WO2008017376A1

WO2008017376A1 - Focusing apparatus for electron beam with coil winding, ferromagnetic core and permanent magnet

Info

Publication number: WO2008017376A1
Application number: PCT/EP2007/006510
Authority: WO
Inventors: Marco Erler
Original assignee: Carl Zeiss Industrielle Messtechnik Gmbh
Priority date: 2006-08-05
Filing date: 2007-07-21
Publication date: 2008-02-14
Also published as: DE102006036694A1

Abstract

The invention relates to a focusing apparatus (9) for an X-ray tube (1), wherein the focusing apparatus (9) has an electrical conductor, which is shaped to form at least one coil winding (10), and a ferromagnetic core (11), which is designed such that it runs in and around the at least one coil winding (10), wherein the core (11) has two ends (25, 27), which are oriented with respect to one another in such a way that an electron beam (4) running coaxially with respect to the coil winding axis (40) can be focused with magnetic lines of force (60) emerging from the ends, wherein at least one permanent magnet (31; 32; 35) is provided in the core (11).

Description

FOCUSING SYSTEM FOR ELECTRON BEAM WITH ROLLING, FERROMAGNETIC CORE AND PERMANENT MAGNET

The invention relates to a focusing device and an X-ray tube with a 5 such focusing and a computed tomography with such an X-ray tube. Furthermore, the invention relates to a method for bundling electron beams with such a focusing device.

X-rays can be used to illuminate objects and examine the resulting image more closely. A particularly efficient fluoroscopy and examination can be achieved with a computer tomograph. The result of these images compared to conventional X-ray images has the advantage that no overlapping of image features occurs and the object can be viewed and evaluated in individual cross-sectional layers. An evaluation option 15 is the measurement of irradiated objects. The measurement uncertainty is the smaller, the sharper an X-ray emitting site can be formed as a point source ("focal spot") This can be achieved, inter alia, by focusing electron beams on a target Such focusing is usually done by means of at least one copper coil Depending on the geometry of pole pieces of the pure iron core and a current through the coil windings, the focal length of the electron beam can be determined.

However, during the focusing of an electron beam in an X-ray tube, it is inevitable that such a coil and the surrounding housing will heat up due to the electrical current flowing in the coil windings and the power dissipated therewith. This causes the geometry of the x-ray tube to change. Since it may take several minutes to half an hour to compose images of an object by means of computed tomography, 30 high demands are placed on temperature equilibrium and unchanged geometry of the x-ray tube. During this time, the position of the focal spot changes relative to the alignment between object and detector of a computer tomograph, which on the one hand by a "wandering" of the focal spot on On the other hand, as a result of the deformation of the entire tube mounting, the measurement uncertainty in the evaluation of the computed tomography images significantly increases.

If the acceleration voltage of the electron source is changed, the magnetic field must be changed by adjusting the current flowing through the coil of the focusing device. Due to the ohmic resistance of the coil, this leads to a changing power loss in the focusing unit, which leads to heating, which is likewise accompanied by a change in geometry of the X-ray tube. A reliable and long-term stable measurement of a transilluminated object from the totality of all projection images is only possible again when a new temperature equilibrium has been established, which remains constant even in the required measuring time.

It is therefore an object of the invention to provide a focusing device and a method for focusing, so that a focal point for an electron beam in an X-ray tube can be kept constant in position and in diameter over a long time, whereby the X-ray tube is simple, compact and easy to build.

The object is achieved by a device having the features of claim 1 and by a method having the features of claim 7. Advantageous embodiments emerge from the features of the dependent claims.

The focusing device according to the invention comprises an electrical conductor which is formed into at least one coil turn, and a ferromagnetic core. is formed so that it extends in and around the at least one coil turn, wherein the core has two ends, which are oriented to each other, that can be bundled with a magnetic field emerging from the ends of a coaxial with the coil winding axis electron beam, wherein in the core at least one

Permanent magnet is provided. By using a permanent magnet in the core, it is possible to generate such a magnetic field strength that coaxially with the coil axis extending electron beams are already bundled, while through the coil no Electricity or only a very small stream is sent. This results in no or a significantly smaller power loss in the coil, so that no or only a small heat generation is generated in the X-ray tube. As a result, there are no mechanical deformations of the X-ray tube due to temperature fluctuations and no displacement of a focal spot on a target. This has the effect that consistent X-ray images can be recorded with a computer tomograph even during a long measuring time.

