Reducing electron emission from an electron emitter

Abstract

The invention relates to a method for reducing electron emission of an electron emitter, an associated computer program product, the electron emitter and an X-ray source. The method according to the invention for reducing electron emission from an electron emitter comprises the steps: - Emitting electrons from the first current path as a function of an emission voltage between the gate electrode and the emission surface by means of the at least one first field effect emitter element, - Determining the nature of the first current path, - Activating the first current limiting unit depending on the determined nature of the first current path for reducing the electron emission of the electron emitter.

Description

The invention relates to a method for reducing electron emission of an electron emitter, to an associated computer program product, to the electron emitter and to an X-ray source.X-ray sources typically generate x-rays by means of electrons, which are generated by an electron emitter in a vacuum and, after acceleration thereof by means of high voltage, are decelerated during an interaction with an anode. The electron emitters regularly used for this purpose can be distinguished in particular into so-called thermionic electron emitters and cold electron emitters. Thermionic electron emitters generate the electrons in particular according to the Edison-Richardson effect with heating of the emitter elements. Cold electron emitters emit the electrons in particular without a comparable heating, but for example according to the field effect by means of field effect emitter elements configured accordingly thereto.Conventional electron emitters with field effect emitter elements are known in particular from U.S. Pat. No. 9,748,071 B2 or EP 3 933 881 A1. The former describes in particular a construction of the field effect emitter elements from emitter needles comprising transistor structures, in order to preferably enable switching of independent groups of the field effect emitter elements. The latter, on the other hand, relates to protective devices for preventing charged particles from being deposited in the field effect emitter elements during operation of such electron emitters.U.S. Pat. No. 5,075,595 A also discloses a field effect emitter. EP 3 075 000 B1 is concerned with protecting a cold cathode against ion impacts.DE 10 2009 011 642 A1 relates to an improved X-ray tube with a plurality of cathodes in an evacuated region. Furthermore, a plurality of wirelessly controllable elements are arranged in the evacuated region, which elements are each assigned to a cathode or a group of cathodes and which, upon receiving a control signal from outside the evacuated region, produce an electrically conductive connection of this cathode or group of cathodes to a cathode control voltage line.WO 2013 / 136 299 A1 relates to an image capturing device comprising an electron receiving construct and an electron emitting construct and further comprising an inner gap providing an unobstructed space between the electron emitting construct and the electron receiving construct. The image sensing device further comprises a resistive layer disposed between the field emission type electron-emitting construct and the emitter seat.Electron emitters with field effect emitter elements are usually more susceptible to damage or even destruction due to electrical discharge processes, in particular high-voltage sparkovers between the cathode device and the anode, within the X-ray source, in comparison with thermionic emitter elements. Such high voltage breakdowns can cause, for example, a short circuit between the field effect emitter elements and the gate electrode of the electron emitter. See, for example, R. F. Asadi, T. Zheng, J. Da Silva, G. Rughoobur, A. I. Akinwande and B. Gnade, "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 with respect to the related effects.The object of the invention is to specify a method for reducing electron emission of an electron emitter, an associated computer program product, the electron emitter and an X-ray source, in which an operating duration of the electron emitter can be extended.The object is achieved by the features of the independent claims. Advantageous embodiments are described in the dependent claims.Regardless of the grammatical sex of a certain term, individuals with male, female or other sex identity are included.The method according to the invention for reducing an electron emission of an electron emitter, wherein the electron emitter isa plurality of parallel-oriented field effect emitter elements for forming an emission surface on the upper side of the plurality of parallel-oriented field effect emitter elements,a gate electrode arranged above the emission surface, anda first current limiting unit,wherein the first current limiting unit and at least one first field effect emitter element of the plurality of field effect emitter elements are part of a first current path,wherein the at least one first field effect emitter element is arranged electrically between the first current limiting unit and the gate electrode in the first current path,comprising the steps of:emitting electrons from the first current path as a function of an emission voltage between the gate electrode and the emission surface by means of the at least one first field effect emitter element,determining a condition of the first current path,activating the first current limiting unit in dependence on the determined nature of the first current path for reducing the electron emission of the electron emitter.The electron emitter according to the invention is configured to carry out the method according to the invention for reducing the electron emission.The cathode device for an X-ray source according to the invention comprisesthe electron emitter andan emitter seat.The X-ray source according to the invention comprisesthe cathode device,an anode andan evacuated housing,wherein the cathode means and the anode are disposed within the evacuated housing.The anode can be, in particular, a rotating anode or a standing anode. In principle, it is conceivable for the anode to rotate together with the evacuated housing.The electrons generated by means of the cathode device are accelerated in particular by the cathode device in the direction of the anode by means of an acceleration unit. The acceleration unit comprises in particular a high-voltage source or a high-frequency source. Depending on the type of acceleration unit, the X-ray source is typically an X-ray emitter, in particular for imaging applications in the keV range, or a linear accelerator, in particular for imaging or therapeutic applications in the MeV range.The cathode device forms in particular a cathode for the X-ray source. The electrical potential of the cathode is typically more negative compared to the electrical potential of the anode.The electron emitter is configured in particular to generate the electrons by means of the field effect emitter elements. The electron emitter is in particular a closed component, for example an electron emitter chip. The electron emitter can consist, for example, of the multiplicity of parallel-oriented field effect emitter elements, the gate electrode and the plurality of first contact elements.The term plurality of field effect emitter elements means