US2703373A - X-ray tube - Google Patents

Description

`March l, 1955 H. R. cuMMlNGs 2,703,373l

X-RAY TUBE Filed June 21, 1949 v @M @new 7a/24%, :1%

United States Patent() X-RAY TUBEr Harold R. Cummings, Waterford, Wis., assigner to General Electric Company, a corporation of New York Application June 21, 1949, Serial No. 100,364 7 Claims. (Cl. 313-149) The present invention relates in general vto electron ilow devices, and has more particular reference to ow dev1ces, including X-ray generators, especially those adapted for high voltage operation.

Devices of the character mentioned may comprise an electron emitting cathode and a cooperating anode enclosed in a sealed envelope of glass. Such electron ilow devices operate as the result. of application of electrical potential difference between the anode and the cathode for the purpose of driving electrons, emitted by the cathode, toward and causing the same to mpinge'upon the anode. Where the flow device is constituted as an X-ray generator, X-rays are produced at the anode in response to the impingement thereon of electrons emitted by the cathode. Some of the cathode emittedrelectrons may deviate from the desired path, between cathode land anode, and impinge upon the envelope walls instead of on the anode, thus subjecting thev walls of the envelope to electrostatic charges of greater or lesser intensity, depending upon the number of such stray electronswhich hit the envelope walls. Such charges may build up sufciently to cause rupture of the envelope, thus ending the useful life of the device. The higher the operatinganodecathode voltage employed, the greater is the` danger of thus rupturing the envelope, the incidence of envelope rupture increasing markedly in devices designed for ultra high voltage operation.

In the interests of operating eciency, X-ray tubes arev desirably designed for operation at as high a voltage as possible, subject, of course, to limits determined by the electrical strength against rupture of the material embodied in the envelope. At the same time, tube dimensions are desirably maintained at minimum values in the interests of size, weight, andv consequent cost of the tube, its enclosing casing, and tube supporting mechanism for mounting the same in operative position. These considerations each tend to mutually limit theother.l A maximinn size limitation very definitely limits-the anode-cathode voltage at which the device may be operated without undue danger of envelope rupture; and, conversely, a required high operating voltage imposes a minimum limit below which the size of the device-may not be reduced without danger of break-down and consequent short service life of the device.

An important object of the present invention 1s to pro- -vide an electron ilow device having means incorporated therein for preventmg envelope rupture when the device is operated at unusually high voltage between anodeand cathode, thereby materially .improving the eiciency and X-ray output capacity of an X-ray tube embodying the present invention, without correspondingly increasing the size of the device. .p .l

A further object of the invention is to thus increase the operating capacity of the device by applying a resinous coating upon the outer surfaces of the glass envelope, whereby to increase the over-all dielectric strength ofthe envelope and its resistance to electrostatic puncture.

Another important object is to employ an outer. resinous coating baked upon,` and thus vintegrated with, the material of the glass envelope; afurther object being to employ a phenolic type resin. i

Another important object is to incorporate ai conducting material, such as graphite, distributed uniformly throughout the resinous envelope coating in quantities tol provide the coating with a resistance characteristic of from 500 to 1,000 megohms per square, whereby to .constitute the resinous coating as a conductor of relatively 2,703,373 Patented Mar. 1, 1955 ICC high resistance value well adapted continuously to drain ott or dissipate such electrostatic charges as may build up on the envelope, the conductive character of the coating thus preventing the building up of electrostatic voltages at any point on the outside of the tube, thereby eliminating high local voltage gradients or peaks on the tube, such as may be caused by the particular configuration of a tube enclosing casing, or auxiliary equipment, near which the tube is mounted for operation.

The foregoing and numerous other important objects, advantages, and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment of the invention.

Referring to the drawings:

Fig. l is a sectional view taken longitudinally through an X-ray tube provided with a conductive coating in accordance with the present invention;

Fig. 2 is a perspective view of a contact clip for making electrical connection with the conductive coating, as shown in Fig. l; and

Fig. 3 is a perspective view of the coated tube.