If the magnetic field of the permanent magnet is still amplified by a magnetic field generated by the coil, a coil with a smaller number of turns or due to reduced current flow, a smaller cross-section and a lighter, because more compact core can be used compared to prior art solutions. While a maximum required magnetic flux density causes a certain cross section of the core, however, the length of the magnetic circuit can be reduced by a smaller coil. This makes it possible, with the same acceleration voltage and the same focal length of an electron beam to build a microfocus X-ray tube, which is made lighter and more compact than in the prior art. A permanent magnet is also an inexpensive component, so that an X-ray tube with the focusing device according to the invention can be manufactured inexpensively.

In a preferred embodiment, the at least one permanent magnet is arranged at a radial distance from the at least one coil turn. Due to the radial distance of the generally two-piece core can be easily mounted. In this case, it is possible to place a permanent magnet on a base part of the core and then to put on this formed parting plane an upper end part of the core. Particularly preferably, the permanent magnet is provided in a part of the core, which is arranged outside around the coil turn around. Due to the distance to the axially extending electron beam, the influence of the residual magnetic field not guided in the iron core on the electron beam can be significantly minimized. The bundling of the electron beam is therefore effected only by the magnetic field emerging at the open ends of the core or the pole piece. Preferably, the permanent magnet is annular. In such a construction, a high symmetry of the magnetic field is achieved, so that the electron beam can be bundled in high quality.

According to a further embodiment, the magnetic field of the permanent magnet can be at least partially short-circuited by an additional part. This allows variable adjustment of the magnetic field present at the ends of the core to focus the electron beam. Regardless of the current supplied to the coil, the magnetic field at the open ends of the core can be set directly by a corresponding position of the additional part. In the case of a current-carrying coil, the proportion of the permanent magnet in the entire magnetic field can be variably set by the additional part. This means that a basic magnetic field can be generated by the permanent magnet, which supports the magnetic field of the coil, so that an optimal focusing of the electron beam is achieved.

A simple way of variably changing the magnetic field contributed by the permanent magnet is that the additional part is arranged displaceably on the core. Preferably, this is infinitely possible, so that a fine focusing of the electron beam is achieved only by appropriate positioning of the additional part. Of course, the fine focusing can be additionally supported by a suitable energizing the coil.

Another possibility for changing the magnetic field introduced by the permanent magnet is that the additional part is movable toward or away from the core. For a digital connection or short-circuiting of the magnetic field generated by the permanent magnet is possible.

It should be noted that bundling of electron beams is possible solely by the magnetic field of the at least one permanent magnet arranged in the core. In addition, bundling of the electron beams may also be effected by a magnetic field generated by energizing a coil. In the latter case, the coil can be used to achieve a fine focusing of the electron beam. The object is further achieved by an X-ray tube with a focusing device described above and by a computed tomography with such an X-ray tube.

Advantages and developments of the invention will be explained with reference to the following figures, in which:

1 shows a schematic, greatly simplified representation of a computer tomograph with an x-ray tube; Fig. 2 is a schematic representation of an X-ray tube for a

Computed tomography; 3 is a partial perspective sectional view of a focusing apparatus for condensing an electron beam according to a first embodiment of the invention; 4 is a cross-sectional view of a focusing apparatus for condensing an electron beam according to the first embodiment of the invention;

Fig. 5 is a cross-sectional view of a focusing apparatus for condensing an electron beam according to a second embodiment of the invention; Fig. 6 is a cross-sectional view of a focusing apparatus for condensing an electron beam according to a third embodiment of the invention; Fig. 7 is a cross-sectional view of a focusing device for bundling a

Electron beam according to a fourth embodiment of the invention; and FIGS. 8A to 8C are cross-sectional views of an upper half of a core for a core

Focusing device according to the third embodiment of the invention with different positions of an additional part for shorting a magnetic field.

FIG. 1 shows a schematic representation of a computer tomograph. The computer tomograph 100 has an X-ray source 101, from which X-rays 102 are emitted in the form of a cone in the direction of an object 103 to be examined. The cone-shaped beams 102 strike the object 103, which is movably mounted on a manipulator 104 relative to the x-ray source 101, see reference numeral 105, and are partially absorbed, scattered, or transmitted. A behind the object 103 arranged detector 106 detects the through the Object 103 passed X-rays 102. In this case, a plurality of images are recorded by the detector 106 as a function of an angular position of the object 103 to the X-ray source 101. With a data processing 107, these images can be processed in such a way that a three-dimensional structure and cross sections of the object 103 can be created ("reconstruction").

