X-ray tube with improved cooling of the anode head

Abstract

An X-ray tube (1), comprising a source (4) for the release of electrons and an anode head (6) with a central axis (ZA), wherein a target (11) is formed on an end face (10) of the anode head (6), onto which the electrons in an excited region (9) impinge during operation, wherein the anode head (6) provides a flow path (16) for a cooling fluid, which leads from at least one inlet port (14a) via a radially outer section (20), further via a cooling gap (21), and further via a radially inner section (22) to at least one outlet port (15a), is characterized in that the excited region (9) of the target (11) is essentially annular, and that in a region (26) of the anode head (6) opposite the excited region (9) of the target (11) a local height (H1, H2) of the cooling gap (21) extends from radially outside to radially steadily increases towards the inside.The X-ray tube according to the invention can be operated with higher power and / or less wear.

Description

The invention relates to an X-ray tube comprising a source for releasing electrons and an anode head having a central axis, wherein a target is formed on an end face of the anode head, onto which target the electrons impinge in an excited region during operation, wherein the anode head provides a flow path for a cooling fluid which leads from at least one inflow connection via a radially outer section, further via a cooling gap, and further via a radially inner section to at least one outflow connection.An X-ray tube of this type is known from EP 4 141 905 A1.X-ray radiation is used in many ways for investigating chemical and physical properties of samples and bodies of all kinds. For example, the atomic composition of samples can be determined qualitatively and quantitatively using X-ray fluorescence. X-ray radiation is also capable of penetrating the interior of a body and radiating it without destroying the body. Thereby, compositions inside the body can be determined.In many cases, it is desirable to use high intensity x-ray radiation to allow more accurate and / or faster x-ray measurements.X-ray radiation is typically generated using a so-called X-ray tube. In an evacuated region of the X-ray tube, a source for releasing electrons is arranged, for example an incandescent filament. The source is connected as a cathode. In addition, a target is arranged in the evacuated region, which is connected as an anode. The target can consist, for example, of copper, rhodium, chromium, molybdenum or silver. Electrons from the source are accelerated by an electric field onto the target and strike it there. The electrons impinging thereon are decelerated and bremsstrahlung radiation is thereby produced. In addition, the impinging electrons eject other electrons from the atoms of the material of the target; when the free electron shells are filled up again, characteristic x-ray radiation arises. The electrons striking the target heat the target strongly, so that it usually has to be actively cooled. If the target becomes too warm, it may melt, which would destroy the X-ray tube. In practice, therefore, often cooling the target limits the power of the X-ray tube.EP 4 141 905 A1 discloses an X-ray tube in which a substantially cylindrical anode head forms a target on its end face in a cathode housing. Electrons released from a glow cathode in the cathode housing are accelerated toward the target. A coolant supply line is connected to a first channel in the anode head, which channel comprises an inclined section and subsequently a section running along a central axis of the anode head. The first channel leads into a cooling gap below the target, which is perpendicular to the central axis of the anode head and has a uniform height. The cooling gap merges into an annular channel located radially on the outside in the anode head. From this annular channel, a second channel leads away for the coolant, to which a return line is connected. A flow of coolant (water) centrally and axially through the first channel, via the cooling gap from radially inside to outside, and outside in the annular gap axially back into the second channel cools the target or the anode head as a whole. Alternatively, the water flow can also be reversed.With this structure, an X-ray tube having X-ray power sufficient for many purposes can be provided.Depending on the heating power of the electrons striking the target, the water in the cooling gap below the target can partially evaporate and bubbles form. It should be noted that although the boiling bubbles (i.e. the water vapor) can absorb a significant amount of heat during their formation as a result of the phase transition during the evaporation, the water vapor in the boiling bubbles can then only absorb and transport away a significantly smaller amount of heat than liquid water. If the boiling bubbles are not transported away fast enough, the cooling capacity in the anode head is greatly reduced and the target can melt.In addition, the resulting boiling bubbles can collapse again shortly after their formation (so-called cavitation), which leads to mechanical stress on surrounding material. Although cavitation does not impair cooling performance, it increases wear on the anode head.The power of the X-ray tube known from EP 4 141 905 A1 is selected such that melting of the target is avoided and also wear due to cavitation remains in an acceptable small range.An X-ray tube is known from DE 10 2017 216 059 A1, in which the target is arranged on a solid, hump-shaped base body.US 2021 / 0249214 A1 describes an X-ray tube with cooling by a cooling fluid, which in one design flows via a central feed chamber via a curved gap below the target into an annular, radially outer return chamber. The curved gap has a constant width.DE 10 2016 000 033 B4 has disclosed an X-ray tube which has a target at a converging end of a carrier body. The support body is coupled via a connection body, a heat dissipation body, and a portion of insulation material to a radially outer cooling unit, which may be water cooled.DE 10 2017 217 181 B3 describes a stand anode for an X-ray radiator, having an anode base body and a nozzle projecting into the anode base body. Running centrally in the nozzle axially is a cooling channel for supplying cooling fluid, which at the end merges into a funnel-shaped outlet opening. The anode base body forms a conical heat exchange surface opposite the target, which is situated opposite the outlet opening of the nozzle. Between the heat exchange surface and the outlet opening of the nozzle there is a gap of uniform height. The cooling fluid can flow off at a radial outer side of the nozzle.U.S. Pat. No. 6,580,780 B1 discloses a cooling system for X-ray tubes with a stationary anode, wherein a coolant injection arrangement is placed in a stationary anode. Axially centrally in the injection arrangement, there is a passage for the incoming coolant which narrows towards an aperture to accelerate the coolant. Behind the aperture, a surface enlargement structure with flow gaps running radially outwards in a star shape and a flow deflection device are arranged. The flow-diversion device is designed to be corrugated and lies on the inside of the stationary anode opposite the target surface. Radially outward of the coolant injection assembly, the coolant flows back.From JP 2021-44 155 A an X-ray tube is known in which an anode is formed with an inner tube and an outer tube. The outer tube is closed at a front end with an end wall on which a target surface is formed. Coolant flows in the inner tube, impinges on the rear side of the end wall and is deflected radially outwards, and flows back between the outer tube and the inner tube. The inner tube is thickened at its target-side end, wherein a channel for the coolant of constant height is formed between the end-side end of the inner tube and the rear side of the end wall.Object of the InventionIt is an object of the present invention to provide an X-ray tube that can be operated with a higher power and / or lower wear.DESCRIPTION OF THE INVENTIONThis object is achieved according to the invention by an X-ray tube of the type mentioned at the beginning, which is characterized in that the excited region of the target is formed substantially annularly, and in that a local height of the cooling gap continuously increases from radially outside to radially inside in a region of the anode head opposite the excited region of the target.With the X-ray tube according to the invention, improved cooling can be achieved. Due to the improved cooling, the X-ray tube can be operated with a higher X-ray power or, with the same X-ray power, can be operated with less wear and thus longer service life.In the X-ray tube according to the invention, the cooling fluid flows from radially outside