High voltage cable for a miniature x-ray tube
In a miniature x-ray tube, which may be on the order of approximately 1 mm in diameter or even less, a high voltage cable is provided in various embodiments for conducting current to the cathode of the x-ray tube and for conducting high voltage to the cathode and anode of the tube. In various embodiments of the cable, two conductors occupy a center region of the cable, packed as closely together as possible, in various shapes that are compact and present as smooth as possible an external shape for maximizing dielectric properties against the exterior high voltage ground, surrounding and generally concentric with the inner conductors. The inner conductors, which carry high voltage in opposition to the outer ground, can be in opposed D shapes, coaxial, two flattened conductors side by side, or simply a pair of cylindrical wires positioned as closely as possible. The space between the inner conductors and the outer ground can be occupied by a glass insulator, polymer, successive layers of polymers and adhesive, air, gas, vacuum or other dielectrics. A partially conductive region can surround the inner conductors.
1. A high voltage cable for a miniature x-ray source, for carrying high voltage potential for a cathode and an anode of the source and also for carrying current for a low voltage cathode heating circuit, comprising:
the cable having an outside diameter not larger than about 4 mm,
the cable having two inner conductors and an outer conductor, and carrying a voltage greater than about 10 kV between the inner conductors and the outer conductor,
the inner conductors being electrically isolated from one another and also carrying a low differential voltage cathode filament heater current,
including a dielectric material insulating the inner conductors from the outer conductor, and
the conductors and the dielectric material being of sufficient flexibility such that the cable can pass through curves having a radius of curvature at least as small as about 8 times the outside diameter of the cable.
2. A high voltage cable according to claim 1 wherein the outside diameter of the cable is at least as small as about 1 mm.
3. A high voltage cable according to claim 1 wherein the outside diameter of the cable is at least as small as about 0.5 mm.
4. A high voltage cable according to claim 1 wherein the voltage carried in the cable is at least about 20 kV.
5. A high voltage cable according to claim 1 wherein the voltage carried in the cable is at least about 40 kV.
6. A high voltage cable according to claim 1 wherein the voltage carried in the cable is at least about 50 kV.
7. A high voltage cable according to claim 1 , wherein the conductors and the dielectric material are of sufficient flexibility that the cable can pass through curves having radius of curvature at least as small as about 5 times the outside diameter of the cable.
8. A high voltage cable according to claim 1 , wherein the outer diameter of the cable is about 3 mm and the cable is of sufficient flexibility to pass through curves having a radius of curvature at least as small as about 20 mm.
9. A high voltage cable according to claim 1 , wherein the dielectric material comprises drawn glass substantially of fiber optic quality.
10. A high voltage cable according to claim 1 , wherein the inner conductors are spaced apart and surrounded by the dielectric material, including between the inner conductors.
11. A high voltage cable according to claim 10 , wherein the dielectric is a polymer material.
12. A high voltage cable according to claim 1 , wherein one of the two inner conductors is coated with an insulative coating, the two conductors being essentially in contact but separated by the insulative coating and the conductors being generally round in cross section.
13. A high voltage cable according to claim 12 , wherein said insulative coating comprises glass, and wherein the dielectric material comprises drawn glass substantially of fiber optic quality.
14. A high voltage cable according to claim 13 , wherein the glass dielectric has been drawn into contact with the two inner conductors.
15. A high voltage cable according to claim 13 , wherein the glass dielectric insulator comprises a glass tube and wherein the two conductors are within the inner diameter of the glass tube and are packed therein with a dielectric potting material.
16. A high voltage cable according to claim 13 , further including in the glass dielectric material a resistive/partially conductive region around the conductors, with a substantially cylindrical configuration at an outer diameter of the region, to improve the dielectric characteristics of the cable.
17. A high voltage cable according to claim 13 , further including a resistive/partially conductive region surrounding the inner conductors, comprising a conductive doping in the glass insulation, the doping being in a graded pattern such that with increasing radius conductivity is reduced, substantially to zero at the outside diameter of the glass insulator.
18. A high voltage cable according to claim 1 , wherein one of the two inner conductors is coated with an insulative coating, the two conductors being essentially in contact but separated by the insulative coating and the inner conductors being generally D-shaped and side-by-side and close together so as to present a substantially round outer surface of the two inner conductors to improve dielectric strength of the cable.
19. A high voltage cable according to claim 18 , wherein one of the D-shaped inner conductors is pre-coated with an insulative coating and the inner conductors are very close together, essentially in contact.
20. A high voltage cable according to claim 18 , further including in the glass dielectric material a resistive/partially conductive region around the conductors, with a substantially cylindrical configuration at an outer diameter of the region, to improve the dielectric characteristics of the cable.
21. A high voltage cable accordingly to claim 18 , further including a resistive/partially conductive region surrounding the inner conductors, comprising a conductive doping in the glass insulation, the doping being in a graded pattern such that with increasing radius conductivity is reduced, substantially to zero at the outside diameter of the glass insulator.
