US2890358A - X-ray tube - Google Patents

Description

June 9, 1959 H. R. cUMMlNGs 2,890,358

x-RAY TUBE Filed F b. 1956 igROLD R. CUMMINGS ATTORNEY United Harold'R-TCummings, Waterford,Wis., assignor to General Electric Company, a corporatipn or New York The' present invention relates in rgeneral to electronics and has more particular reference to electron liow devices such as X-ray generating tubes, especially those adapted for high voltage operation.v

Electron ilow devices of the character mentioned commonlycomprise an electron emitting; cathode and a cooperating anode enclosed in a sealed envelope of glass. Byfapplication, of an electrical potential difference between` the anode and cathode, electrons emitted by and at rthercathode vmay be caused to travel toward and to impinge upon the anode. Where the flow device is constitutedk as an YX-ray generatinggtube, the anodemay be conditioned as a source of X-rays as the result of electron` impingement thereon. When the device is in operation, someH of the cathode emitted electrons may deviate from the desired electron flow path between the cathodeand anode and impinge on the walls of the enclosing envelope, thus subjecting the same to electrou staticcharges of magnitude corresponding with the frequency of-stra-y electron impact upon the envelope walls. Such-charges may buildup sulticientlyto cause rupture of the envelope, thus ending the useful life'of the device. The incidence of such envelope rupture appearsto be a function 'of the operating voltage applied for electron driving purposes lbetween the anode and cathode of the device,` the-,higher the operating voltage the greater is the `likelihood of-envelope rupture.

arent Rotating anode- X-raygenerating tubes, in which the Y anode element is turnablymounted within the envelope, in a sleeve-likeanodeenclosing envelope portion, and constituted as the rotor of an induction motor, the assembly-including a motor stator encircling the sleeve-like envelope portion and the rotor enclosed therein, are especiallylsusceptible to envelope puncture, particularly in theanodeenclosing envelope portions which extend withinthe. stator of the anode driving motor.

Inrthe .interests rof operating efficiency, X-ray tubes are desirablyadesigned for operation at as' high anodecathode'voltage-as possible, subject, of course, to limitations determined by the electrica-l strengthy against rupture of the envelope material. At the same time, tube dimensions are desirably maintained at minimum values in the-interests .of size,.weight and consequent cost of the tube,` its enclosing casing and .the supporting structure employed for mounting the same in operative position.

Heretofore, as disclosed more especiallyin U.S. Letters Patent 2,703,373, which issued-March l, 1955, on the vention Voil-larold R. Cummings in X-Ray Tube, it has beenproposedto minimize the tendencytoward envelope rupture in X-ray generati-ng tubes designed for operation at highv` anode-cathode potential values, by providing glass envelope walls ofoptirnumy puncture resisting thickness `andhaving a high resistance, electrically conductive coatingI applied upon theputer surfaces of the envelope. Rotatinganodev X-ray. generating tubes of the sort hitherto provided for operation at relatively high voltagesphave embodied Venvelopes-of substantial wall thickness opposite the envelope portion, encircled by the rotor driving motor windings., Because of the thickness of glass employed in the envelope Walls', a very substantial temperature differencek may be developed in the envelope material on opposite sides of the wall, as ythe result of rapid, heating or cooling of the device, when in operation, such temperature differential producing severe mechanical stress in and consequent likelihood of rupture ofthe envelope portions thus exposed to rapid heating or cooling eiects.

An important object of the present invention is to provide an electron ow vdevice embodying new and irnproved means for minimizing the likelihood of envelope rupture whenV the device is in operation, thereby materially improving the efficiency and output capacity of devices embodying the present invention, without correspondingly increasing sie, and withoutisubjecting the envclopeto the danger of-rupture due to rapid differential heating-orl cooling of relatively thick envelope walls. i

Another important-object is toprovide a rotating anode X-ray generating tube having a tubular anode enclosing envelope portion embodying a relatively thinwall that is substantially immune to-rupture as theresult of rapid differentialheating` and cooling thereof, including an insulatingsleeve associated withsaid tubular anode enclosing .envelopeportion and adapted to provide adequate electrical insulation, at Said anode enclosing portion, between .theenclosed anode and the encircling stator windingsof, the anode driving motor; a further object being to utilize a relatively thin insulating sleeve of glass embracing theanode enclosing-envelope portion to provide adequatevelectrical insulation between the rotating anode andthe anodedriving motor windings; a still lfurther oblject-bengto provide the insulatingsleeve with a coating comprising high resistance,V electrical conductingl materia-i.

