniques. Copies of these two technical information memoranda are attached hereto as appendices and are hereby incorporated by reference. 
     The cut-away view of the cable configuration is shown in FIG. 7. The seven elements which comprise the cable are discussed below. 
     Center Conductor: The center conductor 1 comprised of 1330 30 gauge nickel plated copper strands. In its present configuration it has a nominal diameter of 12.2 mm (0.480 in). The core portion of the strands are counter-wound from the outer strands for improved flexibility. The total cross-sectional area is 68 mm 2  (or a current carrying cross-section of 130,000 circular mil area). 
     Inner Dielectric: The inner dielectric 2 is extruded perfluoroalkoxy, (PFA) TEFLON with a nominal wall thickness of 5.1 mm. The nominal outside diameter is 22.2 mm (0.875 in). 
     Outer Conductor: The outer conductor 3 is comprised of two counter-wound layers of stranded nickel plated copper wire. Each layer is formed from 48 stranded wires which have been made from nineteen 30-gauge strands. The total cross-sectional area is 93 mm 2  (155,000 circular mils). 
     Outer Dielectric: The outer dielectric 4, made of extruded PFA TEFLON, is utilized to hold the outer conductor in place since it is not braided. It has a nominal wall thickness of 1.6 mm and a nominal outside diameter is 31 mm (1.220 in). 
     KEVLAR Braid: A reinforcing mesh 5 is woven over the outer dielectric to aid in the containment of the magnetic burst forces. The mesh is manufactured from the arimid fiber KEVLAR. 
     Outer Jacket: The outer jacket 6 is made of a flame retardant polyether based polyurethane. 
     At each end of the cable a connector is required for interconnecting the cable to other equipment. This necessitates removal of the insulating material and concurrently the magnetic force containment. As a result, a connector is needed which provides both good electrical contact and mechanical support against magnetic forces. 
     At the high current per contact required in the use of the high energy cable, three important design criteria must be met, if contact arcing is to be avoided. First, the contact must supply mechanical support to the cable at the termination, to prevent magnetic forces from moving the conductor in any direction which would loosen the contact. Second, the connector must be installed with sufficient force to meet the Holm resistance criteria, and force must be maintained during the expected life of the cable and termination. Third, a smooth, well-defined surface is needed at the surface of the connector where it interfaces with the mating system contact. 
     The mating system contacts at the power source and at the pulsed power load should have cylindrical holes into which the smooth surfaces of the connector are inserted, and bolted or otherwise fastened to provide high forces. 
     During development of the high current flexible cable interface, three different termination assembly techniques were used. Each technique was found to produce satisfactory results primarily on availability of equipment to accommodate a particular technique. The parts for the connectors may be of a suitable conductive material, such as copper or brass. 
     The different techniques are shown in the drawings. Each configuration uses different techniques to meet the first and second design criterion, i.e. to provide mechanical support to the cable and to provide sufficient pressure to maintain the contact at non-arcing pressure. Cable support is provided to the center conductor 1 by counter-boring the connector at a diameter similar to that of the center conductor insulator 2, as shown in FIGS. 1, 2 and 3. In each case, the outer layers 3-6 of the cable are removed at the end, and the insulator 2 is removed for a lesser distance to leave a bare portion 1a of the center conductor. The bare portion of the conductor then passes through a smaller diameter and mechanical force is applied to produce a desired contact pressure. 
     In the embodiment of FIG. 1, the mechanical force is provided by use of an 8-jaw hydraulic powered swager. This tool produces a precise but intense pressure which deforms a thick walled connector 10 inward. The connector 10 has a counter bore to provide an insulator support region which is fitted over the inner insulator 2. The bare portions 1a of the center conductor passes through a smaller diameter. The hydraulic force is then applied to the connector 10 as shown. The final design criterion of a precise, smooth surface then requires that the deformed conductor 10 be threaded, and a mating threaded sleeve 40 be screwed in place and torqued to high pressure as shown in FIG. 4. The threaded sleeve 40 has the smooth contact surface 41. While this technique produces highly desirable results, connector design is complex, and assembly requires the use of specialized hydraulic equipment. 
     In the embodiment of FIG. 2, the connector 20 is manufactured to have two different outside diameters. The connector 20 has a large counter bore at the end having the large outer diameter to provide an insulator support region 22 which fits over the inner insulator 2. The remainder of the connector has a bore the size of the inner conductor. The smaller thin walled section is designed such that use of an intense pulsed magnetic force (magnetic swaging) deforms the conductor inward, producing the desired high pressure contact, while the larger and thicker portion of the conductor is unaffected by the swaging process and provides the smooth surface 21 defined by design criterion three. This technique simplifies design requirements on the connector and minimizes assembly time, but requires the availability of relatively specialized magnetic swaging equipment. 