in particular that so many field effect emitter elements are part of the emission surface that the emission surface has an electron current density of at least 0.1 A / cm^2, preferably at least 3 A / cm^2, particularly advantageously at least 10 A / cm^2. The number of field effect emitter elements required for this is typically at least 1000, regularly more than 10000. Advantageously, the emission surface has dimensions of at least 0.1 to 0.1 cm^2 and / or at most 10 to 10 cm^2.The field effect emitter elements can be embedded in an insulating matrix. The insulating matrix preferably holds the plurality of field effect emitter elements together.The field effect emitter elements are aligned in particular parallel and / or flush with respect to the emission surface. In this case, the emission surface is advantageously as planar as possible. The emission surface can basically be finished in order to be planar.The field effect emitter elements typically have an emission point or an emission section at one end of the respective field effect emitter elements. The emission surface consists in particular of the emission points or the emission sections of the field effect emitter elements. The emission point is, for example, the tip of a field effect emitter element designed as a field effect emitter needle. The emission section comprises, for example, the emission point and an adjacent region lying around the emission point.The emission surface forms in particular the upper side of the plurality of field effect emitter elements. An upper side of the electron emitter can correspond to the upper side of the plurality of field effect emitter elements, in particular when the gate electrode is not viewed and / or when the gate electrode is fully integrated into the volume of the field effect emitter elements.The sides of the plurality of field effect emitter elements define in particular the surfaces of the plurality of field effect emitter elements and are fundamentally synonymous in the present description. The sides of the plurality of field effect emitter elements comprise in particular an upper side, an underside and a cladding side. The surfaces of the plurality of field effect emitter elements accordingly comprise in particular a surface which typically corresponds to the emission surface, a lower surface and a lateral surface which typically corresponds to the lateral surfaces.The upper side and the lower side of the field effect emitter elements typically have the same dimensions and / or the same geometric shape. The geometric shape can be angular, in particular quadrangular, preferably rectangular, or round.The side surfaces of the plurality of parallel-oriented field effect emitter elements connect in particular the upper side and the lower side. The side surfaces of the plurality of parallel-oriented field effect emitter elements are formed in particular by the longitudinal sides of the outermost field effect emitter elements and / or by the matrix surrounding the outermost field effect emitter elements.The side surfaces cover in particular the circumference of the field effect emitter elements completely, i.e. by 360°. If the geometric shape of the upper side of the field effect emitter elements is round, a side surface is, by definition, a lateral surface with a maximum of 90° of the circumference. If the geometric shape of the upper side of the field effect emitter elements is angular, a side surface spans a lateral surface from one edge to an adjacent edge, wherein the edges respectively connect the respective corners of the upper side and lower side.It is conceivable for the field effect emitter elements to be grown on a substrate. The substrate can in principle have been removed, for example ground, after the growth of the field effect emitter elements. The substrate is typically arranged on a bottom side of the field effect emitter elements.The field effect emitter elements are designed, for example, as field effect emitter needles, the tips of which form the emission points and thus the emission surface. The field effect emitter needles are in particular nanotubes. Alternatively, it is conceivable for the field effect emitter elements to form at least one spindt cathode.It is regularly possible to integrate transistor structures, for example, into the field effect emitter needles, in particular if the field effect emitter needles consist of a semiconductor such as silicon, carbon or molybdenum, for example.The field effect emitter elements can be switched in particular individually, in groups or all together. Electron emitters which comprise field effect emitter elements that can be switched individually or in groups are typically so-called pixelated or segmented emitters. The segmentation of the circuit of the field effect emitter elements and thus of the electron emitter can be effected by means of different first or second contact elements and / or a segmentation of the gate electrode.In the present application, current paths are defined in such a way that all field effect emitter elements, which can only be turned on or off together, form their own current-carrying current path. Depending on the interconnection, the plurality of field effect emitter elements can therefore comprise a plurality of current paths for the individual field effect emitter elements, some current paths for field effect emitter elements switchable into groups or a single current path if all field effect emitter elements can only be connected together. A separately switchable current path, i.e. one which can be connected or disconnected by means of a contact element, is typically a current path which conducts current independently of other current paths.Typically, each current path is assigned exactly one segment of the electron emitter. Such a segment of the electron emitter forms in particular a pixel.The gate electrode can be designed in particular as a grid. The gate electrode is arranged in particular above the emission surface in such a way as to cause electron emission in the respective field effect emitter elements by means of the emission voltage between the gate electrode and the emission surface in accordance with the field effect. In this context, the term above includes the fact that the gate electrode extends as close as possible to the respective field effect emitter elements, for example is arranged directly at the height of the ends of the field effect emitter elements forming the emission surface and / or surrounds these ends of the field effect emitter elements.The gate electrode is advantageously arranged in such a way as to minimize e.g. an electron emission in or on the gate electrode and / or thermal effects. Alternatively or additionally, the gate electrode is advantageously arranged in such a way as to maximize, for example, the electron emission in or onto the anode and / or a mechanical stability and / or a robustness with respect to a high-voltage breakdown.The emission voltage is present in particular between the emission point or the emission section of the respective field effect elements and the gate electrode. For electron emission, the current path from which the electrons for electron emission originate is typically closed.The potential of the gate electrode is usually more positive than the potential of the field effect emitter elements. For example, the gate electrode can be at constant