US2836748A - Electron discharge device - Google Patents

Description

May 27,1958 2. J. ATLEE 2,336,743

ELECTRON DISCHARGE DEVICE Filed April 20, 1956 INVENTOR. ZED :r. ATLEE ATTORNEYS ELECTRGN DESCHARGE nnvicn Zed 3. Atlas, Chicago, l., assignor to Dunlee Ger-3a., tlhicago, EL, a corporation of lllinors Applica ion A ril 26, $56, Serial No. 579,631

6 Claims. (Cl. 313-58) T he present invention relates to a new and improved construction of electron discharge devices and particularly to a new and improved construction of an X-ray generator.

"I he present invention is concerned with electron discharge devices such as X-ray generators having an electron emitting cathode and an electron receiving anode mounted in spaced relation within elongated envelope formed of glass or similar material having high electrical resistance. To accelerate the electrons from the cathode to the anode, electrical potential is applied therebctwee imposing electrical stress upon the envelope. Unless means are provided for distributing the potential evenly along the envelope, the potential gradient tends to (3 off rapidly at each of the opposite ends thereof imposing relatively high stress on the end portions of the envelope. This phenomena poses major problems in the design of devices for operation at high voltages, say 150 kilo volts and greater, since the potential appliedto the electrodes must be limited to values that will not cause over stressing and electrical breakdown of the end portions of the envelope. Go the other hand, the lightly stressed intermediate part of the envelope could withstand a much higher potential gradient than is imposed thereon in conventional low voltage tubes, and if the potential applied to the electrodes can be graded substantially evenly along the length of the envelope, the operating potential or" the generator can be increased substantially. Also, during the operation of such devices, electrons unavoidably impinge upon the walls of the envelope. The impinging electrons, if not drawn oi, tend to collect and establish charges which may also produce high potential gradients so as to efiect a rupture of the envelope wall or deleteriously efiect the operation of the device. Various measures have been suggested to grade the potential along the envelope of an electron discharge device and obviate the difiiculties mentioned above but the constructions proposed heretofore have been prohibitively expensive or not practical for other reasons.

It is a principal object of the present invention to provide an electron discharge device having a new and improved envelope construction by reason or" which the potential between the electrodes of the device will be substantially evenly distributed along t-e envelope therebetween.

More particularly it is an oblect to provide a relatively inexpensive envelope construction in which he electrode potential will be graded substantially evenly along the length of the envelope.

it is a further object of the invention to provide an envelope construction for an electron discharge device which effects a distribution of the charges collecting upon the envelope wall so as to prevent the development of an excessive charge at any point thereon.

ther objects and advantages of the invention will become more apparent hereinafter.

In accordance with the illustrated form of the inven-' tented may 2?, 1958 iii L1 tion, an element of greater con activity than the envelope material is electrically connected between the electrodes of a discharge device and arranged in such a pattern as to distribute the ditlercnce of potential applied to the electrodes substantially uniformly over the length or" the envelope.

For a more detailed description of the invention, reference to the accompanying drawings wherein the invention is illustrated in connection with a particular embodiment thereof.

In the drawings,

Fig. 1 is a side elevational view of an X-ray generator constructed in accordance with the invention;

Fig. 2 is a longitudinal, sectional view of the generator; and

Fig. 3 is an enlarged, fragmentary view of a portion of the envelope wall of the generator.

To illustrate the invention, there is shown in Figs. 1 to 3, inclusive, an X-ray generator or tube comprising an anode unit lit and a cathode unit 12 mounted in spaced apart relation within an envelope R4 of suitable material such as glass. The envelope 14 is formed at one end with a re-entrant, cylindrical portion 16 to define with the outer wall of the envelope a pocket 13. The anode it) is supported from the re-entrant portion 16 by a metal sleeve 2% sealed to the end of the re-entrant portion 16 by a glass-to-metal seal indicated at 22.

