US2112746A - X-ray tube - Google Patents

Description

March 29, 1938. Y A. WOLFEL Y 2,112,746

X-RAY TUBE Filed April 16, 1935 Inventor:

ttorney.

Albrecht Wdlfel Patented Mar. 29, 1938 UNITED STATES X-RAY TUBE Albrecht Wolfe], Rudolstadt, Germany, assignor to General Electric Company, a corporation of New York Application April 16, 1935, Serial No. 16,673 In Germany May 15, 1934 1 Claim.

In order to adapt an X-ray tube to more than one purpose, for example, therapeutic and radiographic, which necessitate different load conditions, i. e. different sizes of focal spots, etc., one may provide the tube with two or more filamentary cathodes of difierent electron-emitting capacities. Obviously, the cathodes require different amounts of heat. Consequently, when changing the operation of the tube from one use to the other, as for example, when substituting the cathode of smaller electron-emitting or heating capacity for the larger cathode, and assuming that it is desired toenergize the smaller cathode from the same source of electromotive force as r the larger cathode, it is desirable to reduce the heating voltage impressed across the smaller cathode.

In order automatically to reduce the energy supply to the smaller filament under these conditions, it has' been proposed to insert a resistance in the heating current circuit of the oathode and in case the cathode is energized by an alternating current source, a choke coil or a transformer may be used. The choke coil, transformer or resistance is usually connected in the filament circuit external to the tube and in order to make the structure of a unitary design, it has been proposed to enclose the coil or resistance in the base cap of the tube. However, when the voltage-reducing device is enclosed within the base cap, certain difiiculties arise. In the first place, the cap on the cathode side of the tube must be enlarged to accommodate the extra element which may present some difficulty in fitting the tube and its cap into existing apparatus and protective housings. In addition, more especially when using a resistance, but also in the case of a choke coil or transformer in lesser degree, there is the further disadvantage in that the voltage-changing device may give off considerable heat and deleteriously affect the material of the cap.

An object of the present invention is to provide an improved all-purpose X-ray tube containing two or more filamentary cathodes of different electron-emitting or heating capacities, and in which one cathode may be substituted for the other cathode when desired, and is adapted to be energized from a common source of electromotive force. A more specific object is to em.- ploy in a tube of this character, an improved form of voltage-reducing device or element in connection with the cathode of smaller electronemitting or heating capacity and also an improved position of the device. In carrying out these objects, my invention contemplates in brief, the use of a resistance or inductance in series with the smaller filament, i. e. the cathode of lower heating capacity, and mounting the resistance or inductance within the confines of the envelope so as to be surrounded by an evacuated space in case the envelope is highly evacuated. The invention will be clearly understood when reference is made to the following description and the accompanying drawing in which Fig. 1 represents a view, partly in section, of an X-ray tube containing a cathode construction improved in accordance with the present invention; Fig. 2 is an end view of the focusing member and associated cathodes; while Fig. 3 is an enlarged diagrammatic view of the cathode structure per se including the voltage-reducing element.

Referring more particularly to Fig. l, numeral l designates the envelope of an electrical discharge device containing an anode 2, which in the case of an X-ray tube may have an inclined face and be provided with a target. The anode is supported Within the envelope in any suitable manner, for example, by means of a rigid rod 3 secured to a reentrant stem 4 which terminates one end of the envelope. The other end of the envelope is also provided with a reentrant stem 5 in which are fused several rigid rods serving as combined leading-in conductors and supports for the improved cathode structure. This structure includes filaments 6, 1, one of the filaments,

e. g. filament 6, being of greater heating capacity and therefore of greater electron-emitting capacity than the other filament. There may also be provided a hemispherical focusing member 8, supported within the envelope by means of a rod 9 which is secured to one of the conductors passing through the stem 5. The conductor 9 may be connected to one or both of the filamen tary cathodes or as is more usually the case, may be taken out through the envelope as a separate lead to which any desired potential may be applied.

