US2816241A - Electron targets and means for and method of cooling the same - Google Patents

Description

1957 M. J. ZUNICK ETAL ,81

ELECTRON TARGETS AND MEANS FOR AND METHOD "OF COOLING THE SAME Filed Sept. 27, 1951 q I K K T C N D m mw m 26 W L A H 60 MH ATT'Y nitedhtates atent @fiice filfiill Patented Dec. 1.0, 1957 arem ELECTRQN TARGETS AND MEANS FOR AND METHQD F COOLING THE SAME Michael .F. Zunick, Greenfield, and Howard W. G ond rick, Brooidield, Wis, assignors to General Electric (,ompany, a corporation of New York Application September 27, 1951, Serial No. 248,472 12 Claims. (Cl. 3l332) The present invention relates in general to electronics, and has more particular reference to electron targets, more especially targets for use in electron flow devices, such as X-ray generators.

An X-ray generator commonly comprises an assembly of elements including an electron emitter, an electron target, conventionally comprising a block or plate of metal, such as tungsten, and a sealed envelope enclosing the emitter and the target. Such an assembly is commonly referred to as an X-ray tube. X-rays are produced in an X-ray tube by applying electrical potential between the electron emitter and the target for the purpose of driving electrons from the emitter and causing the same to impinge at relatively high velocity upon the target, thereby constituting the emitter and target respectively as cathode and anode, X-rays being generated at the target as the result of electron impingement thereon.

The impingement of electrons on the anode target at high velocity results in the generation of substantial quan tities of heat at and in the target. Such heat, especially in high power tubes, tends to deteriorate and ultimately to destroy the target. In the interests of eflicient X-ray production, the generator tube is usually operated under maximum safe loadings limited so that the resulting temperature, developed in the anode target when the tube is in operation, will not exceed a limit, short of the melting temperature of the target material, above which deterioration of the anode target takes place at an uneconomically high rate.

The power ratings at which an X-ray tube may be operated are thus limited to the temperature at which the anode may safely operate without undue deterioration, and numerous expedients are commonly employed for artificially cooling the anode target in order to hold the same within safe operating temperatures, to thereby allow the tube to be operated at higher power ratings than would be possible without artificial cooling. Presently available cooling expedients for X-ray tubes, however, impose definite limitations upon the electrical loadings under which such devices may be safely operated, it being necessary to dissipate, as heat, approximately 95% of the electrical energy which is applied to the tube for the operation thereof. This energy has to be dissipated from within the relatively thin layer of metal constituting the surface portions of the electron target area of the anode structure. Such heat energy, especially in a tube designed for continuous operation, must be removed as rapidly as it is generated. Otherwise, the target, if operated at its rated loading, may reach a temperature approaching or exceeding the melting temperature of its constituent material, thus resulting in the destruction of the target and consequent failure of the tube.

Tungsten is presently considered to be the most suitable available material for electron target purposes, the same being highly resistant to deterioration at the high temperatures imposed by electron bombardment at the velocities required for X-ray generation. As a consequence, for optimum performance, X-ray tubes embodying tungsten targets are commonly designed for operation under maximum energy loadings allowed by the limits of the physical characteristics of tungsten targets.

For artificial cooling purposes in X-ray tubes, water, oil or air are commonly employed as media for the transfer of heat away from the target, cooling by air being employed in low power tubes in which the required cooling effect is relatively low. Tubes designed for operation at higher power ratings are usually cooled by circulating oil or water. In such tubes, means is commonly provided for delivering the cooling medium continuously in heat exchange relationship with the target, as by circulating the medium continuously between a chamber formed in the anode structure behind the target and an external cooler, so that heat may be absorbed from the target into the medium and then extracted from the medium at the external cooler to condition the medium for recirculation into heat exchange relation with the target being cooled. Water and oil have relatively low boiling points, and these media, together with air, have relatively poor heat storing capacity, which limits their effectiveness for heat absorbing and transferring purposes.

- Modern X-ray design technology thus is limited by presently available cooling expedients, and the peak efliciency and cooling capacity of these coolants have already been attained, so that no substantial increase in tube ratings can be anticipated in X-ray tubes employing conventional cooling facilities.

An important object of the present invention is to provide an improved electron target structure from which heat may be readily dissipated.