22. A high voltage cable according to claim 1 , wherein the two inner conductors are each flattened, generally oval or elliptical in shape, one of the two conductors being pre-coated with an insulative coating.
23. A high voltage cable according to claim 22 , wherein said insulative coating comprises glass, and wherein the dielectric material comprises drawn glass substantially of fiber optic quality.
24. A high voltage cable according to claim 23 , further including in the glass dielectric material a resistive/partially conductive region around the inner conductors, with a substantially cylindrical configuration at an outer diameter of the region, to improve the dielectric characteristics of the cable.
25. A high voltage cable according to claim 23 , further including a resistive/partially conductive region surrounding the inner conductors, comprising a conductive doping in the glass insulation, the doping being in a graded pattern such that with increasing radius conductivity is reduced, substantially to zero at the outside diameter of the glass insulator.
26. A high voltage cable according to claim 1 , wherein the two inner conductors are coaxial.
27. A high voltage cable according to claim 26 , wherein the dielectric material comprises a polymer.
28. A high voltage cable according to claim 26 , wherein the dielectric material comprises drawn glass substantially of fiber optic quality.
29. A high voltage cable according to claim 28 , further including in the glass dielectric material a resistive/partially conductive region around the conductors, with a substantially cylindrical configuration at an outer diameter of the region, to improve the dielectric characteristics of the cable.
30. A high voltage cable according to claim 28 , further including a resistive/partially conductive region surrounding the inner conductors, comprising a conductive doping in the glass insulation, the doping being in a graded pattern such that with increasing radius conductivity is reduced, substantially to zero at the outside diameter of the glass insulator.
31. A high voltage cable according to claim 1 , wherein the dielectric material is coextruded with the inner conductors.
32. A high voltage cable according to claim 1 , wherein at least one of the two inner conductors is pre-coated with insulative coating and the two inner conductors are nested closely together, and wherein the inner conductors are co-extruded with PFA polymer, by extruding the PFA material over the conductors such that essentially no residual entrained air remains between the conductors and the PFA material, the PFA material serving as the dielectric material or insulator, wherein said outer conductor is a ground and comprises a braided or wrapped series of conductive wires covering most of the outer surface of the PFA insulator, and including a protective jacket covering the ground conductor.
33. A high voltage cable according to claim 32 , wherein the ground conductor comprises wires helically wound in two opposite directions around an outer surface of the PFA insulator.
34. A high voltage cable according to claim 32 , wherein the protective outer jacket of the high voltage cable comprises FEP or urethane material.
35. A high voltage cable according to claim 32 , wherein the protective outer jacket of the high voltage cable comprises a TEFLON class material.
36. A high voltage cable according to claim 32 , wherein the outer diameter of the cable, including the protective jacket, is about 3.2 mm.
37. A high voltage cable according to claim 36 , wherein the heat shrink material comprises a TEFLON grade heat shrink material.
38. A high voltage cable according to claim 36 , including at least four layers of adhesive and four layers of heat shrink material.
39. A high voltage cable according to claim 1 , wherein the dielectric is a polymer material, and including in the polymer dielectric material a resistive/partially conductive region around the conductors, with a substantially cylindrical configuration at an outer diameter of the region, to improve the dielectric characteristics of the cable.
40. A high voltage cable according to claim 1 , wherein the dielectric material insulating the inner conductors from the outer conductors comprises a series of layers of polymer materials surrounding the inner conductors, comprising alternating layers of heat shrink material.
41. A high voltage cable for a miniature x-ray source, for carrying high voltage potential for a cathode of the source and also for carrying a low differential voltage cathode heating current, comprising:
the cable an outside diameter not larger than about 3 mm,
the cable having two inner conductors and an outer conductor and carrying a voltage greater than about 40 kV between the inner conductors and the outer conductor,
the inner conductors being electrically isolated from one another and also carrying a low differential voltage cathode filament heater current,
a dielectric material isolating the inner conductors from the outer conductor to prevent arcing and breakdown, the dielectric material comprising drawn glass substantially of fiber optic quality, and
the conductors and the dielectric material being of sufficient flexibility such that the cable can pass through curves having a radius of curvature at least as small as about 10 times the outside diameter of the cable.
42. A high voltage cable according to claim 41 , wherein the outer diameter is at least as small as about 1 mm.
43. A high voltage cable according to claim 41 , wherein the voltage carried in the cable is at least about 50 kV.
44. A high voltage cable according to claim 41 , wherein the conductors and the dielectric material are of sufficient flexibility that the cable can pass through curves having radius of curvature at least as small as about 5 times the outside diameter of the cable.
45. A high voltage cable according claim 44 , wherein the outside diameter of the cable is at least as small as about 1 mm.