     In the embodiment of FIG. 3, connector manufacturing is straightforward and desired contact is provided by separating wire bundles and flaring them between two contact surfaces. The copper connector 30 is similar to the connector 10 of FIG. 1, but it has screw holes drilled and tapped at the end. A copper clamping plate 34 has screw holes matching those of the connector 30. The inner conductor has the conductor bundles 1b separated and flared to go between the end of the connector 30 and the plate 34. Contact force is provided by use of screws 36 to produce desired clamping force between two contact surfaces. The connector contact surface 31 remains smooth. This technique produces satisfactory results without the use of specialized equipment, but requires considerable assembly time and is more susceptible to failures due to personnel error in assembly. 
     The techniques described above are also used to provide support and non arcing contact at the cable outer conductor, as shown in FIGS. 5 and 6. FIG. 5 shows the assembly needed for both swaging techniques of FIGS. 1 and 2, while FIG. 6 shows one configuration for the mechanical assembly technique of FIG. 3. 
     FIG. 5 shows the embodiment of FIGS. 1 and 4 for the termination of the inner conductor (except that the insulator support region is omitted), plus a termination for the outer conductor. The cable comprises the center conductor 1, the inner dielectric 2, the outer conductor 3 (two layers), the outer dielectric 4, the KEVLAR braid 5 and the outer jacket 6, as in FIG. 7. 
     The basis for attaching connectors to both the inner and outer conductors of the High Power Coaxial Cable (HPCC) is crimping soft copper cylinders over the conductors. Both connection points are 11/8 inch right circular cylinders with a spacing of 2.5 inches between the inner and outer connections. Not shown in FIG. 5 is an insulating support sleeve (2 inches in length, 7/8&#34; ID, 11/8&#34; OD) which could be used to cover the exposed inner dielectric and provide additional strength to the cable end. 
     To assemble the outer conductor terminal connector, as shown in FIG. 5, a brass sleeve 50 is fitted under the outer conductor 3, which is then crimped in place with a copper crimp ring 52. A thin walled steel tube 54 is crimped over the copper ring 52 and the end of the outer cable jacket 6 for support and added mechanical strength. The mechanical force for the crimping is provided by use of the 8-jaw hydraulic powered swager. Note that the 2.2 inch portion of the brass sleeve 50 remains smooth. If the embodiment of FIG. 2 is used for the center conductor termination, the use of an intense pulsed magnetic force (magnetic swaging) applied over the crimp ring 52 deforms the conductor inward, as shown in FIG. 5a. 
     To assemble the center conductor terminal connector of FIG. 5, a right circular cylinder of UNS/C1100 copper is cut to a length of 1.3 inches and drilled to an inside diameter of 35/64 inch. The raw 2/0 gauge wire is inserted into the copper which is then swaged. The outside diameter of the copper decreases to less than one inch, the inside diameter decreases to less than 0.4 inches and the length increases to approximately one and a half inches. The copper cylinder is then cut with a 1&#34;-8 UNC die for mating to an 1.125 inch UNS-C3300 brass sleeve 40 as shown in FIG. 5. The 0.125 inch wall thickness brass tube 40 is also threaded to 1&#34;-8 UNC. The brass sleeve 40 is tightened with a strap wrench to insure good contact between the threaded pieces. A very light coating of &#34;electronic joint compound&#34; was applied to all surfaces which are in electrical contact. 
     Not shown in FIG. 5 is an insulating support sleeve (two inches in length, 7/8&#34; ID, 11/8&#34; OD) which could be used to cover the exposed inner dielectric and provide additional strength to the cable end. 
     FIG. 6 shows the embodiment of FIG. 3 for the center conductor, plus a similar mechanically clamped termination using screws for the outer conductor. A copper connector 60 has screw holes drilled and tapped at the end. A copper clamping plate 64 has screw holes matching those of the connector 60. The outer conductor has the conductor bundles 3`b separated and flared to go between the end of the connector 60 and the plate 64. Contact force is provided by use of screws 66 to produce desired clamping force between two contact surfaces. The connector contact surface 61 remains smooth. 
     Fabrication Steps 
     The fabrication steps for the application of the cable end connectors for the embodiment of FIG. 5 are detailed in the following checklist. 
