US1946288A - Electron discharge device - Google Patents

Description

Feb. 6, 1934. w KEARSLEY 1,946,288

ELECTRON DISCHARGE DEVICE Original Filed Sept. 19. 1929 Inventor: William K. Kearsley.

H is Att. ornqy.

Patented Feb. 6, 1934 1,946,288 ELECTRON DISCHARGE DEVICE William K. Kearsley,

Schenectady, N. Y., as-

signor to General Electric Company, a corporation of New York Original application September 19, 1929, Serial No. 393,819. Divided and this application May 12, 1932. Serial No. 610,866

5 Claims.

The present application is a division of my ap-' plication Serial No. 393,819, filed September 19,

The present invention relates to the focussing of an electron beam by a magnetic field. It comprises in particular an arrangement whereby the focussing of the electrons on the anode is automatically broadened or sharpened in accordance with the energy being expended so as not to damage the anode or other part upon which the electron beam impinges.

Heretofore, it has been common practice to so construct X-ray tubes so that the electrons, or cathode rays, impinge on the anode, or target, on a spot of fixed area, known as the focal spot.

Only a certain amount of energy input can be safely used for a given size of focal spot as otherwise the metal of the anode becomes fused or even vaporized, thereby injuring or even destroying the X-ray tube. To secure sharp definition in radiographs the size of the focal spot must be small. When large X-ray output is desired some sacrifice in definition is required as a larger focal spot necessarily must be used to avoid overheating.

As the requirements of the radiographer in definition of radiographs and intensity of X-ray production, are variable, a well-equipped radiographer heretofore kept in stock ready for use several difierent kinds of X-ray tubes, some constructed to operate with a small focal spotand others with a larger focal spot.

As a consequence of my invention, I have provided an improved X-ray equipment whereby the size of the focal spot is automatically determined so that with an increase in energy the size of the focal spot is increased in corresponding degree and vice versa, thereby always securing for the use of the radiographer the most favorable focussing conditions.

. In accordance with the invention covered by the present divisional application, the focussing of the beam of electrons is varied by a magnetic field which in turn is responsive to the flow of electrical energy through the electron tube. As will be hereinafter more fully explained, an increase of electric energy flowing through the electron tube will result in an automatic broadening of the focus of the electron beam and conversely a decrease of such energy will result ma sharpening of such focus.

In the accompanying drawing, Fig. 1 is a diagram of an embodiment of my invention which includes a thermionic X-ray tube which is provided with a magnetic focussing coil and Fig. 2

is an enlarged detail view showing in section the cathode portion of the X-ray tube shown in Fi 1.

In the system illustrated in Fig. 1, a magnetic focussing coil 3 surrounds the X-ray tube 4 and is positioned about the path of the electrons passing from the cathode to the anode to cause focussing of the space current flowing in the X-ray tube. The X-ray tube here shown is of the type described in United States Patent 1,203,495 W. D. Coolidge. The cathode 5 and the anode 6 of the X-ray tube 4 are connected by the supply conductors 7, 8, to the secondary winding of a transformer 9, the primary winding of which is connected to the mains 10, 11, by the conductors 12 containing a switch 13. The magnetic coil 3 is connected to the secondary winding of the transformer 15, the primary of which is connected to the mains 10, 11, in series with the variable resistor 16. Included in the supply circuit 17 of the winding 3 is the secondary of a transformer 18. The primary of this transformer 18 is connected in series with the secondary of the high potential supply transformer 9 by the conductors 19 and therefore is traversed by current flowing through the X-ray tube. The secondary of the transformer 18 is connected in opposition with the secondary of the transformer 15 so that an increase of current through the X-ray tube produces a decrease of current through the focussing coil 3. The cathode filament 5 of the X-ray tube is supplied by the conductors 20, 21 with heating current from the secondary winding of a transformer 22 supplied from the mains 10, 11 through a variable resistor 23.

The cathode of the X-ray tube, as shown in detail in Fig. 2 comprises a spiral filament 5, consisting preferably of tungsten, which is connected at one end to a conductor 25 and at its opposite end to a ring 26 carried by a conductor 27. The cathode conductors 25, 2'7 are sealed into a stem 28 of glass or the like and by electric contact devices (not shown) are connected to the conductors 20, 21, Fig. 1. Surrounding the cathode filament 5 is a focussing device 29 consisting of a dished tubular member which is supported by the conductors 30, 30'. These parts are mounted in turn on a split metal tube 31 which is carried by the glass tube 32. The latter with the stem 28 constitutes a reentrant member in the cathode arm of the envelope of the X-ray tube 4.

When the current through the focussing coil 3 increases, the convergence of the electron stream or cathode rays isincreased, or in other words, the focus of the X-ray tube is-sharpened. Conversely, a decrease of current through the coil 3 broadens the focus of the X-ray tube. In the operation of the arrangement shown. in Fig. 1, when the current through the X-ray tube is increased, either by an increase of voltage of the supply lines 10, 11, or if the value of the resistance 23 is decreased, thereby heating the cathode to a' higher temperature, the consequent increase of current through the primary of the transformer 18 causes an increase in the voltage opposing the voltage impressed upon the secondary of the transformer 15 and produces a resultant decrease in the current flowing through the supply circuit 17 and the coil 3. This widens the focussing of the cathode rays and increases the focal area on the anode. It is apparent that the coil 3 should be positioned longitudinally with respect to the envelope of the X-ray tube to give the best focussing operation throughout the entire range of currents employed in the X-ray tube.

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

1. The combination of an X-ray device which is provided with a thermionic cathode, means for producing a magnetic field about said cathode for focussing an electrical discharge emanating therefrom, and means for automatically varying the strength of said field inversely in response to variations of current in said discharge device.

2. In combination, a thermionic discharge device, a coil for producing a magnetic field encircling the discharge path in said device, a main transformer having a secondary winding connected to said discharge device, an energizing circuit for said coil and a second transformer having a primary winding connected in circuit with the secondary .winding of said main transformer and having a secondary winding connected in series with the energizing circuit for said coil.

3. In combination, a vacuum tube having eleca in, a winding arranged to induce a magnetic field for exerting a focussing effect on the discharge in said tube, a main electric supply circuit for said tube, an electric supply circuit for said winding, and means in said supply circuit for said winding for varying the current in said winding inversely with the value of the current in said main supply circuit, whereby the focussing of said electron discharge is widened with an increase of current and sharpened with a decrease of current.

4. The combination of an X-ray tube which is provided with a thermionic cathode, an anode and an electrostatic focussing electrode surrounding said cathode, a magnetic coil encircling said tube about the path of an electron discharge therein, main energy supply conductors connected to the electrodes of said tube, auxiliary energy supply conductors connected to said coil and means responsive to the flow of current in said main conductors for reducing the current in said auxiliary conductors whereby the focal area on the anode of said tube is varied inversely with the value of the electron discharge therein.

5. An X-ray apparatus comprising the combination of an X-ray tube which is provided with a thermionic cathode, a magnetic coil encircling the discharge path of said tube and being arranged to exert a focussing effect upon an electron discharge in said tube, an electric alternatw ing current source connected to said X-ray tube, a transformer connected to derive current from said source and having a secondary winding connected to said coil, a second transformer also connected to derive current from said source and having a secondary winding connected in opposition to the secondary winding of said first-mentioned transformer.

WILLIAM K. KEARSLEY.

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