Another important-object issto providefa coating-of high resistance electrical conducting material upon *the outer-surfacesL of at least-.one of a plurality of concentric sleeve` portions extendingv withinthe Vanode driving stator of ,a rotating anode electron flow. device; a further obiect being to apply the coating upon theiouter suria'ceof'a `saidsleeve portion; anotherobject being-.to provide a resinouskcoating upon the outer surfaces of the envelope', especialiythe-,anode enclosingv tubularv portions` thereof, in ordertoincreasethe dielectric strength of the envelope audits-resistance tofpuncture; yet a further object being to applya conductive coating'upon the .outer surfaces of both .oi-a pair ofA concentrically mounted tubular 'glass portionsv and to vprovide contact means, for electrically interconnecting sadcoatings, as vby means of connector clips of electrical conducting material applied upon an endedge ofthe outer of saidftubular portions in position ymaking electrical contact with both of said resinous coatings.

Anothenimportant object is to employ a resinous coating-.baked upon and-thus integrated with the material of the-.glass envelope lor of; an auxiliary sleeve snugly encircling thehtubularanode enclosing portion of the envelope; -a further object being to apply a phenolic type resin as a coatingon the envelope orA thesleeve, or both.

Anotherimportant object is to incorporate an electrical conducting material, suchas.graphite,..in finely divided, preferably powdery or dust-like, condition, distributed uniformly throughout the coatingmaterial in quantities constitutingthe coating-as a highresistance electrical ,conductor having an electrical resistance characteristic of the order of 750x250 megohrns-per square, to thereby constitute the coating-as means for-continuously draining off and thus dissipating \suc h electrostatic charges asmay accumulate ,uponwthe .envelopel or-onthel auxiliary con- Cenf-fCaUY mounted Slwaf The. collductive,character of the coatingsfprevents the Aaccuniulation otlhigh electristatic voltagezchargesat anyvpoint ,upon theouter surfaces of the envelope of the device, or on the auxiliary concentrically mounted sleeve, thereby preventing the development of local high voltage peaks, such as otherwise might be caused by the particular configuration ofthe tube enclosing casing, or by the presence of aux1liary equipment near where the device is mounted for operat1on.

Briefly stated, in accordance with one aspect of the 1nvention, an X-ray generator having a rotating anode may be provided with a sealed glass envelope embodylng a tubular portion snugly enclosing a portion of the rotatable anode structure which forms the rotor of an anode driving motor. The stationary windings of the anode driving motor may be disposed outwardly of and in posltlon encircling the anode enclosing envelope portion, said portion comprising glass of such thickness as to substantially inhibit the development of envelope puncturing mechanical strains therein as a result of differential heating of its constituent material, an auxiliary insulating sleeve of glass being mounted concentrically upon and in position snugly embracing said anode enclosing envelope portion. The auxiliary sleeve may extend between the anode enclosing envelope portion and the stationary windings of the anode driving motor. In accordance with the present invention, a coating of high resistance conducting material is applied upon the auxiliary sleeve and also upon said anode enclosing envelope portions, if desired, such coatings preferably comprising resinous material baked upon and hence integrated with the glass on which applied, said resinous material having relatively high resistance conducting characteristics and being electrically interconnected with the grounded core of the stationary motor windings.

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 drawings, discloses a preferred embodiment of the invention.

Referring to the drawings:

Fig. 1 is a sectional view taken longitudinally through an X-ray tube embodying the present invention; and

Figs. 2 and 3 are perspective views of contact clips employed in the device.

