Filament and cathode for an x-ray tube

Abstract

The invention relates to a filament for generating electrons, in particular for an X-ray tube, the filament having an emission body (24) which has an input (27) and an output (28) for a current connection (29), having an emission surface (25) which is formed on the emission body (24), is flat and comprises at least one slot (31, 32) in order to guide at least one current path line (37) from the input (27) along the emission surface (25) to the output (28), and a cross-section of the emission body (24) lies in a sectional plane perpendicular to the flat emission surface (25), which a size of the cross-section of the emission body (24) increases from the input (27) to the middle of the emission surface (25) and a size of the cross-section of the emission body (24) decreases from the middle of the emission surface (25) to the output (28), wherein at least one meandering current path (33), along which the at least one current path line (37) runs, extends from the input (27) to the output (28) of the emission surface (25) of the emission body (24), and the emission surface (25) of the emission body (24) has the shape of an ellipse as seen in plan view.

Description

[0001] Filament and cathode for an X-ray tube

[0002] The invention relates to a filament for generating electrons, in particular for generating an electron beam in an X-ray tube, and a cathode for an X-ray tube, in particular for an X-ray tube for X-ray fluorescence analysis.

[0003] In an X-ray source for generating X-rays, a heated cathode is used, which is arranged in a directional cylinder. This cathode comprises a filament made of wire. By applying a heating voltage, the filament is heated, causing electrons to be emitted through thermionic emission. The directional cylinder directs these electrons toward an anode. These electrons strike an anode material and generate X-rays, which then exit the X-ray source through an optical window. Such X-rays can be used for X-ray fluorescence analysis for non-destructive material testing or layer thickness measurement.

[0004] From JP S 60200447 A, an X-ray irradiation device is known which comprises a heated cathode for generating an electron beam, the cathode being formed with a ribbon- or strip-shaped filament of constant width and thickness.

[0005] A computed tomography (CT) scanner is known from CN 109065430 A. This X-ray tube comprises a flat filament to form the heated cathode. This flat filament, which can be square or round, has one or more slots through which a current path is formed, extending from an inlet for a power connection to the center of the filament and from the center to the outlet for a power connection. In the case of a square flat filament, the current path has a constant cross-section from the inlet to the outlet. In the case of a round flat filament, the current path tapers in width from the inlet to the center of the filament and then widens again from the center to the outlet.

[0006] From DE 100 29 253 CI, a directly heated thermionic flat emitter is known. This flat emitter comprises an emission surface with meandering conductive traces. The conductive traces are formed by slots in the emission surface. The slots are arranged according to a cutting pattern consisting of a plurality of straight step slots extending at least transversely to the direction of the main current, which are arranged in several rows offset from one another.

[0007] From DE 27 27 907 A1, an electrically heated annealing emitter for an X-ray tube is known. This annealing emitter comprises an elongated sheet metal plate with terminals at its longitudinally oriented ends. The plate has a uniform width along its entire length and features alternating transverse cuts from opposite sides. These embodiments have the disadvantage that, analogous to a heating filament, there is an uneven temperature distribution from the inlet, through the middle, to the outlet of the filament.

[0008] The invention is based on the objective of proposing a filament for a cathode for generating electrons in an X-ray tube, as well as a cathode for an X-ray tube that enables an improved temperature distribution.

[0009] This problem is solved by a filament which has an emission body having an inlet for a current connection and an outlet for a current connection, wherein the inlet and the outlet merge into the emission body and with an emission surface formed flat on the emission body, into which slots are provided to form at least one current path from the inlet along the emission surface to the outlet of the emission body, and wherein the emission body has a cross-sectional area in a section plane perpendicular to the emission surface, wherein from an inlet to a center of the emission surface an increase in the cross-section of the emission body is provided and from the center to the outlet a decrease in the cross-section of the emission body.The emission surface comprises at least one meandering current path extending from the inlet to the outlet of the emission surface, along which at least one current path runs. Viewed from above, the emission surface of the emitter is elliptical, particularly an elongated ellipse. This design results in a nearly homogeneous or homogeneous temperature field from the inlet to the outlet along the emission surface of the emitter. This homogeneous temperature field has the advantage of increasing the intensity of electron emission towards the anode, thereby enhancing the monochromatization of the X-rays.