ground potential and the field effect emitter elements can be at negative potential. Alternatively, the field effect emitter elements can be at constant ground potential.It is conceivable that the gate electrode can provide different emission voltages to the emission surface, in particular if the gate electrode is designed as a grid. In this case, in particular the gate electrode can be segmented, so that the electron emitter is a segmented electron emitter. For example, the emission voltage between groups of field effect emitter elements can be varied by means of the segmented gate electrode, for example in that the electrical potential of the segments of the gate electrode differs.The electron emitter can in particular have a plurality of first contact elements for at least two current-carrying current paths of the electron emitter which are independent of one another. The emitter seat can have a plurality of second contact elements, which can be connected to the plurality of first contact elements at the end of the current paths. Advantageously, the plurality of first contact elements are arranged on a side of the plurality of parallel-oriented field effect emitter elements facing away from the emission surface.The plurality of first contact elements and the plurality of second contact elements are in particular designed to establish a reliable electrical connection by means of mutual contacting. In particular, an electrical connection can be made by a first contact element and a second contact element. This electrical connection typically connects the current path at this point. The plurality of second contact elements are in particular configured as part of the emitter seat in such a way that they can be connected to the plurality of first contact elements.The contact elements may comprise contact points or contact surfaces. The contact points typically have a contact surface which is as small as possible and is still electrically secure. It is conceivable that a first contact element has a contact point and a second contact element has a contact surface, wherein the contact point of the first contact element and the contact surface of the second contact element can be connected to one another, or vice versa. A contact surface typically has dimensions greater than a contact point, so that these two contact elements have a certain play relative to one another in one plane.The field effect emitter elements can connect directly to the plurality of first contact elements. The connection between the plurality of first contact elements and the field effect emitter elements can be effected through the substrate.The field effect emitter elements are connected to a current source in particular by means of electrically connected contact elements. In this case, a current path extends in particular from the current source via a second contact element, a first contact element connected thereto, as far as the field effect emitter element.It is conceivable that a first contact element is connected to one or more field effect emitter elements. A second contact element may be connectable to one or more first contact elements. The number of the first contact elements and the second contact elements may be identical or may vary.A plurality of contact elements means in particular that typically at least one order of magnitude fewer first contact elements or second contact elements are present than field effect emitter needles. In principle, it is conceivable that the number of first contact elements corresponds to the number of field effect emitter needles.Typically, the number of first contact elements correlates with the number of current paths. The plurality of first contact elements for the at least two current-carrying current paths of the electron emitter independently of one another are electrically conductively connected in particular to the at least two current paths. The at least two current paths of the electron emitter can comprise exclusively current paths of the field effect emitter elements or, in addition to the current paths of the field effect emitter elements, further current paths, for example a further current path of the gate electrode, of the electron emitter.Connectable means in particular electrically connectable, i.e. establishing an electrical connection. Electrically connectable means in particular electrically contactable. Electrically connected means in particular electrically contacted.Depending on the configuration of the cathode device, the connectable contact elements can already be electrically connected. Non-connected contact elements have in particular not closed an associated current path, but rather the latter is open. In principle, the first contact elements and the second contact elements can be connected equally or mutually exchanged.The emitter seat typically has fastening means for fastening and / or aligning the electron emitter on the emitter seat by means of the fastening means. The emitter seat has, in particular, a carrier body to which the second contact elements are fastened. The carrier body and the second contact elements are usually galvanically separated. The second contact elements are arranged in particular on a side of the emitter seat facing the electron emitter, in particular of the carrier body.The emitter seat, in particular the carrier body, can be designed as a focus head for the emitted electrons. The emitter seat, in particular the carrier body, can be configured to be metallic, in particular additionally electrically conductive, for example to be at a negative high-voltage potential or ground potential. The emitter seat can be configured in such a way as to lengthen the current paths of the field effect emitter elements as far as the current source in order to enable connection of the field effect emitter elements to the current source. For this purpose, the emitter seat can have, for example, at least one line which connects a field effect emitter element to the current source.The side facing away from the emission surface is in particular not the surface or top side. The side facing away from the emission surface can be, in particular, the underside and / or a jacket side. In particular, the plurality of second contact elements does not face the emission surface, but rather the plurality of first contact elements. When the plurality of first contact elements face the plurality of second contact elements, the first contact elements and the second contact elements are typically connectable.The first current limiting unit advantageously enables operation of the electron emitter despite a change in nature and / or because of a particular original nature. The method according to the invention advantageously takes into account the instantaneous nature of the first current path and / or of the electron emitter for the operation thereof.According to the invention, in particular the operation of the electron emitter with a reduced electron emission can advantageously be continued and / or carried out. The reduced electron emission can advantageously be adjusted by means of the first current limiting unit. The reduced electron emission is in particular synonymous with a reduced electron current.The invention is advantageous in particular for a pixelated or segmented electron emitter. In particular, the first current limiting unit can switch off the pixel with