To illustrate the invention, the drawings show an X-ray tube 11` comprising an anode 12 and a cooperating cathode 13 enclosed in a sealed envelope 14 comprising glass.

The anode 12 in the illustrated embodiment comprises a rotating structure suitably journaled on a stem 15 sealingly supported on and extending outwardly of the envelope, at one end thereof, for connection with a suitable. high voltage power source for applying operating power between anode and cathode. The rotary anode 12 provides a preferably annular target 16 in position opposite the cathode, to receive the impingement of electrons emitted by the cathode during the operation of the with a pocket 18, in which is mounted an electron emission filament 19 in position, when energized, to emit electrons in the direction of the target 16. The lament, at its opposite ends, may be mounted on, electrically connected to, and supported by suitable conductor means 20, which may extend outwardly of the envelope, at the end thereof remote from the anode, for connection with a suitable source of filament energizing power, and with the high voltage source of anode-cathode operating power.

The anode and cathode structures are respectively supported at the opposite ends of the envelope 14, which is preferably of generally cylindrical configuration. The anode and cathode structures may be like those illustrated and described in U. S. Patent No. 2,336,769, of December 14, 1943, on the invention of Z. I. Atlee; and the anode` and Cathode structures may be sealed in the envelope 14 as taught more particularly in U. S. Patent No. 2,311,725, of February 23, 1943, on the invention of Z. I. Atlee.

It should be understood, of course, that an X-ray tube functions to generate X-rays as a result of the impingement on the anode target 16 of electrons emitted by the cathode filament 19. X-rays produced at the target 16 may pass therefrom outwardly through a Window 21, comprising an area of reduced thickness formed in the wall of the envelope 14, on one side thereof, and opposite the target 16.

In order to rotate the anode 12, the stator 22 of an induction motor may be mounted around the envelope opposite the body of the anode, which thus acts as a rotor within the stator 22, torque being developed in the rotor through the envelope wall in response to energy suppliedl to the stator from a suitable power source.

It should be understood, of course, that X-ray generators are normally enclosed in grounded, shock-proof casings, as a safety measure. These casings are ordinarily of metal and relatively closely enclose the X-ray tube which is mounted therein. The presence of such an enclosing. casing, or of other metal structures near the tube if the same be not cased, creates conditions resulting in the establishment of localized electrostatic charges on the envelope in various areas, depending upon the shape and configuration of the surrounding casing, or adjacent metal structures. These charges in part are due to irnpingement of stray electrons upon the inner surface of the envelope and frequently result in the formation of minute hair-like puncture of the envelope at the place therein where such internal and external electrostatic charges accumulate, this being particularly the case where the operating potential between anode and cathode is relatively high.

The phenomenon of electrostatic envelope puncture appears to take place in a direction inwardly from the outer surface of the envelope. lt is apparently due to the development of electrostatic voltage peaks as a result of stray electron impingement on the inner surfaces of the envelope, in combination with conditions established by external metal bodies, such as the enclosing tube casing, disposed near the tube. Puncturing appears to occur at or near the envelope area which is in closest proximity rounds the envelope and extends from near the cathode end of the envelope to near the anode end thereof, including the envelope portions encircled by the stator 22. The coating 23 preferably comprises a resinous substance capable of being integrated with the glass of the envelope, as by applying the resin to the envelope in fluid form as a coat of substantially uniform thickness and then baking the resinous coating in order to solidify it in place and integrate the same with the envelope material. To this end, it is preferable to utilize a coating material comprising a heat hardening and curing resin adapted in cured condition to adhere, in preferably substantially integral fashion, with the glass on which coated, a suitable resin for the purpose being a phenolic reaction product known to the trade as Rescon With the resin is uniformly incorporated a conducting material, such as graphite, in amorphous form and uniformly distributed throughout the resin, the conducting material being present preferably in quantities such that the final coating has a resistance of the order of 500 to 1,000 megohms per square. Accordingly, the coating is thus preferably constituted as a conductor of relatively high resistance value, being sufliciently conductive only to drain off slowly any electrostatic charges that may be set up in localized areas of the envelope, to thereby prevent accumulation of electrostatic voltage effects in any portion of the envelope in sufficient intensity to cause puncture of the envelope. In order to thus drain olf electrostatic charges and thereby control the voltage gradient across the tube, conductor clips 24 may be provided to electrically connect the coating with the core of the stator v 22, said core being grounded, as on the tube enclosing casing. As a consequence, any electrostatic voltage tending to build up in any localized area on the envelope will be dissipated through the semi-conductive coating 23 and through the contact clips 24, thereby controlling the voltage gradient across the tube and maintaining the same more or less uniform.