[0010] Preferably, the emission surface of the emission body comprises only a single current path extending from the inlet to the outlet of the emission surface. This allows for optimization of the emission body's design to achieve a homogeneous temperature distribution from the inlet to the outlet along the emission surface.

[0011] Preferably, the cross-section of the inlet and the cross-section of the outlet of the emission body are identical. This allows for a simplified filament design.

[0012] In particular, it is provided that the inlet and outlet of the emission body, as well as the area between them, have the same height or thickness. Preferably, the filament is made of a flat strip material with a constant thickness. This simplifies the manufacturing and / or forming of the filament. Advantageously, the current path is continuous, extending from the inlet across the center of the emission body to the outlet, along which the current lines are guided. This single continuous current path, with its increasing cross-sectional area from the inlet to the center of the emission surface and decreasing cross-sectional area from the center to the outlet of the emission body, enables a homogeneous temperature field to be distributed across the entire emission surface.

[0013] Furthermore, the cross-sectional enlargement and / or reduction can be continuous. Alternatively, the enlargement and / or reduction of the cross-section can be stepwise. Specifically, the cross-sectional enlargement and / or reduction of the emission surface is determined by an optimization algorithm such that the resulting temperature difference between individual points on the emission surface approaches zero. A target parameter of the filament is prioritized. This parameter is the degree of homogeneity of a temperature field within the emission surface. An objective function of this optimization algorithm is independent of this parameter.The maximum homogeneity of the filament's emission surface is determined by the objective function, which minimizes the squared difference between the variable point on the filament's emission surface responsible for the maximum achievable temperature and the point responsible for the minimum achievable temperature. A minimum or optimum is achieved when the results of the squared difference asymptotically approach zero. The objective function is: f = min (Tpmax - Tpmin). 2 , where Tpmax represents the maximum temperature on the emission surface of the emission body and Tpmin represents the minimum temperature on the emission surface of the emission body.

[0014] According to a preferred embodiment, it is provided that the

[0015] The emission surface of the emission body has a contour that is several times longer than it is wide, with the length of the emission surface being formed along a longitudinal axis that lies in a connecting line from the entrance through the center of the emission surface to the outlet of the emission body.

[0016] In particular, it may be provided that the outer surfaces of the emission surface, which extend between the inlet and outlet of the power connection of the emission body, are rotationally symmetric to the longitudinal axis.

[0017] This elliptical shape of the filament is particularly advantageous because the anode lies at an angle to the electron beam axis of the electrons emitted by the filament, thereby creating a circular spot for the X-ray radiation at the exit of the X-ray tube to strike a measuring object.

[0018] The emission surface of the emission body has slots, particularly longitudinal slots, extending from its outer edges into the emission surface along its longitudinal axis. These slots are designed as open slots, i.e., slots open to the outside. This allows a continuous current path to be formed between the inlet and outlet of the emission body, thus enabling a defined current path. The cross-sectional area can be increased or decreased by adjusting the positioning of the slots along the longitudinal axis and / or the length of the slots from the outer edge towards the longitudinal axis of the emission body. Advantageously, the slots are oriented perpendicular to the longitudinal axis of the emission body. The emission surface of the emission body can also have slots located within the emission surface itself, i.e., so-called closed slots.By combining open and closed slots, two or more current paths can be formed, which are preferably of the same length from the input to the output.

[0019] Preferably, the open slots introduced into the emission body, alternately opposite each other when viewed from the entrance towards the exit, extend from the respective outer sides into the emission surface in order to form a current path on the emission surface.

[0020] Furthermore, slots may be designed to have a constant width. In particular, the base of the slots may be rounded.

[0021] Furthermore, to preferably form two current paths on the emission surface, it can be provided that two open slots are arranged on a common axis perpendicular to the longitudinal axis of the emission surface and opposite each other, and a closed slot is positioned spaced apart from these in the emission surface, the longitudinal axis of which is oriented perpendicular to the longitudinal axis of the emission surface, wherein preferably the closed slot is oriented mirror-symmetrically to the longitudinal axis of the emission surface. This arrangement is repeated successively along the length of the emission surface.