. Another important object is toutilize metal in liquid condition, as a heat exchanging medium, for the dissipation of heat from electron target structures; a further object being to employ liquid mercury as a preferred heat transfer medium, or liquid sodium, potassium, lead, hismuth, lithium, tin, or other low melting point metal for anode target cooling purposes; a still further object being to employ low melting point mixtures or alloys of metals, especially potassium-sodium, or lead-bismuth eutectic alloys as cooling media for electron targets.

Another important object is to provide an improved target structure formed particularly to enable the circulation of a liquid metal cooling medium in heat exchange relation with the electron target portions of the anode structure.

Another important object is to provide for continuously circulating the liquid metal coolant between the anode structure and suitable external cooling apparatus, whereby to continuously extract heat from the cooling medium and thus condition it for recirculation in heat absorbing relation with the target structure to be cooled; a further object being to form the anode structure with ducts and cavities, including a chamber immediately behind the target area of the anode, to thus form a circulating system capable of retaining a liquid metal coolant in said ducts and cavities; a still further object being to construct the anode in suitable fashion and of selected refractory materials capable of retaining the liquid metal cooling medium in the circulating system and of resisting the corrosive action of the liquid metal coolant upon the materials defining the circulating system in the anode structure.

The foregoing and numerous other 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 conjunction with the'accompanying drawing, discloses a preferred embodiment of the invention for the purpose of demonstrating the same.

Referring to the drawings,

Fig. 1 is a side view of an X-ray generating tube, shown partially in section, and having an anode target structure embodying the present invention;

Fig. 2 is an enlarged sectional view taken through the anode target structure shown in Fig. 1;

Fig. 3 is a sectional view taken substantially along the line 33 in Fig. 2;

Fig. 4 is a partially sectionalized side elevation view of a modified anode target structure embodying the invention; and

Fig. 5 is a sectional view taken substantially along the line 55 in Fig. 4.

To illustrate the invention, the drawings show an X-ray generating tube 11- comprising a cathode-12 emb0dying an electron emitting element 13', an anode 14'-providing anv electron target 15, and. a sealed andevacuated envelope 16 enclosing and supporting 'the anode and cathode in spaced apart facing relationship within the tube.

While the invention isnot necessarily limited toany particular form, construction or configuration of theenvelope 16,. or of the cathode12, the envelope may comprise a tubular glass element having reentrant end portions 17 and 17 at the opposite ends of the envelope. These envelope end portions may be circumferentially sealed respectively to the anode and cathode structures, in order to hermetically seal the envelope and to mechanically support the anode and cathode structures respectively upon the reentrant portions 17 and 1-7', with portions of the. anode'andcathodestructures forming outwardly exposed envelope parts within the reentrantJ-portions. The emitter element 13' mayv comprise a filament suitably supported on and. insulated from the cathode structure 12, the filament being connected with lead conductors 18 adapted for connection with a suitable power source outwardly of the'envelope for the purpose of energizing the filament 13 for electron emission, the filament being thus connected with the conductors 18 through suitable envelope seals, preferably formed in the out wardly exposed envelope forming part of the cathode structure, at the inner end of the reentrant portion 17'.

It will, of course, be understood that the generator tube 11 may be operated for the production of X-rays, at the target structure 15, by energizing the filament 13 for electron emission, as by connecting the conductors 18 with a suitable source of emitter energizing power, while simultaneously applying electron. driving potential between the cathode and the target structure, as by connecting the target structureqand one of the cathode connected conductors 18 with a suitable source 'of electron driving potential outwardly of the envelope. To this'end, the necessary electrical connection with the anode may be made at or through the outwardly exposed envelope forming part thereof, at the inner end of the reentrant portion 17. Electrons emitted by the filament 13, when so energized, may travel as an. electron beam 19 from the filament 13, under the influence of the anode-cathode electron driving potential, andimpinge upon the facing surface of the target 15, thereby constituting the target as a source of X-rays which-may be projected from thetarget as an X-ray beam 20. The X-ray beam thus generated at the targetmay pass thence, outwardly of the envelope, through an X-ray transmitting window 21 formed inthe envelope opposite the target.

