Improved X-ray tube with field-effect emitter

Abstract

An X-ray tube (1) has a tube body (2) that gas-tightly encloses a tube volume (3). An emitter electrode (4), a gate electrode (5), and an anode (6) are arranged within the tube volume (3). The emitter electrode (4) is designed as an unheated electrode and has a plurality of emitter needles (7) arranged on a substrate (8) in a region facing the gate electrode (5). The arrangement of the emitter electrode (4) and the gate electrode (5) is such that, when an emission voltage (U') is applied between the emitter electrode (4) and the gate electrode (5), electrons (10) are emitted from the emitter needles (7) at the tips (9) of the emitter needles (7) due to the resulting electric field.The gate electrode (5) has recesses (11) under which a portion of the emitter needles (7) is arranged, so that the electrons (10) emitted from the tips (9) of the emitter needles (7) arranged under the respective recess (11) are accelerated through the recesses (11) towards the anode (6) due to a high voltage (U) applied between the emitter electrode (4) and the anode (6). The gate electrode (5) is divided into a main region (14) and several auxiliary regions (15) by insulating structures (13) incorporated into the gate electrode (5). The emission voltage (U') is applied to the main region (14) relative to the emitter electrode (4). The auxiliary regions (15) are electrically connected to the main region (14) only via connecting bridges (16). Each auxiliary region (15) has at least one recess (11).

Description

[0001] The present invention relates to an X-ray tube, - wherein the X-ray tube has a tube body that gas-tightly encloses a tube volume, - wherein an emitter electrode, a gate electrode and an anode are arranged within the tube volume, - wherein the emitter electrode is designed as an unheated electrode which has a plurality of emitter needles arranged on a substrate in a region facing the gate electrode, - wherein the arrangement of the emitter electrode and the gate electrode are such that, by applying an emission voltage between the emitter electrode and the gate electrode, electrons escape from the emitter needles at the tips of the emitter needles due to the electric field thereby formed, - wherein the gate electrode has recesses under which a portion of the emitter needles is arranged, so that the electrons emerging from the tips of the emitter needles arranged under the respective recess are accelerated through the recesses towards the anode due to a high voltage applied between the emitter electrode and the anode.

[0002] Such an X-ray tube is generally known. WO 2013 / 136 299 A1 and EP 3 748 667 A1 can be cited as purely examples.

[0003] It is known to design the emitter electrode of an X-ray tube as a field-effect emitter. With such a design, a high current density can be achieved. As a purely illustrative example, reference can be made to the article "Silicon Field Emitter Arrays With Current Densities Exceeding 100 A / cm² at Gate Voltages Below 75 V" by Guerrera et al., published in IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 1, JANUARY 2016.

[0004] DE 10 2020 206 939 B4 relates to an X-ray source and an X-ray device. The X-ray source according to the invention has an evacuated X-ray tube housing rotatably mounted about an axis of rotation, wherein the X-ray tube housing has an anode and an electron source, wherein the anode is arranged within the X-ray tube housing in a rotationally fixed manner relative to the X-ray tube housing and is designed to generate X-rays by means of electrons incident on a focal spot of the anode, wherein the electron source is mounted within the X-ray tube housing in a substantially fixed position relative to the axis of rotation, wherein the electron source has a main emitter and at least one secondary emitter for the emission of electrons, and wherein the electron emission of the main emitter and / or the at least one secondary emitter is controllable in such a way that a spatial movement of the focal spot based on a movement of the electron source is reduced.

[0005] When using a field-effect emitter as the emitter electrode, excessive gate current can occur for various reasons. For example, individual emitter needles can make electrical contact with the gate electrode due to manufacturing defects or mechanical damage, resulting in a short circuit. Contamination or excessive currents through individual emitter needles can also cause such a short circuit. Another possible cause is arcing between the anode and the emitter electrode.

