US2349468A - Electron emission device - Google Patents

Description

T Ma 23,- 1944. c. M. SLACK ET AL 5 3 ELECTRON EMISSION :DEVICE I Filed July 22, 1942 INVENTORS 'ATTORNEY Patented May 23, 1944 ELECTRON EMISSION DEVICE -Charles M. Slack, Glen Ridge, and Louis F. Ehrke,

Newark, N. J assignors to. Westinghouse Electrio & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 22, 1942, SerialNo. 451,932

7 Claims.

The present invention relates to electron discharge devices and particularly to such devices as a-high voltage X-ray tube of the type shown in the patent to C. M. Slack, No. 2,311,705,'issued February 23, 1943, and assigned to the same assignee as the present invention.

As therein shown and described, the discharge device operates upon what is termed field emission of electrons and is accordingly designed to pass high currents of the magnitude of amperes. In other words, there is no initial thermionic source of electronsgbut on the contrary a high potential gradient is employed to initiate an arc dischargebetween an auxiliary electrode and the cathode of the device solely by field emission of electrons from the cathode, which are discharge thus becomes the source of electrons for the high currentfiow between the cathode and anode. However, this high current flow is necessarily of extremely short duration being of the order of but a fraction to a few microseconds as' otherwise the device would be destroyed.

Also, as shown in the above noted pending application, the leading-in conductors for the various electrodes are spaced from each other the customary distance for the high voltage applied to prevent spark-over therebetween by ionization of the air-gap. This accordingly limited the dimensions of the device which formerly required an increase in size as-the operating voltage increased.

It is accordingly an object of the present invention to provide an electron discharge device wherein the air-gap spacing between the leading-in conductors and all metal parts of opposite instantaneous polarity are considerably less thanthat normally required to prevent sparkover'at the voltage applied 'therebetween.

Another object of the present invention is the provision of an electron discharge device wherein the metallic parts of opposite instantaneous polarity are spaced from each other a reduced distance to enable the device to be greatly reduced in size and without spark-over occurring between such parts by ionization of the air-gap therebetween regardless of the applied voltage.

Another object of the present invention is the provision of an electron discharge device such as an X-ray tube which is operable by field emission of electrons, and wherein the air-gap spacing between metallic parts carrying opposite instantaneous polarity is greatly reduced to thereby reduce the overall dimensions of the tube without'attendant spark-over between such metallic entrant sleeve. "it will be seen that this auxiliary electrode l0,

parts, regardless of thevoltage applied therebetween.

Another object of the present invention is the provision of a field emission X-ray tube wherein variouspartsof the tube may be assembled in correctly spaced relation prior to insertion and sealing into the tube for complete fabrication thereof.

Still further objects of the present invention will become obvious to those skilled in the art by reference to the accompanying drawing where- 1n:

Fig. 1 is a fragmentary view partly in crosssection of an electron discharge device constructed in accordance with the present invention;

Fig. 2 is an end view of the cathode electrode as viewed at the line II-II of Fig. 1 and looking in the direction indicated by the arrows;

Fig. 3 is a sectional view taken on the line IIIIII of Fig. 2" and looking in the direction indicated by the arrows;

Fig. 4 is an enlarged fragmentary sectional view of the cathode end of the device taken on the line IVIV of Fig. 2;

Fig. 5 is a fragmentary View partly in crosssection similar-to Fig. 1 showing a slight modification which the structure of the device may take;

Fig. 6 is a sectional view taken on the line VI-VI of Fig. 5;

Fig; 7 is a fragmentary view of a still further modification which the electrode construction may take, and

Fig. Bis a fragmentary view partly in crosssection of yet another modification which the electrode structure may take.

Referring now to the drawing in detail, the electron discharge device shown in Fig. 1 comprises an X-ray tube of the type operable by field emission of electrons, such as shown and described in the above-mentioned pending application. Such X-ray tube comprises an evacuated envelope 5 havingan anode 6 of a refractory metal, such as tungsten or the like, and supported by a leading-in conductor or rod 1 secured to a metallic'terminal 8,-the latter of which forms an hermetic seal with the envelope 5 as hereinafter described.