The present invention broadly contemplates the utilization of metal in its liquid phase for the purpose of cooling the target in order thus to dissipate heat energy generated in the anode target as the result of'electron impingement thereon; and the inventive concept includes the formation of the anode with a chamber or cavity 22 therein immediately behind the target '15, through. which chamber liquid metal may be circulated. as a coolant medium for extracting heat from thetarget 15 as rapidly as such heat is generatedin, the target as-the result'of electron impingement. The invention further provides for the construction of the anode of selected refractory materials capable of resisting the. destructive actiomthereon of the liquid metal coolant medium, in heated condition, while traveling to and from and in the chamber 22, and includes structural features adapted to promote the coolant retaining ability of the anode structure against the pressure and corrosive action of liquid metal cooling media circulated therein, during an extended service life of the device.

To these ends, the anode structure may comprise a base 23 carrying means for sealing the structure upon the reentrant envelope portions 17 and formed with ducts 24 and 24/ for the circulation of the cooling medium to and from the chamber 22, said base carrying chamber forming means preferably comprising sheet metal elements sealed thereto in position forming the chamber 22 and the target 15, atone end of the structure, with the chamber 22 in open communication with the ducts 24 and 24'.

As shown more particularly in Figs. 2 and 3 of the drawings, the base 23 may comprise a preferably cylindrical block of refractory metal, such as stainless steel, iron, molybdenum or nickel, the block being formed with annular seats 25 and 26 in position to sealingly receive the open ends of sheet metal shells 27 and 28, one within the other, said shells having cylindrical skirt portions adapted to be sealed upon the base portion 23, in

the seats '25 and 26, as by brazing the same with a suitablebrazing medium, such as the alloy of manganese and nickel. The shells 27 and 28 may have integral wall portions 29 and 30 closing the base remote ends of the shells, said wall portions being spaced apart sutficiently to form the chamber 22 therebetween.

If desired, these wall portions may be disposed in planes inclined at an angle with respect to the longitudinal axis of the anode structure, as shown, in order to constitute the wall 30 of the outer shell as an inclined target portion of the anode. Alternately, the wall 30 may be disposed at right angles with respect to the longitudinal axis of the target structure.

The inner shell 27 may comprise iron, nickel, molybdenum, stainless steel, or other refractory material, not readily attacked by a liquid metal cooling medium, such as mercury, or such other medium as may be employed for cooling the anode structure. The outer wall 28 may comprise tantalum, tungsten, or other suitable metal, adapted not only to function as an electron target, but being also of refractory character capable of containing a liquid metal cooling medium within the chamber 22 defined'lbetween the walls 29 and 30 of the shell elements.

Theducts24 and 24., asshown in Fig. 2, may comprise pipes 31 and 31- made of metalsuch as iron, nickel, molybdenum, stainless steel, or other metal that is inert to the metal coolant medium. These pipes may be sealed, as by brazing the same, with manganese-nickel alloy as a brazing medium, in sockets 32 formed in the base 23 and opening in the surface thereof which extends at the inner end of the reentrant portion 17, remote from the shells 27 and-28, the socket mounted ends of the pipes 31- and 31' communicating with the interior of the shell 29 through ducts 33 formed in the base member 23. The iron-nickel brazing material forms a joint highly resistant to the action of mercury and other liquid metal coolants which may be employed. One of the ducts 33 is preferably connected directly with the chamber 22, as by means of a pipe 34 of iron, molybdenum, nickel, stainless steel, or other refractory material, said pipe being sealed at one end in an opening 35 formedv metal cooling medium, such as mercury, sodium, potassium, lead, bismuth, lithium, tin, or alloy mixtures of said metals, may be delivered as through the pipes 24 and 31 into the chamber 22 from a source disposed outwardly of the X-ray tube envelope, and may be caused to flow through the chamber 22 immediately behind and in heat exchange relationship with the wall portion 30 which constitutes the electron target 15. After thus passing in heat exchange relation with the wall portion 30 and absorbing heat therefrom, the liquid metal coolant may pass from the chamber 22 through the opening 36 into the space within the shell 27, whence the cooling medium, in heated condition, may pass from the X-ray tube through the pipe 31'. The outer ends of the pipes 31 and 31' may be connected with any suitable cooling or refrigerating apparatus, including pump means for circulating the liquid metal cooling medium through the pipe 31, from the chamber 22 to the cooling apparatus, and thence back through the pipe 31 and into the chamber.