the first current path in the manner of a predetermined breaking point in the event of damage, but the rest of the electron emitter can advantageously continue to emit electrons. In principle, the first current limiting unit in the manner of a self-healing function of the electron emitter also makes it possible to burn out a pixel with a short circuit.The condition relates in particular to a technical composition of the first current path and / or of the electron emitter. The nature of the first current path and / or of the electron emitter can change, regularly deteriorate, in particular on account of aging and / or operation and / or damage acted upon from the outside. The condition can in principle remain unchanged over a certain period of time. The determined characteristic is in particular the instantaneous characteristic of the first current path and / or of the electron emitter at the time of the determination.The instantaneous and thus the determined characteristic depends in particular on a technical property of the first current path and / or of the electron emitter. The quality of the characteristic correlates in particular with the quality of the technical property.The technical property is in particular the capacity of the first current path and / or of the electron emitter to generate a specific and / or predefined electron current. The technical property is thus in particular the degree of functionality with respect to the electron current.Since the electron emitter at least partially comprises the first current path, the condition of the first current path influences the electron emitter. The nature of the first current path typically does not influence the nature of a further current path of the electron emitter, but regularly, if at all, only a technical property thereof in the form of the embodiments of the present invention.Reducing the electron emission means in particular reducing the originally provided and / or theoretically possible electron current. For example, the technical composition of the first current path and / or of the electron emitter can be designed by a manufacturer of the electron emitter in such a way that it is possible to generate a specific and / or predefined electron current. According to the invention, this determined and / or predefined electron current is reduced by means of the first current limiting unit.Alternatively or additionally, the reduction of the electron emission can mean, in particular, a reduction of an electron current already achieved during the operation of the electron emitter. For example, a performance of the electron emitter may be impaired completely, for example due to damage which has taken place only partially. According to the invention, such an impairment is addressed by means of the first current limiting unit in such a way that the functioning part of the electron emitter can continue to generate electrons.Reducing the electron emission can relate to the first current path and / or additionally to a further current path or all other current paths of the electron emitter. Electron emission is reduced, for example, if a single current path generates fewer or no electrons at all. Advantageously, a further current path can continue to generate electrons.Reducing the electron emission may mean shutting off the electron emission. In this case, for example, the first current path and all other current paths can be switched off. Alternatively, only the first current path can be switched off, while at least one further current path emits electrons.When determining the nature of the first current path, the entire technical composition or only a part of the technical composition can be determined. For example, the visually visible proportion and / or the electrically measurable proportion of the technical composition can be determined.The ascertainment of the condition of the first current path and / or of the electron emitter takes place, for example, by means of a sensor unit and / or a measuring unit and / or a storage unit and / or a computing unit. In particular, the storage unit and / or the computing unit can be part of the sensor unit and / or of the measuring unit.The sensor unit can, for example, determine with camera assistance how the instantaneous visual condition of the first current path and / or of the electron emitter is, and / or provide sensor signals for the memory unit and / or arithmetic unit, for example. The sensor signals include, in particular, a signal value describing the condition and / or the raw sensor data.The instantaneous visual condition can relate in particular to a topology of the emission surface. The instantaneous visual nature of the first current path and / or of the electron emitter may depend, for example, on the state of the emission surface. Typically, the state of the emission surface is the worse the more irregularities the emission surface has. The irregularities can in particular become more caused by damage acting externally on the emission surface. Alternatively or additionally, the irregularities may be present on account of the production method of the electron emitter, for example in the form of field effect emitter elements which are not completely exactly identical and / or are aligned flush. In this case, the topology already has the irregularities due to the manufacturer.The measuring unit can ascertain, for example, the electrical condition of the first current path and / or of the electron emitter and / or provide, for example, measurement signals for the storage unit and / or computing unit. The measurement signals comprise in particular a measurement value describing the condition and / or the measurement raw data.The electrical condition can in particular denote a conductivity of the first current path and / or of all other current paths of the electron emitter. The electrical conductivity can deteriorate in particular due to damage and / or overcurrent conduction acting externally on the emission surface. The electrical conductivity typically collapses or at least reduces in the event of a short circuit. For example, the measurement unit may measure a short-circuit current.It is conceivable in principle that sensor signals and / or measurement signals are stored in the memory unit for retrieval, in particular by the computing unit. It is conceivable for the computing unit to determine the condition by means of the sensor signals and / or the measurement signals.The computing unit can in particular have program code means which can determine the condition from the sensor signals by means of image algorithms. The image algorithms can comprise in particular a filtering and / or segmentation and / or transformation. Furthermore, the computing unit can have, in particular, program code means which can determine the condition from the measurement signals by means of data algorithms. The data algorithms can comprise in particular a filtering and / or a threshold value comparison. The program code means can preferably process sensor signals, sensor values, raw sensor data, measurement signals, measurement values and / or raw measurement data.The ascertainment of the characteristic can comprise, in particular, a comparison of the characteristic with a reference characteristic, in particular of the sensor signals and / or measurement signals with reference signals. For example, the