[0022] The length of the successive slots, which are alternately inserted from each side edge, and / or their spacing relative to the longitudinal axis, and / or the length of the slots themselves, are advantageously matched such that the current path on the emitter exhibits a uniform temperature distribution from the inlet to the center of the emitter and from the center to the outlet. In particular, the current path's profile with respect to its cross-section can be determined by the objective function of the optimization algorithm, ensuring a uniform temperature distribution across the entire emission surface of the emitter, or that the maximum temperature difference between two points on the emission surface is less than 10 °C, and particularly less than 1 °C.

[0023] Preferably, a simulation iteration is performed starting from an initial model that does not yet include an optimized shape of the emission surface and / or an optimized position and / or length of the slots. Subsequently, the temperature field on the new emission surface is determined. This is done for an emission surface shape, position, and / or slot length modified by the simulation. This iteration continues until the temperature difference between two points or two areas on the emission surface of the emitting body is minimized. This optimizes and determines the shape of the emission surface and the position and / or slot length.

[0024] Advantageously, the distance between the inlet and outlet of the emission body, as well as the course of the outer edges of the emission body provided between them, is adapted to the size of an optical spot of the emerging X-ray radiation and an angular alignment of the anode to the beam axis of the emitted electrons.

[0025] The object underlying the invention is further solved by a cathode for an X-ray source with a filament and a direction cylinder associated with the filament, which has an aperture with an aperture opening for arranging the filament, wherein a filament is positioned in the aperture opening according to one of the embodiments described above.

[0026] In an advantageous embodiment of the cathode, the aperture is adapted to an outer contour of the filament's emission surface. This allows for a small gap between the outer contour of the filament's emission surface and the aperture, thus reducing the proportion of peripheral electrons emitted from the back of the emission surface.

[0027] According to a further preferred embodiment of the cathode, the emission surface of the emission body facing the outlet of the directional cylinder is set back relative to a surface of the aperture facing the outlet of the directional cylinder. This has the advantage that electrons emitted laterally from the emission body, which travel towards the anode, are reduced. The invention, as well as further advantageous embodiments and developments thereof, are described and explained in more detail below with reference to the examples shown in the drawings. The features that can be derived from the description and the drawings can be applied individually or in any combination according to the invention. The drawings show:

[0028] Figure 1 is a schematic sectional view of an X-ray tube,

[0029] Figure 2 shows a perspective view of a filament according to the invention.

[0030] Figure 3 shows a schematic side view of the filament according to Figure 2.

[0031] Figure 4 shows a schematic top view of a filament made from a strip material.

[0032] Figure 5 shows a schematic view of the filament according to Figure 2 with streamline lines.

[0033] Figure 6 shows a perspective view of a cathode of the X-ray tube according to Figure 1.

[0034] Figure 7 shows a schematic sectional view of the cathode according to Figure 6, and

[0035] Figure 8 shows a schematic view of an alternative embodiment of the filament compared to Figure 5.

[0036] Figure 1 shows a schematic view of an X-ray tube 11. This X-ray tube 11 comprises a housing 12, within which a vacuum is maintained. An anode 14 and a cathode 16 are arranged in the housing 12, opposite the anode 14. The anode 14 advantageously includes a cooling element 17. The cathode 16 is advantageously designed as a heated cathode and comprises a filament 18. A filament voltage, also referred to as the heating voltage, is applied to the cathode 16. An anode voltage is applied between the cathode 16 and the anode 14. This is also referred to as the accelerating voltage, as it accelerates electrons from the cathode 16 to the anode 14. The electrons emitted by the cathode 16 or the filament 18 are emitted in the form of an electron beam 19.The electron beam 19 can be focused by a focusing cylinder 20, in particular a Wehnelt cylinder, so that the electrons strike the anode 14. Fluorescence radiation, also known as X-ray radiation 21, is emitted by the material of the anode 14. This X-ray radiation 21 exits through a window 22 in the housing 12. This X-ray radiation 21 can be directed to strike a sample comprising an object and / or at least one layer of an object. For example, an X-ray fluorescence analysis can be performed, in which the material(s) of the sample and / or the thickness of at least one layer of the sample are analyzed. This X-ray radiation can also be used in the medical field.