[0006] A flashover can have various consequences. One possible consequence is that the gate electrode evaporates in a section. In this case, the emitter needles located below the evaporated area no longer have a gate electrode available to generate the electric field that excites the electrons to exit the emitter needles. However, the remaining area of ​​the emitter electrode is still functional. Another possible consequence is that the gate electrode does not evaporate in a section, but melts, sealing the recesses in that section, and then solidifies again. In this case, the gate electrode is still available to the emitter needles below the melted and resolidified section, but the electrons exiting the emitter needles in this section can no longer reach the anode, but are absorbed by the gate electrode.This results in a current flowing through the gate electrode. This can either cause the gate electrode to simply heat up, which is harmless. Alternatively, the gate electrode can melt again and later solidify. Alternatively, the gate electrode can also vaporize.

[0007] Another possible consequence of a flashover is that the gate electrode melts, forming an electrically conductive connection to one or more of the emitter needles. In this case, a short circuit occurs between the gate and emitter electrodes. Such a short circuit can affect the entire field-effect emitter array. After such a short circuit, the array may only function to a limited extent, if at all. The X-ray tube is no longer fully functional.

[0008] Theoretically, a large portion of the gate electrode would still be functional. However, this is not possible with a unified control of the gate and emitter electrodes—that is, without dividing the gate and / or emitter electrodes into independently operable sub-areas. This problem is explained, for example, by RF Asadi, T. Zheng, J. Da Silva, G. Rughoobur, AI Akinwande, and B. Gnade in their conference paper “Failure Mode of Si Field Emission Arrays based on Emission Pattern Analysis,” 2021 34th International Vacuum Nanoelectronics Conference (IVNC), Lyon, France, 2021, pp. 1–2, doi: 10.1109 / IVNC52431.2021.9600740.

[0009] It is already known to divide the gate electrode and / or the emitter electrode into independently operable sub-sections, see the aforementioned WO 2013 / 136 299 A1 and the also previously mentioned EP 3 748 667 A1. This approach ensures that in the event of a short circuit, only a sub-section of the gate electrode and / or a sub-section of the emitter electrode fails. However, a disadvantage of this solution is that deactivating the non-functional sub-section must be done actively. Therefore, appropriate sensors with downstream logic are required to detect defective sub-sections. Furthermore, control of the sub-sections based on this is necessary. This significantly increases the complexity of the design and also entails higher costs.

[0010] The object of the present invention is to provide a simple and cost-effective way by which, in the event of a short circuit between the gate electrode and the emitter electrode, only partial areas fail.

[0011] The problem is solved by an X-ray tube having the features of claim 1. Advantageous embodiments of the X-ray tube are the subject of dependent claims 2 to 4.

[0012] According to the invention, an X-ray tube of the type mentioned above is designed by: - that the gate electrode is divided into a main area and several additional areas by insulating structures inserted into the gate electrode, - that the emission voltage is applied to the main area relative to the emitter electrode, - that the additional areas are only electrically connected to the main area via connecting bridges and - that at least one exception is arranged in each of the additional areas.

[0013] This design ensures that in the event of a short circuit in one of the auxiliary sections, the connecting bridge – essentially like a fuse – "burns out," thereby electrically isolating the corresponding auxiliary section from the main section. Consequently, the auxiliary section is no longer at the potential applied to the main section. Instead, due to the short circuit, it is at the potential applied to the emitter electrode. However, because of the electrical isolation of the auxiliary section from the main section and thus from the other auxiliary sections, this only affects the affected auxiliary section. The main section and the other auxiliary sections can still be connected to the potential applied to the main section. Therefore, the remaining portion of the gate electrode can continue to be used.

[0014] The number of cutouts in a given sub-area of ​​the gate electrode can be determined as required. Typically, a given sub-area of ​​the gate electrode will contain n1 x m1 cutouts, where n1 and m1 can be any natural numbers. However, n1 and m1 are usually approximately equal in size.

[0015] Similarly, the number of emitter needles located under a given recess can also be determined as needed. Typically, there will be n² x m² emitter needles under each recess of the gate electrode, where n² and m² can be any natural numbers. However, n² and m² are usually approximately equal.

[0016] Due to the design according to the invention, it is not necessary to divide the emitter electrode into different sub-areas that can be controlled independently of one another. However, this remains possible. In this case, the emitter needles form several individually controllable groups. When forming groups, it is also possible to adjust the groups to the sub-areas. However, this too is not necessary.