storage unit can have the reference condition and / or reference signals. The reference signals typically have comparison values, threshold values and / or comparison images. The reference condition and / or the reference signals can describe, in particular, an idealised model of the electron emitter and / or a delivery state of the electron emitter and / or a previous state of the electron emitter, in particular on the basis of the previously determined condition.The ascertainment of the condition can comprise a repetition of the ascertainment of the condition, in particular in the case of no or slight change in the condition in comparison with previous ascertainment results, preferably without the first current limiting unit being activated. Alternatively or additionally, the determination of the condition can comprise a rough grading of the technical property, for example solely on the basis of progressive ageing and / or operating time. The flat grading may be reducing the technical property by a fixed percentage.When ascertaining the condition, a control signal can be calculated which describes the comparison result and / or the global grading. The control signal can advantageously be transmitted to the first current limiting unit.Activating the first current limiting unit means that the first current limiting unit is activated for the purpose of reducing electron emission, in particular according to the control signal. Depending on the configuration of the invention, the activation can comprise a switching on of the first current limiting unit or a switching off of the first current limiting unit. Activation thus means in particular a change of the operating state of the first current limiting unit to the effect that after activation the electron emission is reduced. Typically, therefore, at least the theoretically possible and / or already achieved electron current before the activation of the first current limiting unit is higher than the electron current generated after the activation.The activation of the first current limiting unit takes place in particular as a function of a comparison result of the determined condition with the reference condition and / or on the basis of the rough grading. The first current limiting unit is activated in particular on the basis of the control signal. The control signal can alternatively specify that the first current limiting unit is not activated.The first current limiting unit can be activated multiple times, which can cause, in particular, a consecutive reduction in the electron emission. The repeated activation in particular causes a constant reduction of the electron emission.One embodiment provides that the determination of the condition takes place before the electron emission. The determination of the condition is preferably carried out before an initial electron emission, in particular after the production of the electron emitter. The first current limiting unit is also advantageously activated before the electron emission. This embodiment is particularly advantageous in order to prevent damage to the first current path during the (first time) electron emission. It is also conceivable in principle for this embodiment to be carried out after an earlier electron emission has already taken place. In this case, according to this embodiment, electron emission is performed again after the condition is determined.An advantageous development of the preceding embodiment provides that the ascertainment of the condition comprises ascertaining a geometric shape of the at least one first field effect emitter element with respect to the emission surface and assigning an electrical potential to the geometric shape. The geometric shape can be, in particular, a relative height of a tip of the first field effect emitter element with respect to the emission surface. Alternatively or additionally, the geometric shape can be a radius of the tip of the first field effect emitter element. The radius of the tip correlates in particular with a peak of the tip. The radius of the tip is the smaller the more pointed the tip is. The geometric shape can be determined, for example, by means of the sensor unit, in particular visually, and / or by means of the measuring unit, in particular comparing it with other measured values. The assigned electrical potential depends in particular on the geometric shape. For example, electrical potential values can be stored in the memory unit, to which different geometric shapes are assigned or can be assigned. The assignment is carried out, for example, by means of the arithmetic unit. The electrical potential can be zero, in particular if the geometric shape with respect to the emission surface, that is to say with respect to other field effect emitter elements, is not deviating, in particular is the same, for example that is to say the relative height and / or a difference of the radius from the average radius is zero. The average radius may depend, for example, on the geometric shape of the other field effect emitter elements. Typically, the greater the difference of the first field effect emitter element in the geometric shape from the other field effect emitter elements, the higher the amount of electrical potential. The ascertainment of the geometric shape of the first field effect emitter element can comprise, in particular, ascertaining the geometric shapes of the first field effect emitter element and of at least one field effect emitter element adjacent to the first field effect emitter element. In this case, the geometric shape of the first field effect emitter element is, for example, an average value of the first and of the at least one adjacent field effect emitter element.An advantageous development of the preceding embodiment provides that the activation of the first current limiting unit comprises applying the associated electrical potential to a first potential-controllable electrode of the first current limiting unit in order to reduce the emission voltage with respect to the at least one first field effect emitter element. The first potential-controllable electrode is in particular a part of the first current-limiting unit. Typically, the emission voltage is lower the more positive the associated electrical potential is compared to the original, standard reference value for the electrical potential. In this case, the assigned electrical potential can generally be electrically negative or positive overall. Because the at least one first field effect emitter element is arranged electrically between the first current limiting unit and the gate electrode in the first current path, increasing the assigned electrical potential in the current path at the location of the first current limiting unit reduces the emission voltage and thus the electron emission.An advantageous development of the preceding embodiment provides that the associated electrical potential corresponds to the gate potential for zeroing the emission voltage. In this case, the emission voltage is advantageously zero and thus the electron emission from this at least one first field effect emitter element is switched off.An advantageous development of the preceding embodiments provides that after the activation of the first current limiting unit, electrons are emitted from a second current path, wherein a second field effect emitter element is part of the second current