By way of comparison, it has been found that tubes constructed with conductive coatings, as taught in the present invention, can be operated continuously and successfully, without increased puncture incidence, at voltages of the order of 50% higher than identical tubes omitting the high resistance coating. By providing the conductive coating, therefore, tubes may be operated at appreciably higher voltage, with the many advantages deriving therefrom, without increasing the dimensions of the tube. Conversely, it is possible, if necessary or desirable, to make a tube of physically smaller size for successful puncture-free operation at the same operting voltage for which uncoated tubes of larger physical size have heretofore been employed.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and, it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts Without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

1. A device of the character described having an anode, a cathode, an enclosing glass envelope, a coating on and in intimate electrical contact with said envelope and forming an electrical conducting layer on the outer surfaces thereof, and contact clip means for electrically connecting the layer to ground.

2. A device of the character described having an anode, a cathode, and enclosing glass envelope, a coating on and in intimate electrical contact with said envelope and forming an electrical conducting layer on the outer surfaces thereof, said coating comprising a thermo-plastic material in heat hardened condition, and contract clip means for electrically connecting the layer to ground.

3. A device of the character described having an anode, a cathode, an enclosing glass envelope, a resinous coating on and in intimate electrical contact with said envelope and forming an electrical conducting layer on the outer surfaces thereof, said resinous coating comprising a phenolic resin in heat hardened condition on and integrated with the material of the envelope, and contact clip means for electrically connecting the layer to ground.

4. A device of the character described having an anode, a cathode, an enclosing glass envelope, a resinous coating on and in intimate electrical contact with said envelope and forming an electrical conducting layer on the outer surfaces thereof, said resinous coating embodying electrical conducting material in finely divided condition and uniformly distributed throughout said coating, and contact clip means for electrically connecting the layer to ground.

5. A device of the character described having an anode, a cathode, an enclosing glass envelope, a resinous coating on and in intimate electrical contact with said envelope and forming an electrical conducting layer on the outer surfaces thereof, said resinous coating embodying graphite in finely divided condition and uniformly distributed throughout said coating, and contact clip means for electrically connecting the layer to ground.

6. A device of the character described having an anode, a cathode, an enclosing glass envelope, a resinous coating on and in intimate electrical contact with said envelope and forming an electrical conducting layer on the outer surfaces thereof, said resinous coating contains electrical conducting material uniformly distributed throughout the coating to constitute the same as an electrical conducting layer having electrical resistance of the order of 500-1000 ohms per square, and contact clip means for electrically connecting the layer to ground.

7. A device of the character described having an anode, a cathode, and an enclosing glass envelope, said anode being mounted for rotation within the envelope, a motor stator surrounding a portion of the envelope for turning the anode, a coating adhering upon and in intimate electrical contact with the outer surfaces of said envelope and forming an electrical conducting layer of relatively high resistance thereon, and contact clip means secured on and Within said stator and thereby held in contact with said layer and connected to said stator.

References Cited in the file of this patent UNITED STATES PATENTS 1,626,693 Rentschler May 3, 1927 1,685,928 Morrison Oct. 2, 1928 2,024,332 Bouwers Dec. 17, 1935 2,121,630 Gross et al June 21, 1938 2,256,229 Atlee et al Sept. 16, 1941 2,307,027 Davie et al. Ian. 5, 1943 2,343,630 Atwood, Ir. Mar. 7, 1944 2,449,397 Lamphere Sept. 14, 1948 2,516,663 Zunick July 25, 1950

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