EP1158562B1 - X-ray tube with a flat cathode - Google Patents

X-ray tube with a flat cathode Download PDF

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Publication number
EP1158562B1
EP1158562B1 EP01000176A EP01000176A EP1158562B1 EP 1158562 B1 EP1158562 B1 EP 1158562B1 EP 01000176 A EP01000176 A EP 01000176A EP 01000176 A EP01000176 A EP 01000176A EP 1158562 B1 EP1158562 B1 EP 1158562B1
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EP
European Patent Office
Prior art keywords
electron emitter
ray tube
anode
electrode
electron
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Expired - Lifetime
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EP01000176A
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German (de)
French (fr)
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EP1158562A1 (en
Inventor
Robert Dr. Hess
Frank Demuth
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Philips Intellectual Property and Standards GmbH
Koninklijke Philips NV
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Philips Intellectual Property and Standards GmbH
Koninklijke Philips Electronics NV
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
    • H01J35/06—Cathodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
    • H01J35/06—Cathodes
    • H01J35/064—Details of the emitter, e.g. material or structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
    • H01J35/06—Cathodes
    • H01J35/066—Details of electron optical components, e.g. cathode cups
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/14—Arrangements for concentrating, focusing, or directing the cathode ray
    • H01J35/147—Spot size control
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/14—Arrangements for concentrating, focusing, or directing the cathode ray
    • H01J35/153—Spot position control