to radially inside due to the connections provided (inlet connection and outlet connection) in the cooling gap. During operation, boiling bubbles form in the cooling gap. Due to their formation (more precisely due to the phase conversion of liquid water to water vapor or correspondingly for other cooling fluids), the boiling bubbles achieve a noticeable cooling effect. The cooling gap becoming higher in the direction of flow of the cooling fluid improves the removal of boiling bubbles from the cooling gap. Even large amounts of boiling bubbles can then be flushed out quickly with the cooling water in a reliable manner. Accordingly, good heat dissipation can be ensured even at higher X-ray power (and thus higher heat load on the target). The X-ray tube according to the invention can thus then be operated with a higher power than conventional X-ray tubes.Due to the cooling fluid flowing from the outside to the inside, the cross-sectional area of the flow path along the flow direction can be kept comparatively small despite the increase in the height of the cooling gap along the flow direction. In contrast, in the case of a flow direction from the inside to the outside, the cross-sectional area would inevitably increase greatly with the height of the flow path increasing in the flow direction. Because the cross-sectional area along the flow direction can be kept comparatively low, the pressure in the coolant can be kept high, in particular wherein the pressure in the coolant in the cooling gap remains the same or even increases along the flow direction. This prevents a collapse of boiling bubbles (or of the cavitation) in or near the cooling gap. Accordingly, a slight wear in the anode head is achieved.The excited region of the target is substantially ring-shaped. In the associated, opposite region of the anode head (i.e. below or behind the excited region), the cooling gap can then be set up with the height increasing in the flow direction, from which the boiling bubbles can be easily transported away. A center of the end face of the anode head remains without directly impinging electrons, so that the latter also does not heat up as strongly, and no boiling bubbles or at most few boiling bubbles are generated in adjacent parts of the flow path. Typically, a pin is located opposite the center of the end face (see below).The local height of the cooling gap can be measured in a direction perpendicular to the target-facing wall of the cooling gap (any micro-waviness of the target-facing wall is neglected). This direction usually corresponds substantially to the axial direction (direction of the central axis of the anode head).The end face is typically oriented at least substantially perpendicular to the central axis of the anode head. The central axis and the annular excited region are typically concentric. Typically, the outer shape of the anode head is designed rotationally symmetrical to the central axis at least in the region of the end face and a circumferential side wall. The anode head can in particular have a substantially circular cylindrical outer shape.The region in which the local height of the cooling gap increases from radially outside to radially inside typically comprises at least 15% of the radius of the anode head, preferably at least 20%, particularly preferably at least 25%.Preferred Embodiments of the InventionAn embodiment of the X-ray tube according to the invention is preferred in which a local cross-sectional area of the cooling gap in the region of the anode head opposite the excited region of the target continuously decreases from radially outside to radially inside, remains the same or continuously increases by a maximum of 15%. By means of a cross-sectional area decreasing in the cooling gap along the flow direction, the pressure in the cooling fluid can be held or increased, in particular in the region of the cooling gap lying radially further inward. The average flow velocity of the cooling fluid then remains the same or increases radially inward in the cooling gap. This counteracts cavitation and reduces macroscopic recirculation. Typically, a decrease in the cross-sectional area is about 5-20 %. Since the volume of the cooling fluid increases along the flow direction (i.e. radially inward in the cooling gap) as a result of formation of boiling bubbles and also thermal expansion, a constant cross-sectional area or even a slight increase in the cross-sectional area can also be acceptable without increased cavitation occurring. The reduced cavitation improves the durability of the X-ray tube. Lower recirculation improves cooling performance. The local cross-sectional area can be measured as the partial area of a conical jacket or cylinder jacket, wherein this jacket is oriented along a direction perpendicular to the target-facing wall of the cooling gap (any micro-corrugation of the target-facing wall is neglected in this case). This direction usually corresponds substantially to the axial direction.An embodiment is advantageous in which a local cross-sectional area of the cooling gap in the region of the anode head opposite the excited region of the target continuously decreases from radially outside to radially inside, in particular continuously decreases by a maximum of 20%. A decreasing cross-sectional area ensures that the pressure in the cooling fluid always increases from the outside to the inside along the flow path in the region of the cooling gap (i.e. even if no or only a few boiling bubbles should yet be formed and / or thermal expansion effects should be low), and cavitation is correspondingly minimized. If the decrease in the cross-sectional area is 20% or less, and accordingly the flow rate of the cooling fluid does not increase too much, a mechanical flushing of material of the anode head is counteracted.An embodiment is also preferred in which the flow path in the anode head is formed at least substantially rotationally symmetrical with respect to the central axis. As a result, a particularly uniform cooling of the excited region of the target can be achieved. Note that the flow path is typically not completely strictly rotationally symmetric. In particular, a deviation in the region of the outer section of the flow path due to distribution structures can be present in a rotationally symmetrical, circumferential radial gap, or else at the transitions to the inflow connection or to the outflow connection. In the region of rectifying elements, there is typically a rotational symmetry with respect to the central axis with a count (usually high count, e.g. 8 or higher), and in the region of swirl elements there can be, for example, a rotational-translational symmetry (e.g. a screwing, with the central axis as screw axis) or, in turn, a rotational symmetry with a count (usually high count, e.g. 8 or higher) with respect to the central axis. Such deviations are still considered to be consistent with a substantially rotationally symmetrical flow path.An embodiment is also advantageous in which the radially outer section of the flow path is formed at least substantially around the entire circumference of the anode head. This contributes to particularly uniform cooling of the excited region of the target. Preferably, the flow path in the radially outer portion is formed completely around the entire circumference of the anode head.Particularly preferred is an embodiment in which distribution structures are formed in the radially outer section of the flow path, with which a cooling fluid flow from the at least one inlet connection to the cooling gap can be distributed and equalized over the circumference of the anode head. This in turn achieves a particularly uniform and efficient cooling of the excited region of the target. The distribution structures can be formed in particular by a plurality of axially and azimuthally spaced blades or guide plates.A development of this embodiment is advantageous, which provides that the distribution structures comprise a set of rectifying elements for the cooling fluid distributed over the circumference of the anode head in a subsection of the radially outer section close to the target, which rectifying elements run between an inlet-side annular gap and an outlet-side annular gap or the cooling gap, and with which at least substantially parallel partial flows of the cooling fluid can be set up and separated from one another. The rectifying elements ensure that a fluid flow which locally flows in at the circumference is distributed in the circumferential direction over axial partial flows, and cannot continue to flow axially in an unbended manner at its location in the circumferential direction. In particular, a swirl introduced upstream in the fluid stream can be eliminated again or at least minimized in the resulting fluid stream. The ("inlet-side") annular