path, as a function of an emission voltage between the gate electrode and the emission surface by means of the second field effect emitter element. In this case, in particular the emission voltage for the first field effect emitter element and the emission voltage for the second field effect emitter element can differ, in particular on the basis of a different assigned electrical potential. This embodiment is particularly advantageous in order to enable the operation of different field effect emitter elements.One embodiment provides that the determination of the condition takes place after the beginning of the electron emission. The determination of the condition can be effected during the electron emission or after the electron emission. This embodiment is advantageous in order to be able to monitor the operation of the electron emitter and to be able to adapt it if necessary.An advantageous development of the preceding embodiment provides that the ascertainment of the condition comprises checking the first current path for a short circuit. The short circuit can be checked in particular by means of the measuring unit, alternatively or additionally by means of the sensor unit. The short circuit can be determinable, for example, as an irregularity in the emission surface and / or as a reduced or zeroed conductivity of the first current path. This embodiment advantageously allows a short circuit to be detected in the first current path.An advantageous development of the preceding embodiment provides that, in the presence of the short circuit, the activation of the first current limiting unit comprises the reduction of the emission voltage and an output of a short-circuit signal. In particular, in this case, the emission voltage can be set to zero. The electron emitter can be deactivated, for example, by means of the short-circuit signal.An advantageous development of the preceding embodiments provides that, in the presence of the short circuit, the activation of the first current limiting unit comprises triggering an overcurrent protection device of the first current limiting unit for irreversibly interrupting the first current path. This embodiment is particularly advantageous in order to deactivate the segment with the short circuit by means of the triggered overcurrent protection device.An advantageous development of the preceding embodiment provides that after the activation of the first current limiting unit, electrons are emitted from a second current path, wherein a second field effect emitter element is part of the second current path, as a function of an emission voltage between the gate electrode and the emission surface by means of the second field effect emitter element. The triggering of the overcurrent protection device causes in particular only an irreversible interruption of the first current path. Advantageously, other current paths, in particular the second current path, continue to be functional.The computer program product can be a computer program or comprise a computer program. The computer program product has, in particular, the program code means which depict the method steps according to the invention. As a result, the method according to the invention can be carried out in a defined and repeatable manner and control over a forwarding of the method according to the invention can be carried out. The computer program product is preferably configured such that the computing unit can carry out the method steps according to the invention by means of the computer program product. The program code means can be loaded in particular into a memory of the arithmetic unit and can typically be executed by means of a processor of the arithmetic unit with access to the memory. If the computer program product, in particular the program code means, is executed in the computing unit, typically all embodiments of the described method according to the invention can be carried out. The computer program product is stored, for example, on a physical, computer-readable medium and / or digitally stored as a data packet in a computer network. The computer program product may represent the tangible, computer readable medium and / or the data packet in the computer network. Thus, the invention can also proceed from the physical, computer-readable medium and / or the data packet in the computer network. The physical, computer-readable medium can usually be directly connected to the computing unit, for example by the physical, computer-readable medium being inserted into a DVD drive or being plugged into a USB port, as a result of which the computing unit can access the physical, computer-readable medium in particular in a reading manner. The data packet may preferably be retrieved from the computer network. The computer network can have the computing unit or be indirectly connected to the computing unit by means of a wide area network (WAN) or a (wireless) wireless area network connection (WLAN or LAN). For example, the computer program product can be digitally stored on a cloud server at a storage location of the computer network, can be transmitted by means of the WAN via the Internet and / or by means of the WLAN or LAN to the computing unit, in particular by calling up a download link which points to the storage location of the computer program product.Features, advantages or alternative embodiments mentioned in the description of the device are likewise to be transferred to the method and vice versa. In other words, claims can be further developed on the method with features of the device and vice versa. In particular, the device according to the invention can be used in the method.The invention is described and explained in more detail below with reference to the exemplary embodiments shown in the figures. In principle, structures and units that remain substantially the same are denoted by the same reference sign as when the respective structure or unit occurs for the first time in the following description of the figures.The following are shown: FIG. 1 shows a first variant of the method according to the invention, FIG. 2 shows a second variant of the method according to the invention, FIG. 3 shows a first exemplary embodiment of the first variant, FIG. 4 shows a second exemplary embodiment of the first variant, FIG. 5 shows a first exemplary embodiment of the second variant, FIG. 6 shows a second exemplary embodiment of the second variant, FIG. 7 shows an electron emitter according to the invention, FIG. 8 shows a cathode device according to the invention, and FIG. 9 shows an X-ray source.FIG. 1 shows a first variant of a method according to the invention for reducing an electron emission of an electron emitter in a flow chart with steps S 100 to S 102.The electron emitter hasa plurality of parallel-oriented field effect emitter elements for forming an emission surface on the upper side of the plurality of parallel-oriented field effect emitter elements,a gate electrode arranged above the emission surface, anda first current limiting unit,wherein the first current limiting unit and at least one first field effect emitter element of the plurality of field effect emitter elements are part of a first current path,wherein the at least one first field effect emitter element is arranged electrically between the first current limiting unit and the gate electrode in the first current path,It is characteristic