[0017] The field-effect emitters can be made of any suitable material. Preferably, the emitter needles are made of silicon (Si), or alternatively, molybdenum (Mo). This design allows for high current densities of up to 100 A / cm². The use of silicon offers the additional advantage that the material itself intrinsically acts as a current limiter.

[0018] The emitter needles preferably form a regular, especially periodic, grid.

[0019] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

[0020] The properties, features, and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more readily understandable in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawings. These drawings show, in schematic representation: Fig. 1 an X-ray tube, Fig. 2 a small section of an electrode arrangement from the side, Fig. 3 a larger section of the electrode arrangement of Fig. 2 from the top and Fig. 4 a block diagram.

[0021] According to Fig. Figure 1 comprises an X-ray tube 1 with a tube body 2. The tube body 2 can be made of glass, for example. The tube body 2 encloses a tube volume 3 in a gas-tight manner. The tube volume 3 is evacuated. Within the tube volume 3 are (among other things) an emitter electrode 4, a gate electrode 5, and an anode 6. The gate electrode 5 is located between the emitter electrode 4 and the anode 6, in close proximity to the emitter electrode 4. The gate electrode 5 has a grid-like design. The anode 6 can be a conventional rotating anode. The electrodes 4 to 6 are connected via electrical conductors (not shown) to electronics located outside the tube volume 3.

[0022] According to the Fig. 2 and Fig. In section 3, the emitter electrode 4 is designed as an unheated electrode, i.e., as a so-called cold emitter. It has a plurality of emitter needles 7 in a region facing the gate electrode 5. The emitter needles 7 are in turn arranged on a substrate 8. The emitter needles 7 preferably consist of silicon (Si) or molybdenum (Mo). As can be readily seen, they form a regular, in particular periodic, grid.

[0023] The arrangement of the emitter electrode 4 and the gate electrode 5 is coordinated such that by applying an emission voltage U' (see Fig. 4) between the emitter electrode 4 and the gate electrode 5, due to the electric field formed therein, electrons 10 are emitted at the tips 9 of the emitter needles 7 (see Fig. 1) emerge from the emitter needles 7. Provided the emission voltage U' between the emitter needles 7 of the emitter electrode 4 and the gate electrode 5 is high enough, the emitter needles 7 emit free electrons at their tips 9 into the region between the emitter needles 7 and the gate electrode 5. The emission voltage U' is less than 1 kV, usually even less than 100 V, for example about 50 V.

[0024] Due to the lattice-like structure of the gate electrode 5, the gate electrode 5 exhibits, according to the Fig. 2 and Fig. Three recesses 11 are provided. A portion of the emitter needles 7 is arranged beneath each recess 11. The electrons 10 emerging from the tips 9 of the emitter needles 7 arranged beneath the respective recess 11 are therefore initially accelerated slightly by the emission voltage U' and leave the immediate vicinity of the tip 9 of the respective emitter needle 7. The electrons 10 are further accelerated by a high voltage U applied between the emitter electrode 4 and the anode 6 (see Fig. 4) Electrons are drawn from the immediate vicinity around the emitter needles 7 and accelerated through the recesses 11 towards the anode 6. The high voltage U is in the double-digit, sometimes even the low triple-digit kilovolt range. The electrons 10 striking the anode 6 generate X-rays 12 there. The anode 6 can, for example, be designed as a rotating anode.

[0025] According to Fig. 3 - partly also from Fig. As can be seen in Figure 2, insulating structures 13 are incorporated into the gate electrode 5. These insulating structures 13 divide the gate electrode 5 into a main region 14 and several auxiliary regions 15. The insulating structures 13 ensure that the auxiliary regions 15 are electrically connected to the main region 14 only via connecting bridges 16. Each auxiliary region 15 also contains at least one recess 11. Of the insulating structures 13, the auxiliary regions 15, and the connecting bridges 16, Fig. 3 only a few are marked with their reference symbol. The same applies to the emitter needles 7 and their tips 9.

[0026] The emission voltage U' relative to the emitter electrode 4 is determined according to Fig. 4 attached to the main area 14.