To illustrate the invention, there is shown in Figs. 1 to 3, inclusive, an X-ray generator or tube ccmprisinfi an anode unit ill and a cathode unit l2 mounted in spaced apart relation within an envelope i i of suitable material such as glass. The envelope 14 is formed at one end wi h a re-entrant, cylindrical portion 16 to define with the outer wall or" the envelope a pocket 13. The anode 1b is supported from the reentrant portion 16 by a metal sleeve 24 sealed to the end of the reentrant portion 16 by a glass-to-metal seal indicated at 22.

To prevent the impingement of stray electrons upon the seal 22, a cylindrical metal skirt 24 may be mounted on the anode by suitable means such as screws 26, the skirt extending in spaced relation around the seal 22 as best shown in Fig. 2.

The skirt 24 is shown formed with an annular groove 27 on the inner wall thereof near the free end and in which a getter element in the form of a split ring 28 is mounted. T he getter element 23 may consist of a length of tubular wire filled with a suitable gettering material such as barium or magnesium, or a mixture of such metals, and may be flashed after the tube has been evacuated by inductive heating, as more particularly described in the Atlee at al. Patent No. 2,502,070, whereupon a deposit of gettering material will condense upon the Walls of the pocket 13 and over the glass-to-metal seal 22 and sleeve 2%, as indicated at S The cathode i2 is illustrated as comprising an elongate sleeve portion 5%, one end of which fits telescopically over the end of a re-entrant, cylindrical glass stem portion 52 of the envelope 14-, the sleeve being secured by suitable means such as wire wrappings 54. Mounted in the opposite end of the sleeve so is a cathode head block 56 formed with an electron focussiug cup 58 in which is mounted a filament 6% adapted to be energized by leads 62 extending outwardly of the envelope through a pinch stem 64. The sleeve 5% also supports a shirt 66 provided v 'th an annular groove 68 in its inner surface in which a gettering element 23 may be mounted. Flashing of tr e gettering element 28 in the cathode sleeve may be accomplished by induction heating and will cause a deeosit 72 cf gettering material to form on the sleeve 55) and on the walls of the pocket at the cathode end of the tube.

' As isknown, in the operation of X-ray tubes such as 'that'illustrated, current is'supplied to heat the filament 6t), whereupon the electrons are driven off from the same, and potential is applied between the anode 10 and the cathode 12 to accelerate the electrons and cause them 7 to impinge upon the X-ray generating target of the anode.

The stressing efiect of this potential upon the envelope has been discussed above. 'While the majority of the electrons 'fiow directly from the filament'to the anode target, someelectrons deviate from the path and impinge upon the envelope 14. Also, during the operation of the .X-ray tube, some of the other metallic parts may become the usefulness of the tube. V The collected charges may 7 also create field conditions; which disturb the desired flow of electrons from the filament to the target.

In accordance with the present invention, the area of the inner surface of the envelope on which such charges normallytend to collect is provided with a narrow, elongate element or film ofrnaterial of high electrical resistance but of greater electrical conductance than that of'the glass of the envelope, the film being arranged in a path having componentsextending circumferentiallyand lengthwise of the envelope so that the lengthof the film. is sufficient to resist breakdown under the potential applied to the electrodes. One end of the film is electrically connected to the anode and the other end of the film to the cathode, which causes the potential gradient imposed upon the envelope between the electrodes to be distributed substantially evenly along the length of the envelope and also causes the charges collecting on the interior surface of the envelope to .be distributed in an even gradient lengthwise of the envelope so as to minimize the possibility of charges building up to a detrimental value. the film necessary and its thickness so that the film, as stated, may withstand the electrical potential imposed thereon and at the same time carry suf- The actual length of 'ficient current flow to distribute the collecting charges.

The glass of the envelope preferably has slight conductivity, as. is conventional, so that the potential between the'inner and outer surface of the envelope will tend to equalize and prevent puncture.

