Patent Application
20140168832
This application claims the benefit of German Patent Application No. DE 102012223569.8, filed Dec. 18, 2012, the entire contents of which are hereby incorporated herein by reference.
The present teachings relate generally to X-ray tubes with vacuum housings.
An x-ray tube may include a vacuum housing having at least one cathode and at least one anode disposed therein. The at least one cathode and the at least one anode may be insulated by at least one insulation element. The cathode (e.g., a flat panel emitter, a filament) emits electrons that strike the anode as an electron beam when a high voltage is applied to the cathode.
The electron beam is accelerated towards the anode and strikes the surface of the anode, thereby creating x-ray radiation in the anode material. The x-ray radiation exits as useful x-ray radiation from an x-ray beam exit window in the vacuum housing. The x-ray radiation may be used for imaging processes in the medical or non-medical fields.
With rotating anodes (e.g., rotating anode x-ray tubes or rotating envelope x-ray tubes), compensation may be made for rotation of the anode. The compensation is achieved using deflection electrodes. The electron beam may be focused even in small spaces using deflection electrodes that are arranged close to the cathode (e.g., on the focus head). The deflection electrodes may apply and maintain variable deflection voltages to the cathode voltage. The deflection electrodes may be insulated from the cathode (e.g., insulated from the focus head). The insulation elements may be glass or ceramic passthroughs. The insulation elements may have a reference to the cathode voltage (e.g., HV potential of the cathode).
Because of the space available in the area of the cathode, the size of the insulation elements may be configured for only normal operation although this is not problematic.
A drop in potential that affects the cathode may occur in the event of a technically unavoidable “arcing.” The term “arcing” refers to voltage flashovers and voltage discharges (e.g., exceeding tolerance range of the rated voltage) that occur as transient events (e.g., at random and, therefore, unpredictable times).
Temporally resolved, the potential of at least one of the deflection electrodes and/or the potential of the focus head is reduced by the above-described drop in potential. The deflection electrodes disposed as insulated electrodes briefly remain at full potential. The deflection voltage may also be present at the deflection electrodes.
Since the high-voltage is not generated directly at the cathode, a delay may occur while the focus head and the deflection electrodes adapt to the same potential. During the interim, almost all of the voltage drops across the insulation elements of the deflection electrodes. Further discharges may result shortly after the arcing and may lead to an accelerated destruction of the sensitive insulation elements of the deflection electrodes. Due to the energy-rich discharge, discharge traces on the insulation elements and material deposits on the insulation elements may occur. The material deposits are detrimental to the vacuum in the vacuum housing and, therefore, to operation of the x-ray tube.
The scope of the present invention is defined solely by the appended claims, and is not affected to any degree by the statements within this summary.
Other insulated functional parts in the vacuum housing of the x-ray tube in addition to the cathode may be subject to a problem, as described above. These additional parts include, for example, anodes, backscattered electron collectors, and deflection devices.
The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, in some embodiments, an x-ray tube including functional parts that are reliably protected against overvoltages over service lifetimes is provided.
An x-ray tube in accordance with the present teachings includes a vacuum housing. At least one cathode and at least one anode are disposed in the vacuum housing. The at least one cathode and at least one anode are insulated by at least one insulation element. On application of high voltage, the cathode emits electrons that strike the anode as an electron beam. In some embodiments, the x-ray tube includes a voltage arrester device with an insulation path that has a higher field strength than the field strength at the insulation element. Thus, when a voltage flashover occurs, a voltage leakage takes place by the voltage arrester device.
The higher field strength of the insulation path of the voltage arrester device as compared to the field strength at the insulation element results in a higher discharge probability at the voltage arrester device. As a result, the insulation element is protected against damage.
The destructive discharge mechanisms of the insulated functional parts in the vacuum housing (e.g. focus head) are reliably prevented. The voltage arrester device provides an “electrical break point” between the respective functional parts and the associated insulation elements. Large differences in potential may lead to destructive discharge mechanisms. The electrical stresses from the insulation elements are taken up by the break point. The break point flashes over or arcs more quickly than the insulation elements.