The frame 23 may be formed with annular seats 37 and 38 for respectively receiving and supporting a metal seal element 39 and seal enclosing guard members 40. The seal element 39 may comprise a sleeve of tapering sectional configuration and formed of metal having a coefficient of thermal expansion substantially equal to that of the glass comprising the reentrant envelope portion 17, upon which the anode structure is mounted. This sleeve may be sealingly secured to the frame 23, as by brazing the relatively thick end of the sleeve to the frame 23 at the seat 37; and a glass-to-metal seal 41 may be formed at the junction of the reentrant envelope portion 17, and the thin edge of the element 39, to thus mount the anode structure on and seal the same in and as a part of the envelope, the outer end of the frame 23 being exposed outwardly of the envelope, at the inner end of the envelope portion 17, through which the pipes 31 and 31' may extend for connection with the external cooling and pumping equipment. The seal enclosing members 40 may comprise curved sheets of metal secured in the seat 38, as by means of fastening screws 42, in position enclosing the glass-metal seal 41 for the purpose of screening the same against the impact thereon of stray electrons.

As shown more particularly in Figs. 4 and 5 of the drawings, the frame 23 may comprise a generally cylindrically formed block of metal, such as iron, molybdenum, nickel, stainless steel, or other refractory metal, substantially inert to the action of mercury or other liquid metal coolant medium. Longitudinal ducts, forming the channels 24 and 24, may be provided in the frame 23, which, at one end, may be formed with stepped shoulders 43. A cover structure 44, comprising inner and outer cup-shaped portions 45 and 46 and a target plate 47, set in the bottom of the outer cup-shaped portion, may be sealingly secured to the frame 23 at the shoulders 43. The inner portion 45 may comprise a relatively thin shell of molybdenum. The outer portion 46 may comprise copper, while the target plate 47 may comprise a button of tungsten, tantalum, or other suitable target material. The target button 47 and the shell portion 45 may comprise pre-formed elements, and the cover structure 44 may be formed by casting copper, in a suitable form or mold, upon the preformed elements 45 and 47 which, during the casting operation, may be suitably secured in the mold in proper relative position, as by means of molybdenum binding wires, in order to form the portion 46 as a copper casting integrated with the molybdenum cup and the target member 47.

After formation of the cover structure 44, it may be secured on the end'of the frame 23 by means of silver solder 48 joining the copper portion to the member 23 at the shoulders 43, thereby forming the chamber 22within thestructure 44, at the inner end of the frame 23 and in open communication with the ducts 24and 24', If desired, a battle 49 of refractory metal may be applied with-' in the chamber 22, in position between the open ends of the ducts 24 and 24' for the purpose of directing fluid flow in the chamber 22 behind the target member 47. This bafiie may be formed as an integral part of the member 23, as shown in Figs. 4 and 5, or may be formed as a separate element and attached in place in aiiy convenient manner, as by welding or brazing. A seal member 39 and seal guarding means 40 may, of course, be formed upon the body of the frame 23 for the purpose of mounting the anode structure on and within the reentrant portion 17 of an X-ray tube envelope; and the target remote ends of the ducts 24 and 24' may be connected outwardly of the X-ray tube with suitable coolant circulating and cooling apparatus.

The anode construction and constituent materials heretofore specified are of importance in liquid metal cooled anodes made in accordance with the present invention. Since mercury will readily attack copper, silver and gold, such metals, if used, have to be isolated from contact with the cooling medium, as is accomplished in the embodiment shown in Figs. 4 and 5, wherein the copper target mounting portions of the cover structure 44 are isolated from the cooling medium by means of the molybdenum shell 45. A limiting factor also is the rate of heat transfer through the target material to the liquid metal coolant. This is directly proportional to the thickness of metal through which the heat must travel in reaching the coolant. It is, therefore, desirable that the target be made as thin as possible, limited only to the mechanical strength required to safely retain the liquid metal coolant within the chamber 22.