[0027] As a rule, some of the electrons 10 do not pass through the recesses 11 and therefore do not reach the anode 6. Instead, this portion of the electrons 10 strikes the gate electrode 5. This is unavoidable in a typical design and can be tolerated within certain limits. The crucial point is that the gate electrode 5 is not excessively heated by the electrons 10 and the resulting current flow. This applies equally to the prior art and the present invention. Insofar as the electrons 10 strike the gate electrode 5, they induce a current there. For example, in Fig. Figure 3 indicates a possible current flow, indicated by arrows 17. As long as the current flow through a specific connecting bridge 16 does not exceed a limit, this has no further consequences. However, if the current flow exceeds the limit—for example, due to a short circuit between one of the emitter needles 7 and the gate electrode 5—the gate electrode 5 melts or vaporizes in the area of ​​the connecting bridge 16 carrying the current flow. This interrupts the corresponding connecting bridge 16. Fig. Figure 3 shows, by way of example, a small area 18 in which a single connecting bridge 16 has evaporated, and a larger area 19 in which two adjacent connecting bridges 16 have evaporated.

[0028] As a result of the inventive design, if an excessively high current flows through a respective connecting bridge 16, the auxiliary area 15 surrounded by the respective insulating structure 13 is electrically isolated from the main area 14. After the corresponding connecting bridge 16 is interrupted, this auxiliary area 15 can no longer be supplied with the potential applied to the main area 14. The main area 14 and the unaffected auxiliary areas 15, however, can continue to function properly.

[0029] It is possible that the emitter needles 7 are always controlled uniformly. Preferably, however, the emitter needles 7 form a configuration as shown in Fig. 4 several individually controllable groups. Each group of emitter needles 7 can be supplied with its own potential. The in Fig. The number of groups of emitter needles 7 shown in section 4 is purely exemplary. In general, there are considerably more than just the two groups of emitter needles 7 shown. Likewise, the number shown in Fig. The number of emitter needles shown per group is purely exemplary. In reality, the groups contain considerably more emitter needles.

[0030] The present invention has many advantages. In particular, it provides an X-ray tube in which, due to the structure of the gate electrode 5, additional areas 15 of the gate electrode 5, in which a short circuit with the emitter electrode 4 exists, are automatically separated from the main area 14 and the other additional areas 15 of the gate electrode 5. So-called pixelation of the emitter electrode 4 is not required, although this remains possible.

[0031] Although the invention has been further illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived by the person skilled in the art without leaving the scope of protection of the invention.

Claims

[1] X-ray tube, - wherein the X-ray tube has a tube body (2) which gas-tightly encloses a tube volume (3), - wherein an emitter electrode (4), a gate electrode (5) and an anode (6) are arranged within the tube volume (3), - wherein the emitter electrode (4) is designed as an unheated electrode which has a plurality of emitter needles (7) arranged on a substrate (8) in a region facing the gate electrode (5), - wherein the arrangement of the emitter electrode (4) and the gate electrode (5) are such that by applying an emission voltage (U') between the emitter electrode (4) and the gate electrode (5) electrons (10) are emitted from the emitter needles (7) at the tips (9) of the emitter needles (7) due to the electric field formed therein, - wherein the gate electrode (5) has recesses (11) under which a portion of the emitter needles (7) is arranged, so that the electrons (10) emerging from the tips (9) of the emitter needles (7) arranged under the respective recess (11) are accelerated through the recesses (11) towards the anode (6) due to a high voltage (U) applied between the emitter electrode (4) and the anode (6), characterized by , - that the gate electrode (5) is divided into a main region (14) and several additional regions (15) by insulating structures (13) introduced into the gate electrode (5), - that the emission voltage (U') is applied to the main area (14) relative to the emitter electrode (4), - that the additional areas (15) are only electrically connected to the main area (14) via connecting bridges (16) and - that at least one exception (11) is arranged in each of the additional areas (15). [2] X-ray tube according to claim 1, characterized by , that the emitter needles (7) form several individually controllable groups. [3] X-ray tube according to claim 1 or 2, characterized by , that the emitter needles (7) are made of silicon or of molybdenum. [4] X-ray tube according to claim 1, 2 or 3, characterized by , that the emitter needles (7) form a regular, in particular periodic, grid.

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