The opposite end of the envelope 5 is provided with a reentrant tube 9 of the same vitreous material as the envelope and an auxiliary electrode, shown generally at H], is supported by this re- By particular reference to Fig. 4

as shown, comprises a split metallic sleeve [2 fitting over the vitreous reentrant tube 9 and held in place by an annular collar or clamp l3. The end of this metallic sleeve is closed by a metallic block or the like It telescopically engaging the sleeve l2 and held therein by set screws l (Fig. 3). This block i4 is provided with an elliptical opening IS in the wall thereof and in the direction of the anode, the open end of the block converges to form parallel side walls I! extending normal to the longitudinal axis of the tube, as can be appreciated from Figs. 2, 3, and 4.

A cathode electrode l8 which, as shown, is in the form of a rectangular plate and of a low work function material, such as thorium, molybdenum or the like, is disposed in the rectangular opening I6 with the edge thereof spaced but a few millimeters from the edge formed by the converging side walls [1. Such cathode is supported by a leading-in conductor in the form of a rod or the like l9 connected to a metallic terminal 26, the latter of which is hermetically sealed, in the same manner as the terminal 8, to an annular wall 22 formed interiorly of the reentrant tube 9. In order to conduct energy to the auxiliary electrode ID, the latter is provided with a lug 23 formed on the periphery of the metallic sleeve l2 which has a recess 24 telescopically engaged by a leading-in conductor or rod 25 likewise secured to a metallic terminal 26 sealed to the envelope '5.

As shown more clearly in Fig. 1, the various terminals 8, 20 and 26 are of cup-shape with the peripheral edge thereof tapered and sealed to the vitreous envelope. Such metallic terminals may be of copper with the leading-in conductors or rods soldered to the interior thereof or, if desired, they may be formed of an alloy having a coefficient of expansion closely approximating that of the vitreous envelope.

Due to the close spacing between the cathode l3 and the auxiliary electrode 10, and as disclosed more in detail in the above-mentioned pending application, a high potential gradient results which causes field emission of electrons from the unheated metallic cathode. Apparently metallic particles are pulled out of the oathode, causing a low voltage metallic arc discharge between the cathode and auxiliary electrode. This results in the generation of positive ions and in reduction in the space charge independent of any residual gas. Also, since thi are discharge occurs between the auxiliary electrode and the cathode at a point nearest the anode, the electrostatic field readily influences the arc discharge which, together with reduction in space charge, results in the flow of electrons to the anode with substantially no accompanying flow of metallic particles or positive ion bombardment of the anode and the electron discharge alone serving for the flow of the entire available energy of a magnitude of amperes.

In order for the discharge device to withstand currents of the magnitude of hundreds and even thousands of amperes, the period of energization is necessarily of extremely short duration, being of but a few micro-seconds to milli-seconds duration. To supply such energy a circuit similar to that described in the above noted application is shown in Fig. 1 which comprises a high tension transformer 30 provided with a primary winding 32 and a high voltage secondary Winding 33 which generates a voltage ranging from 20 to several hundred kilovolts. The center point of the secondary winding is connected to ground high potential gradient is present which draws auxiliary electrode the air at the applied potential.

34 and one end of this winding is connected through a rectifying valve tube 35 to one side of a bank of condensers 36 and to the metallic terminal 8. Similarly, the other end of the secondary winding is connected through a sound rectifying valve 39, to the opposite side of the bank of condensers 36 and to one terminal of a spark-gap 31, the other terminal of which connects with the cathode l8.

The bank of condensers 36 are grounded at 40 and a spark-gap 4| is interposed between a portion of the condenser bank with a control device 38 being connected thereon and to ground. The condensers are charged by the transformer 30 during the full wave of the alternating circuit to the maximum energy delivered by the transformer which, however, is insufficient to cause breakdown of the spark-gap 31. When it is desired to energize the X-ray tube or discharge device 5, the control device 38 is set into operation. As shown in the above-mentioned pending application, this applies an additional charge to the bank of condensers 36 which exceeds the breakdown value of the spark-gap lit with the spark-gap 4| preventing the control device being connected to ground at 40. Since the spark-gap connects with the terminal 20, the discharge circuit of the bank of condensers is thus across the cathode l8 and anode 6 of ;the tube 5.

electrode It is impressed with the same high potential as the anode 6, and the total voltage available from the source (condensers 36) is impressed across the cathode l 8 and. auxiliary electrode l0.