Referring to the drawings, in accordance with the-illustrated embodiment of the invention the.interiorsurface' of the envelope .14 is coated with an elongate, narrow.

ribbon or film 89 of a material of high resistance but hav ing greater conductivity. than glass, the film being arranged in a helix or spiral or evenly spaced convolutions extending lengthwise of the envelope. Preferably, the intermediate portion of the envelope 14-, that is, that portion which is a subject to the collection of electron charges, is formed with undulations extending inwardly and outwardly to define, relative'to the interior of the envelope, an alternate groove 82 and a ridge 84 which spiral lengthwise of the envelope in a threadlike manner, the film 30 being positioned at thebottom of the groove throughout the length, A suitable film' thereof, as indicated in Figs. 2 and 3. 89 may be positioned in the groove 32 by placing a small tube and abruptly end,

and width is selected electron device under consideration. suitable construction, I have secured desirable results in an X-ray tube such as shown in the drawings wherein .X-ray tube.

from the end of the groove 82 along the interior surface 7 of the envelope to a position wherein it is'overlappedby the gettering deposit 30 which, being conductive and being in contact with the anode supporting sleeve 20, acts as a means to electrically connect that end of the film to the ly connect that end of the film to the cathode 12. -"Ihe spiralling filrn 80 helps to even out the potential gradient along the envelope and bleeds ofif thecharges collecting on the envelope 14 and prevents a build up of the charges to the point Where they interfere with the operation of the As a consequence, provision of an X-ray tube with a threaded envelope and spiralling, conductive film, as described above, permits the tube to be operated at a rating considerably. above that which would be permissible if the tube were provided straight walled envelope.

As has beensuggested hereinbefore, such criteria as the width, thickness and length of the conducting film strip will be dependentupon the characteristics of the As an indication of a an aquadag film carried a current flow of about 5 milliampere where the accelerating voltage appl ed to the elec trodes was about 250 kilovolts.

Having illustrated and described a preferred embodii ment of'the invention, it should be apparent to those skilled in the art thatthe invention permits ofmodification in arrangement and detail. I claim as my lnvention all such modifications as come within the true spirit and scope;

of the appended claims. 7

Iclaim: V 1. An electron discharge device comprising an elongate tubular envelope formed'of material of a high electrical resistance, an electron emissive electrode unit mounted in one end of said envelope and an electron receptive electrode unit mounted at the other end of the envelope, the portion of said envelope intermediate said electrodes being of substantially uniform thickness and undulating in- Wardly and outwardly to define, relative 'to the interior of said envelope, an alternate groove and a ridge spiralling lengthwise of the envelope, a thin, narrow film of a material having high electrical resistance but an electrical conductivity greater than that of said envelope material coated on the interior surface of said envelope in the bottom of said groove throughout the length thereof, 'said groove being of a depth substantially greater than the thickness of said film'so that'said film will be protected from electron bombardment, and means electrically connecting each of the ends of said film to the electrode unit at the corresponding end of the envelope. i

2. An electron discharge device comprising an elongate tubular envelope formed of glass, a pair of electrode units mounted in said envelope in axially spaced relation, the

portion of said envelope intermediate said electrode units being of substantially uniform wall thickness and undu-I.

lating inwardly and outwardly to define, relative to the interior of said envelope, an alternate groove and a ridge spiralling lengthwise of said envelope, a ribbon of graphite coated on the interior surface of said envelope in the botamount of aquadag in the groove, prior to assembling i the groove to leave a film of graphite when the water is driven off. The positioning of the film 8G in the groove.