To illustrate the invention, the drawings show an X-ray generating tube 11, comprising an anode 12 and a cooperating cathode 13, enclosed in a sealed envelope 14 of glass. The anode 12 comprises a rotating structure 15 suitably journaled upon a stem 16 sealingly supported on and extending outwardly of the envelope, at an end thereof, for connection with a suitable high voltage anodecathode power source, the stem 16 having a portion 17 extending within the envelope for turnably mounting the anode structure therein, as by means of roller bearings 18. The rotary anode structure provides a preferably ring-like target 19 in position, opposite the cathode, to receive the impingement of electrons emitted by the cathode during operation of the device as an X-ray generator.

The cathode 13 preferably comprises a head 20 formed with a pocket 21 in which an electron emission element 22 is mounted in position to emit electrons when electrically energized. The filament 22, at its opposite ends, may be mounted on, electrically connected with and supported by suitable conductors which may extend thence outwardly of the envelope, at the end thereof remote from the anode, said conductors being adapted for connection with a suitable source of filament energizing power, and with the high voltage source of anode-cathode operating potential.

The anode and cathode may be of any suitable or preferred construction and may be supported respectively at opposite ends of the envelope 14, which is preferably of generally cylindrical configuration. As shown, the envelope may comprise a tubular portion 23, at the anode end of the envelope, said portion being sized to closely encircle the anode structure 15. Medially of the ends of the envelope, the tubular portion 23 may merge with an enlarged envelope portion 24 into which the target forming portions of the anode extend. If desired, the anode structure may be like those illustrated and described in U.S. Letters Patents 2,242,101 and 2,336,769 of May 13, 1941, and December 14, 1943, respectively. The cathode may be like those shown in U.S. Letters Patents 2,345,723 and 2,348,184 of April 4 and May 9, 1944, respectively; and the cathode and anode structures may be sealed in the envelope 14 as taught more particularly in U,S. Letters Patents 2,250,323 and 2,311,725, of July 22, 1941 and February 23, 1943.

It should be understood that an X-ray generating tube produces X-rays as the result ofthe impingement on the anode target 19 of electrons emitted by and at the cathode filament 22, and driven thence toward and caused to impinge upon the anode target, at high velocity, under the influence of electron driving potential applied between the cathode and anode. X-rays thus produced at the target 19 may pass thence outwardly of the envelope 14 through a window 25, comprising an area of reduced thickness, formed in the wall of the envelope 14, opposite the target 19, as in the enlarged envelope portion 24. In order to rotate the anode 12, the stator 26 of an induction motor may be mounted in position encircling the tubular envelope portion 23 opposite the main ybody of the anode 15, which may be constituted as the rotor of an induction motor, within the stator 26,

said sta-tor comprising a core 27 of magnetic material and windings 28 applied on and supported by the core in conventional fashion. Anode driving torque may be developed in the anode body 15, operating as the rotor of an induction motor, in response to energy supplied to the stator windings 28 from a suitable motor energizing power source.

X-ray generators are usually enclosed in grounded, shockproof casings as a safety measure, the core 27 of the anode driving motor being conventionally grounded by connecting the same with the grounded casing. Such casings are ordinarily made of metal and sized to relatively closely enclose the X-ray generating tube which is mounted therein. The presence of such an enclosing casing, or of other metal structures near the envelope of the generator, if the same be not encased, may result in the establishment of localized electrostatic potential charges on the envelope, depending upon the configuration of the surrounding casing, or on the shape and location of adjacent metal bodies. These charges, at least in part, are due to the impingement of stray electrons upon the inner surface of the envelope, and frequently cause minute hair-like puncture of the envelope at places where internal and external electrostatic charges may happen to accumulate, such puncture being particularly likely to occur when the operating potential between anode and cathode is relatively high.