gap lying upstream of the set of rectifying elements with respect to the flow direction and, if present, the ("outlet-side") annular gap following the set of rectifying elements in the flow direction each form a smoothing zone. The cooling gap typically adjoins the outlet-side annular gap. The at least substantially parallel partial flows typically have an intermediate angle of 20° or less, usually 10° or less (with respect to a mean flow direction in the respective partial flow).Another advantageous development of the above embodiment provides that the distribution structures comprise a set of rectifying elements for the cooling fluid flow distributed over the circumference of the anode head in a subsection of the radially outer section of the flow path remote from the target, said rectifying elements running between an inlet-side annular gap and an outlet-side annular gap, and with which at least substantially parallel partial flows of the cooling fluid can be set up and separated from one another. The rectifying elements once again ensure that a fluid stream flowing locally at the circumference is distributed over axial partial flows in the circumferential direction, and cannot continue to flow axially in an unbended manner at its location in the circumferential direction. In particular, a swirl introduced upstream in the fluid stream can be eliminated again or at least minimized in the resulting fluid stream. The at least one inlet connection typically opens into the inlet-side annular gap to the rectifying elements of the partial section remote from the target; alternatively, a swirling cooling fluid flow from upstream swirling elements can also flow into the inlet-side annular gap. The at least substantially parallel partial flows typically have an intermediate angle of 20° or less, usually 10° or less (with respect to a mean flow direction in the respective partial flow).A subvariant is also advantageous in which sets of rectifying elements are provided both in a subsection close to the target and in a subsection (or more remote from the target) of the radially outer section of the flow path, wherein it is provided that the ring gap on the outlet side to the rectifying elements of the subsection remote from the target is at the same time the ring gap on the inlet side to the rectifying elements of the subsection close to the target, and that the set of rectifying elements of the subsection close to the target is offset with respect to one another in the azimuthal direction with respect to the central axis of the anode head in relation to the set of rectifying elements of the subsection remote from the target. As a result, a minimum deflection is introduced into the coolant flow in the central annular gap, and a particularly good homogenization of the cooling performance in the azimuthal direction is achieved.A subvariant is also preferred in which the rectifying elements are aligned at least substantially in the axial direction and the partial flows of the cooling fluid are aligned at least substantially in the axial direction accordingly. This design is simple in construction and reliably minimizes swirl in the cooling fluid. Typically, a deviation from an exact axial alignment is a maximum of 10°, usually a maximum of 5°, or even 0°.A preferred subvariant provides that the rectifying elements are formed at least in part byparallel lamellae, in particular straight parallel lamellae, and / ortrapezoidal or triangular lamellae, and / ordrop-shaped or diamond-shaped lamellae, wherein the lamellae are formed on a radially outwardly directed wall side and / or a radially inwardly directed wall side of the anode head, wherein the radially outwardly directed wall side and the radially inwardly directed wall side also delimit the radially outer section of the flow path. These designs have proven themselves in practice and are comparatively simple to produce.A subvariant is also advantageous in which the rectifying elements are formed at least in part by a plurality of parallel hollow structures which are arranged in the radially outer section of the flow path, in particular wherein the plurality of parallel hollow structures form a honeycomb structure. The hollow structures are simple to construct structurally. By means of a honeycomb structure, the hollow structures can be packed particularly tightly, and a particularly large cross-sectional area can be set up for the partial flows.In an advantageous development, it is provided that the distribution structures comprise one or more swirl elements in a subsection of the radially outer section of the flow path that is remote from the target, said swirl elements being arranged between the at least one inlet connection and an annular gap on the outlet side, and with which swirl can be introduced with respect to the central axis into the cooling fluid flow. The swirl of the cooling fluid provides uniform distribution of the cooling fluid (and the cooling fluid flow) over the circumference of the radially outer part of the flow path. The swirl causes a velocity component of the cooling fluid in the circumferential direction of the anode head, so that the cooling fluid rotates about the anode axis. Typically, the partial section away from the target, in which the swirl elements are arranged, is at the same time the partial section away from the target of the radially outer part of the flow path.A development of this embodiment is preferred, in which the at least one swirl element extends helically about the central axis of the anode head, as a result of which at least one helical line channel is set up for the cooling fluid flow. This construction is proven in practice and can be used well even with a high coolant flow in order to reliably introduce swirl into the coolant flow. A helical swirl element preferably runs at least 0.8 times, particularly preferably at least 1.0 times, very particularly preferably at least 2.0 times, around the central axis.In a preferred development, it is provided that a plurality of swirl elements run helically about the central axis of the anode head, as a result of which a plurality of helical line channels offset azimuthally and / or axially with respect to one another are set up for partial flows of the cooling fluid. By means of a plurality of helical line channels, the coolant flow can be divided along the circumference and each provided with a swirl, and thus contributes to the uniform distribution of the cooling power.A development of the embodiment with distribution structures in the radially outer section of the flow path is preferred, wherein a homogenization zone for the cooling fluid flow is set up between distribution structures of a partial section of the radially outer section remote from the target and distribution structures of a partial section of the radially outer section close to the target, in particular wherein the homogenization zone is set up as an annular gap. In a smoothing zone between two (adjacent) sections, the flow rate of the cooling fluid can be smoothed. In particular, the axial flow velocity (with respect to the direction of flow of the cooling fluid) behind the circumferential homogenization zone may have less scattering width than before the homogenization zone. In addition, smoothing zones can be provided in front of distribution structures of a subsection that is furthest away from the target and / or behind distribution structures of a subsection that is closest to the target of the radially outer section of the flow path.In a preferred embodiment, the anode head is formed with a first flow element and a second flow element which are inserted into one another and which form at least a part of the flow path between them. This is simple in construction and has proven successful in practice.An embodiment is particularly preferred in which at least the radially inner section of the flow path, and optionally also the cooling gap, are partially delimited by a pin which protrudes away from the end face of the anode head into the interior of the anode head. In other words, the pin protrudes from a target-facing wall of the flow path into the flow path in the interior of the anode head into the radially inner section. The pin typically lies on the central axis. The pin prevents direct impingement (in the radial flow direction) of the confluent cooling fluid from different radial directions in the transition region of the cooling gap and radially inner section of the flow path. The cooling fluid can be deflected with the pin, in particular in the axial direction, and the flow of the cooling fluid can in particular be approximately parallelized. In addition, the pressure curve and the flow rate in the cooling fluid can be held or mitigated by the journal.Also included within the scope of the present invention is an X-ray