2 shows a schematic view of an X-ray tube 1 of an X-ray source 101. The X-ray tube 1 has a cathode with a hairpin filament 2 and a Wehnelt cylinder 3 in the form of a Abschirmgitterblende. The electrons emitted by the filament 2 are repelled by the negatively poled Wehnelt cylinder 3. The electron beam 4 emerging from the Wehnelt cylinder is deflected in a direction such that it experiences an attractive force due to a hole anode 6 lying at positive potential and concentrates at a crossing point 5 lying in the plane of the hole anode 6. The anode 6 is followed by a plurality of deflection coils 7, 8, which align the electron beam 4. This is followed by a focusing device 9 with a coil 10 and a core 1 1 with a pole piece 12, wherein the core 1 1 encloses the coil 10 on all sides or encapsulates. The magnetic field generated by this focusing device 9 allows a strong focusing of the electron beam 4 by means of a short focal length of the focusing device 9, so that the electron beam 4 is directed to a target 13 and focused. The electron beam 4 bounces on the target 13 in such a way that a focal spot 14 is formed, from which X-rays 15 are emitted into a full space. They arrive as a useful beam cone via an exit region 16 of the X-ray tube 1 addition. The X-ray tube is oriented in the room so that the X-rays are directed to an object to be examined.

The focusing device 9 according to the invention is shown in a perspective partial sectional view in FIG. 3 and in a cross-sectional view in FIG. 4. To the coil 10 is a core 1 1 associated, which extends with an inner ring 21 in the coil and with an outer ring 22 around the coil. The rings 21 and 22 are coupled together by a connecting part 23 on one side of the core 1 1, wherein the two rings 21, 22 and the connecting part 23 may be integrally formed. On the opposite side of the connecting part 23, the core 11 has a core end portion 24 oriented to the coil axis 40, which continues the inner ring 21. Of the Kernendabschnitt 24 is conical in the first embodiment of the focusing device 9 and has an open end 25, see Fig. 4, through which coaxially to the coil axis 40 of the electron beam 4 (in Fig. 3 and 4 symbolically represented by an arrow) can happen. The core 1 1 further includes a core end portion 26 which continues the outer ring 22, wherein the core end portion 26 at the first

Embodiment of the focusing device is conical and has an open end 27, see Fig. 4. so that the electron beam 4 can pass through the core coaxially to the coil axis 40. The ends 25 and 27 of the two core end portions 24 and 26 form a pole piece 12 on which a core magnetic field 60 can emerge, see FIGS. 8A to 8C, to focus the electron beam 4 coaxial with the coil axis 40.

The outer ring 22 is designed in two parts in the embodiment shown in Fig. 3 and 4, to allow mounting of the focusing device 9. The outer ring 22 has a base part 28 and a head part 29, which can be placed on the base part 28. Thus, there is a parting plane 30 between the two parts 28 and 29. The outer ring 22 of the core 1 1 is provided according to the first embodiment of the invention with a permanent magnet 31 which is annular. The permanent magnet 31 can be inserted into the outer ring 22 or the base part 28 or the head part 29 or placed on this. For a symmetrical course of the magnetic field in the core 1 1, the annular geometry is advantageous.

The poles of the permanent magnet 31 are oriented such that the magnetic field lines emanating from the permanent magnet from the ferromagnetic core 1 1 on the one hand by the base part 28 of the outer ring 22, the connecting part 23 and from there to the inner ring 21 to the Kernendabschnitt 24 and on the other hand from the head portion 29 of the outer ring 22nd be passed over the Kernendabschnitt 26. At the ends 25 and 27 exit the field lines and widen in the air gap of the pole piece 12, see Fig. 8 A to 8C.

The magnetic field can be increased in the core 1 1 by not just one

Permanent magnet 31, but an additional permanent magnet 32 is provided. It is arranged, for example, in the inner ring 21 of the core 1 1 along the parting plane 30. In this embodiment, the inner ring 21 has a base part 33 and a head part 34, so that the additional permanent magnet 32 can be inserted therebetween. In this embodiment, however, care must be taken to ensure that an edge magnetic field of the additional permanent magnet 32 does not impair the electron beam 4 in its shape or only insignificantly, in order then to achieve a precise adjustment in the region of the pole piece. For a simple and compact construction, the inner walls of the inner ring 21 at the same time represent the walls of a guided in vacuum electron beam tube (not shown) and are therefore made vacuum-tight and vacuum-clean. This can be achieved, for example, by a vacuum-clean permanent magnet 32 with vacuum-tight joint edges. Another possibility is to keep the permanent magnet 32 away from the vacuum and to provide appropriate seals. The magnetic field emanating from the permanent magnet 32 can be magnetically shielded by suitable materials so that the electron beam 4 is hardly affected by the edge magnetic field.

