WO2019192686A1

WO2019192686A1 - Easy-to-install cathode geometry for x-ray tubes

Info

Publication number: WO2019192686A1
Application number: PCT/EP2018/058584
Authority: WO
Inventors: Michael Chabior; Arnaud Sottas; Christoph Walther
Original assignee: Comet Ag
Priority date: 2018-04-04
Filing date: 2018-04-04
Publication date: 2019-10-10
Also published as: CN214203603U; DE212018000413U1

Abstract

The present invention proposes a novel X-ray cathode head design, thanks to which the alignment of the filament with the focusing cup is facilitated. In particular, it proposes an X-ray cathode head in which filament and focusing cup are self-aligning upon assembly. The goal of the invention is achieved with an X-ray cathode head comprising a filament, supported by a filament support part, a focusing cup with an extraction slit integrated its center. Upon assembly of the focusing cup with the filament support part, the filament and the extraction slit are translationally and rotationally self-aligning, resulting in the alignment of both short and long axes of the filament and of the extraction slit.

Description

Easy-to-install cathode geometry for X-ray tubes

Technical Field

The present invention relates to the field of X-ray tubes, more particularly to X-ray tube cathode heads, and even more particularly to easy-to- install X-ray cathode heads. Specifically, the present invention relates to X-ray cathode heads, wherein upon assembly the extraction slit, the focusing cup and the filament are self-aligning. The present invention relates furthermore also to X-ray tube apparatus comprising an X-ray cathode head according to the present invention.

Background of the invention

X-ray tubes have many different industrial and medical applications.

In particular, they form the essential part of X-ray tube apparatuses that are employed for the medical purpose of patient imaging. Normally, such

apparatuses possess an electron generating part, called the cathode head or assembly, and an X-ray generating part called the anode or target. During operation, the electrons generated at the cathode are accelerated by a high electric field towards the anode onto which they eventually impinge. The loss of kinetic energy of the electrons due to their interaction with the anode material atoms results in the generation of X-ray radiation. The most important parameters in an X-ray tube apparatus for medical imaging application is the created X-ray flux and the size of the virtual X-ray source. Ideally, one would like to achieve a very high flux that permits to have a very short patient exposure time to the potentially damaging X-ray radiation while maintaining a very small virtual source size. Both would lead to an increased resolution in X-ray images. However, it is difficult to improve both parameters at the same time. Since the number of X-rays generated at the anode is directly proportional to the numbers of electrons impinging onto it, the simplest way to increase the number of generated X-rays is to increase the number of electrons generated at the cathode. However, because of the phenomenon called coulomb blockade, the number of generated electrons per surface area of the cathode filament cannot be increased arbitrarily. That implies that an extended electron source is normally required to obtain a large number of emitted electrons. However, if the electrons can freely propagate between the cathode and the anode, a filament of large size would imply a large electrons landing area on the anode and consequently a large source of X-rays. In order to reduce the size of the electrons landing area, one normally places between the cathode and the anode or behind the filament a focusing element that has the purpose to focus the electrons onto the anode and to reduce the size of the X-ray source.

A well-known design for X-ray tubes cathode heads consists of a filament as electron source and a focusing cup as focusing element. Usually, a high temperature melting metal, as for instance tungsten, is used for the filament. That permits to reach high temperature, and consequently a high yield of thermally produced electrons, without any deformation of the filament or translation of its relative position towards the focusing cup. The later consists of a plate with a hemispherical indentation, which size and form correspond to the size and form of the filament. In operation, the focusing cup is held at a lower potential with respect to the anode. Due to the hemispherical form of the focusing cup that is placed in close proximity to the filament, the electrons experience an electrical field, which is collimating the electron beam towards the anode. Unfortunately, it is difficult to obtain an important focusing effect and a small X-rays source size with this kind of cathode geometry. Therefore, modern cathode assemblies normally incorporate two separate filaments in one cathode head, a large and a small one, the large one being used when high flux of X-rays is desired and the small one when highest resolution is required.

Another drawback of the above-described cathode head design is given by the fact that in order to attain a focusing effect the hemispherical indentation of the focusing cup must be very precisely produced. Normally, the indentation is eroded, which is a very expensive and time-consuming process.

A not perfectly produced indentation in such a cathode assembly would result in a spreading more than in a focusing of the electrons due to spurious stray fields. Furthermore, the filament must be very precisely positioned with respect to the focusing cup. A misalignment of the filament towards the focusing cup would once again results in a large electrons landing area on the anode.