82 tends to shield the film 80 from direct exposure'to the impingement of electrons and which, if they struck the film,

might cause injuryto the same. The film 3'3 adjacent the end of the anode is extended, as shown at 86 in Fig.1,

tom of said groove throughout the length thereof, said groove being of a depthsubstantially greater than the thickness of said film' so that said film will be protected from electron bombardment, and means electrically connecting the ends of said ribbon to the electrode unit at the corresponding end of the envelope. 7 7

3. An X-ray generator comprising an elongate tubular envelope formed of glass of substantially uniform thick:

ness, a pair of spaced apart electrode units mounted in said envelope at the opposite ends thereof, the opposite end portions of said envelope including re entrant sleeve porwith the conventional,

tions defining a pocket at the corresponding envelope end, said anode unit being mounted on one of said sleeve portions, said cathode unit being mounted on the other of said sleeve portions, the intermediate portion of said envelope undulating inwardly and outwardly to define, relative to the interior of said envelope, an alternate groove and a ridge spiralling lengthwise of said envelope, a narrow film of graphite coated on the interior surface of said glass in the bottom of said groove throughout the length thereof, said groove being of a depth substantialiy greater than the thickness of said film, the opposite ends of said film extending into the pocket of the corresponding envelope end, and means electrically connecting the ends of said film to the electrode unit at the corresponding end of the envelope.

4. An X-ray generator comprising an elongate tubular envelope formed of glass of substantially uniform thickness, a pair of spaced apart electrode units mounted in said envelope at the opposite ends thereof, the opposite end portions of said envelope including re-entrant sleeve por tions defining a pocket at the corresponding envelope end, said anode unit being mounted on one of said sleeve portions, said cathode unit being mounted on the other of said sleeve portions, the intermediate portion of said envelope undulating inwardly and outwardly to define, relative to the interior of said envelope, an alternate groove and a ridge spiralling lengthwise of said envelope, a ribbon of material of greater conductance than said glass coated on the interior surface of said glass in the bottom of said groove throughout the length thereof, said groove being of a depth substantially greater than the thickness of said film, the opposite ends of said ribbon extending into the pocket of the corresponding envelope end, and means electrically connecting the ends of said ribbon to the electrode unit at the corresponding end of the envelope.

5. An X-ray generator comprising an elongate tubular envelope formed of glass of substantially uniform thickness, 2. pair of spaced apart electrode units mounted in said envelope at the opposite ends thereof, the opposite end portions of said envelope including re-entrant sleeve portions defining a pocket at the corresponding envelope end, said anode unit being mounted on one of said sleeve portions, said cathode unit being mounted on the other of said sleeve portions, the intermediate portion of said envelope undulating inwardly and outwardly to define,

relative to the interior of said envelope, an alternate groove and a ridge spiralling lengthwise of said envelope, a narrow film of aquadag coated on the interior surface of said glass in the bottom of said groove throughout the length thereof, the opposite ends of said film extending into the pocket of the corresponding envelope end, and a deposit of gettering material at each of the envelope ends on the envelope wall portions defining said pockets in electrical contact with the adjacent film end and electrode unit whereby to electrically connect said film to such units.

6. An electron discharge device comprising an elongate tubular envelope formed of material at a high electrical resistance, an electron emissive electrode unit mounted in one end of said envelope and an electron receptive electrode unit mounted at the other end of the envelope, the portion of said envelope intermediate said electrodes being of substantially uniform wall thickness and undulating inwardly and outwardly to define, relative to the interior of said envelope, an alternate groove and a ridge spiralling lengthwise of the envelope, a narrow film of a material having a conductivity greater than that of said envelope material coated on the interior surface of said envelope in the bottom of said groove throughout the length thereof, said groove being of a depth substantially greater than the thickness of said film, and means electrically connecting the ends of said film to the electrode unit at the corresponding end of the envelope, said means comprising a first deposit of gettering material overlapping said electron emissive electrode unit and the end of said film adjacent thereto and a second deposit of gettering material overlapping said electron receptive electrode unit and the end of said film adjacent thereto.

References Cited in the file of this patent UNIT ED STATES PATENTS 1,860,210 Spannor et al May 24, 1932 2,005,021 Brasch et a1 June 10, 1935 2,297,305 Kerst Sept. 29, 1942 2,516,663 Zunick July 25, 1950 2,597,562 Bladgett May 20, 1952 2,640,167 Altee et al May 26, 1953 2,703,373 Cummings Mar. 1, 1955 2,714,679 Van De Graafi et a1. Aug. 2, 1955

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