[0037] Furthermore, at least one deflection device can be provided in the X-ray tube 11, which is positioned between the direction cylinder 20 and the anode 14. Such a deflection device allows the electron beam 19 to be deflected to strike different areas on the anode 14.

[0038] The anode 14 can, for example, be made of tungsten, molybdenum, rhodium, or chromium. A combination of these elements can also form an impact surface at the anode.

[0039] The filament 18 can advantageously be made of tungsten or molybdenum.

[0040] Figure 2 shows a perspective view of filament 18.

[0041] Figure 3 shows a side view of filament 18 according to Figure 2.

[0042] This filament 18 is designed as a so-called flat filament. The filament 18 comprises an emission body 24, which has an emission surface 25. This emission body 24 advantageously has a constant thickness, as can be seen in Figure 3. The emission body 24 has an elongated emission surface 25, which extends along the longitudinal axis 26. The width of the emission surface 25, which is oriented perpendicular to the longitudinal axis 26, is a multiple smaller than the length of the emission surface 25. At one end of the emission surface 25, an inlet 26 for a power connection 29 is provided. At the opposite end, an outlet 28 for a power connection 29 is provided. The inlet 26 and the outlet 28 for the respective power connection 29 are preferably angled relative to the emission surface 25 of the emission body 24. This angle can, for example, be rectangular.

[0043] Figure 4 shows a manufacturing step of a flat filament 18 before it is transformed into the form shown in Figures 2 and 3, which is exemplary and ready for installation. In the top view of the flat filament 18 shown in Figure 4, the outer contour for the inlet 27 and the outlet 28 of the power connector 29 and the elongated shape of the emission surface 25 are first produced from a ribbon-shaped material, advantageously with a constant thickness. This can be done, for example, by laser cutting or wire EDM. The emission surface 25 comprises several slots 31. These slots 31 are also produced by laser cutting or wire EDM. These slots 31 preferably have a constant width. The lengths of the slots 31 vary from one another.After completion of the manufacturing step according to Figure 4, the electrical connections 29 at the input 27 and output 28 are angled accordingly, as shown in Figures 2 and 3.

[0044] The design of the contour of the emission surface 25 of the emission body 24, as well as the arrangement of the slots 31 in terms of their spacing and / or length, are coordinated to achieve, in particular, a homogenized temperature field along the entire emission surface 25. This homogenized temperature field also ensures consistent aging of the flat filament 18. Consequently, this flat filament 18 has a significantly longer lifespan than, for example, a filament.

[0045] The flat filament 18 (Figures 2 and 4) has an elliptical contour. This elliptical contour of the emission surface 25 is advantageously dependent on an anode angle, or rather, an angle of the anode 14's impact surface relative to the electron beam 19, and a spot size of the X-ray beam 21 at the window 22 of the housing 12. To determine the elliptical contour of the emission surface 25 of the emission body 24, a spot size at the window 22 of the housing 12, or the diameter of the X-ray beam 21, is used as a starting point. This spot size is projected onto the angled anode 14. Based on this, an oval contour is obtained on the impact surface of the anode 14. To achieve this impact surface at the anode 14, the outer contour of the emission surface 25 of the emission body 24 is scaled and adjusted with respect to the distance from the cathode 16 to the anode 14. This results in an elliptical emission surface 25.This elliptical shape of the emission surface 25 allows the desired spot of X-ray radiation 21 to be maintained at the exit window. At the same time, the amount of electrons striking the anode 14 can be increased compared to a filament.

[0046] To achieve a uniform temperature distribution across the entire emission surface 25 of the emission body 24, slots 31 are provided. These slots 31 are each oriented from the outside of the emission surface 25 towards the longitudinal axis 26. The slots 31 are preferably provided alternately from the respective opposite outside of the emission surface 25. The slots 31 are advantageously arranged such that a double rotationally symmetric arrangement can be applied to them.

[0047] This arrangement of the slots 31 and their length create and define a single current path 33 from the inlet 27 to the outlet 28 within the emission surface 25. Figure 5 schematically illustrates, for example, the current paths 37 along the current path 33. The current paths 37 extend from the inlet 27 along the longitudinal axis 26 to the outlet 28. The course of the current path 37 is determined by the alternating slots 31. This results in the temperature distribution along the current path 33. It is preferably provided that, starting from the inlet 27 to the center of the emission surface 25, the cross-sectional area of ​​the current path 33 increases, and, starting from the center of the emission surface 25 towards the outlet 28, the cross-sectional area of ​​the current path 33 decreases. The cross-section of the current path 33 is understood to be a cross-sectional area perpendicular to the longitudinal axis 26 of the emission body 24.