Definitions

  • the invention relates to an X-ray tube with an anode and with a cathode arrangement which has a cathode pot for electron focusing, a flat, apertured electron emitter and an electrode which is arranged on the side facing away from the anode side of the electron emitter.
  • a cathode pot for electron focusing a cathode pot for electron focusing
  • a flat, apertured electron emitter an electrode which is arranged on the side facing away from the anode side of the electron emitter.
  • the electrode emitter is a flat, flat and meandering metal band. Between the reciprocating webs of this metal band so breakthroughs are present.
  • the potential of the cathode pot is variable with respect to the electron emitter, so that errors in the manufacturing process have no effect on the dimensions of the focal spot. If the potential at the cathode pot is more positive by a certain amount than at the electron emitter, electrons from the lateral regions or from the back of the electron emitter can reach the cathode pot and heat it up. Therefore, in one embodiment, a small distance from the electron emitter, an electrode is provided which shields the back and the lateral regions of the electron emitter and whose potential corresponds at least approximately to the potential of the electron emitter.
  • the advantage of such a flat electron emitter over an electron emitter made of a helically wound wire is that the electron trajectories can be better focused so that a focal spot with a more favorable electron density distribution is produced on the anode. Nevertheless, the achievable electron density distribution in the focal spot does not match that of an ideal flat emitter.
  • the ideal flat emitter is a flat emitter with zero thickness and no openings.
  • Object of the present invention is to provide an X-ray tube of the type mentioned in such a way that the characteristic of the ideal flat emitter at least Object of the present invention is to provide an X-ray tube of the type mentioned in such a way that the characteristic of the ideal flat emitter results at least approximately.
  • the electrode in the operating state of the X-ray tube leads to the electron emitter negative potential of such magnitude that the amount of electric field strength in the space between the electrode and the electron emitter at least 20% - preferably at least 100% - of the amount the field strength is on the anode-facing side of the electron emitter.
  • the invention is based on the finding that in the known X-ray tube, the electric field reaches into the apertures, so that the equipotential lines are drawn into the apertures at the surface of the electron emitter facing the anode. In the area of the openings, therefore, electron paths are obtained which deviate from those of an ideal flat emitter and prevent the characteristic of this ideal flat emitter from being achieved. As a result of the fact that the electrode has a negative potential on the rear side of the electron emitter remote from the anode, the equipotential lines are forced back out of the openings. With a suitable choice of the potential can be achieved that the equipotential surfaces on the anode facing the front of the electron emitter are almost flat. The electron paths then run in the vicinity of the electron emitter everywhere straight and perpendicular to its surface.
  • Another advantage resulting from the invention is that the position and / or size of the Make it easier to control focal spots.
  • a preferred embodiment is specified in claim 2.
  • the electron emitter may have a different shape than that of a meander (e.g., the shape of a spiral), a meander is easier to manufacture.
  • a better penetration of the electrode located at the back of the electron emitter is achieved on the front of the electron emitter. At a constant distance between the electron emitter and the electrode, the electrical voltage between these parts can be reduced.
  • a preferred embodiment is specified in claim 4.
  • another form is possible, for example. a curved shape of the electron emitter.
  • the electrode would have to be adapted to this curvature.
  • position of the electron emitter electron emitter and cathode pot can lead the same potential.
  • An X-ray device with an X-ray tube according to claim 1 is specified in claim 7.
  • the embodiment according to claim 8 has the effect that the bias voltage of the electrode is varied in dependence on the tube voltage (that is to say the voltage between anode and cathode) so that the optimal field profile always results in the region of the electron emitter.
  • the rotating anode X-ray tube shown has an anode disc 1 rotating in the operating state and a cathode assembly 2.
  • the cathode assembly 2 is connected via an insulator 3 to the metal housing 4 of the X-ray tube.
  • the anode 1 may be connected via an insulator to the housing 4 or lead the potential of the (grounded) housing.
  • the electrons emitted from the cathode strike the anode in a focal spot and generate there X-radiation, which can leave the X-ray tube through a window 5.
  • the invention is also applicable to x-ray tubes with fixed anodes or x-ray tubes used in the non-medical field.
  • Fig. 2 shows the cathode assembly in a cross section. It can be seen a cathode pot 201, which is provided with a die 202, which serves to focus the electron beam. At the bottom in the middle of the die is a slot 204, the longitudinal direction of which extends radially to the axis of rotation of the anode disk 1.
  • slot is a flat, flat electron emitter 203, the front side of which (that is, the side facing the anode 1) is in one plane with the bottom of the die.
  • the electron emitter has the shape of a meander whose individual ridges are perpendicular to the Plane of the Fig. 3 - And thus extend in the longitudinal direction of the slot 204.
  • the openings between adjacent lands have a dimension of about 0.1 mm, while the width of the lands (that is, the dimension in the vertical direction in the plane of the drawing) is about 0.2 mm.
  • the webs 203 may also extend perpendicular to the longitudinal direction of the web 204 - similar to the U.S. Patent 4,344,011 , They can then be easier to produce.
  • the electron emitter 203 is heated by an electric current flowing through it in the operating state, so that it can emit electrons.
  • an electrode 205 is arranged in the slot on the rear side of the electron emitter 203, which leads to a negative potential with respect to the electron emitter 203.
  • FIG. 2 shows a highly simplified, schematic block diagram of an X-ray device with the X-ray tube according to the invention. This is connected to the output of a first rectifier 91, which supplies a DC voltage in the range between 40 and 125 kV.
  • a second rectifier 92 to whose positive output terminal the electron emitter 203 and to its negative output terminal the electrode 205 are connected, supplies the negative kV of the electrode 205 to the electron emitter 203.
  • a heating current source heats the electron emitter so that electrons are emitted therefrom can not be shown for the sake of simplicity.
  • the negative bias of the electrode 205 with respect to the electron emitter 203 is now chosen so that on the front of the electron emitter - and also in the region of the openings between the webs - an approximately flat course of Equipotential surfaces results.
  • the x-ray generator feeding the x-ray tube 100 may also have a different structure. It is essential that it contains an (additional) DC voltage source for generating a DC voltage between electron emitter 203 and electrode 205, which preferably changes proportionally in accordance with the high voltage between the anode and the cathode. (This condition is taken from the in Fig. 4 simplified circuit shown with the connected with its primary winding 81 to an inverter transformer 8, at the Sekundärväddungm 82 and 83, the rectifier 91, 92 are connected, met only with restrictions.)
  • the electric field at the backside of the electron emitter must be stronger than at the front side. How much it needs to be stronger depends on the thickness of the webs (these are the ones in the drawing plane of Fig. 3 horizontally extending dimensions), from their width and from their mutual distance.
  • One way to improve the penetration of the electric field generated by the electrode 205 on the front of the electron emitter is to bevel the side surfaces of the individual webs of the electron emitter, so that they taper towards the electrode 203 or the openings to the electrode expand.
  • the electric field strength behind the electron emitter is just as large as the electric field strength in front of the electron emitter, then complete compensation of the fissures caused by the apertures in the electron emitter is not possible, but there is still a positive effect.
  • the negatively biased electrode 205 is practically ineffective.
  • Fig. 5a shows the electron trajectories in a helically wound wire 203 'as an electron emitter (whose cross section appears elliptical because of the distorted representation).
  • the course of the electron orbits depends on where the electron emitters the electron emit. Despite focusing (not shown in detail), the electrons therefore meet in a comparatively large cross-section.
  • Fig. 5b are compared with the conditions in an ideal surface emitter. All electron orbits start perpendicular to the surface of the emitter until they meet under the effect of a focusing field in a focal spot of minimum size.
  • Fig. 5c shows the conditions in a real meandering electron emitter.
  • the electron paths are curved, which, in spite of the focusing, leads to an enlargement of the focal spot (in comparison to the ideal surface emitter).
  • the focal spot is significantly smaller than in a helically wound electron emitter.