Mercury is considered to be the most desirable material for anode cooling purposes, since the same is a liquid at normal atmospheric temperature and hence does not require that the system be heated in order to retain the cooling medium in liquid condition. Mercury moreover is readily attainable at reasonable cost, it is not susceptible to excess oxidation even at relatively high temperatures, and has a relatively high heat transferring coefficient. The heat transfer coefficient of sodium and lithium is thought to be slightly better than that of mercury. These metals, together with the others named except mercury, have the disadvantage of being solid at normal temperatures, thus requiring the continuous operation of means for keeping the media in liquid state, or the draining of the coolant from the anode when the tube is not in operation for X-ray production. In this connection, the invention contemplates the incorporation of an electrical heater 59 in the anode, with means for energizing same from a suitable external source of power, to hold the cooling medium in liquid condition in the anode during periods when the tube is inactive as an X-ray generator. The heater, of course, may be disposed and secured in any suitable retaining cavity formed in the anode structure. All of the metals and alloys herein mentioned as suitable for anode cooling purposes have from six to ten times the cooling ability of water and oil. Accordingly, X-ray generators embodying the teachings of the present invention may be operated at substantially and significantly higher ratings than is possible with like structures embodying conventional cooling facilities.

It is thought that the invention and its numerous attendant advantages will be fully understood from the fore going 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 forms herein being preferred embodiments for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

1. An anode comprising a metallic structure forming an electron target and having means forming a channel for the circulation of a metal medium in its liquid phase in said channel in heat exchange relationshipwithsaid 7 tar-get, and an electrically energized heating coil-enclosed in said structure in heat exchange :relation with 'ICSPCCt to Y the metal medium in said channel.

2. Ananode comprising a metallic structure forming an electron target and having means forming ac-hannel for circulating a metal medium inits liquid phase in heat exchange'relationship'with said target, said structurehaving a pocket formed therein in heat exchange relation with'r'especttothe metal mediumin said channel, and heating means disposed in said pocket to maintain the medium in'liquid condition in the anode.

3. An anode comprising a metal struoture embodying a base, a shell portion sealed-on said base-in positio'n forming an electron target and defining a wall of a chamber in the anode behind said target, said base being formed with channels for eircula'ting a inetal medium in its liquidtphase in said chamber :inheat exchange relationship with said WtllL said base being also formed with a'lpockot in "heat exchange'r'elation with respect to the metal medium in said channels, and heating meansdisposed in'said pocket to maintain the medium in liquid condition in'the anode.

'4. A nanode comprising a metallic-structure including a base of refractory'metal, and-a'she'll of refractory metal sealedon said base in' po'sition enclosing a chamber at an end of the base, said shell providing an electron target in front of said chamber, said base being formed with ducts 'therethrough and opening into said chamber for'the circulation, to and "from, and in *said chamber, of a metal medium in its liquid phase, in heat exchange relationship with the target forming "wall of said shell, and heating means on said base, in heat exchange re'lation with'respect to the metal medium in said ducts for maintaining the medium in liquid condition therein.

-5. 'An anode comprising a metallic structure including a base of refractory metal, and a cup-shaped end cover having a bottom providing an electron target and peripheral side walls-sealedon said base in position enclosing a chamber behind said target, said end cover comprising metal of the class consisting of tantalum and tungsten said base being formed with duct means therein 'communicating with said chamber for the-circulation, to and from, and in said chamber, of a metallic medium in its liquid phase, in heat exchange relationship with said target, and heating means on said base, in heat exchange relation with respect to the metallic medium in said duct means for maintaining the medium in liquid condition therein. I

6. An anode comprising a metallic structure including a base of refractory metal selected from the class consisting of iron, molybdenum, nickel, and stainless steel, a cupshaped metal shell having a bottom providing support for an electron target and having integral side walls embracing and sealed upon a peripheral seat forming portion of said base in position enclosing a chamber at the end of said base and behind said target, said base being formed with duct means therein communicatingwith said chamber for the circulation of a metallic medium in its liquid phase within said chamber in heat exchange relationship with the target carrying bottom of said shell, and heating means on'said base, in'heat exchange relation with respect to the metallic medium in said duct means and chamber, for maintaining themediuminliquid condition therein.

