X-ray tube with improved control electrode arrangement

[57] ABSTRACT

Electron emissive device such as an x-ray tube having a cathode for generating electrons and an anode for receiving the same is provided with an auxiliary electrode insulated from the focusing cup electrode and positioned behind the cathode filament. A positive potential may be applied to such auxiliary electrode relative to the filament so that such auxiliary electrode attract electrons and prevent their flow to the anode.

Claims, 6 Drawing Figures

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PATENTED JAN 11974

3,783,333

SHEET ,1 OF 2

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PATENTED·)* ΉΜ

3,783.333

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3,783,333

X-RAY TOBE WITH IMPROVED CONTROL ELECTRODE ARRANGEMENT

BACKGROUND OF THE INVENTION

In electron emissive devices such as x-ray tubes, utilizing tungsten filaments, it has been possible to maintain the electron emissive level by simply changing the current level supplied to the filament. The response of a pure tungsten filament is rapid enough both in heating and cooling and thus in emission to give the control which is desired. Where additional control has been necessary, conventional wire grids positioned between the anode and the cathode have been utilized in some circumstances. However, the grid wires distort the electron beam which is undersirable in many circumstances such as, for example, in the production of x-rays where an anode focal spot of regular confirmation and even distribution of electrons thereover is desired.

Thoriated tungsten filaments have many desirable features and many efforts have been made to utilize them in x-ray tubes and other electron emissive devices. In come circumstances the thoriated tungsten filaments are adaptable, but in others, such as x-ray tubes, they are generally undesirable under control arrangements which have heretofore been available. Since thoriated tungsten filaments are operated at a lower temperature than pure tungsten filaments the cooling rate of a thoriated tungsten filament is slower than that of a tungsten filament and thus control of emission level by a decrease in filament current is frequently too slow in the case of a thoriated tungsten filament to permit its use. The use of a grid for control is undesirable for the reasons outlined above.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with th present invention, an x-ray tube or similar electron emissive device having an electron generating cathode filament, a focusing cup electrode and an anode is provided with an auxiliary electrode that is positioned behind the cathode filament with respect to the anode. By applying a positive potential to such auxiliary electrode, that is, positive with respect to the filament, electrons can be drawn from the filament to the electrode thus reducing the number of electrons which flow from the cathode to the anode. Conversely, reducing the potential will permit more electrons to flow to the anode. The auxiliary electrode thus provides a means for controlling the level of electron flow to the anode.

The auxiliary electrode arrangement can also be utilized to control operation of x-ray tubes having dual filaments which are provided in order to produce focal spots of different sizes on the anode. In such tubes, it is frequently desirable to place a bias upon the cathode itself in order to cut off flow of electrons from the filament to the anode. X-ray tubes have been provided heretofore with cathodes having two filaments, one being longer than the other and arranged so as to form on the anode, depending upon the filament which is operated, a large or a small focal spot. The larger focal spot is utilized, for example, for spot film radiography whereas the small focal spot is utilized for fluoroscopic examination. In some instances it is desirable to be able to switch rapidly from one filament to another. In accordance with a modified embodiment of the present invention, such rapid switching is enabled by constructing a cathode with two filaments mounted in electri cally insulated focusing cup eletrodes. This enables both filaments to be operated continuously and the electron flow from the respective filaments cut off by selectively applying a negative bias to the respective focusing electrode, thereby cutting off the flow by reason of the space charge effect accross the mouth of the focusing cup recess. Each cathode portion may also be provided with an auxiliary control electrode behind the filament such as discussed above.

Other objects and advantages of the invention will become more apparent hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation of an x-ray tube constructed in accordance with the invention;

FIG. 2 is a fragmentary view of the anode target looking in the direction of the arrows 2—2 of FIG. 1;

FIG. 3 is an enlarged end view of the cathode head taken generally along the lines 3—3 of FIG. 1;

FIG. 4 is a sectional view taken substantially along the lines 4—4 of FIG. 3;

FIG. 5 is a view similar to FIG. 3 of further embodiment of the invention; and

FIG. 6 is a sectional view taken substantially along lines 6—6 of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is illustrated in connection with an x-ray generator comprising an envelope 10 of glass or other suitable material, having a cathode unit 12 suitably mounted in one of the envelope and a rotary anode unit 14 suitably mounted in the other end of the envelope.