Description

Die Erfindung betrifft eine Röntgenröhre mit einer Anode und mit einer Kathodenanordnung die einen Kathodentopf zur Elektronenfokussierung, einen flachen, mit Durchbrüchen versehenen Elektronenemitter und eine Elektrode aufweist, die auf der von der Anode abgewandten Seite des Elektronenemitters angeordnet ist. Eine solche Röntgenröhre ist aus der US-PS 4 344 011 bekannt. Bei einer der dort angegebenen Ausführungsformen ist der Elektrodenemitter ein ebenes, flaches und mäanderförmig verlaufendes Metallband. Zwischen den hin- und hergehenden Stegen dieses Metallbandes sind also Durchbrüche vorhanden.The invention relates to an X-ray tube with an anode and with a cathode arrangement which has a cathode pot for electron focusing, a flat, apertured electron emitter and an electrode which is arranged on the side facing away from the anode side of the electron emitter. Such an X-ray tube is from the U.S. Patent 4,344,011 known. In one of the embodiments specified therein, the electrode emitter is a flat, flat and meandering metal band. Between the reciprocating webs of this metal band so breakthroughs are present.

Bei der bekannten Röntgenröhre ist vorgesehen, dass das Potential des Kathodentopfes gegenüber dem Elektronenemitter variabel ist, so dass Fehler beim Fertigungsprozess keinen Einfluss auf die Abmessungen des Brennflecks haben. Wenn das Potential am Kathodentopf um einen bestimmten Betrag positiver ist als am Elektronenemitter, können Elektronen aus den seitlichen Regionen oder aus der Rückseite des Elektronenemitters auf den Kathodentopf gelangen und diesen aufheizen. Deshalb ist bei einer Ausführungsform in geringem Abstand von dem Elektronenemitter eine Elektrode vorgesehen, die die Rückseite und die seitlichen Bereiche des Elektronenemitters abschirmt und deren Potential zumindest annähernd dem Potential des Elektronenemitters entspricht.In the known X-ray tube is provided that the potential of the cathode pot is variable with respect to the electron emitter, so that errors in the manufacturing process have no effect on the dimensions of the focal spot. If the potential at the cathode pot is more positive by a certain amount than at the electron emitter, electrons from the lateral regions or from the back of the electron emitter can reach the cathode pot and heat it up. Therefore, in one embodiment, a small distance from the electron emitter, an electrode is provided which shields the back and the lateral regions of the electron emitter and whose potential corresponds at least approximately to the potential of the electron emitter.

Der Vorteil eines solchen flachen Elektronenemitters gegenüber einem Elektronenemitter aus einem helixförmig gewickelten Draht besteht darin, dass sich die Elektronenbahnen besser fokussieren lassen, so dass auf der Anode ein Brennfleck mit einer günstigeren Elektronendichteverteilung erzeugt wird. Gleichwohl reicht die erzielbare Elektronendichteverteilung im Brennfleck nicht an die eines idealen Flachemitters heran. Als idealer Flachemitter wird ein ebener Emitter mit der Dicke Null und ohne Durchbrüche bezeichnet.The advantage of such a flat electron emitter over an electron emitter made of a helically wound wire is that the electron trajectories can be better focused so that a focal spot with a more favorable electron density distribution is produced on the anode. Nevertheless, the achievable electron density distribution in the focal spot does not match that of an ideal flat emitter. The ideal flat emitter is a flat emitter with zero thickness and no openings.

Aus US 5 633 907 ist eine elektronenfokussierende Kathode für eine Röntgenröhre bekannt, wobei die Kathode eine große Kavität aufweist, in welcher eine Elektronenwolke erzeugt wird, welche vor dem primären elektrischen Feld zwischen der Kathode und der Anode geschützt ist.Out US 5 633 907 For example, an electron-focusing cathode for an X-ray tube is known, wherein the cathode has a large cavity in which an electron cloud is generated, which is protected from the primary electric field between the cathode and the anode.

Aufgabe der vorliegenden Erfindung ist es, eine Röntgenröhre der eingangs genannten Art so auszugestalten, dass sich die Charakteristik des idealen Flachemitters wenigstens Aufgabe der vorliegenden Erfindung ist es, eine Röntgenröhre der eingangs genannten Art so auszugestalten, dass sich die Charakteristik des idealen Flachemitters wenigstens näherungweise ergibt.Object of the present invention is to provide an X-ray tube of the type mentioned in such a way that the characteristic of the ideal flat emitter at least Object of the present invention is to provide an X-ray tube of the type mentioned in such a way that the characteristic of the ideal flat emitter results at least approximately.