Obtaining and maintaining the desired alignment between the filament and the focusing cup can be challenging upon assembly of these components together and during the mounting of the cathode head in the X-ray tube apparatus. In addition, the relative position between focusing cup and filament shall also be maintained constant during the lifetime of the cathode head; in particular, it should not change with repetitions of warming and cooling cycles of the filament.

It is therefore a goal of the present invention to propose a novel X-ray cathode head design, thanks to which the alignment of the filament with the focusing cup is facilitated. In particular, it is a goal of the present invention to propose an X-ray cathode head in which filament and focusing cup are self aligning upon assembly. With an X-ray cathode head according to the present invention, it is possible to obtain an easy-to-install cathode with a high flux of electrons and a small electron landing area on the anode.

Summary of the invention

Thus, the object of the present invention is to propose a novel X-ray cathode head, with which the above-described drawbacks of the known systems are completely overcome or at least greatly diminished.

An object of the present invention is in particular to propose an X-ray cathode head, whose assembly and in particular, the alignment of the filament with the focusing cup and with the extraction slit is facilitated.

An object of the present invention is also an X-ray tube apparatus comprising an X-ray cathode head according to present invention.

According to the present invention, these objects are achieved in particular through the elements of the two independent claims. Further advantageous embodiments follow moreover from the dependent claims and the description. In particular, the objects of the present invention are achieved by an X-ray cathode head comprising:

^■ a filament supported by a filament support part; and

^■ a focusing cup with an extraction slit integrated its center; wherein upon assembly of the focusing cup with the filament support part, the filament and the extraction slit are translationally and rotationally self aligning, resulting in the alignment of both short and long axes of the filament and of the extraction slit.

Thanks to the present invention, the assembly of the X-ray cathode head is particularly simple while enabling a good alignment of filament and extraction slit and consequently focusing cup. The filament, which is pre aligned on the support part, is self-aligning with the extraction slit upon assembly. This can be achieved by means known by a person skilled in the art, for example by using alignment marks on the focusing cup and on the filament support part. It is therefore not necessary to align the filament with respect to the extraction slit and the focusing cup after the assembly.

In one preferred embodiment, the focusing cup further comprises on the opposite side of the filament a first cavity acting as first electron focusing element. During operation, an appropriate voltage can be applied to the focusing cup, this voltage having a focusing effect on the electrons emitted from the filament and accelerated through the slit by the high electrical field between filament and target. Thanks to this focusing effect, the size of the electrons landing area on the target can be reduced, increasing the performance of the X- ray tube apparatuses in which X-ray cathode heads according to the present invention are used.

In another preferred embodiment, the focusing cup comprises a second cavity surrounding the filament and acting as second electron focusing element. Thanks to the presence of this cavity, the flux of electron that can be extracted through the extraction slit and therefore the flux of electrons landing on the target can be increased, improving the performance of the X-ray apparatuses in which X-ray cathode heads according to the present invention are used. The electrons emitted from the filament in a direction other than the direction of the extraction slit can be re-directed towards the slit by the voltage applied to the focusing cup. A person skilled in the art would understand that simulation could be performed to determine the optimal form and size of the cavity in order to maximize the flux of electrons exiting the extraction slit in dependence upon the form and size of the filament, of the slit and upon other relevant geometrical parameters of the cathode head. In another preferred embodiment, the width of the extraction slit is smaller than the width of the filament. This relaxes the alignment requirement of the filament with the cavity of the focusing cup acting as first focusing element. The slit reduces the size of the virtual electron source in the direction of the minor axis of the extraction slit and a minor misalignment of the filament with the focusing cup in this direction does not play an important role for the performances of the X-ray cathode head.

In another preferred embodiment, the length of the extraction slit is smaller than the length of the filament. This further relaxes the alignment requirement of the filament with the cavity of the focusing cup acting as first focusing element. The slit reduces the size of the virtual electron source in the direction of the major axis of the extraction slit and a minor misalignment of the filament with the focusing cup in this direction does not play an important role for the performances of the X-ray cathode head.

In another preferred embodiment, the focusing cup and the filament support are attached together by gluing. This represents a very simple way of holding both parts together and facilitates the assembly of the X-ray cathode head according to the present invention.