In Fig. 6, a third embodiment of the invention is shown, wherein the magnetic field of a permanent magnet 31 is partially short-circuited by an additional part in the form of a displaceable ring 50. The ring 50 is displaceable by means of a mechanism in the axial direction of the focusing device 9 such that it is not, partially or completely flooded by the magnetic field of the permanent magnet 31, see FIGS. 8A to 8C. Thus, the proportion of the magnetic field of the permanent magnet 31 for

Total magnetic field, which includes the magnetic field by an energized coil 10, are set variably, so that can be fine-tuned at the pole piece 12 of the passing electron beam 4.

In FIGS. 8A to 8C, only the upper half of the core 1 1 is simplified in each case without the

Spool 10 is shown. The drawn magnetic field lines represent the magnetic field that is generated in the core by the permanent magnet 31 as a function of the position of the ring 50. In the position of the ring 50 shown in FIG. 8A, a magnetic field B 1 is present. The amount of the magnetic field Bl is higher than that of the magnetic field B2 at the position of the ring 50 shown in FIG. 8B, at which a part of the magnetic field emitted from the permanent magnet 31 is short-circuited. An even smaller amount than the magnetic field B2, the magnetic field B3, see Fig. 8C. In this position of the ring 50, almost the entire magnetic field of the permanent magnet 31 is short-circuited.

The fine adjustment of the total magnetic field can alternatively or in addition to the position of the ring 50 by changing the current flowing in the coil 10 current. As a measure of the fine adjustment of the magnetic field and thus the concentration of the electron beam 4 and the size of the focal spot 14, among other things, the achieved resolution of a recorded image using a calibration object can be used.

A fourth embodiment of the focusing device 9 according to the invention is shown in FIG. The focusing device 9 has a permanent magnet 35 within the connecting part 23, wherein a perforated disc 51 is arranged on an end face of the focusing device 9 parallel to the connecting part 23. The disk 51 can be used as an armature which, for example, bears completely against the permanent magnet 35, thereby short-circuiting the magnetic field emanating from it. With appropriate dimensioning of the disk 51, the outgoing from the permanent magnet 35 magnetic field can be only partially shorted. The disk 51 can also be placed at a distance from the permanent magnet 35 so that it does not influence the course of the magnetic field of the permanent magnet 35. By an axial movement of the disk 51, a connection or disconnection of a short-circuit effect for the magnetic field of the permanent magnet 35 can be achieved. In this embodiment, it is possible to perform a sudden increase or decrease in the magnetic field at the pole piece 12.

Claims

claims

A focusing device (9) for an X-ray tube (1), comprising:

an electrical conductor formed into at least one coil turn (10), a ferromagnetic core (11) designed to pass in and around the at least one coil turn (10), the core (11 ) has two ends (25, 27) which are oriented towards one another in such a way that an electron beam (4) extending coaxially with the coil winding axis (40) can be bundled with a magnetic field (60) emerging from the ends, wherein in the core (11) at least one permanent magnet (31; 32; 35) is provided.

2. Focusing device (9) according to claim 1, wherein the at least one permanent magnet (31; 32) in a radial distance to the at least one coil winding

(10) is arranged.

The focusing device (9) according to claim 2, wherein said at least one permanent magnet (31; 32) is provided in a part of said core disposed outside around said coil turn.

4. focussing device (9) according to one of claims 1 to 3, wherein the at least one permanent magnet (31; 32) is annular.

5. focusing device (9) according to one of claims 1 to 4, wherein the magnetic field (60) of the permanent magnet (31; 32; 35) can be at least partially shorted by an additional part (50; 51).

6. focusing device (9) according to claim 5, wherein the additional part (50; 51) on the core

(1 1) is arranged displaceably.

7. focusing device (9) according to claim 5 or 6, wherein the additional part (51) to the core (1 1) towards or away from the core (1 1) is movable away.

8. A method for bundling an electron beam (4) with a focusing device (9) according to one of claims 1 to 7, wherein the bundling of the electron beam (4) by means of a magnetic field (60) is performed, which of the at least one permanent magnet (31; 32, 35) is generated.

9. The method of claim 8, wherein the bundling of the electron beam (4) by means of a magnetic field (60) is performed, which is additionally generated by at least one current-carrying coil winding (10).

10. X-ray tube (1) with a focusing device according to one of claims 1 to 7.

1 1. Computed tomography with an X-ray tube (1) according to claim 10.