Diese Aufgabe wird erfindungsgemäß dadurch gelöst, dass die Elektrode im Betriebszustand der Röntgenröhre ein gegenüber dem Elektronenemitter negatives Potential von solcher Größe führt, dass der Betrag der elektrischen Feldstärke im Raum zwischen der Elektrode und dem Elektronenemitter mindestens 20% - vorzugsweise mindestens 100% - des Betrags der Feldstärke auf der der Anode zugewandten Seite des Elektronenemitters beträgt.This object is achieved in that the electrode in the operating state of the X-ray tube leads to the electron emitter negative potential of such magnitude that the amount of electric field strength in the space between the electrode and the electron emitter at least 20% - preferably at least 100% - of the amount the field strength is on the anode-facing side of the electron emitter.

Die Erfindung basiert auf der Erkenntnis, dass bei der bekannten Röntgenröhre das elektrische Feld bis in die Durchbrüche hineingreift, so dass die Äquipotentiallinien an der der Anode zugewandten Oberfläche des Elektronenemitters in die Durchbrüche hineingezogen werden. Im Bereich der Durchbrüche ergeben sich daher Elektronenbahnen, die von denen eines idealen Flachemitters abweichen und verhindern, dass die Charakteristik dieses idealen Flachemitters erreicht werden kann. Dadurch, dass die Elektrode auf der von der Anode abgewandten Rückseite des Elektronenemitters ein negatives Potential führt, werden die Äquipotentiallinien aus den Durchbrüchen zurückgedrängt. Bei geeigneter Wahl des Potentials kann erreicht werden, dass die Äquipotentialflächen auf der der Anode zugewandten Vorderseite des Elektronenemitters nahezu eben sind. Die Elektronenbahnen verlaufen dann in der Nähe des Elektronenemitters überall geradlinig und senkrecht zu seiner Oberfläche.The invention is based on the finding that in the known X-ray tube, the electric field reaches into the apertures, so that the equipotential lines are drawn into the apertures at the surface of the electron emitter facing the anode. In the area of the openings, therefore, electron paths are obtained which deviate from those of an ideal flat emitter and prevent the characteristic of this ideal flat emitter from being achieved. As a result of the fact that the electrode has a negative potential on the rear side of the electron emitter remote from the anode, the equipotential lines are forced back out of the openings. With a suitable choice of the potential can be achieved that the equipotential surfaces on the anode facing the front of the electron emitter are almost flat. The electron paths then run in the vicinity of the electron emitter everywhere straight and perpendicular to its surface.

Durch diese Maßnahmen vergrößert sich das Verhältnis zwischen der Fläche des Elektronenemitters und der Fläche des Brennflecks. Man kann also eine bestimmte Brennfleckgröße mit einem größeren Elektronenemitter erreichen. Um in dem Brennfleck eine bestimmte Elektronendichte zu erzielen, kann der Elektronenemitter auf einer niedrigeren Temperatur gehalten werden, wodurch sich seine Lebensdauer verlängert. Ein weiterer aus der Erfindung resultierender Vorteil ist, dass sich Lage und/oder Größe des Brennflecks leichter steuern lassen.These measures increase the ratio between the area of the electron emitter and the area of the focal spot. So you can achieve a specific focal spot size with a larger electron emitter. In order to achieve a certain electron density in the focal spot, the electron emitter can be kept at a lower temperature, which prolongs its life. Another advantage resulting from the invention is that the position and / or size of the Make it easier to control focal spots.

Eine bevorzugte Ausgestaltung ist in Anspruch 2 angegeben. Der Elektronenemitter kann zwar auch eine andere Form als die eines Mäanders haben (z.B. die Form einer Spirale), doch ist ein Mäander einfacher herzustellen. Durch die Ausgestaltung nach Anspruch 3 wird ein besserer Durchgriff der an der Rückseite des Elektronenemitters befindlichen Elektrode auf die Vorderseite des Elektronenemitters erreicht. Bei gleichbleibendem Abstand zwischen Elektronenemitter und Elektrode kann dadurch die elektrische Spannung zwischen diesen Teilen verringert werden.A preferred embodiment is specified in claim 2. Although the electron emitter may have a different shape than that of a meander (e.g., the shape of a spiral), a meander is easier to manufacture. By the embodiment according to claim 3, a better penetration of the electrode located at the back of the electron emitter is achieved on the front of the electron emitter. At a constant distance between the electron emitter and the electrode, the electrical voltage between these parts can be reduced.

Eine bevorzugte Ausgestaltung ist in Anspruch 4 angegeben. Grundsätzlich ist auch eine andere Form möglich, zB. eine gekrümmte Form des Elektronenemitters. In diesem Fall müsste die Elektrode an diese Krümmung angepasst sein.A preferred embodiment is specified in claim 4. In principle, another form is possible, for example. a curved shape of the electron emitter. In this case, the electrode would have to be adapted to this curvature.

Bei der im Anspruch 5 angegebenen Lage des Elektronenemitters können Elektronenemitter und Kathodentopf dasselbe Potential führen.In the specified in claim 5 position of the electron emitter electron emitter and cathode pot can lead the same potential.