The phenomenon of electrostatic envelope puncture appears to occur in a direction inwardly from the outer surface of the envelope. The puncture appears to be the result of the accumulation of electrostatic voltage peaks upon the inner surface of the envelope through stray electron impingement thereon, in combination with conditions established by metal bodies located externally of the envelope. Envelope puncture usually occurs at or adjacent the envelope portion which is located in closest proximity to an external metallic body; and it is presumed that the location, and perhaps also, the configuration of the envelope facing portions of such external metal body, determines the place in the envelope at which puncturing is most likely to occur. It is thought, however, that the actual puncture occurs as the result of the building up of puncturing voltage on the inner surface of the envelope through the impingement of stray electrons.

The metallic body which is normally disposed nearer to the envelope of a rotating anode electron ow device than any other, is almost invariably the core-'27r ofthe anode'driving motor. `Although the envelope portion 23 `immediately within said core 27 is substantially shielded against stray electron impact bythe target'portion of Ythe anode, experience shows that there is 4considerable likelihood of envelope puncture at the portions 23, especially those that lieimmediate'ly Within the stator 26; and it has beenl customary, in the past, to provide relatively thick massive envelope walls, in the tubular envelope portions 23 whi'chextendfwithin the stator 26, in order to minimize the'likelihoodlfof envelope puncture in said portions. l

Electronic ir'n'pingenentv upon 'the target 19 for the `Vproduction of X`r`ays`"1"'e"s'ults"'in 'the 'generation of rela-` tively largeV quantities( ofheat`at"the target, which 1s,

of necessity, dissipated 'thence' through the body of the anode 15, through the stem 16 and also outwardly of the envelope through the wall portions 23. When the generato-r is in operation, the anode ybody may attain temperatures of the order of 500 C., so that the inner surface of the anode enclosing envelope portions 23 may become heated to a substantially higher temperature than that which prevails at the outer surfaces of said envelope portion which is normally exposed in a cooling medium, such as insulating loil contained in the tube enclosing casing. Such differential heating of the inner and outer portions of the envelope wall produces severe mechanical stresses therein which frequently result in rupture of the glass, the effect of such stresses being intensified as the result of alternate heating and cooling of the envelope as the generator is alternately started and stopped.

In order to minimize the tendency toward envelope rupture through differential heating of relatively thick envelope wall portions, the present invention contemplates the provision of a plurality of concentrically arranged, relatively thin wall members between the anode body 15 and the stator 26 of the anode driving motor, at least one of said concentric wall members forming the sleeve-like envelope portion 23. To this end, the wall portion 23 of the envelope 14 preferably is of unusually small thickness, as compared with conventional anode enclosing envelope portions. Puncture inhibiting means comprising one or more sleeves 23 of suitable insulating material may be applied in position concentrically encircling the tubular envelope portion 23 within the stator 26, said sleeve or sleeves 23 being sized to fit snugly upon and about the tubular envelope portion 23 and within the core 27 of the stator 26. As shown, but a single sleeve 23 may be adequate for puncture inhibiting purposes, the same, as well as the envelope portion 23, preferably comprising relatively thin Walls of glass, although the sleeve 23 may comprise any suitable or preferred ceramic material. Indeed, the invention contemplates the possibility of making the envelope portion 23, as well as the sleeve 23', of ceramic material. The invention also contemplates the possibility of making the sleeve 23 as a molded plastic element. The sleeve 23 may comprise a portion formed vto snugly embrace the tubular wall portion 23 within the stator 26 and a ared extension 24' adapted to overlie the correspondingly ared envelope portion which interconneots the tubular portion 23 with the enlarged medial envelope portion 24.

The employment of a relatively thin sleeve 23 in position snugly embracing a relatively thin anode encircling envelope wall portion 23 will provide puncture iesistant characteristics superior even to those of a thick massive envelope wall; and the arrangement will also allow each of the relatively thin wall members to expand independently when exposed to heat developed in the anode as the result of operation of the device, thereby reducing the danger of envelope rupture as the result of thermally reduced mechanical stresses.