[0048] The elliptical contour of the emission surface 25 and the slots 31 incorporated therein enable, for example, a nearly uniform temperature distribution along the entire surface from the inlet 27 to the outlet 28 of the emission surface 25. For instance, the temperature difference along the entire flat filament 18 is less than 10 °C, and in particular less than 0.5 °C. In comparison, the temperature difference in a filament is approximately 950 °C.

[0049] This homogeneous temperature distribution also makes it possible to transform the distribution of emitted electrons from the Gaussian shape of a conventional filament into a rectangular function. This results in a quantitatively and qualitatively smaller electron spot shadow area. The evaluation of the X-ray radiation 21 after the exit window, in this case, does not have to be at 50% of the photon quantity, as is the case with a filament, but at approximately 90%. This allows for an increase in the monochromatization of the X-ray radiation 21.

[0050] Figure 6 shows a perspective view of the cathode 16 according to Figure 1. Figure 7 shows an enlarged sectional view along a longitudinal axis of the cathode 16 according to Figure 6. The directional cylinder 20 comprises an aperture 34 with an aperture opening 35. The filament 18 is positioned in the aperture opening 35. The geometry and size of the aperture opening 35 are adapted to the elliptical outer contour of the filament 18. This allows for a very small gap between the aperture opening 35 and the elliptical contour of the emission surface 24. This enables the reduction of electrons exiting the emission body 24 laterally and from the rear, or a reduction in emission towards the anode 14.

[0051] Furthermore, it is preferably provided, as shown in Figure 7, that the emission surface 25 of the emission body 24 is set back relative to an end face 36 of the aperture 34 facing the anode 14. For example, the emission surface 25 of the emission body 24 can be set back 0.1 to 0.5 mm relative to the end face 36 of the aperture 34. This positioning of the filament 18 within the aperture 33 has the additional advantage that cross-electrons from the lateral end face of the emission body 24 are further reduced. Furthermore, by means of this aperture opening 35, which is adapted to the outer contour of the filament 18, and the setback of the filament 18 relative to the end face 36 of the aperture 34, peripheral electrons from the rear of the emission body 24 towards the anode 14 can also be significantly reduced.This ensures that essentially only the electrons emitted from the emission surface 25 reach the anode 14 at high speed and almost without interference.

[0052] Figure 8 shows an alternative embodiment of a filament 18 compared to the embodiment shown in Figure 5. The embodiment shown in Figure 8 comprises, for example, two current paths 33, which are formed and defined by the length of the laterally open slots 31, each extending from the outside of the emission surface 25 towards the longitudinal axis 26, and closed slots 32 located within the emission surface 25. The slots 32 located within the emission surface 25 are closed, whereas the slots 31 are open towards the outside of the emission surface 25. Preferably, two open slots 31 are provided, oriented symmetrically to the longitudinal axis of the emission surface 25, followed at a distance by a closed slot 32. This combination or successive arrangement is repeated multiple times along the longitudinal axis of the emission surface 25.The successive arrangement of the open slots 31 and the closed slots 32 forms two current paths 33 along which the current lines 37 run. The length of the slots 31 and 32, their position and distance from each other, as well as the successive arrangement of the open slots 31 and the closed slot 32, result in two current paths 33 of equal length. This ensures a homogeneous temperature distribution on the surface of the emission surface 25. In particular, achieving current paths 33 of equal length prevents an inhomogeneous temperature distribution.

Claims

Claims 1. Filament for generating electrons, in particular for an X-ray tube, with an emission body (24) having an inlet (27) and an outlet (28) for a current connection (29), with an emission surface (25) formed on the emission body (24), which is flat and includes at least one slot (31, 32) to guide at least one current path (37) from the inlet (27) along the emission surface (25) to the outlet (28), and a cross-section of the emission body (24) lies in a plane perpendicular to the flat emission surface (25), which has an increase in the cross-section of the emission body (24) from the inlet (27) to the center of the emission surface (25) and a decrease in the cross-section of the emission body (24) from the center of the emission surface (25) to the outlet (28), characterized in that - that at least one meandering stream path (33) extends from the inlet (27) to the outlet (28) of the emission surface (25) of the emission body (24), along which at least one stream path line (37) runs, and - that the emission surface (25) of the emission body (24) is in the shape of an ellipse when viewed from above.