In another preferred embodiment, the focusing cup and the filament support are attached together by soldering. This represents another very simple way of holding both parts together and facilitates the assembly of the X- ray cathode head according to the present invention. In another preferred embodiment, the filament is a coil filament. Coil filaments have the advantage that the ratio of the surface area out of which electrons can be emitted to the length of the filament is maximized while requiring only a relatively small amount of heating current due to self-heating of the coil windings. Therefore, the flux of electrons can also be maximized.

In another preferred embodiment, the filament is a strip filament. A strip filament has the advantage that it can be easily pre-aligned on the filament support part and that the electrons are emitted only from the top and bottom surfaces of the filament. This permits to have a smaller electron virtual source that eventually increased the performances of the X-ray cathode head.

In another preferred embodiment, the extraction slit is mechanically milled. Mechanical milling is easy and cost inexpensive. Therefore, the machining of the focusing cup is simple for a person skilled in art and the overall costs for the fabrication of the X-ray cathode can be reduced. In another preferred embodiment, the ends of the extraction slit have the form of a portion of a circle. This slit geometry has the advantage that spurious electrical field that would otherwise be created at sharp edges of the extraction slit are suppressed.

Objects of the present invention are also achieved by an X-ray tube apparatus characterized in that it comprises an X-ray cathode head according to the present invention. By using an X-ray cathode head according to the present invention, the performances of the X-ray tube apparatus are improved and the assembly of the apparatus is simplified.

Brief description of the drawings Figure 1 is a perspective section view of a preferred embodiment of an X-ray cathode head according to the present invention taken along the short axis of the extraction slit. Figure 2 is a section view of a preferred embodiment of an X-ray cathode head according to the present invention taken along the long axis of the extraction slit.

Figure 3 is a section view of a preferred embodiment of an X-ray cathode head according to the present invention taken along the short axis of the extraction slit.

Detailed description of a preferred embodiment

Figure 1 illustrates a perspective section view of a preferred embodiment of an X-ray cathode head according to the present invention taken along the short axis of the extraction slit. The X-ray cathode head 1 comprises a focusing cup 2 with extraction slit 3 in its center and a filament 5 supported by a filament support part 4 and pre-aligned along the line connecting the holes 5’. In order to facilitate the alignment of the filament 5 on the support part 4, guiding lines (not shown here) can be added on the surface of the support part 4.

In this preferred embodiment, the filament 5 is a coil filament but it could also be a flat strip or any appropriate filament form. In any case, a metal or a metal alloy with a high melting temperature and a high electrical resistance as for instance tungsten is preferred. In order to reduce the work function of the filament 5, the latter can additionally be coated with a low work function material as for instance an alkali metal or manufactured from a specialized alloy with optimized work function (like thoriated tungsten).

The filament support part 4 is made out of an insulating material that is compatible with a vacuum environment, as for instance but not exhaustively Torlon, Ceramic or PEEK. Filament 5 is connected to the contacts 6, which are attached by appropriate means, for instance by gluing, to the filament support part 4. The contacts 6 are fabricated out of a low electrical resistance metal or a low electrical resistance metal alloy, as for instance but not exhaustively copper, aluminum, nickel, iron-nickel-cobalt alloy or beryllium-coper. The contacts 6 are used to apply the electrical potential difference across the filament that eventually results in an electrical current and an increase of the filament temperature and finally in thermionic electrons emission.

The focusing cup 2 comprises a first cavity 2’ on the opposite side of the filament 5. During operation of the X-ray cathode head 1 , the divergent electron beam extracted out of the filament 5 by thermionic emission and accelerated through the extraction slit 3, by means of a large electrical filed between the filament 5 and a target (not shown here), can be collimated and focused onto the target by applying an appropriate voltage V on the focusing cup 2. The cavity 2’ acts therefore as a first focusing element. It has to be noted that the X-ray cathode head 1 can be used either in a mode where the filament 5 is held at a high negative voltage and the target at ground potential or in a mode where the filament is kept at a potential close to ground and the target at a high positive voltage. In order to avoid stray fields than could result in a reduction of the performance of the X-ray cathode head 1 , the edges 2” of the cavity 2’ are rounded off as illustrated in Figure 1.

The focusing cup 2 further comprises a second cavity 3’ on the side of the filament 5. After assembly of the X-ray cathode head 1 , the cavity 3’ surrounds the filament 5 and acts as a second focusing element on the thermionically emitted electrons. The person skilled in the art would understand that optimum form and size of the cavity 3’ could be determined by simulation.