The anode unit 14 comprises a rotating target element 16 preferably formed of a metal of tungsten or other refractory metal. The anode 16 is provided with a frusto-conical target surface 18 defining a target area for the reception of electrons upon superimposed focal spots 22, 24 respectively.

Turning now more particularly to FIGS. 3 and 4, the cathode 12 therein illustrated comprises a metal body 30 which is supported within a tubular nonconductive sleeve 32 supported from the envelope 10. The body 30 is formed within an indented face adjacent the end facing the anode unit 14 defined by a pair of planar face portions 34—36. The face portion 34 has an elongated focusing cup recess 38 formed therein and the face portion 36 has a shorter focusing cup recess 40 formed therein. Mounted within the recess 38 is a filament 42 for generating electrons which are focused upon the anode target portion 18 so as to form the focal spot 24 thereon. Mounted within the smaller recess 40 is a shorter filament 44 for generating electrons which are focused upon the target portion 18 to form the focal spot 22 thereon. The focusing cup recesses 38-44 are each formed with a relatively narrow bottom slot or gap 46,48 respectively, extending through the back face of the body 30 which in this instance is formed in a cup shape with a hallowed out back portion 50. Leads for connecting the filaments to a source of electrical energy are provided including a lead 52 connected to one end of the filament 42 and a lead 54 connected to the corresponding end of the filament 44. The leads extend through circular openings formed at the opposite ends of the slots 46,48 from the cups to the back face of the body 30. The opposite ends of the filament 42,44 are interconnected to a common lead 56. Thus the fila

3,783,333 merits 42, 44 can be selectively energized to generate electrons for focusing on the respective portions of the anode target.

Mounted within the hollowed out base back portion 50 is an auxiliary electrode 60 comprising a body made of conductive material such as metal, the body being mounted in closely spaced but electrically insulated relation with respect to the cathode body 30. The body 60 is cylindrical and is secured to the body 30 by suitable means such as a screw 62 threaded into the body 30 and extending through an axial opening 64 in the body 60, but insulated therefrom by washers 66. The screw 62 provides a means for connecting the body 30 to a source of electrical potential through a lead 68 which is secured to the screw by suitable means. Means are provided for connecting the electrode 60 to a source of electrical potential comprising a lead 70 secured in position by a setscrew 72. It is to be understood that all such leads extend outwardly from the envelope 10 through a suitable conventional arrangement for connection to sources of electrical energy and potential such as shown in U. S. Pat. No. 2,862,107 of H. R. Cummings granted Nov. 25, 1958.

As will be understood by those skilled in the art, upon energization of a filament 42 or 44 and the application of potential between the cathode body 30 and the anode 14, electrons will be accelerated from the energized filament to the anode. In accordance with the invention the flow of electrons from an energized filament to the anode can be controlled by changing a positive voltage (with respect to the energized filament) applied to the auxiliary electrode 60. The application of such a potential to the electrode will cause some of the electrons which are being boiled off the filament to be attracted to the auxiliary electrode thus diminishing the flow of the electrons to the anode. By increasing or decreasing the potential of the electrode 60, the number of electrons diverted from the flow to the anode can be changed. Since the electrode 60 is out of the high voltage field of the anode, the potential applied to the electrode will not affect the focal spot size or electron distribution thereover.

This invention has particular advantage in the operation of x-ray tubes in which the filaments are thoriated tungsten. Such filaments are operated at such a low temperature that the usual control by decrease of the current applied to the filament can not be utilized since the cooling rate of the filament is too slow. Control by means of the application of potential to the auxiliary electrode 60 on the other hand is instantaneous and thus the emission level of the filament to the anode can be maintained substantially uniform.