An x-ray tube in accordance with the present teachings may provide one or more of the following: a voltage arrester device that is highly susceptible to high vacuums over the operating range of the x-ray tube (e.g., 20° C. to 2,000° C. at 10−8 mbar to 10−4 mbar); a voltage arrester device that is short-circuit proof under normal operation (e.g., grating lock operation at the focus head, focus voltages of about 6 kV); a voltage arrester device that in the event of arcing is “weaker” in high voltage terms than the insulation elements.
In some embodiments, the voltage arrester device “fires” more quickly than the insulation elements, thereby leading to a lower incidence of wear and degradation in the insulation elements.
In some embodiments, an x-ray tube does not need any insulation elements for effective protection of its functional parts. The functional parts may be configured for potential overvoltages, and may be overly large and overly heavy. In some embodiments of an x-ray tube, volume and weight of the insulation elements increase only insignificantly.
In some embodiments, the voltage arrester device may protect different functional parts arranged insulated in the vacuum housing of the x-ray tube against overvoltages.
In some embodiments, the voltage arrester device may be provided on a focus head of a cathode having at least one deflection electrode, thereby protecting the insulation elements of the cathode against damage from overvoltages.
In some embodiments, the voltage arrester device includes at least one first protection electrode and at least one second protection electrode. The at least one first protection electrode and the at least one second protection electrode are at a predetermined distance from one another. This distance defines the insulation path of the voltage arrester device.
In some embodiments, at least one first protective electrode is arranged on the focus head, and at least one second protective electrode is arranged on at least one deflection electrode. In some embodiments, the focus head forms at least one first protective electrode. Alternatively or in addition, in some embodiments, at least one deflection electrode may form a second protective electrode.
In some embodiments, there is only vacuum in the space between the first protective electrode and the second protective electrode, such that an arc arising in a voltage flashover or during a voltage surge may extinguish itself automatically.
In some embodiments, the voltage arrester device is provided between the cathode and the vacuum housing.
In some embodiments, the voltage arrester device is provided between the anode and the vacuum housing.
In some embodiments, the voltage arrester device may be provided between the cathode and the anode.
In some embodiments, molybdenum may be used as a vacuum-resistant metallic electrode material for the first protective electrode and for the second protective electrode.
Depending on the operating conditions of the x-ray tube and/or the type and the number of the functional parts to be protected, different contours (e.g., symmetrical or non-symmetrical arrangements) may be used for the first protective electrode and the second protective electrode.
In some embodiments, at least one first protective electrode has a spherical contour.
Alternatively or in addition, in some embodiments, at least one second protective electrode has a spherical contour.
In some embodiments, at least one first protective electrode has a plate-shaped contour.
In some embodiments, at least one second protective electrode has a plate-shaped contour.
Since the first protective electrode and the second protective electrode do not have any micro tips, different combinations of electrode shapes may be used depending on the application in order to prevent or greatly reduce arcing. In some embodiments, only small signs of wear and degradation may appear in the insulation elements of the functional parts.
Other contours besides the contours of the two above-described protective electrodes may be used. Examples of other contours are Borda and Rogowski profiles.
The above-described electrode shapes may lead to a weak, inhomogeneous electrical field. In normal operation of the x-ray tube, preliminary discharges of the protective electrodes may be avoided.
Upon application of a cathode voltage UC (high voltage), the cathode 2 emits electrons that strike the anode 3 as an electron beam 5. An anode voltage UA is present at the anode 3. The electrons of the electron beam 5 create x-ray radiation 6 at a focal point in the material of the anode 3. The x-ray radiation 6 exits the vacuum housing 1 as useful x-radiation from an x-ray radiation exit window 7.
The cathode 2 includes a focus head 8. A number of deflection electrodes 9 are disposed on the focus head 8 above the insulation elements 4. Once again, for simplicity, only two of the deflection electrodes 9 are shown. A deflection voltage UD is present at the deflection electrodes. By application of the cathode voltage UC with a deflection voltage±UD, the electron beam 5 may be influenced.