Die bessere Steuerbarkeit der Lage und/oder der Größe des Brennflecks lässt sich durch die in Anspruch 6 angegebenen Maßnahmen ausnutzen. Durch Variation der Ströme in der Quadrupoleinheit können die Abmessungen des Brennflecks stufenlos variiert werden.The better controllability of the position and / or the size of the focal spot can be exploited by the measures specified in claim 6. By varying the currents in the quadrupole unit, the dimensions of the focal spot can be varied steplessly.

Ein Röntgeneinrichtung mit einer Röntgenröhre nach Anspruch 1 ist im Anspruch 7 angegeben. Die Ausgestaltung nach Anspruch 8 bewirkt dabei, dass die Vorspannung der Elektrode in Abhängigkeit von der Röhrenspannung (d. h. der Spannung zwischen Anode und Kathode) so variiert wird, dass sich im Bereich des Elektronenemitters stets der optimale Feldverlauf ergibt.An X-ray device with an X-ray tube according to claim 1 is specified in claim 7. The embodiment according to claim 8 has the effect that the bias voltage of the electrode is varied in dependence on the tube voltage (that is to say the voltage between anode and cathode) so that the optimal field profile always results in the region of the electron emitter.

Die Erfindung wird nachstehend anhand der Zeichnungen näher erläutert. Es zeigen:The invention is explained below with reference to the drawings. Show it:

Fig 1Fig. 1
eine Röntgenröhre, bei der die Erfindung anwendbar ist, in schematischer Darstellung,an X-ray tube to which the invention is applicable, in a schematic representation,
Fig. 2Fig. 2
die Kathodenanordnung einer solchen Röhre,the cathode arrangement of such a tube,
Fig. 3Fig. 3
eine vergrößerte Darstellung eines Teils dieser Anordnung,an enlarged view of a part of this arrangement,
Fig. 4Fig. 4
ein Blockschaltbild einer Röntgeneinrichtung mit einer erfindungsgemaßen Röntgenröhre unda block diagram of an X-ray device with an inventive X-ray tube and
Fig. 5aFig. 5a
bis b die Elektronenbahnen verschiedener Elektronenemitterto b the electron orbits of different electron emitters

Die in Fig 1 dargestellte Drehanoden-Röntgenröhre besitzt eine im Betriebszustand rotierende Anodenscheibe 1 und eine Kathodenanordnung 2. Die Kathodenanordnung 2 ist über einen Isolator 3 mit dem Metallgehäuse 4 der Röntgenröhre verbunden. Auch die Anode 1 kann über einen Isolator mit dem Gehäuse 4 verbunden sein oder das Potential des (geerdeten) Gehäuses führen. Die aus der Kathode emittierten Elektronen treffen in einem Brennfleck auf die Anode auf und erzeugen dort Röntgenstrahlung, die durch ein Fenster 5 die Röntgenröhre verlassen kann.In the Fig. 1 The rotating anode X-ray tube shown has an anode disc 1 rotating in the operating state and a cathode assembly 2. The cathode assembly 2 is connected via an insulator 3 to the metal housing 4 of the X-ray tube. Also, the anode 1 may be connected via an insulator to the housing 4 or lead the potential of the (grounded) housing. The electrons emitted from the cathode strike the anode in a focal spot and generate there X-radiation, which can leave the X-ray tube through a window 5.

Die in Fig. 1 dargestellte Röntgenröhre ist eine Drehanoden-Röntgenröhre, wie sie bei medizinisch diagnostischen Untersuchungen verwendet wird Die Erfindung ist jedoch auch bei Röntgenröhren mit feststehenden Anoden bzw. bei Röntgenröhren anwendbar, die im nicht medizinischen Bereich verwendet werden.In the Fig. 1 The invention is also applicable to x-ray tubes with fixed anodes or x-ray tubes used in the non-medical field.

Fig 2 zeigt die Kathodenanordnung in einem Querschnitt. Man erkennt einen Kathodentopf 201, der mit einem Gesenk 202 versehen ist, das der Fokussierung des Elektronenstrahls dient. Am Boden in der Mitte des Gesenkes befindet sich ein Schlitz 204, dessen Längsrichtung radial zur Rotationsachse der Anodenscheibe 1 verläuft. Fig. 2 shows the cathode assembly in a cross section. It can be seen a cathode pot 201, which is provided with a die 202, which serves to focus the electron beam. At the bottom in the middle of the die is a slot 204, the longitudinal direction of which extends radially to the axis of rotation of the anode disk 1.