tube arrangement comprising an X-ray tube according to the invention described above and a provision device for a cooling fluid, wherein the provision device provides fresh cooling fluid at a delivery outlet and the delivery outlet is connected to the at least one inlet connection, in particular wherein the provision device further receives cooling fluid heated at a recirculation inlet and the outlet connection is connected to the recirculation inlet. The cooling fluid is provided and typically also conveyed (e.g. pumped) with the provision device, and the cooling fluid flow in the anode head is adjusted from radially outside to radially inside. The X-ray tube achieves particularly good cooling.Also within the scope of the present invention is the use of an X-ray tube according to the invention as described above or an X-ray tube arrangement according to the invention as described above, wherein the X-ray tube is in operationelectrons from the electron-releasing source strike an annular, excited region of the target and x-ray radiation is thereby generated, andin the cooling gap, cooling fluid flows from radially outside to radially inside in relation to the excited region. The cooling by the cooling fluid in the cooling gap is highly efficient. Boiling bubbles can contribute to the absorption of heat and are transported away well by the cooling gap which becomes higher in the direction of flow. At the same time, cavitation can be kept low.In a preferred variant of the use according to the invention, it is provided that an average flow speed of the cooling fluid in the cooling gap remains the same or increases, in particular increases by a maximum of 25%, compared to the excited region from radially outside to radially inside. The increasing flow rate ensures that the pressure in the cooling fluid does not decrease, and cavitation remains particularly low. Accordingly, the wear on the anode head is also slight.Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those set out further below can be used according to the invention individually or together in any desired combinations. The embodiments shown and described are not to be understood as a final enumeration, but rather have exemplary character for describing the invention.DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGFIG. 1 shows a schematic longitudinal section through an exemplary embodiment of an X-ray tube according to the invention; FIG. 2 ashows a schematic longitudinal section through an exemplary anode head according to a first design for the invention, with swirl elements in a lower subsection remote from the target and rectification elements in an upper subsection close to the target of a radially outer section of the flow path; FIG. 2 bshows a schematic cross section through the anode head of FIG. 2 a, on the plane B-B of FIG. 2 a; FIG. 2 cshows a schematic cross section through the anode head of FIG. 2 a, on the plane C-C of FIG. 2 a; FIG. 2 d shows a schematic top view of the anode head of FIG. 2 a; FIG. 2 e shows a schematic, semi-open perspective view of the anode head of FIG. 2 a, with the inner flow element; FIG. 2 f shows a schematic, semi-open perspective view of the anode head of FIG. 2 a, without an inner flow element; FIG. 2 g shows an enlarged view of the anode head of FIG. 2 ain the region close to the end face; FIG. 3 ashows a schematic longitudinal section through an exemplary anode head according to a second design for the invention, with rectifying elements in a lower subsection remote from the target and rectifying elements in an upper subsection close to the target of a radially outer section of the flow path; FIG. 3 bshows a schematic cross section through the anode head of FIG. 3 a, on the plane B-B of FIG. 3 a; FIG. 3 cshows a schematic cross section through the anode head of FIG. 3 a, on the plane C-C of FIG. 3 a; FIG. 3 d shows a schematic cross section through the anode head of FIG. 3 a, on the plane D-D of FIG. 3 a; FIG. 3 e shows a schematic, semi-open perspective view of the anode head of FIG. 3 a, with the inner flow element; FIG. 3 f shows a schematic, semi-open perspective view of the anode head of FIG. 3 a, without an inner flow element; FIG. 3 g shows an enlarged view of the anode head of FIG. 3 ain the region close to the end face; FIG. 4 ashows a schematic longitudinal section through an exemplary anode head according to a third design for the invention, with swirl elements in a lower subsection remote from the target, rectification elements in a centrally located subsection remote from the target, and rectification elements in an upper subsection close to the target of a radially outer section of the flow path; FIG. 4 bshows a schematic cross section through the anode head of FIG. 4 a, on the plane B-B of FIG. 4 a; FIG. 4 cshows a schematic cross section through the anode head of FIG. 4 a, on the plane C-C of FIG. 4 a; FIG. 4 d shows a schematic cross section through the anode head of FIG. 4 a, on the plane D-D of FIG. 4 a; FIG. 4 e shows a schematic, semi-open perspective view of the anode head of FIG. 4 a, with the flow element inside; FIG. 4 f shows a schematic, semi-open perspective view of the anode head of FIG. 4 a, without an inner flow element; FIG. 4 g shows an enlarged view of the anode head of FIG. 4 a in the region close to the end face; FIG. 5 shows a schematic, half-open side view of an anode head for the invention in the region of the radially outer section of the flow path, with helical swirl elements and drop-shaped rectification elements; FIG. 6 shows a schematic, half-open side view of an anode head for the invention in the region of the radially outer section of the flow path, with swirl elements formed as obliquely running lamellae and trapezoidal rectifying elements; FIG. 7 shows a schematic, half-open side view of an anode head for the invention in the region of the radially outer section of the flow path, with hollow structures as rectifying elements and trapezoidal rectifying elements.FIG. 1 shows a schematic longitudinal section of an exemplary embodiment of an X-ray tube 1 according to the invention.In its upper part in FIG. 1, a housing 2 (also called cathode housing) encloses an evacuated space 3. The incandescent filament can be heated with an electric current via the connections 5. An anode head 6 also projects into the evacuated space 3.The source 4 is brought to a negative electrical potential (in comparison with the anode head 6) via the connections 5, i.e. is connected as a cathode. The anode head 6 is brought to a positive electrical potential (in comparison to the source 4) via an electrical connection 7, that is to say is connected as an anode. Electrons released at the source 4 are then accelerated toward the anode head 6 through the evacuated space 3 by the potential difference between the source 4 and the anode head 6. Typically, the potential difference (also called acceleration voltage) is between 1 kV and 100 kV.By suitably configured deflection electrodes 8 at a suitable potential, the trajectory of the electrons is configured such that the electrons strike the end face 10 of the anode head 6 in an annular region 9 (indicated by dotted lines in FIG. 1, see also FIG. 2 d hereto). A target 11 is formed on the end face 10 of the anode head 6. The target 11 here consists of an applied disk of rhodium (or alternatively, for example, copper, molybdenum, chromium or silver, depending on the desired characteristic X-ray radiation), which extends here over the entire surface of the end face 10 which is planar here.The electrons which strike the target 11 in the ring-shaped, excited region 9 penetrate into the material of the target 11 and are decelerated in the process. This produces x-ray radiation in the form of bremsstrahlung radiation. In addition, electrons are ejected from the electron shells of the atoms of the material of the target 11. If these electron shells are filled again with electrons from higher shells, characteristic x-ray radiation is produced. The X-ray radiation generated in this way on the target 11 emerges largely through an X-ray window 12 of the housing 2 and is then used for an application, for example an X-ray fluorescence experiment (application not shown in more detail). The X-ray window 12 is formed here by a beryllium disk.The housing 2 and the anode head 6 are arranged here on an insulation body 13. The insulation body 13 can be formed, for example, as in EP 4 141 905 A1.The electrons impinging on the target 11 during operation heat up the anode head 6 very strongly. The anode head 6 is therefore actively cooled with a cooling fluid. The cooling fluid can be water, for example.An inlet line 14 for fresh (cool) cooling fluid leads through the insulation body 13 to an inlet connection 14 aof the anode head 6. Furthermore, an outlet line 15 for used (heated) cooling fluid runs from an outlet connection 15 aof the anode head 6 through the insulation body 13. Inside the anode head 6, a flow path 16 for the cooling fluid leads from the inlet connection 14 ato the outlet connection 15 a(to the flow path 16, see in particular FIGS. 2 a- 2 gon the bottom).The X-ray tube 1 is connected to a supply device 17 for cooling fluid. Fresh cooling fluid is discharged from the supply device 17 at a delivery outlet 14 band fed into the anode head 6 via the feed line 14. Heated cooling fluid passes from the anode head 6 via the outlet line 15 to the return inlet 15 bof the supply device 17. the supply device 17 can comprise a cooling unit and a pump for the cooling fluid (not shown in more detail). The totality of X-ray tube 1 and connected supply device 17 for cooling fluid is also referred to as X-ray tube arrangement 40.FIGS. 2a to 2g illustrate an exemplary anode head 6 in a first embodiment of the invention. Fig. 2a shows a longitudinal section, and Figs. 2b and 2c show cross sections at the planes B-B and C-C. FIG. 2 d shows a plan view of the anode head 6 on its end face 10. FIGS. 2 eand 2 f show semi-open perspective views of the anode head 6, with the inner flow element 18 (FIG. 2 e ) and without the inner flow element (FIG. 2 f ). FIG. 2 g finally shows an enlarged view from the longitudinal section of FIG. 2 ain the region close to the end face 10 of the anode head 6.The anode head 6 of FIGS. 2 a- 2 gcorresponds to the anode head of FIG. 1 (see also there). In addition, parts of the inlet line 14 and the outlet line 15 for the coolant are also illustrated, cf. FIGS. 2 a, 2 e, 2 f. The coolant flows along the flow direction FR. In the top view of FIG. 2 d, the excited, ring-shaped region 9 (boundary shown in dashed lines) of the target 11 can also be clearly seen. The target 11 is here perpendicular to the central axis ZA of the anode head 6. the anode head 6 is generally circular cylindrical in shape, here with two small, encircling shoulders.In the embodiment shown, the anode head 6 is formed by a first, inner flow element (or partial component) 18 and a second, outer flow element (or partial component) 19. The second, outer flow element 19 comprises here a cap 19 awhich rests on a foot 19 band is soldered or welded onto the latter. The flow path 16 for the cooling fluid is formed within the inner flow element 18 and between the flow elements 18, 19. The flow path 16 runs from the inlet connection 14 ato the outlet connection 15 avia an inlet channel 14 c(here running axially but eccentrically), a radially outer section 20, a cooling gap 21, a radially inner section 22 (here running axially and centrally) and an outlet channel 15 c(here inclined to the central axis).The radially outer section 20 extends between the radially outer side of the inner flow element 18 and the radially inner side of the outer flow element 19, and the cooling gap 21 lies behind the target 11, opposite the annular, excited region 9 of the target 11 (which can be seen well in FIG. 2 g ). The cooling gap 21 is delimited at the top by the outer flow element 19 and at the bottom by the inner flow element 18. A pin 23 of the outer flow element 19 protrudes into the radially inner section 22 in an upper part, so that in this upper part the radially inner section 22 is bounded by the outer flow element 19 and the inner flow element 18. In a lower part of the radially inner section 22 of the flow path 16, only the radially inner flow element 18 delimits the radially inner section 22, and the axial, radially inner section 22 merges into the discharge channel 15 c, which is inclined here.The cooling fluid flows axially upward in the flow path 16 along the radially outer section, from radially outward to radially inward in the region of the cooling gap 21, and axially downward in the region of the radially inner section 22.As can be seen well in FIG. 2 g, in the design shown, the upper wall 24 of the cooling gap 21 facing the target lies perpendicular to the central axis ZA. The lower wall 25 of the cooling gap 21 facing away from the target extends straight here, but with a slight inclination with respect to the direction perpendicular to the central axis, wherein the wall 25 facing away from the target slopes downwards radially inward. In a region 26 opposite the ring-shaped, excited region 9 of the target 11, a height of the cooling gap 21 (measured in a direction perpendicular to the wall 24 facing the target, i.e. measured here in the axial direction) increases steadily from radially outside to radially inside (i.e. along the flow direction FR in the cooling gap 21). A first height H 1 is shown by way of example further radially outward and a second height H 2 further radially inward, in each case at the edge of the region 26. The region 26, in which the height of the cooling channel 21 increases steadily, extends here over approximately 30% of the radius of the anode head 6 (at the axial position of the cooling gap 21).In addition, in the region 26, the cross-sectional area available to the cooling fluid in the cooling gap 21 also changes along the flow direction FR, i.e. from radially outside to radially inside. The cross-sectional area here corresponds to a cylinder jacket surface in the cooling gap 21, which at the respective radial position lies perpendicular to the upper wall 24 facing the target. By way of example, a first surface F 1 is marked further radially outward and a second surface F 2 is marked further radially inward, in each case dotted at the edge of the region 26. Note that the cross-sectional area is calculated according to the product of the circumference and the height of the cooling gap at the corresponding radial position. In the embodiment shown, the cross-sectional area decreases slightly from radially outside to radially inside, here approximately F2=0.8*F1 (note that in FIG. 2 g, the radius decreases more rapidly towards the inside than the height of the cooling gap increases, therefore the cross-sectional area decreases towards the radially inside although the height increases). The cross-sectional area decreasing in the flow direction FR in the cooling gap 21 ensures that the pressure in the cooling fluid in the cooling gap 21 increases somewhat radially inward; this prevents the boiling bubbles (cavitation) from collapsing.By means of the pin 23, the fluid flow of the cooling fluid, which flows radially inward from the cooling gap 21, is deflected axially downward. The partial flows of cooling fluid coming from different azimuthal positions, after deflection by the pin 23, flow substantially parallel to one another along the axial direction until they are finally joined at the lower end of the pin with respect to the radial direction. This provides for efficient flow of the cooling fluid.In the illustrated design, the radially outer section 20 comprises two partial sections 27 a, 27 bin which distribution structures 28 for the cooling fluid are respectively formed. In the flow direction FR, in front of, between and behind the subsection 27 a, 27 b, annular gaps 29 a, 29 b, 29 c(also referred to as annular channels) are arranged in each case here. The cooling fluid can spread over the entire circumference in the annular gaps; no distribution structures are arranged in the annular gaps. With the distribution structures 28 in the sections 27 a, 27 band the annular gaps 29 a- 29 cacting as smoothing zones 32, the cooling fluid, which here flows into the anode head 6 via a single inlet connection 14 aand inlet channel 14 c, is distributed over the circumference of the anode head 6, so that a substantially equal coolant flow in the axial direction is achieved at all circumferential positions directly upstream of the cooling gap 21.The inlet channel 14c, which is formed near the right edge in the lower part of the anode head 6 in FIG. 2a, opens into the lower annular gap 29a. This runs here around the entire circumference of the anode head 6. The lower annular gap 29a acts as a smoothing zone 32 and does not contain any distribution structures.The lower annular gap 29 ais adjoined by the subsection 27 a, which is remote from the target. In this embodiment, a helical swirl element 30 is provided as the first distribution structure 28 remote from the target, which is formed here on the radially outer side of the inner flow element 18, as can be seen easily in FIG. 2 e. The helical twisting element 30 wraps around the central axis ZA about 2 times here. The helical swirl element 30 projects as far as the radially inner side of the outer flow element 19, so that a helical line channel 31 is formed in the subsection 27 a(in the radial gap between the flow elements 18, 19). When flowing through the helical line channel 31, the cooling fluid receives a velocity component in the circumferential