In order to further minimize the tendency toward envelope puncture, when the ray generating tube is operated Yupon the surfaces of the sleeve 23 which face outwardly upon thetsurrounding stator 26. This coating preferably terminates, at its opposite ends, a distance of the order ofl one-eighth inch inwardly of the end of the extension 274" and of the extension remote end of the sleeve 23. The coating 29 preferably comprises a resinous substance capable of being integrated with the material of the sleeve 23', as by applying the substance thereto in Afluid form, as a coat of substantially uniform thickness, and then baking the resinous material in order to solidify the coating in situ andv to integrate the same-with the material of the sleeve. To this end, it is preferable to utilize a coating material comprising a heat hardening resin adapted to adhere, in substantially integral fashion, with the glass on which coated, a suitable resin for such purpose being the phenolic reaction product known to the trade as Rescon.

A conducting material, such as graphite, in amorphous, powdery form is mixed with and substantially uniformly distributed throughout the resin in quantities such that the nal coating will have a resistance of the order of SOO-1,000 megohms per square. The coating, accordingly, is thus constituted as an electrical conductor having relatively high resistance characteristics, the same being only conductive enough to drain off slowly such electrostatic charges as may appear on the surface of the sleeve 23 by virtue of the adjacency of the stator core 27 and thereby prevent the accumulation of electrostatic voltage of suicient intensity to cause puncture of the sleeve 23 and the envelope portion 23.

ln order to thus drain off electrostatic charges from the coating 29, conductor clips 30 may be provided to electrically connect the coating with the grounded core of the stator. As a consequence, any tendency of electrostatic voltage to build up in any localized area of the sleeve 23 will become dissipated through the high resistance coating 29 and through the contact clips 30.

In order to further minimize the tendency toward puncture of the envelope 14, a resinous coating 29 may also be applied upon the external surfaces of the glass envelope 14, said coating preferably completely surrounding the envelope and extending from near the cathode end thereof to near the anode end of the envelope, including the envelope portions enclosed by the sleeve member 23. The coating 29 may conveniently comprise the same material used for the high resistance conductive coating 29; and means may be provided for connecting the coating 29' to ground, in order to continuously drain off any electrostatic charges that may accumulate on the coating and thereby control the voltage gradient between the opposite ends of the envelope. To this end, the coating 29 may be electrically con nected to ground through the coating 29, by means of a connection clip or clips 30' applied at either or both of the opposite ends of the sleeve 23 in position electrically interconnecting the coatings 29 and 29.

Devices embodying the present invention and including the sleeve 23 of insulating material surrounding an envelope wall portion 23 of minimal thickness within an anode driving stator, show unexpectedly low envelope puncture incidence, even in the absence of high resistance conductive coatings applied upon the envelope and the auxiliary sleeve or shield 23'. Assemblies in which a resinous coating is applied, in accordance with the present invention, upon either the envelope or the shield, show even lower puncture incidence, envelope puncture being almost entirely eliminated by utilizing high resistance coatings upon the envelope as well as on the shield 23'.

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 ar- 7 rangement of the several parts without departing from the spirit and 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:

An X-ray generating tube comprising an electron emitting cathode, a cooperating rotary anode and an envelope enclosing the anode and cathode, said anode having a portion forming a motor rotor and being supported on and within the envelope for turning movement about an axis of rotation, said envelope being formed with a tubular portion of insulating material sized to snugly enclose said rotor, and a separate sleeve of electrical insulation closely embracing said tubular envelope portion, said sleeve being adapted to tit within the stator of an anode driving motor disposed in position encircling said sleeve and said tubular envelope portion opposite the said rotor, a coating of high resistance electrical conducting material formed on the outer surfaces of said tubular envelope portion and on the outer surfaces of said sleeve, connection means at an end of said sleeve for electrically interconnecting said coatings, and means to electrically ground the outer coating on said sleeve to the said stator.

References Cited in the le of this patent UNITED STATES PATENTS 2,119,069 Bouwers May 31, 1938 2,216,888 Machlett Oct. 8, 1940 2,233,194 Atlee et al. Feb. 25, 1941 2,703,373 Cummings Mar. 1, 1955

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