Wie Fig 3, die diesen Schlitz vergrößert darstellt, deutlich zeigt, befindet sich in dem Schlitz ein flacher, ebener Elektronenemitter 203, dessen Vorderseite (das ist die der Anode 1 zugewandte Seite) mit dem Boden des Gesenks in einer Ebene liegt. Der Elektronenemitter hat die Form eines Mäanders, dessen einzelne Stege senkrecht zur Zeichenebene der Fig. 3 - und damit in Längsrichtung des Schlitzes 204 verlaufen. Die Durchbrüche zwischen benachbarten Stegen haben eine Abmessung von ca. 0,1 mm, während die Breite der Stege (das ist die Abmessung in der senkrechten Richtung in der Zeichenebene) ca. 0,2 mm beträgt. - Die Stege 203 können aber auch senkrecht zur Längsrichtung des Steges 204 verlaufen - ähnlich wie bei der US-PS 4 344 011 . Sie lassen sich dann leichter herstellen. Der Elektronenemitter 203 wird durch einen im Betriebszustand durch ihn hindurch fließenden elektrischen Strom aufgeheizt, sodass er Elektronen emittieren kann.As Fig. 3 clearly showing that slot is a flat, flat electron emitter 203, the front side of which (that is, the side facing the anode 1) is in one plane with the bottom of the die. The electron emitter has the shape of a meander whose individual ridges are perpendicular to the Plane of the Fig. 3 - And thus extend in the longitudinal direction of the slot 204. The openings between adjacent lands have a dimension of about 0.1 mm, while the width of the lands (that is, the dimension in the vertical direction in the plane of the drawing) is about 0.2 mm. - But the webs 203 may also extend perpendicular to the longitudinal direction of the web 204 - similar to the U.S. Patent 4,344,011 , They can then be easier to produce. The electron emitter 203 is heated by an electric current flowing through it in the operating state, so that it can emit electrons.

Das im Betriebszustand erzeugte Feld zwischen Anode und Kathode greift in das Gesenk 202 und in die Durchbrüche zwischen den Stegen hinein. Ohne Kompensation würden die Äquipotentialflächen also in die Durchbrüche zwischen den Stegen des Elektronenemitters 203 hineingezogen, was zu den eingangs erläuterten negativen Konsequenzen führen würde. Um diese zu vermeiden, ist in dem Schlitz auf der Rückseite des Elektronenemitters 203 eine Elektrode 205 angeordnet, die ein gegenüber dem Elektronenemitter 203 negatives Potential führt.The field generated in the operating state between the anode and cathode engages in the die 202 and in the openings between the webs. Without compensation, the equipotential surfaces would thus be drawn into the openings between the webs of the electron emitter 203, which would lead to the negative consequences explained in the introduction. In order to avoid this, an electrode 205 is arranged in the slot on the rear side of the electron emitter 203, which leads to a negative potential with respect to the electron emitter 203.

Fig. 4 stellt ein stark vereinfachtes, schematisches Blockschaltbild einer Röntgeneinrichtung mit der erfindungsgemäßen Röntgenröhre dar. Diese ist an den Ausgang eines ersten Gleichrichters 91 angeschlossen, der eine Gleichspannung im Bereich zwischen 40 und 125 kV liefert. Eine zweiter Gleichrichter 92, an dessen positiven Ausgangsanschluss der Elektronemitter 203 und an dessen negativen Ausgangsanschluss die Elektrode 205 angeschlossen sind, liefert die einige kV betragende negative Vorspannung der Elektrode 205 gegenüber dem Elektronenemitter 203. Eine Heizstromquelle die den Elektronenemitter aufheizt, sodass daraus Elektronen emittiert werden können, ist der Einfachheit halber nicht dargestellt. Fig. 4 FIG. 2 shows a highly simplified, schematic block diagram of an X-ray device with the X-ray tube according to the invention. This is connected to the output of a first rectifier 91, which supplies a DC voltage in the range between 40 and 125 kV. A second rectifier 92, to whose positive output terminal the electron emitter 203 and to its negative output terminal the electrode 205 are connected, supplies the negative kV of the electrode 205 to the electron emitter 203. A heating current source heats the electron emitter so that electrons are emitted therefrom can not be shown for the sake of simplicity.

Die negative Vorspannung der Elektrode 205 gegenüber dem Elektronenemitter 203 ist nun so gewählt, dass sich auf der Vorderseite des Elektronenemitters - und zwar auch im Bereich der Durchbrüche zwischen den Stegen - ein annähernd ebener Verlauf der Äquipotentialflächen ergibt.The negative bias of the electrode 205 with respect to the electron emitter 203 is now chosen so that on the front of the electron emitter - and also in the region of the openings between the webs - an approximately flat course of Equipotential surfaces results.