direction about the central axis ZA.The helical line channel 31 opens into the central annular gap 29b. The annular gap 29 balso runs around the entire circumference of the anode head 6. The central annular gap 29b also acts as a smoothing zone 32 and does not contain any distribution structures. The swirl introduced into the cooling fluid distributes the cooling fluid flow in the annular gap 29 b very uniformly over the entire circumference of the anode head 6.The middle annular gap 29 bis adjoined by the partial section 27 btoward the target. In the partial section 27b close to the target (in the radial gap between the flow elements 18, 19), a set 33 of rectifying elements 34 is provided. The rectifying elements 34 are designed here as straight lamellae 34 aextending parallel to the central axis ZA. The straight lamellae 34 aare here formed as radially inwardly directed projections on the radially inner side of the outer flow element 19, as can be seen clearly in FIG. 2 f. The swirl is removed from the cooling fluid again by the rectifying elements 34. The coolant flows axially upward in the section 27 close to the target in the intermediate spaces 35 between the lamellae 34 a.Adjoining the partial section 27b close to the target is the upper annular gap 29c. The intermediate spaces 35 open into this annular gap 29c. The annular gap 29c here likewise runs around the entire circumference of the anode head 6. This annular gap 29c also acts as a smoothing zone 32 and does not contain any distribution structures.The upper annular gap 29c merges at its upper end into the cooling gap 21. The distribution structures 28 in the sections 27 a, 27 bin cooperation with the homogenization zones 32 (annular gaps 29 a, 29 b, 29 c) ensure that cooling fluid flows from all locations along the circumference of the anode head 6 approximately as quickly radially inward into the cooling gap 21 and through the cooling gap 21. This achieves uniform cooling of the anode head 6, in particular in the (radial) region 26 which lies opposite the excited region 9 of the target 11 and close to the target 11.FIGS. 3a to 3g illustrate an exemplary anode head 6 in a second embodiment of the invention. FIG. 3 ashows a longitudinal section, and FIGS. 3 b, 3 cand 3 dshow cross sections at the planes B-B, C-C and D-D. FIGS. 3 eand 3 f show semi-open perspective views of the anode head 6, each with and without an inner flow element 18, FIG. 3 g finally shows an enlargement from the longitudinal section of FIG. 3 ain the region near the end face 10 of the anode head 6, The second design is largely similar to the first design of FIGS. 2 a- 2 g(in particular with respect to the cooling channel 21 and the radially inner section 22 of the flow path 16), so that only the essential differences (above all in the region of the radially outer section 20 of the flow path 16) are explained below.The lower annular gap 29 ais adjoined by the subsection 27 a, which is remote from the target. In this, a first set 33a of rectifying elements 34 is provided. The rectifying elements 34 are designed here as straight lamellae 34 aextending parallel to the central axis ZA. The straight lamellae 34 aare formed here as radially inwardly directed projections on the radial inner side of the outer flow element 19. The cooling fluid flows axially upward in the section 27 btoward the target in the intermediate spaces 35 between the fins 34 a.The middle annular gap 29 bis connected to the lower partial section 27 a, which is remote from the target. The annular gap 29 balso runs around the entire circumference of the anode head 6. The central annular gap 29b acts as a smoothing zone 32.The middle annular gap 29 bis in turn adjoined by the partial section 27 btoward the target. In this, a second set 33b of rectifying elements 34 is provided. The rectifying elements 34 are here likewise designed as straight lamellae 34 aextending parallel to the central axis ZA. The straight lamellae 34 aare here formed as radially inwardly directed projections on the radial inner side of the outer flow element 19, as can be seen easily in FIG. 3 f. The cooling fluid flows axially upward in the section 27 btoward the target in the intermediate spaces 35 between the fins 34 a.The second set 33 bof rectifying elements 34 is arranged offset in azimuth with respect to the first set 33 aof rectifying elements 34, as can be seen clearly in FIGS. 3 cand 3 d. At the azimuthal location of a respective blade 34a in the first set 33a there is an aligned space 35 in the second set 33b and vice versa. This ensures that a partial flow of cooling fluid from an intermediate space 35 in the first set 33 acannot continue in a straight (axial) direction into an intermediate space 35 of the second set 33 b, but rather must be deflected transversely thereto. This transverse deflection is further carried by the central annular gap 29b.Adjoining the partial section 27b close to the target is the upper annular gap 29c. The intermediate spaces 35 of the second set 33b open into this annular gap 29c. The annular gap 29c here likewise runs around the entire circumference of the anode head 6. This annular gap 29c also acts as a smoothing zone 32.FIGS. 4a to 4g illustrate an exemplary anode head 6 in a third embodiment of the invention. FIG. 4 ashows a longitudinal section, and FIGS. 4 b, 4 cand 4 dshow cross sections at the planes B-B, C-C and D-D. FIGS. 4 eand 4 f show semi-open perspective views of the anode head 6, each with and without an inner flow element 18. FIG. 4 g finally shows an enlargement from the longitudinal section of FIG. 4 ain the region near the end face 10 of the anode head 6.In the design of FIGS. 4 a- 4 f, the radially outer section 20 of the flow path 16 comprises three sub-sections, namely the lower sub-section 27 a, which is remote from the target (and remote from the target), a middle sub-section 27 c, which is likewise remote from the target, and an upper sub-section 27 b, which is close to the target. In front of the subsection 27 a, behind the subsection 27 band between the subsection 27 a, 27 c, 27 b, there are in each case annular gaps 29 a, 29 b, 29 c, 29 d, which in each case simultaneously represent smoothing zones 32.The configuration of the lower annular gap 29 a, of the lower partial section 27 a, which is remote from the target, with a helical swirl element 30 as a distribution structure 28 and the adjoining central annular gap 29 bcorresponds to the design of FIGS. 2 a- 2 g.The middle subsection 27 cis connected to the middle annular gap 29 b. Since this middle subsection 27 cis not that subsection which lies closest to the target 11 in the axial direction, it is also regarded as being remote from the target.In the middle subsection 27 c, a first set 33 aof rectifying elements 34 is provided here. The rectifying elements 34 are designed here as straight lamellae 34 aextending parallel to the central axis ZA. The straight lamellae 34 aare here formed as radially inwardly directed projections on the radial inner side of the outer flow element 19, as can be seen easily in FIG. 4 f. The coolant flows axially upward in the middle subsection 27 cin the intermediate spaces 35 between the plates 34 a.The middle subsection 27 cis adjoined by the further, middle annular gap 29 d. The annular gap 29 dalso runs around the entire circumference of the anode head 6. The annular gap 29d acts as a smoothing zone 32.The middle annular gap 29 dis adjoined by the partial section 27 btoward the target. In this, a second set 33b of rectifying elements 34 is provided. The rectifying elements 34 are here likewise designed as straight lamellae 34 aextending parallel to the central axis ZA. The straight lamellae 34 aare here formed as radially inwardly directed projections on the radially inner side of the outer flow element 19, as can be clearly seen in FIG. 4 f. The cooling fluid flows axially upward in the partial section 27 btoward the target in the intermediate spaces 35 between the fins 34 a.The second set 33 bof rectifying elements 34 is again arranged offset in the azimuthal direction with respect to the first set 33 aof rectifying elements 34, as can be seen clearly in FIGS. 4 cand 4 d. At the azimuthal location of a respective blade 34a in the first set 33a there is an aligned space 35 in the second set 33b and vice versa. This ensures that a partial flow from an intermediate space 35 in the first set 33 acannot continue in a straight (axial) direction into an intermediate space 35 of the second set 33 b, but rather must be deflected transversely thereto. This transverse deflection is further carried through the central annular gap 29d. Note that the blades 34 aand the gaps 35 may have different widths in the azimuthal direction.Adjoining the partial section 27b close to the target is the upper annular gap 29c. The