of the first variant that the ascertainment of the characteristic according to S 101 takes place before the electron emission.Method step S 101 identifies a condition of the first current path.Method step S 102 identifies activating the first current limiting unit depending on the determined nature of the first current path for reducing the electron emission of the electron emitter.Method step S 100 identifies emitting electrons from the first current path as a function of an emission voltage between the gate electrode and the emission surface by means of the at least one first field effect emitter element.FIG. 2 shows a second variant of a method according to the invention for reducing an electron emission of an electron emitter in a flow chart with steps S 100 to S 102.It is characteristic of the second variant that the ascertainment of the characteristic according to S 101 takes place after the beginning of the electron emission.FIG. 3 shows a first exemplary embodiment of the first variant in a flow chart.Method step S 103 identifies that the ascertainment of the condition comprises ascertaining a geometric shape of the at least one first field effect emitter element with respect to the emission surface.Method step S 104 identifies that the ascertainment of the characteristic comprises an assignment of an electrical potential to the geometric shape.Method step S 105 identifies that activating the first current limiting unit comprises applying the associated electrical potential to a first potential-controllable electrode of the first current limiting unit in order to reduce the emission voltage with respect to the at least one first field effect emitter element.FIG. 4 shows a second exemplary embodiment of the first variant in a flow chart.Method step S 106 identifies that the ascertainment of the characteristic comprises an assignment of an electrical potential to the geometric shape, the assigned electrical potential corresponding to the gate potential for zeroing the emission voltage.Method step S 107 identifies that after the activation of the first current limiting unit, electrons are emitted from a second current path, wherein a second field effect emitter element is part of the second current path, as a function of an emission voltage between the gate electrode and the emission surface by means of the second field effect emitter element, wherein during the electron emission a different emission voltage is applied with respect to the second field effect emitter element than with respect to the at least one first field effect emitter element.FIG. 5 shows a first exemplary embodiment of the second variant in a flow chart.Method step S 108 identifies that the ascertainment of the condition comprises checking the first current path for a short circuit.Method step S 109 identifies that, in the presence of the short circuit, activating the first current limiting unit comprises reducing the emission voltage.Method step S 110 identifies that, in the presence of the short circuit, the activation of the first current limiting unit comprises an output of a short circuit signal.FIG. 6 shows a second exemplary embodiment of the second variant in a flow chart.Method step S 111 identifies that, in the presence of the short circuit, the activation of the first current limiting unit comprises triggering an overcurrent protection device of the first current limiting unit for irreversibly interrupting the first current path.Method step S 112 identifies that after the activation of the first current limiting unit, electrons are emitted from a second current path, wherein a second field effect emitter element is part of the second current path, as a function of an emission voltage between the gate electrode and the emission surface by means of the second field effect emitter element.FIG. 7 shows an electron emitter 10 according to the invention in a schematic sectional view.The electron emitter 10 has a plurality of parallel-aligned field effect emitter elements 11 for forming an emission surface 12 on the upper side of the plurality of parallel-aligned field effect emitter elements 11. The field effect emitter elements 11 are arranged on an optional substrate. The emission surface 12 is perpendicular to the image plane of FIG. 7 and is indicated by a dashed line.The electron emitter 10 further comprises a gate electrode 13 which is arranged above the emission surface 12. The gate electrode 13 is formed as a grid. The grating is arranged as close as possible to the ends of the field effect emitter elements 11.A first current path in FIG. 7 comprises four field effect emitter elements 11 and a second current path comprises four further field effect emitter elements 11. The electron emitter 10 further comprises a first current limiting unit 15 and a second current limiting unit 16.The first current limiting unit 15 and at least one first field effect emitter element of the plurality of field effect emitter elements 11 are part of a first current path. The at least one first field effect emitter element is arranged electrically between the first current limiting unit 15 and the gate electrode in the first current path. The first current limiting unit 15 has a first potential-controllable electrode for reducing the emission voltage with respect to the at least one first field effect emitter element.The second current limiting unit 16 and at least one further field effect emitter element of the plurality of field effect emitter elements 11 are part of a second current path. The at least one second field effect emitter element is arranged electrically between the second current limiting unit 16 and the gate electrode in the second current path. The second current limiting unit 15 has a second potential-controllable electrode for reducing the emission voltage with respect to the at least one second field effect emitter element.Alternatively or additionally, the first current limiting unit 15 can have an overcurrent protection device for irreversibly interrupting the first current path. It is conceivable for the second current limiting unit 16 alternatively or additionally to have an overcurrent protection device for irreversibly interrupting the second current path. Overcurrent protection devices can be configured in particular upstream of the field effect emitter elements or as part of the field effect emitter elements.FIG. 8 shows a cathode device 30 according to the invention in a schematic sectional view.The cathode device 30 comprises the electron emitter 10 and an emitter seat 20. The first current limiting unit 15 and the second current limiting unit 16 are not shown in FIG. 8.In addition, the electron emitter 10 has a plurality of first contact elements 14 for the at least two current-carrying current paths of the electron emitter 10 independently of one another. The plurality of first contact elements 14 are configured to be stationary in FIG. 8.The first current path and the second current path can each be connected to a separate second contact element 21 via a separate first contact element 14. The electron emitter 10 of FIG. 8 is thus a so-called segmented or pixelated emitter.The emitter seat 20 has a plurality of second contact elements 21, which can be connected to the plurality of first contact elements 14 at the end of the current paths and are electrically connected in FIG. 8. Depending on an emission