Der die Röntgenröhre 100 speisende Röntgengenerator kann auch einen anderen Aufbau haben. Wesentlich ist, dass er eine (zusätzliche) Gleichspannungsquelle zur Erzeugung einer Gleichspannung zwischen Elektronenemitter 203 und Elektrode 205 enthält, die sich vorzugsweise proportional entsprechend der Hochspannung zwischen Anode und Kathode ändert. (Diese Bedingung wird von der in Fig. 4 stark vereinfacht dargestellten Schaltung mit dem mit seiner Primärwicklung 81 an einen Wechselrichter angeschlossenen Transformator 8, an dessen Sekundärväddungm 82 und 83 die Gleichrichter 91, 92 angeschlossen sind, nur mit Einschränkungen erfüllt.)The x-ray generator feeding the x-ray tube 100 may also have a different structure. It is essential that it contains an (additional) DC voltage source for generating a DC voltage between electron emitter 203 and electrode 205, which preferably changes proportionally in accordance with the high voltage between the anode and the cathode. (This condition is taken from the in Fig. 4 simplified circuit shown with the connected with its primary winding 81 to an inverter transformer 8, at the Sekundärväddungm 82 and 83, the rectifier 91, 92 are connected, met only with restrictions.)

Da der Elektronenemitter den Durchgriff des zwischen ihm und der Elektrode bestehenden elektrischen Feldes behindert, muss das elektrische Feld an der Rückseite des Elektronenemitters stärker sein als an der Vorderseite. Um wie viel es stärker sein muss, hängt von der Dicke der Stege (das sind die in der Zeichenebene von Fig. 3 horizontal verlaufenden Abmessungen), von ihrer Breite und von ihrem gegenseitigen Abstand ab. Eine Möglichkeit, um den Durchgriff des von der Elektrode 205 erzeugten elektrischen Feldes auf die Vorderseite des Elektronenemitters zu verbessern, besteht darin, die Seitenflächen der einzelnen Stege des Elektronenemitters abzuschrägen, so dass diese sich zur Elektrode 203 hin verjüngen bzw. die Durchbrüche sich zur Elektrode hin erweitern.Since the electron emitter impedes the penetration of the electric field existing between it and the electrode, the electric field at the backside of the electron emitter must be stronger than at the front side. How much it needs to be stronger depends on the thickness of the webs (these are the ones in the drawing plane of Fig. 3 horizontally extending dimensions), from their width and from their mutual distance. One way to improve the penetration of the electric field generated by the electrode 205 on the front of the electron emitter is to bevel the side surfaces of the individual webs of the electron emitter, so that they taper towards the electrode 203 or the openings to the electrode expand.

Wenn die elektrische Feldstärke hinter dem Elektronenemitter genauso groß ist, wie die elektrische Feldstärke vor dem Elektronenemitter, dann ist zwar eine vollständige Kompensation der durch die Durchbrüche im Elektronenemitter hervorgerufenen Felche-zernmgen nicht möglich, jedoch ergibt sich immer noch ein positiver Effekt. Bei einem Wert von weniger als 20% der Feldstärke auf der Vorderseite ist die negativ vorgespannte Elektrode 205 praktisch wirkungslos.If the electric field strength behind the electron emitter is just as large as the electric field strength in front of the electron emitter, then complete compensation of the fissures caused by the apertures in the electron emitter is not possible, but there is still a positive effect. At a value of less than 20% of the field strength on the front, the negatively biased electrode 205 is practically ineffective.

Die Wirkung der Erfindung im Vergleich zu anderen Ausführungsformen einer Elektrodenanordnung ergibt sich aus den Fig. 5a bis d. Die Darstellung in diesen Figuren ist dabei in der Weise verzerrt, dass der Maßstab für die vertikalen Abmessungen mehrfach so groß ist wie der Maßstab für die horizontalen AbmessungenThe effect of the invention compared to other embodiments of an electrode arrangement results from the Fig. 5a to d. The illustration in these figures is distorted in such a way that the scale for the vertical dimensions is several times as large as the scale for the horizontal dimensions

Fig. 5a zeigt die Elektronenbahnen bei einem helixförmig gewickelten Draht 203' als Elektronenemitter (dessen Querschnitt wegen der verzerrten Darstellung ellipsenförmig erscheint). Der Verlauf der Elektronenbahnen hängt davon ab, an welcher Stelle des Elektronenemitters die Elektronen austreten. Trotz Fokussierung (nicht näher dargestellt) treffen sich die Elektronen daher in einem vergleichsweise großen Querschnitt. In Fig. 5b sind dem die Verhältnisse bei einem idealen Flächenemitter gegenübergestellt. Alle Elektronenbahnen starten senkrecht zur Oberfläche des Emitters, bis sie sich unter der Wirkung eines fokussierenden Feldes in einem Brennfleck minimaler Größe treffen. Fig. 5a shows the electron trajectories in a helically wound wire 203 'as an electron emitter (whose cross section appears elliptical because of the distorted representation). The course of the electron orbits depends on where the electron emitters the electron emit. Despite focusing (not shown in detail), the electrons therefore meet in a comparatively large cross-section. In Fig. 5b are compared with the conditions in an ideal surface emitter. All electron orbits start perpendicular to the surface of the emitter until they meet under the effect of a focusing field in a focal spot of minimum size.