7. An anode'comprising a metallic structure including abase of refractory metal, and a cup shaped end cover having a bottom providing an electron target and peripheral's ide walls sealed 'on said 'base in position'enclosing a chamber within said target, said end cover comprising a cup-shaped shell of molybdenum enveloped in a copper sheath,whereby the shell serves as a liner defining said chamber Within said shell, said target comprising a plate of metallic electron target material embedded in the copper sheath and thus secured on the bottom of the cups'haped shell, said "base being formed with duct means therein communicating withsaid chamber for the circula- 8 tion, to and from, and in said chamber, of a metallic medium in its liquid phase, heat exchange relationship with-said target, and heatingme'ans on said base, :in heat exchange relation With respect to 'the'metallic medium in said "duct means for maintaining the medium in liquid condition therein.

8. An X-ray tube embodying a cooperating anode 'and cathode enclosed in'a sealed envelope, said anode 'com= prising a metal .structureaproviding an 'electron target'at an end thereof and ia 'cav-ity therein 'behindsaid target, said anode being formed With'ducts in communication with said chamber' and opening outwardly of said envelope for the circulation of a me'tallic' medium in its liquid phase to and from said-chamber and in'heat exchange relationship with said 'targen and 'betw'een saitl chamber and asourcc of said=medium outwardly of said "envelope and a heater mounted' o'n said anode-in heat exchange relation with-said cavity and ducts :and operable to maintain the mediumin-its liquid :phase in said cavity and ducts.

9. An X-ray' tube embodyi'ng :acooperating 'a'node and cathode enclosed in a sealed'envelope, said anode=comprising a structure ofrefractory materi-alsubstantially :inert to metal in the :liquid phase, :said structure-embodying a plate-like portion of metal capable of sustaining electron impact and providing an electron target 'at an end-of the structure, said rplate-like iportion defining a cavity '-inthe anode behind said target, said anode being iormed with ducts incommunica'tion with said chamber and opening outwardly 'of said envelope "at :the opposite end of the anodefor' the circulationof ametallicmedium inits liquid phase 'to and from said chamber and in heat exchange relationship with :said target, and between said chamber and a source of said me'dium outwardly of said envelope and a heater :mounted on said "anode in 'heat exchange relation withsa'id cavity a'nd-ducts-and operable to maintain the medium in its Iliquid Jphase' in said cavity and ducts.

1'0. AnX-ray tube embodying a cooperating anode and cathode enclosed in a sealed envelope, said anode comprising a base ofrefractony materiaL such-as iron, nickel, or stainless steel, substantially linert to metal in liquid condition at-temperatures below the melting point :of such refractory material, and a cup-shaped end cover having a bottom providing an electron target and peripheral side walls'sealed-on'said base in position presen'ting said electron target aban'end of said base-and a'c'avity in'the anode behind :said :target, said base being sealed "on said envelope, between its opposite ends, and being formed with ducts in communication with'said'chamber :and opening outwardly of said envelope a't'the targetrremote end of the base for the circulation ofa metallieirne'diumin its liquid phase to and from said chamber and-in heat exchange relationship with said target, and between said chamber and a source of said medium outwardly of s'aid envelope and a theater mounted on said anode in heat =e'xchange relation with said cavity and ducts and'o'pera'ble tom'aintain the medium -in its liquid phase 'in' said cavity and ducts.

11. An anode comprising a metal structure embodying a base, an electron target supportingsh'ell-sealed on said base and defining a chamber in the anode behind said target, "said base bein'g formed with ducts for circulating'a metal medium'in its liquid phase to and from said chamben a bai'rle plate-disposed withinsaid shell for guiding said'mediu'm in'said chamber behind and in heat exchange relationship'with said target, and an electrically energized heating coil'enclosed in said structure in heat exchange relation with respect to the metal mediu'm in said ducts.

12. An anode comprising a metal structure embodying 'a base, an*electron target supporting 'shell sealed on said base and defining a chamber in the anode behind said target, said base being formed with a'pairof "ducts for circulating a metal medium in its liquid phase to and from said chamber, said base being formed with a bafile projection disposed between said ducts and extending from the base into said chamber, said baflie projection having an edge spaced inwardly of the target supporting portions of said shell to guide said medium within the shell behind and in heat exchange relationship with said target, and an electrically energized heating coil enclosed in said structure in heat exchange relation with respect to the metal medium in said ducts.

References Cited in the file of this patent UNITED STATES PATENTS 1,180,998 Gibson Apr. 25, 1916

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