The x-ray tube shown in
Upon application of a cathode voltage UC (high voltage), the cathode 2 emits electrons that strike the anode 3 as an electron beam 5. An anode voltage UA is present at the anode 3. The electrons of the electron beam 5 create x-ray radiation 6 at a focal point in the material of the anode 3. The x-ray radiation 6 exits the vacuum housing 1 as useful x-radiation from an x-ray radiation exit window 7.
The cathode 2 includes a focus head 8. A number of deflection electrodes 9 are disposed on the focus head 8 above the insulation elements 4. For simplicity, only two deflection electrodes 9 are shown. A deflection voltage UD is present at the deflection electrodes. By application of the cathode voltage UC with a deflection voltage±UD, the electron beam 5 may be influenced.
In the event of technically unavoidable voltage flashovers and voltage surges (e.g., exceeding tolerance range of the rated voltage), there is a drop in potential that affects the cathode 2. The transient (e.g., random and, therefore, unpredictable with regard to time) occurrences of voltage flashovers or voltage surges are also referred to as “arcing.”
Resolved over time, the above-described drop in potential causes the potential UD of at least one of the deflection electrodes 9 and/or the potential UK of the focus head 8 to be lowered. The deflection electrodes 9 disposed as insulated electrodes briefly remain at full potential UC. The deflection voltage UD may also be present at the deflection electrodes 9.
Since the high-voltage is not generated directly at the cathode 2, a delay may occur while the focus head 8 and the deflection electrodes 9 adapt to the same potential. During the interim, almost all of the voltage drops across the insulation elements 4 of the deflection electrodes 9. Further discharges may result shortly after the arcing and may lead to an accelerated destruction of the sensitive insulation elements 4 of the deflection electrodes 9. Due to the energy-rich discharge, discharge traces on the insulation elements 4 and material deposits on the insulation elements 4 may occur. The material deposits are detrimental to the vacuum in the vacuum housing 1 and, therefore, to operation of the x-ray tube.
In the x-ray tube shown in
As shown in
The number and the form of the first protective electrode 10 and the second protective electrode 11 may be readily adapted to the respective constructive circumstances and to the respective application.
As shown in
As shown in
The head of the finger electrode 10 (e.g., the first protective electrode) has a radius r (e.g., a “head radius”) and a distance s (also referred to as “arc width”) to the deflection electrode 9. The selection of the radius r and the distance s (e.g., insulation path of the voltage arrester device) enables the field strength to be set for normal operation. The “sphere-plate” arrangement provides a weakly inhomogeneous electric field. Premature discharges may be reliably avoided in the weakly inhomogeneous electric field.
As shown in
Since molybdenum bars may be used as supply leads to the focus head 8, the supply leads may, for example, be attached at a defined distance from one another, so that the molybdenum bars may provide a spark gap. For this, a sufficient mechanical stability and resistance to degradation against electrical discharges is to be provided.
The field strengths Emax are plotted on the ordinate axis and standardized to the respective ideal homogeneous field strength Ehom (e.g., dimensionless variables).
The head radius r of the first protective electrode 10 is plotted on the abscissa axis in units of millimeters.
The field strengths Emax are plotted as standardized to the respective ideal homogeneous field strength Ehom (e.g., dimensionless variable). The homogeneous field strength Ehom is defined for the ideal plate capacitor by the respective plate distance s (e.g., “surge width”). The head radius r of the first protective electrode 10 determines the respective percentage field increase.
In some embodiments of the voltage arrester device, the electrical fields do not exhibit too great an inhomogeneity and are slightly inhomogeneous. A head radius r of the first protective electrode 10 that is too small would lead to undesired cold emissions or premature discharges in normal operation. A flashover only occurs with an overvoltage at the focus head 8 (e.g. flashover between anode 3 and cathode 2).
While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.
It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding claim—whether independent or dependent—and that such new combinations are to be understood as forming a part of the present specification.
| Number | Date | Country | Kind |
|---|---|---|---|
| DE102012223569.8 | Dec 2012 | DE | national |