intermediate spaces 35 of the second set 33b open into this annular gap 29c. The annular gap 29c here likewise runs around the entire circumference of the anode head 6. This annular gap 29c also acts as a smoothing zone 32.As can be seen well in FIG. 4 g, in the embodiment shown, the upper wall 24 of the cooling gap 21 facing the target is oriented here with a slight inclination to a direction perpendicular to the central axis ZA, wherein the wall 24 facing the target slopes downward radially inward. The lower wall 25 of the cooling gap 21 facing away from the target extends approximately straight with a somewhat more pronounced inclination with respect to the direction perpendicular to the central axis ZA, wherein the wall 25 facing away from the target likewise slopes downward radially inward. In a region 26 opposite the ring-shaped, excited region 9 of the target 11, a height of the cooling gap 21 (measured in a direction perpendicular to the wall 24 facing the target, i.e. in a direction slightly oblique with respect to the central axis ZA) increases from radially outside to radially inside (i.e. along the flow direction FR in the cooling gap 21). A first height H 1 is shown by way of example further radially outward and a second height H 2 further radially inward, in each case at the edge of the region 26, which facilitates removal of boiling bubbles from the cooling gap 21 into the radially inner section 22.In addition, in the region 26, the cross-sectional area available to the coolant in the cooling gap 21 also changes along the flow direction FR, i.e. from radially outside to radially inside. The cross-sectional area here corresponds to a conical jacket surface in the cooling gap 21, which at the respective radial position lies perpendicular to the upper wall 24 facing the target. By way of example, a first surface F 1 is marked further radially outward and a second surface F 2 is marked further radially inward, in each case dotted at the edge of the region 26. The cross-sectional area also decreases in this design from radially outside toward the inside, i.e. F 2<F 1. The cross-sectional area decreasing in the flow direction FR in the cooling gap 21 ensures that the pressure in the cooling fluid increases somewhat radially inward; this prevents the boiling bubbles (cavitation) from collapsing.FIG. 5 illustrates a further exemplary design of an anode head 6 for the invention. Only a part of the anode head 6 is shown in a highly schematic side view, which comprises the radially outer section of the flow path 16. The radially outer flow element 19 is cut open and is removed on the side toward the observer, so that the observer can look at the inner flow element 18 and the distribution structures 28 in the radially outer section 20 of the flow path 16 ("half-open" anode head 6). The essential deviations from the design of FIGS. 2a-2g will be explained.In the lower section 27 a, which is remote from the target, a total of four helical swirl elements 30 are provided here (helical swirl elements 30 on the rear side of the radially inner flow element 18 are illustrated in dashed lines), by means of which a total of four helical line channels 31 are set up.In the upper section 27 b, close to the target, the rectifying elements 34 are provided here as drop-shaped lamellae 34 b. These are all aligned axially (along the central axis ZA). The lamellae 34 bare formed here on the radially outwardly directed wall side 38 of the inner flow element 18 and project into the radial gap 37 between this wall side 38 and the radially inwardly directed wall side 39 of the radially outer flow element 19. Alternatively, the lamellae 34 bmay also be formed on the wall side 39 (not shown in detail, but see e.g. FIG. 2 f hereto). The radial gap 37 substantially forms the radially outer section 20 of the flow path 16.FIG. 6 shows a further exemplary design of an anode head 6 for the invention, again in a half-open illustration as explained with reference to FIG. 5. The essential deviations from the design of FIGS. 2a-2g will be explained.In the lower portion 27a remote from the target, a plurality of swirl elements 36 is provided here, which are each designed as individual blades lying obliquely to the central axis ZA. All swirl elements 36 here have the same inclination ("pitch") to the central axis ZA, and are arranged here distributed in the circumferential direction and also in the axial direction in the section 27 aon the outer side of the radially inner flow element 18. It should be noted that in other designs subsets of the swirl elements may also be inclined differently (not shown in more detail). Each individual plate here overlaps only a small part of the circumference (for example 1 / 10 or less of the circumference). The individual plates are here spaced apart from one another radially and axially.In the upper section 27 b, close to the target, the rectifying elements 34 are provided here as trapezoidal lamellae 34 c. The trapezoidal fins 34 care all axially aligned (along the central axis ZA). However, the orientation of the trapezoidal fins 34 cis alternated here, so that the thin end is arranged alternately at the top and bottom. As a result, the spaces 35 between the blades each have a slight inclination with respect to the central axis ZA, this inclination also alternating in its direction along the circumferential direction. Accordingly, the partial flows of the cooling fluid in adjacent intermediate spaces 35 are then also slightly inclined with respect to one another (with respect to their mean flow direction, cf. also the flow directions FR). This may contribute to a better homogenization of the partial flows of the cooling fluid.FIG. 7 shows a further exemplary design of an anode head 6, again in a half-open illustration as explained with reference to FIG. 5. The essential deviations from the design of FIGS. 3a-3g will be explained.In the lower portion 27a remote from the target, a plurality of rectifying elements 34 is provided here, which are designed as hollow structures 34d in the form of round tubes here. The tubes are all aligned parallel to the central axis ZA. The tubes here completely fill the radial gap 37 between the inner flow element 18 and the outer flow element 19, here with a single layer of the round tubes or hollow structures 34 d; residual spaces ("gusset") are filled with a resin (not shown in more detail). Alternatively, the hollow structures 34 dmay also be formed in a honeycomb shape, for example, and fill the gap 37 with typically a plurality of layers of cells (not shown in more detail).In the upper section 27b close to the target, the rectifying elements 34 are designed here as triangular lamellae 34e. The triangular sipes 34 eare all axially aligned (along the central axis ZA), with the tips upward. Accordingly, the gaps 35 are narrower at the bottom than at the top. As a result, the pressure in the annular gap 29b situated in front can be kept high, which improves the uniformity of the cooling fluid flow.List of reference characters1 X-ray tube 2 housing 3 evacuated space 4 source for releasing electrons, here incandescent filament 5 electrical connections (of the cathode) 6 anode head 7 electrical connection (of the anode) 8 deflection electrodes 9 (annular) excited region 10 end face of the anode head 11 target 12 X-ray window, here beryllium disk 13 insulation body 14 inlet line for cooling fluid 14 a inlet connection of anode head 14 b conveying outlet 14 c inlet channel 15 outlet line for cooling fluid 15 a outlet connection of anode head 15 b return inlet 15 c outlet channel 16 flow path 17 supply device for cooling fluid 18 inner (first) flow element 19 outer (second) flow element 19 a cap of outer flow element 19 bfoot of outer flow element 20 radially outer section of flow path 21 cooling gap 22 radially inner section of flow path 23 tenon 24 wall of cooling gap 25 facing the target, wall of cooling gap 26 region of the anode head, the target is opposed to the excited region of the target by 27a (lower) partial section 27b (upper) partial section 27c (middle) partial section 28 close to the target; distribution structures (generally) 29a-d annular gaps 30 helical swirl element 31 helical line channel 32 smoothing zone 33 set of rectification elements 33a first set of rectification elements 33b second set of rectification elements 34 rectification elements (generally) 34a straight lamellae 34b drop-shaped lamellae 34c trapezoidal lamellae 34d hollow structures (here round tubes) 34e triangular lamellae 35 intermediate spaces 36 swirl element (obliquely located single lamella) 37 radial gap 38 radially outwardly directed wall side (on the inner flow element) 39 radially inwardly directed wall side (on the outer flow element) 40 X-ray tube arrangement fr flow direction ZA central axis

Claims