Since the spacing therebetween is short, a

electrons from the metal of the cathode due to field concentration giving rise to field emission" of electrons. This causes a low voltage metallic arc discharge which, as above explained, becomes the source of electrons for carrying the current from the cathode 18 to the anode 6. This entire phenomenon occurs, as above mentioned, in a period of time ranging from a fraction to a few micro-seconds. Immediately following the metallic arc discharge between the auxiliary electrode 10 and the cathode l8, the current attempts to rise; but because of the resistance 42, the voltage drops and the potential of the I0 almost immediately becomes that of the cathode [8. A high resistance 43 is connected in parallel with the X-ray tube, but since the current flowing through this resistance is but a minute fraction of the several hundreds or more amperes flowing between the anode 6 and cathode l8, such resistance serves to control the uniformity of breakdown voltage of the spark-gap 31 for its particular setting In the above-mentioned pending application the air-gap spacing between the metallic leadingin conductors or terminals for the anode, auxiliary electrode and cathode are shown spaced apart the customary distance required to prevent spark-over therebetween by ionization of We have discovered that in a discharge device of the type herein shown and described which operates on the principle of "field emission of electrons,

the time of energization is short that pre-: vious requirements as to spacing of metallic smears parts of opposite instantaneous" polarity "exteriorly of the device ar unnecessary.

In other words, the entire period" of energization of the discharge device is much shorter than the period required 'forion'ization of the air-gap between metallic parts, which results in sparkover. Under these conditions and as shown in the accompanying drawing",'the spacing between,

however, that, as shown in the drawing-this latter spacing is considerably grea-ter than the spacing between the metallic terminals 20' and 26. This is because the'period of application of the high potential between the terminals 20 and 26 is of the order of a fraction ofa micro-second,-

whereas the electron discharge occurring between the cathode i8 and anode 6 is of longer duration and thus the permissible reduction in air-gap spacing ofthe terminals'Zll and'26 is greater than that permissible between"8a nd 2|]; Accordingly, a tube of much smaller dimensions "results than heretofore believed possible because of the considerable reduction in the air-gap spacing of theterminalsdespite the exceedingly high instantaneous potentialexisting across the terminals. Moreover, due to the short period of energization of the tube, there is no spark-over between the various metallic parts exteriorly of the device.

A slight mechanical modification is shown in Figs. 5 and 6 wherein the metallic terminals and 26 are dispensed with and in lieu thereof the leading-in conductors 44, and 46 are sealed directly to the glass wall of the envelope. In addition, fabrication of the tube of this mocli' fication is facilitated in that the leading-in conductors M and 46 form a threaded engagement with their respective electrode assembly. For example, the anode 6 in this particular modification is supported by a metallic sleeve 41 fitting over a reentrant stem 48 and held in place by a clamp 49. The anode stem is provided with a threaded angular portion 50 which is readily engaged by the leading-in conductor 44 prior to sealing-in the latter to the envelope wall. Similarly, the auxiliary electrode I0 is provided with a metallic strip 52 having a threaded angular portion 53 also readily engageable by the leading-in conductor 45 prior to its being sealed into the glass wall of the envelope.

The modification shown in Fig. '7 difiers slightly from that shown in Figs. 5 and 6 merely in that the leading-in conductor for the auxiliary electrode Ill is sealed into a press formed in a side arm 54 projecting from the wall of the tube adjacent the cathode end, but again spaced from the cathode leading-in conductor 55 which carries a high potential of opposite instantaneous polarity a distance considerably less than that required to prevent spark-over and limited only by mechanical requirements.

The same also applies to the modification such as shown in Fig. 8 wherein the leading-in conductors 56 and 57 for the cathode l8 and auxiliary electrode l0 are sealed together in a single reentrant press portion 58 of the envelope 5, as is -customary with low voltage discharge devices, but again without danger of spark-over despite the close spacing between the leading-in con ductors 'and'the high voltage momentarily existing therebetween during energization of the device.