The cathodes are preferably constituted of the same material, for example, tungsten wire of the same gage, and since one cathode is intended to emit greater amounts of electrons than the other cathode, it follows that the lengths of the respective cathodes are different.

When a tube of this character is operated, it is desirable to be able to switch from one cathode to the other in case the same tube is alternatively serving two Widely different purposes, as for example, when used in therapy and radiography. In case a common source of electromotive force is employed to energize both cathodes, obviously the voltage impressed across the cathode of smaller heating or electron-emitting capacity must be less than the voltage applied to the cathode of larger heating or electron-emitting capacity. The necessary reduction of voltage in case it is desired to substitute the smaller cathode for the larger cathode, is conveniently obtained by means of a resistance or choke coil l connected in series with the smaller filament 1. As shown, the adjacent ends of the filaments 6, I are connected together by a common leading-in conductor and the opposite end of the larger filament is connected to another leading-in conductor, while the free end of the element I0 is connected to the remaining leading-in conductor. The filaments are conveniently arranged side by side within the opening at the bottom of the spherical focusing member 8 as is shown more clearly in Fig. 2. When the filaments 6, l are energized from a common source I I of alternating or direct current, and when a switch I2 is provided in the circuit for selectively connecting the filament 6 or I. to the source, the same voltage may now be applied, without danger to the smaller filament 1, as isapplied to the larger filament 6. Obviously, the voltage actually impressed across the filament 1 represents a reduced voltage obtained by the effect of the resistance or inductance l0 which serves to dissipate or consume the energy of the undesired voltage. It is desirable when the element i0 is constituted of resistance, that the latter should be made of the same wire as the filaments. 6 and I. It is also apparent that the combined resistance of the filament 'l and the resistance II] should be substantially equal to the resistance 'of the filament 6. By placing the voltage-reducing element I0 within the envelope instead of being positioned external to the envelope and contained within a base cap at the cathode end of the tube, the element is in such a position as not to cause any deleterious heating efi'ects, nor does it necessitate a large cathode cap within which to be accommodated. It will be advantageous to mount theresistance close to the cathode, since in this case the heat generated in it will heat only such internal constructional parts of the tube as are heated by the cathode itself and are therefore designed to withstand the heat produced. Moreover, the improved position of the element I0 ofiers a unitary and compact construction.

In the event that the element I0 is required to dissipate or consume considerable energy and thereby is rendered sufiiciently incandescent to emit electrons, it may be desirable to provide a metal shield 13 interposed between the element and the anode. This shield may be given any suitable shape, e. g. an angular configuration in order to provide a mechanical barrier for electrons which tend to leave the end of the element Ill near the envelope. The shield may be supported within the envelope in any suitable manner.

In addition to the shield I3 and as shown in Fig. 3, there may be provided other angular members I4 positioned respectively between the free ends of the filaments 6, l and the envelope. All of these shields or screens serve to prevent any electrons from reaching the anode from undesired parts of either cathode or associated elements.

While I have described my invention more especially in connection with an X-ray tube for the reason that cathodes of different electron-emitting capacity are frequently employed in. this connection, it will be understood that the present invention is not limited to tubes of this type but is sufiiciently broad to embrace all forms of electrical discharge apparatus, highly evacuated or gas-filled, in which a plurality of filamentary cathodes are employed having different electrical characteristics and which are selectively energized from a common source of electrom'otive force.

What I claim as new and desire to secure by Letters Patent of the United States, is,

An electrical discharge device comprising an envelope containing an anode, a main cathode and a secondary cathode, said cathodes being arranged adjacent to one another, a voltage-reducingelement within said envelope and connected to said secondary cathode, the combined resistance of the secondary cathode and said element being substantially equal to the resistance of said main cathode whereby equal voltages may be applied to the main and secondary cathode circuits to produce equal heating currents, and a shield mounted adjacent to said voltage-reducing element for preventing radiation of electrons by said element but permitting radiation by the secondary cathode.

ALBRECHT WOLFEL.

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