X-ray tube (1) comprising a source (4) for releasing electrons and an anode head (6) having a central axis (ZA), wherein a target (11) is formed on an end face (10) of the anode head (6), the electrons impinging on said target in an excited region (9) during operation, wherein the anode head (6) provides a flow path (16) for a cooling fluid which leads from at least one inlet connection (14a) via a radially outer section (20), further via a cooling gap (21) and further via a radially inner section (22) to at least one outlet connection (15a), characterized in that the excited region (9) of the target (11) is formed substantially annularly, and in that a local height (H1, H2) of the cooling gap (21) increases steadily from radially outside to radially inside in a region (26) of the anode head (6) opposite the excited region (9) of the target (11).X-ray tube (1) according to Claim 1, characterized in that a local cross-sectional area (F1, F2) of the cooling gap (21) in the region (26) of the anode head (6) opposite the excited region (9) of the target (11) continuously decreases from radially outside to radially inside, remains the same or continuously increases by at most 15%.X-ray tube (1) according to Claim 1 or 2, characterized in that a local cross-sectional area (F1, F2) of the cooling gap (21) in the region (26) of the anode head (6) opposite the excited region (9) of the target (11) continuously decreases from radially outside to radially inside, in particular continuously decreases by at most 20%.X-ray tube (1) according to one of the preceding claims, characterized in that the flow path (16) in the anode head (6) is formed at least substantially rotationally symmetrical with respect to the central axis (ZA).X-ray tube (1) according to one of the preceding claims, characterized in that the radially outer section (20) of the flow path (16) is formed at least substantially around the entire circumference of the anode head (6).X-ray tube (1) according to one of the preceding claims, characterized in that in the radially outer section (20) of the flow path (16), distribution structures (28) are formed, with which a cooling fluid flow from the at least one inlet connection (14a) as far as the cooling gap (21) can be distributed and equalized over the circumference of the anode head (6).X-ray tube (1) according to claim 6, characterised in that the distribution structures (28) comprise, distributed over the periphery of the anode head (6), a set (33; 33b) of rectifying elements (34) for the cooling fluid in a subsection (27b) of the radially outer section (20) close to the target, said rectifying elements running between an inlet-side annular gap (29b; 29d) and an outlet-side annular gap (29c) or the cooling gap (21), and with which at least substantially parallel partial flows of the cooling fluid can be set up and separated from one another.X-ray tube (1) according to one of claims 6 or 7, characterised in that the distribution structures (28) comprise, distributed over the periphery of the anode head (6), a set (33a) of rectifying elements (34) for the cooling fluid flow in a subsection (27a; 27c) of the radially outer section (20) of the flow path (16) remote from the target, which elements run between an inlet-side annular gap (29a; 29b) and an outlet-side annular gap (29b; 29d), and with which at least substantially parallel partial flows of the cooling fluid can be set up and separated from one another.X-ray tube (1) according to claims 7 and 8, characterised in that the outlet-side annular gap (29b; 29d) to the rectifying elements (34) of the partial section (27a; 27c) remote from the target is at the same time the inlet-side annular gap (29b; 29d) to the rectifying elements (34) of the partial section (27b) close to the target, and in that the set (33b) of rectifying elements (34) of the partial section (27b) close to the target is offset relative to one another in the azimuthal direction with respect to the central axis (ZA) with respect to the set (33a) of rectifying elements (34) of the partial section (33a) remote from the target.X-ray tube (1) according to one of claims 7 to 9, characterised in that the rectifying elements (34) are aligned at least substantially in the axial direction, and correspondingly the partial flows of the cooling fluid are aligned at least substantially in the axial direction.X-ray tube (1) according to one of claims 7 to 10, characterised in that the rectifying elements (34) are formed at least in part by - parallel lamellae (34a; 34b; 34e), in particular straight parallel lamellae (34a), and / or - trapezoidal (34c) or triangular lamellae (34e), and / or - drop-shaped (34b) or diamond-shaped lamellae, wherein the lamellae (34a; 34b; 34c; 34e) are formed on a radially outwardly directed wall side (38) and / or a radially inwardly directed wall side (39) of the anode head (6), wherein the radially outwardly directed wall side (38) and the radially inwardly directed wall side (39) also delimit the radially outer section (20) of the flow path (16).X-ray tube (1) according to one of claims 7 to 11, characterised in that the rectifying elements (34) are formed at least in part by a plurality of parallel hollow structures (34d) which are arranged in the radially outer section (20) of the flow path (16), in particular wherein the plurality of parallel hollow structures (34d) form a honeycomb structure.X-ray tube (1) according to one of Claims 6 to 12, characterized in that the distribution structures (28) comprise one or more swirl elements (30; 36) in a subsection (27a) of the radially outer section (20) of the flow path (16), which subsection is remote from the target, said swirl elements being arranged between the at least one inflow connection (14a) and an annular gap (29b) on the outlet side, and with which swirl with respect to the central axis (ZA) can be introduced into the cooling fluid flow.X-ray tube (1) according to claim 13, characterised in that the at least one swirl element (30) extends helically around the central axis (ZA) of the anode head (6), whereby at least one helical channel (31) is set up for the cooling fluid flow.X-ray tube (1) according to claim 13 or 14, characterised in that a plurality of swirl elements (30) run helically about the central axis (ZA) of the anode head (6), as a result of which a plurality of helical line channels (31) offset azimuthally and / or axially with respect to one another are set up for partial flows of the cooling fluid.X-ray tube (1) according to one of Claims 6 to 15, characterized in that a smoothing zone (32) for the cooling fluid flow is set up between distribution structures (28) of a subsection (27a; 27c) of the radially outer section (20) which is remote from the target and distribution structures (28) of a subsection (27b) of the radially outer section (20) which is close to the target, in particular wherein the smoothing zone (32) is set up as an annular gap (29b; 29d).X-ray tube (1) according to one of the preceding claims, characterized in that the anode head (6) is formed with a first flow element (18) and a second flow element (19), which are inserted into one another and which form at least a part of the flow path (16) between them.X-ray tube (1) according to one of the preceding claims, characterized in that at least the radially inner section (22) of the flow path (16), and optionally also the cooling gap (21), are partially delimited by a pin (23) which protrudes away from the end face (10) of the anode head (6) into the interior of the anode head (6).X-ray tube arrangement (40), comprising an x-ray tube (1) according to one of the preceding claims and a provision device (17) for a cooling fluid, wherein the provision device (17) provides fresh cooling fluid at a delivery outlet (14b) and the delivery outlet (14b) is connected to the at least one inlet connection (14a), in particular wherein the provision device (17) furthermore receives cooling fluid heated at a return inlet (15b) and the outlet connection (15a) is connected to the return inlet (15b).Use of an X-ray tube (1) according to one of Claims 1 to 18 or an X-ray tube arrangement (40) according to Claim 19, wherein, during operation of the X-ray tube (1), electrons from the source (4) strike an annular, excited region (9) of the target (11) in order to release electrons and X-ray radiation is thereby generated, and - cooling fluid flows from radially outside to radially inside in the cooling gap (21) opposite the excited region (9).Use according to Claim 20, characterized in that an average flow speed of the cooling fluid in the cooling gap (21) remains the same or increases, in particular increases by a maximum of 25%, with respect to the excited region (9) from radially outside towards radially inside.

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