' Itthu's becdmesobvious to those skilled in the art-thatan electron discharge device is herein providedwhich is compact and of small dimensions. Inasmuch as'the time'period of-energization' of the device is of a duration less than that required to'cause ionization of air; all'leading-in conductors and metallicparts of opposite instantaneous polarity are 'spacedapart a reduced distance rather than in accordancewith customary electrical requirements for a given applied voltage, "and despite their close spacing no sparkover occurs, even though'such parts are subjected to a high voltage of opposite instantaneous polarity. Moreover, by the provision of threaded engagement of the various parts, fabrication of the device is greatly facilitated since the main and auxiliary electrodes may-be assembled in the correctly spaced relationship outside the envelope andsealed-in as a unit, thus' permitting more accurate and rapid assembly.

Although several embodiments of the present invention have been shown and described, it is to be understood that still further embodiments thereoi may be made without departing from the spirit, and scope of the appended claims.

We claim:

,1. An electron discharge device comprising an envelope, electrodes in said envelope between which a high potential difference occurs during operation of said device, metallic leading-in conductors connected to said electrodes for transmitting energy thereto and having a spacing therebetween exteriorly of said device less than that necessary to prevent spark-over at the applied voltage, and means for energizing said device for a period of time so short that ionization of air with attendant spark-over between the closely spaced metallic conductors does not occur regardless of the high instantaneous potential difference existing between said conductors.

2. An electron discharge device comprising an envelope, electrodes in said envelope between which a high potential difference occurs during operation of said device, metallic leading-in conductors sealed to said envelope and connected to said electrodes for transmitting energy thereto having a spacing therebetween exteriorly of said device below that electrically necessary to prevent spark-over at the applied voltage, and means for energizing said device for a period of time less than that required to cause ionization of the air between the closely spaced metallic conductors to prevent the occurrence of spark-over therebetween regardless of the high instantaneous potential difference existing between said conductors.

3. An electron discharge device comprising an envelope, electrodes in said envelope between which a high potential difference occurs during operation of said device, metallic leading-in conductors connected to said electrodes for transmitting energy thereto and spaced apart exteriorly of said envelope a distance considerably less than the normal electrical requirements necessary to prevent spark-over therebetween at the applied voltage, and means for energizing said device to cause the field emission of electrons accompanied by an electron discharge between said electrodes and for a period of time less than that required to cause ionization of the air with resultant spark-over between said metallic conductors regardless of the high instantaneous potential difference existing between said conductors.

4. An electron discharge device comprising an envelope, electrodes in said envelope between which a high potential difference occurs during operation of said device, metallic leading-in conductors sealed to said envelope and connected to said electrodes for transmitting energy thereto and having a spacing therebetween exteriorly of said device considerably less than that normally electrically necessary to "prevent spark-over therebetween at the applied voltage, and an energizing circuit for supplying energy to said device for a period of time so short that ionization of air with attendant spark-over between the closely spaced metallic conductors does not occur regardless of the high instantaneous potential difference existing between said conductors.

5. An X-ray tube comprising an envelope, electrodes in said envelope between which a high potential difierence occurs during the production of X-rays, metallic leading-in conductors connected to said electrodes for transmitting energy thereto and having a spacing therebetween eX- teriorly of said tube less than that necessary to prevent spark-over at the applied voltage, and means for energizing said tube for a period of time so short that ionization of air with attendant spark-over between the closely. spaced metallic conductors cannot occur regardless of the high instantaneous potential difference existing between said conductors.

6. An X-ray tube comprising an envelope, electrodes in said envelope between which a high potential difierence occurs during the production of X-rays, metallic leading-in conductors sealed to said envelope and connected to said electrodes for transmitting energy thereto having a spacing therebetween exteriorly of said tube below that normally electrically necessary to prevent spark-over at the applied voltage, and means for energizing said tube for a period of time less than that required to cause ionization of the air between the closely spaced metallic conductors to prevent the occurrence of sparkover therebetween regardless of the high instantaneous potential diiTerence existing between said conductors.

7. An X-ray tube comprising an envelope, electrodes in said envelope between which a high potential difference occurs during the production of X-rays, metallic leading-in conductors connected to said electrodes for transmitting energy thereto and spaced apart exteriorly of said envelope a, distance considerably less than the normal electrical requirements necessary to prevent spark-over therebetween at the applied voltage,

and means for energizing said tube to cause field emission of electrons accompanied by the generation of X-rays and for a period of time less than that required to cause ionization of the air with resultant spark-over between said metallic conductors regardless of the high instantaneous potential difference existing between said conductors.

CHARLES M. SHACK.

LOUIS F. EI-IRKE.

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