voltage between the gate electrode 13 and the emission surface 12, in particular also on the electrical connection between the contact elements 14, 21, electrons can be emitted from at least one of the current paths by means of the field effect emitter elements 11.The plurality of first contact elements 14 are arranged on a side of the plurality of parallel-oriented field effect emitter elements 12 facing away from the emission surface 12. In the cathode device 30 of FIG. 8, the plurality of first contact elements 14 are arranged exclusively on the side facing away from the emission surface 12, wherein the side facing away from the emission surface 12 is the underside of the plurality of parallel-oriented field effect emitter elements 11 opposite the emission surface 12 and facing the emitter seat 20.The first contact elements 14 are arranged exclusively in a first plane. The plurality of second contact elements 21 are arranged exclusively in a second plane. The first plane and the second plane are oriented parallel to each other. The plurality of first contact elements 14 are advantageously arranged according to a BGA ball grid arrangement.A contact surface of one of the plurality of first contact elements 14 and a contact surface of one of the plurality of second contact elements 21 have different surface areas. The plurality of second contact elements 21 each have a contact point, so that its surface area is smaller than the surface area of the associated plurality of first contact elements 14.The emitter seat 20 has a carrier body 22, and for the connection, the plurality of second contact elements 21 are configured to be movable relative to the carrier body 22. Alternatively or additionally, the plurality of first contact elements 14 can be configured to be movable relative to the field effect emitter elements 11.A further current path of the at least two current-carrying current paths independently of one another has the gate electrode 13 and one of the plurality of second contact elements 21. This current path additionally has a further first contact element 14.The movable configuration takes place by means of an elastic connecting element, wherein the at least one elastic connecting element has a mechanical force transmitter which is a press pin or a spring pin. The plurality of second contact elements 21 are exclusively connected in a force-fit manner to the plurality of first contact elements 14. Alternatively or additionally, a form-fit connection would be conceivable.FIG. 9 shows an X-ray source 40 in a schematic longitudinal section.The X-ray source 40 includes a cathode device 30, an anode 41 and an evacuated housing 42. The cathode assembly 30 and the anode 41 are disposed within the evacuated housing 42.Although the invention has been illustrated and described in more detail by the preferred exemplary embodiments, the invention is nevertheless not restricted by the disclosed examples and other variations can be derived therefrom by the person skilled in the art without departing from the scope of protection of the invention.

Claims

Method for reducing an electron emission of an electron emitter, wherein the electron emitter - has a multiplicity of field effect emitter elements oriented in parallel for forming an emission surface on the upper side of the multiplicity of field effect emitter elements oriented in parallel, - a gate electrode which is arranged above the emission surface, and - has a first current limiting unit, - wherein the first current limiting unit and at least one first field effect emitter element of the multiplicity of field effect emitter elements are part of a first current path, - wherein the at least one first field effect emitter element is arranged electrically between the first current limiting unit and the gate electrode in the first current path, comprising the steps of: - emitting electrons from the first current path as a function of an emission voltage between the gate electrode and the emission surface by means of the at least one first field effect emitter element, - determining a condition of the first current path, activating the first current limiting unit in dependence on the determined nature of the first current path for reducing the electron emission of the electron emitter.The method of claim 1, wherein the determining of the condition is performed prior to electron emission.The method of claim 2, wherein the determining of the condition comprises determining a geometric shape of the at least one first field effect emitter element with respect to the emission surface and associating an electrical potential with the geometric shape.The method of claim 3, wherein activating the first current limiting unit comprises applying the associated electrical potential to a first potential controllable electrode of the first current limiting unit to reduce the emission voltage with respect to the at least one first field effect emitter element.The method of claim 4, wherein the associated electrical potential corresponds to the gate potential for zeroing the emission voltage.Method according to one of Claims 3 to 5, wherein after the activation of the first current limiting unit, electrons are emitted from a second current path, wherein a second field effect emitter element is part of the second current path, as a function of an emission voltage between the gate electrode and the emission surface by means of the second field effect emitter element.The method of claim 6, wherein during electron emission a different emission voltage is applied with respect to the second field effect emitter element than with respect to the at least one first field effect emitter element.The method of claim 1, wherein the determining of the condition occurs after the beginning of the electron emission.The method of claim 8, wherein determining the condition comprises testing the first current path for a short circuit.The method of claim 9, wherein in the presence of the short circuit, activating the first current limiting unit comprises reducing the emission voltage and outputting a short circuit signal.Method according to one of claims 9 or 10, wherein, in the event of the short circuit occurring, the activation of the first current limiting unit comprises triggering an overcurrent protection device of the first current limiting unit for irreversibly interrupting the first current path.Method according to Claim 11, wherein after the activation of the first current limiting unit, electrons are emitted from a second current path, wherein a second field effect emitter element is part of the second current path, as a function of an emission voltage between the gate electrode and the emission surface by means of the second field effect emitter element.Electron emitter configured to carry out a method according to one of the preceding claims.An X-ray source comprising - cathode means having an emitter seat and an electron emitter according to claim 13, - an anode and - an evacuated housing, wherein the electron emitter, the emitter seat and the anode are arranged within the evacuated housing.Computer program product which can be loaded directly into a memory of a computing unit, having program code means for carrying out a method according to one of Claims 1 to 12 when the computer program product is executed in the computing unit.

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