In order to achieve highest performance of the X-ray cathode head 1 in terms of minimal electron landing area on the target, a perfect alignment of the filament 5 with the extraction slit 3, and consequently with both cavities 2’ and 3’, is essential. According to the present invention, the extraction slit 3 and the filament 5 are upon assembly of the focusing cup 2 with the filament support part 4 self-aligning. The self-alignment can be achieved by means known by a person skilled in the art, for example by using alignment marks on the focusing cup 2 and on the filament support part 4. Filament 5 and extraction slit 3 are, after assembly, translationally and rotationally aligned with both long axes and short axes aligned. Figures 2 and 3 are section views of the preferred embodiment of the X-ray cathode head 1. As can be seen in these Figures, the width W3 of extraction slit 3 is preferably chosen smaller than the width W5 of filament 5. With a slit 3 narrower than filament 5, the performance of the X-ray cathode head 1 is further improved, since slit 3 reduces the size of the virtual source of electrons eventually increasing the performance of the X-ray cathode head. There is a trade-off between the size of the electrons landing area on the target and the electron current impinging on the latter. While reducing the width of the slit 3 permits to have a smaller electron landing area on the target, it reduces also the impinging electron current. Optimal slit width must therefore be determined according to the specific use of the X-ray cathode head. In order to avoid spurious stray electrical field that would reduce in a reduction of the performance of the X-ray cathode head, the ends of the extraction slit are in the form of a portion of a circle. While in Figures 1 to 3 the length L3 of the extraction slit 3 is chosen bigger than the length L5 of the filament 5, the person skilled in the art would understand that the electron virtual source size can be further reduced by choosing an extraction slit length L3 smaller than the filament length L5.

As can be seen in Figures 1 to 3, a space of height H2 is created after as assembly between the focusing cup 2 and the support part 4. From a vacuum technical point of view, this is advantageous, since it is better to separate two adjacent surfaces as keeping them in contact because virtual leaks could otherwise be created. The space between the support part 4 and the focusing cup 2 can conveniently be evacuated through the slit 3 by appropriate means during evacuation of an X-ray tube comprising an X-ray cathode head according to the present invention.

Finally, it should be pointed out that the foregoing has outlined one pertinent non-limiting embodiment. It will be clear to those skilled in the art that modifications to the disclosed non-limiting embodiment can be effected without departing from the spirit and scope thereof. As such, the described non-limiting embodiment ought to be considered merely illustrative of some of the more prominent features and applications. Other beneficial results can be realized by applying the non-limiting embodiments in a different manner or modifying them in ways known to those familiar with the art.

Claims

1. X-ray cathode head (1 ) comprising:

^■ a filament (5), supported by a filament support part (4); and

^■ a focusing cup (2) with an extraction slit (3) integrated its center; characterized in that upon assembly of the focusing cup (2) with the filament support part (4), the filament (5) and the extraction slit (3) are translationally and rotationally self-aligning, resulting in the alignment of both short and long axes of the filament (5) and of the extraction slit (3).

2. X-ray cathode head (1 ) according to claim 1 , characterized in that the focusing cup (2) comprises on the opposite side of the filament (5) a first cavity (2’) acting as first electron focusing element.

3. X-ray cathode head (1 ) according to claim 1 or 2, characterized in that the focusing cup (2) comprises a second cavity (3’) surrounding the filament (5) and acting as second electron focusing element.

4. X-ray cathode head (1 ) according to any one of the preceding claims, characterized in that the width (W3) of the extraction slit (3) is smaller than the width (W5) of the filament (5).

5. X-ray cathode head (1 ) according to any one of the preceding claims, characterized in that the length (L3) of the extraction slit (3) is smaller than the length (L5) of the filament (5).

6. X-ray cathode head (1 ) according to any one of the preceding claims, characterized in that focusing cup (2) and filament support (4) are attached together by gluing.

7. X-ray cathode head (1 ) according to any one of the preceding claims, characterized in that focusing cup (2) and filament support (4) are attached together by soldering.

8. X-ray cathode head (1 ) according to any one of the preceding claims, characterized in that the filament (5) is a coil filament.

9. X-ray cathode head (1 ) according to any one of the claims 1 to 7, characterized in that the filament (5) is a strip filament.

10. X-ray cathode head (1 ) according to any one of the preceding claims, characterized in that the extraction slit (3) is mechanically milled.

11. X-ray cathode head (1 ) according to any one of the preceding claims, characterized in that the ends of the extraction slit (3) have the forms of portions of a circle.

12. X-ray tube apparatus characterized in that it comprises an X-ray cathode head (1 ) according to any one of the claims 1 to 10.