As will be apparent, in the embodiment of FIG. 3 and FIG. 4 the filaments 42-44 can be operated independently of each other only by selectively applying power to the filaments. Thus rapid switching from one focal spot to another is difficult, particularly with thoriated tungsten filaments because of the slow response time of the filaments. Moreover, control of emission by applying a bias to the cup 30 is difficult in the configuration of FIGS. 3 and 4 inasmuch as the application of a bias sufficient to cut off emission from the large filament 42 will place stress across the small gap 48 in the smaller cup 40. These problems are alleviated by the cathode structure shown in FIGS. 5 and 6. The cathode structure therein shown comprises a cup comprising two separate semi-cylindrical sections 80, 82 suitably sup ported from the sleeve 32 and insulated from one another by a diametrically extending insulator 84. The cup section 80 has an elongated focusing cup 86 formed therein having a focusing slot 88 in the bottom thereof with a pair of openings 90, 92 for the passage of lead wires 94, 96 connected to the opposite ends of a filament 98. The section 82 is provided with a shorter focusing cup 100 in the surface thereof facing the anode 14 in the bottom of which cup is provided a focusing slot 102 and suitable openings for passage of leads 104,106 connected to the opposite ends of a filament 108. The rear surfaces, that is the surfaces of the cup sections 80, 82 remote from the anode 14, are coplanar. Mounted closely adjacent to such surfaces, but spaced therefrom, are a pair of auxiliary electrodes 110, 112, each of which is semi-circular in configuration and which are separated by a diametric insulator 114. The electrodes 110,112 are secured to the corresponding electrodes 80, 82, respectively, by means of bolts 116, 118, respectively, which are in electrical contact with the respective cup section but are insulated from the respective electrode by means of insulators 120. An electrical lead 122 is connected to the bolt 116 to provide means for connecting the cup section 80 to a source of electrical potential and lead 124 is connected to the bolt 118 to provide means for connecting the cup section 82 to a source of electrical potential. The various filament leads 94, 96,104,106 extend outwardly through the electrodes 110, 112, each filament lead being encased within a tubular insulator 126, which in turn is surrounded by a metal sleeve 128. Means are provided for connecting the electrodes 110, 112 to source of electrical potential, the plate 110 being connected to a lead 130, extending into the plate and held in place therein by a setscrew 132. The plate 112 is connected to a lead 134, held in place by a setscrew 136.

In the operation of the present device the filaments 98,108 can be simultaneously energized. If it is desired to secure a focal spot from the emission from the filament 98 sufficient bias is applied to the cup section 82 to suppress the emission from the filament of 108. If it is then desired to secure emission from the focal spot generated by emission from the smaller filament 108, the bias can be instantaneously switched from the cup section 82 to cup section 80 thus suppressing emission from the filament 98 while permitting emission from the filament 108 to pass to the anode.

The auxiliary electrodes 110, 112, function to control the amount of available electrons from each of the filaments in a manner similar to the control electrode 60. Thus, if the filament 98 is operating to generate an electron beam focused upon the anode 14, when the desired emission level from the filament is reached a suitable power supply circuit will be energized to apply a positive potential to the auxiliary electrode 110. This will cause electrons to be drawn from the filament 98 through the openings 90 to the plate 110, thereby maintaining the electron flow to the anode at the desired level. In similar fashion when the cup 80 is biased to cut off the emission from the filament 98 and the cup 82 is neutral thereby permitting emission of electrons 108 to the anode 14, the auxiliary electrode 112 can be utilized to control the flow of electrons to the anode and cut off excessive flow when the desired level is reached. That is, when a desired level of current flow through the tube is sensed in the suitable control cir3,783,333 cuit, a voltage will be applied to the auxiliary electrode 112 to attract electrons from the filament 108 through the openings for the lead wires 104, 106.

Having illustrated and described preferred embodiments of the invention, it will be apparaent it permits of modification in arrangement and detail.