     
         ______________________________________                                    
.sub.-- a.                                                                
      Cut cable to finished length.                                       
.sub.-- b.                                                                
      Using a tubing cutter, cut through outer polyethylene               
      jacket 6 and KEVLAR braid 5 at a point 63/4 inches                  
      from each end.                                                      
.sub.-- c.                                                                
      Using a utility knife, cut and remove outer jacket 6                
      and KEVLAR braid 5.                                                 
.sub.-- d.                                                                
      Cut and remove dielectric 4 between KEVLAR                          
      braid 5 and outer conductor 3.                                      
.sub.-- e.                                                                
      Slide four inch long steel tube 54 over outer conductor             
      3 and onto outer jacket 6.                                          
.sub.-- f.                                                                
      Slide copper crimp over outer conductor 3 up to outer               
      jacket 6.                                                           
.sub.-- g.                                                                
      Slide brass sleeve 50 over inner dielectric 2 and under             
      the outer conductor 3.                                              
.sub.-- h.                                                                
      Tap the copper crimp ring 52 with a rawhide mallet                  
      while pushing the brass sleeve 50 toward the bulk of                
      the cable.                                                          
.sub.-- i.                                                                
      Insure that the outer conductor wires are distributed               
      uniformly between the brass sleeve 50 and copper crimp              
      ring 52.                                                            
.sub.-- j.                                                                
      Adjust the position of the brass sleeve 50 to be 3.2                
      inches from the cut portion of the outer jacket 6.                  
.sub.-- k                                                                 
      Position the copper crimp ring 52 to be 2.2 inches from             
      the end of the brass sleeve 50.                                     
.sub.-- l.                                                                
      Crimp the copper crimp ring 52 on to the outer conduc-              
      tor 3 using die set FT 1330-200-8 and a ram set of 980              
      on the crimping machine. The finished diameter of the               
      copper ring 52 should be 1.345 ± 0.005 inches.                   
.sub.-- m.                                                                
      Trim excess length of the outer conductor 3 back to the             
      copper crimp ring 52.                                               
.sub.-- n.                                                                
      Slide the steel tube 54 forward until it is flush with              
      the outer edge of the copper ring 52. Using the same                
      die set as above, and a ram set of 840, crimp the steel             
      tube 54 over the copper ring 52. Perform a second                   
      crimp to tighten the steel tube to the outer jacket 6.              
.sub.-- o.                                                                
      Cut the inner dielectric 2 with a tubing cutter two                 
      inches from the end of the brass sleeve 50. Take care               
      not to cut the inner dielectric 2.                                  
.sub.-- p.                                                                
      Remove the section of inner dielectric 2 by twisting                
      with pliers.                                                        
.sub.-- q.                                                                
      If an additional sleeve is required over the inner                  
      dielectric 2 it should be installed at this time.                   
.sub.-- r.                                                                
      Slide the copper end piece 10 over the center conductor             
      1 up to the cut portion of the inner dielectric 2.                  
.sub.-- s.                                                                
      Crimp the copper end piece 10 onto the center conductor             
      1 using die set FT 1330-200-4 with a ram set of 440.                
      Recrimp as necessary to insure that the entire copper               
      end is approximately round with a final diameter of                 
      0.995 ± 0.005 inches.                                            
.sub.-- t.                                                                
      Trim excess center conductor back to end of copper end              
      piece 10.                                                           
.sub.-- u.                                                                
      Clamp copper end piece 10 in pipe vise leaving 11/8                 
      inches free.                                                        
.sub.-- v.                                                                
      Cut at least eight full threads using a 1&#34;-8 UNC die.               
.sub.-- w.                                                                
      Apply a thin coating of electrical joint compound to                
      the copper threads.                                                 
.sub.-- x.                                                                
      Screw on brass connector 40 and tighten with strap                  
      wrench                                                              
.sub.-- y.                                                                
      Remove all burrs with a fine file.                                  
.sub.-- z.                                                                
      Repeat procedure on opposite end of cable.                          
______________________________________                                    
 
    
     SCOPE OF THE INVENTION 
     Non-arcing contact between two conductors requires a contact pressure as described by Holm. Key elements in meeting the contact pressure criteria include minimizing pressure requirements through the use of good conductor materials such as copper or brass, using sufficiently thick conductor walls that pressure is maintained over a long lifetime, and providing mechanical support to the cable at end terminations. These criteria may be met using various materials and design configurations. Three such designs have been described herein. 
     It is understood that certain modifications to the invention as described may be made, as might occur to one with skill in the field of the invention, within the scope of the appended claims. Therefore, all embodiments contemplated hereunder which achieve the objects of the present invention have not been shown in complete detail. Other embodiments may be developed without departing from the scope of the appended claims.