Planar filament with focused, central electron emission
A planar filament for an x-ray tube can have a different cross-sectional area at different locations. In regions of smaller cross-sectional area, there can be higher current density, and thus increased heating and higher temperature of the wire. In regions of larger cross-sectional area, there can be lower current density, and thus decreased heating of the wire. Regions of larger cross-sectional area can also be stronger, thus reducing early filament failures. Wider regions can have increased area for electron emission. By adjusting the cross-sectional area and width of the wire at different locations, electron emission can be largely confined to a center of the filament, and filament life can increase.
1 . An x-ray tube comprising:
a cathode and an anode electrically insulated from one another, the cathode including a filament configured to emit electrons towards the anode, and the anode configured to emit x-rays out of the x-ray tube in response to impinging electrons from the filament;
the filament comprises an elongated wire in a plane extending non-linearly in between a pair of electrodes, the filament capable of being heated by an electrical current through the elongated wire due to a voltage differential across the pair of electrodes;
the filament includes a low-region and a pair of outer-high-regions;
the low-region is electrically-coupled between the pair of outer-high-regions;
the low-region is configured to have lower current density during operation than the pair of outer-high-regions due to a larger thickness in the low-region than a thickness in the pair of outer-high-regions; and
wherein the filament has a planar face and the larger thickness in the low-region extends perpendicular from the planar face.
2 . The x-ray tube of claim 1 , wherein each outer-high-region has ≥1.1 times as much current density during operation as in the low-region.
3 . The x-ray tube of claim 1 , wherein a width of the wire in the low-region is larger than a width of the wire in the outer-high-regions.
4 . The x-ray tube of claim 1 , wherein W 12 /W 11 ≥1.1, where W 12 is a width of the wire in the low-region and W 11 is a width of the wire in the outer-high-regions.
5 . The x-ray tube of claim 1 , wherein the low-region is located in a central 25% of a length of the wire.
6 . The x-ray tube of claim 1 , further comprising a smooth transition of thickness between each of the outer-high-regions and the low-region.
7 . The x-ray tube of claim 1 , wherein a center of the filament is wider and thinner than any other part of the filament.
8 . The x-ray tube of claim 1 , further comprising:
the low-region includes a pair of low-regions;
the filament includes a central-high-region;
each low-region is electrically-coupled to one of the outer-high-regions at one end and to the central-high-region at an opposite end; and
a smooth transition of cross-sectional area of the wire between each low-region and the outer-high-region adjacent to it and between each low-region and the central-high-region.
9 . The x-ray tube of claim 8 , wherein:
the central-high-region includes a pair of central-high-regions;
the wire further comprises a center-region, the center-region is electrically-coupled between the pair of central-high-regions; and
the center-region is configured to have lower current density during operation, due to a larger cross-sectional area of the wire in the center-region, than the pair of central-high-regions.
10 . The x-ray tube of claim 9 , wherein AL/AHc≥1.1 and Ac/AHc≥1.1, where AL is the cross-sectional area of the wire of each low-region, AHc is the cross-sectional area of the wire of the central-high-region adjacent to the low-region, and Ac is the cross-sectional area of the wire of the center-region.
11 . The x-ray tube of claim 9 , wherein a width of the wire in each low-region is larger than a width of the wire in the central-high-region adjacent to it, and a width of the wire in the center-region is larger than a width of the wire in the pair of central-high-regions.
12 . The x-ray tube of claim 9 , wherein W 12 /W 13 ≥1.1 and W 14 /W 13 ≥1.1, where W 12 is a width of the wire in each low-region, W 13 is a width of the wire in the central-high-region adjacent to the low-region, and W 14 is a maximum width of the wire in the center-region.
13 . The x-ray tube of claim 9 , further comprising a smooth transition of cross-sectional area of the wire between each central-high-region and the low-region adjacent to it and between each central-high-region and the center-region.
14 . The x-ray tube of claim 1 , further comprising:
the low-region includes a pair of low-regions;
the filament includes a central-high-region;
each low-region is electrically-coupled to one of the outer-high-regions at one end and to the central-high-region at an opposite end; and
the wire has the same cross-sectional area throughout the outer-high-regions and the same cross-sectional area throughout the central-high-region.
15 . The x-ray tube of claim 14 , wherein:
the central-high-region includes a pair of central-high-regions;
the wire further comprises a center-region, the center-region is electrically-coupled between the pair of central-high-regions; and
the center-region is configured to have lower current density during operation, due to a larger cross-sectional area of the wire in the center-region, than the pair of central-high-regions.
16 . The x-ray tube of claim 15 , wherein AL/AHc≥1.1 and Ac/AHc≥1.1, where AL is the cross-sectional area of the wire of each low-region, AHc is the cross-sectional area of the wire of the central-high-region adjacent to the low-region, and Ac is the cross-sectional area of the wire of the center-region.
17 . The x-ray tube of claim 15 , wherein a width of the wire in each low-region is larger than a width of the wire in the central-high-region adjacent to it, and a width of the wire in the center-region is larger than a width of the wire in the pair of central-high-regions.
18 . The x-ray tube of claim 15 , wherein T 12 /T 11 ≥1.1, T 12 /THc≥1.1, Tc/T 11 ≥1.1, Tc/THc≥1.1, where T 12 is a maximum thickness of the wire in each low-region, T 11 is a minimum thickness of the wire in the outer-high-region attached to the low-region, THc is a minimum thickness of the wire in the central-high-regions attached to the low-region, and Tc is a maximum thickness of the wire in the center-region.
19 . The x-ray tube of claim 1 , wherein T 12 /T 11 ≥1.1, where T 12 is a maximum thickness of the wire in the low-region, and T 11 is a minimum thickness of the wire in the outer-high-regions.
20 . The x-ray tube of claim 1 , wherein 90% of electrons are emitted from a central 25% of the filament.