Fig. 5c zeigt die Verhältnisse bei einem realen mäanderförmigen Elektronenemitter. In den Randbereichen der Stege des Elektronenemitters sind die Elektronenbahnen gekrümmt, was trotz der Fokussierung zu einer Vergrößerung des Brennflecks (im Vergleich zum idealen Flächenemitter) führt. Allerdings ist der Brennfleck deutlich kleiner als bei einem helixförmig gewickelten Elektronenemitter. Fig. 5c shows the conditions in a real meandering electron emitter. In the edge regions of the webs of the electron emitter, the electron paths are curved, which, in spite of the focusing, leads to an enlargement of the focal spot (in comparison to the ideal surface emitter). However, the focal spot is significantly smaller than in a helically wound electron emitter.

Fig 5d zeigt die Verhältnisse bei der erfindungsgemäßen Kathodenanordnung mit einer negativ vorgespannten Elektrode auf der Rückseite des mäanderförmigen Elektronenemitters. Die Elektronen werden auf zunächst senkrecht zum Elektronenemitter verlaufenden Bahnen beschleunigt, um danach im Brennfleck fokussiert zu werden. Die Verhältnisse sind nicht ganz so günstig wie bei Fig 5b, jedoch deutlich besser als bei den flachen, mäanderförmigen Emitter ohne die Elektrode (Fig. 5c). Der Schlitz ist mit Vorsprüngen 206 versehen, die die Ränder der Elektrode, deren Abmessungen größer sind als die des Elektronenemitters, abschirmen. Fig. 5d shows the conditions in the cathode assembly according to the invention with a negatively biased electrode on the back of the meandering electron emitter. The electrons are accelerated to initially perpendicular to the electron emitter paths, to be focused in the focal spot afterwards. The conditions are not quite as favorable as at Fig. 5b , but significantly better than the flat, meandering emitter without the electrode ( Fig. 5c ). The slot is provided with protrusions 206 which shield the edges of the electrode whose dimensions are larger than those of the electron emitter.

Claims (8)

  1. An X-ray tube which includes an anode (1) and a cathode arrangement (2) that includes a cathode cup (201) for electron focusing, a flat electron emitter (203) that is provided with openings, and an electrode (205) that is arranged on the side of the electron emitter that is remote from the anode, characterized in that the electrode carries a negative potential relative to the electron emitter in the operating condition of the X-ray tube, which negative potential is so high that the value of the electric field strength in the space between the electrode and the electron emitter amounts to at least 20%, but preferably at least 100%, of the value of the field strength on the side of the electron emitter that faces the anode.
  2. An X-ray tube as claimed in claim 1, characterized in that the electron emitter (203) has a meandering shape.
  3. An X-ray tube as claimed in claim 1, characterized in that the openings have a cross-section that becomes wider in the direction of the electrode.
  4. An X-ray tube as claimed in claim 1, characterized in that the electrode (205) and the electron emitter (203) are plane.
  5. An X-ray tube as claimed in claim 4, characterized in that the electron emitter (203) is situated in a slit (204) in the cathode cup (201) whose upper surface is flush with the surface of the electron emitter (203) that faces the anode, and that the cathode cup and the electron emitter carry at least approximately the same potential.
  6. An X-ray tube as claimed in claim 1, characterized in that it includes a quadrupole unit (6) for controlling the size and/or the position of the focal spot formed on the anode.
  7. An X-ray device which includes an X-ray generator and an X-ray tube as claimed in claim 1, characterized in that the X-ray generator includes a voltage source (83, 92) for generating a potential on the electrode (205) that is negative relative to the electron emitter (203).
  8. An X-ray device as claimed in claim 7, including a high-voltage generator for generating a voltage between the anode and the cathode in the operating condition of the X-ray tube, characterized in that the voltage source (83, 92) is coupled to the high-voltage generator (81, 82, 91) in such a manner that a fixed ratio that is independent of the voltage between the anode and the cathode exists between the voltages delivered by the high-voltage generator and the voltage source.
EP01000176A 2000-05-24 2001-05-22 X-ray tube with a flat cathode Expired - Lifetime EP1158562B1 (en)

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DE10025807A DE10025807A1 (en) 2000-05-24 2000-05-24 X-ray tube with flat cathode

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DE50114206D1 (en) 2008-09-25
US20020009179A1 (en) 2002-01-24
DE10025807A1 (en) 2001-11-29
US6556656B2 (en) 2003-04-29
EP1158562A1 (en) 2001-11-28

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