2. Filament according to claim 1, characterized in that only a single current path (33) extends from the inlet (27) to the outlet (28) of the emission surface (25) of the emission body (24).

3. Filament according to claim 1 or 2, characterized in that the cross-section of the emission body (24) is the same at the inlet (27) and at the outlet (28).

4. Filament according to one of the preceding claims, characterized in that the inlet (27) and outlet (28) of the emission body (24) and the intermediate area with the emission surface (25) have the same thickness.

5. Filament according to one of the preceding claims, characterized in that the cross-sectional enlargement and / or the cross-sectional reduction depends on an objective function from f = min (Tpmax-Tpmin) 2 is formed, where Tpmax represents the maximum temperature on the emission surface (25) of the emission body (24) and Tpmin represents the minimum temperature on the emission surface (25) of the emission body (24).

6. Filament according to one of the preceding claims, characterized in that the emission surface (25) of the emission body (24) has a contour which is several times longer than it is wide, wherein a longitudinal axis (26) of the emission surface (25) lies in a connecting line between the inlet (27) and the outlet (28).

7. Filament according to claim 6, characterized in that the outer surfaces of the emission surface (25), which extend between the inlet (27) and the outlet (28), are rotationally symmetric to the longitudinal axis (26).

8. Filament according to one of the preceding claims, characterized in that the emission body (24) has laterally open slots (31), in particular longitudinal slots, extending from its side edges, which extend between the inlet (27) and the outlet (28), into the emission surface (25) and / or has closed slots (32) located within the emission surface (25).

9. Filament according to claim 8, characterized in that the slots (31, 32) are aligned perpendicular to the longitudinal axis (26) and preferably the slots (31, 32) have a constant width and / or their slot base is rounded.

10. Filament according to one of claims 8 to 9, characterized in that the slots (31) for forming the one current path (33) on the emission surface (25) are alternately opposite each other and are provided in the emission surface (25) starting from the side edges of the emission surface (25) from the inlet (27) in the direction of the outlet (28).

11. Filament according to one of the preceding claims, characterized in that, to form two current paths (33) on the emission surface (25), two open slots (31) are provided in an axis perpendicular to the longitudinal axis of the emission surface (25) and opposite each other, and a closed slot (32) is positioned spaced apart therefrom, the longitudinal axis of which is aligned perpendicular to the longitudinal axis of the emission surface (25), and preferably the closed slot (32) is aligned in a mirror-symmetrical manner to the longitudinal axis of the emission surface (25).

12. Filament according to one of claims 8 to 11, characterized in that the length of the open slots (31) increases towards the center of the emission surface (25).

13. Filament according to one of the preceding claims, characterized in that the distance between the inlet (27) and the outlet (28) of the emission surface (25) is adapted to the size of an optical spot of an X-ray radiation (21) at the outlet of a housing (12) of the X-ray tube (11) and to an angular arrangement of the anode (14) relative to the axis of the electron beam (19).

14. Cathode for an X-ray tube (11), with a filament (18), with a direction cylinder (20) to which the filament (18) is assigned, with a diaphragm (34) arranged in the direction cylinder (20), which has a diaphragm opening (35), characterized in that - that the filament (18) is formed according to one of the preceding claims 1 to 13 and is arranged in the aperture opening (35).

15. Cathode according to claim 14, characterized in that the aperture opening (35) is adapted to an outer contour of the filament (18).

16. Cathode according to claim 14 or 15, characterized in that the emission surface (25) of the emission body (24) is arranged recessed relative to an end face (36) of the aperture (35) oriented towards the anode (14).

17. Cathode according to one of claims 14 to 16, characterized in that the entire emission surface (25) of the emission body (24) can be controlled with a homogeneous temperature field, and preferably a temperature difference along the emission surface (25) of less than 10 °C, in particular less than 1 °C.

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