US2567624A - Cathode arrangement for electron discharge devices - Google Patents

Description

Sept; 11, 1951 THOMSON ETAL 2,567,624

CATHODE ARRANGEMENT FOR ELECTRON DISCHARGE DEVICES Filed Sept. 2, 1947 2 Sheets-Sheet 1 Sept. 11, 1951 CATHODE ARRANGEMENT FOR ELECTRON DISCHARGE DEVICES -Filed Sept. 2, 1947 2 Sheets-Sheet 2 5f \r 50 25 y 7/ QTTORNFY iaten ted Sept.

(BATH-ODE ARRANGEMENT FOR ELECTRON DISCHARGE DEVICES Amnf strnximme Thomson, George Paul Wright, and Alexander Rizo Rangabe,

Baldock, England AbplicationSeptember 2, 1947,-Serial 0. 771,734 In Great Britain September 3, 1946 16 Claims.

This invention relates to electron discharge de vices and more particularly to cathode's'of'magnetrons. In prior art magnetrons'serioiis difliculties have been encountered in'centri'rig the cathode, in preventing power loss due to the ca pacity effect between the cathode and its adjacent filamentary heater coil, in preventing overheating oi the filamentary heatercoil during both manufacture andinormal oper'ationof the magnetron, and in delivering at all times the proper amount of neat to the'cathode. This in'vention has for its "primary object the provision of an improved cathode. Other objects are to overcome the above mentioneddifiiculties. Still other objects Will-become apparent to' those skilled in the'a'rt as this description'proceeds.

Prior art magnetrons have employed a centrically located oxide-coated cylinder heated by an adjacent filamentary heater. For best results this cylinder should be accurately positioned'along the axis of the central cavity of the magnetron and difiiculties in centering the cylinder have been encountered.

Our invention employs a large rigid' metalllc cantilever support for one end of the cathode. This large support accurately supports the oathode and overcomes the centering difiiculties here tofore encountered and; also serves to overcome another difficulty encountered in prior art cons't'ruction's in that it conducts excess heat away from'the cathode. H I p 7} Since prior art magnetrons deliver considerable power, they have large filamentary heaters, Furthermore they often are called upon to deliver out put currents which have sharp peaks, or are otherwise modulated at high frequency. Inview of the large size of the filamentary heater and its closeproximity to the cathode, there is a capacitive effect between the filamentary heater and'cathode which permits highifrequ'ency power tojpass from the cathode to the filamentary heater and thence to ground. The power transformer that energizes the filamentary heater does jnot stop this fiow of current since radio frequency energy in the secondary of the transformer easily passes to the primaiy winding in view of the capacity between such primary and secondary v fIfhe high frequency power passing to the primary winding is fed to ground, and since the anode is' grounded there is' an 'efie'ctive loss of power. 7 ,our invention reduces this powerloss in that the filamentary heater is replaced with an electron gun positioned some distance away fromtlie cathode. The cathode'is provided with a siii "bIe 2 and the bombardment of this target "creates enough heat to operate the cathode. By reason of this arrangement, the capacity eiiect tending to transfer power to the filamentary heater-is re duced. Furthermore, more heat can beisupplied to the cathode than heretofore since heating of the prior art cathodes was limited to. thetemperature that the filamentary heater could withstand. During the process of manufacture of a magnetron it is desirable to heat the cathode to temperatures higher than filamentaryheaters could withstand. Such heating, during'processiiig, is for the purpose of converting the carbonates oi' such metals as barium and strontium into oxides; and when a thorium oxide'cathode is used it serves to outgas the cathode member. At times, during operation of the magnetron'as a radio frequency oscillator, back-bombardment of the cathode tndsto overheat it and destroy ordistort the filamentary heater. With our arrange emtm filamentary heater of theelectron gun is not subjected to the high temperature of the cathode,

and hence there is no damage to our device dur:

ing back-bombardment of the cathode or during processing of the cathode. v v

The electron gun referred to above asv one feature of our invention, is, together with its as-v sociated target, located in a-separate evacuated envelope from the one surrounding the cathode. This prevents poisoning of the gun-electrode during outgassing of the magnetron.

In the-drawings: H u

F i g;ure -1 is ,a longitudinal sectional view of -a magnetron embodying certain features of the nv n on; n -Fi gure 2 is a circuit diagramshowing the electrical connections of certain parts of the magnetron to external current-sources.

Figure 3 illustrates the principleof operation of .one type-of magnetron to which the invention maybe applied. 7

, InFigure 3, which illustratesone form of magnetron to which our invention maybe applied, theanode blocl; H) has a plurality of resonator cavities-38 disposed around and opening-into a maincentral cavity 39. The cathode cylinder I l is in the center of the maincavity and magnetic is applied parallel with and along the axis ofthe cathode. Electronsleaving the cathode toward the'po'sitively charged anodeare'defiected from a radial path to a curvilinear path by reas on of the magnetic flux. As theelectrons curve graze the pole pieces 40 which separate the resonator cavities, they become bunched and setup osoilla'tions in the resonator cavities. Power may be extracted from the entire system by a wave guide or other coupling associated with one resonator cavity 38. Magnetrons of this type are now well known and are more fully disclosed in the copending application of John T. Randall and Henry A. H. Boot, Serial No. 407,680, filed August 20, 1941, which issued on February 20, 1951, as Patent No. 2,542,966, as well as in the copending application of James Sayers, Serial No. 577,067, filed February 9, 1945, which issued on March 27, 1951, as Patent No. 2,546,870.

In Figure 1, cathode member I l is shown located in the central cavity 39. A copper shell l2 provided with cooling fins I3 is located in contact with and surrounding the anode ID. A power output channel [4, of any conventional type, may be employed, to extract power from the magnetron. Pole pieces l5 and N5 of the main magnet of the magnetron are provided to apply a magnetic flux parallel to the axis of the cathode I I. These pole pieces are contiguous with the shell l2. A glass envelope portion 11, which is sealed at pip l8 maintains the cathode space evacuated. Another glass envelope portion 36 also serves to maintain the cathode space evacuated.

The cathode member II is accurately centered along the axis of the central cavity by the heavy metallic rigid support 20 which is in turn supported by the heavy copper cylinder 2|. This large rigid cantilever greater accuracy than prior art cathode mounting arrangements, and has the further advantage of serving to hold the cathode in its proper position at all times, there being no warping or bending of the heavy support 20. Support 26 may be composed of any suitable metal, but is preferably either nickel or molybdenum.

During back-bombardment of the cathode during operation of the magnetron, the cathode II is highly heated. The heavy supporting cylinder 26 tends to conduct excess heat away from the cathode l I. The excess heat conducted away from the cathode by metallic member 20 is transferred to copper cylinder 2| where it may be dissipated by forced air or water cooling. Cyldrical shield 23 to electrostatically focus the electrons upon the target 29 in the well known manner.

The electron gun 22 is located in an evacuated space distinct from the one surrounding cathode ll. electron gun 22 is defined by the target 29 at the left, then by the inner walls of copper cone 26, then by the inside wall of copper cylinder 2|, and finally by the glass sealing member 31. The evacuated space surrounding the cathode H is defined at the left by glass envelope portion N, then by the inside wall of pole piece 55, then by copper sealing member [2, then by the inner wall of pole piece It as well as by the outer wall of cone 20 and finally by the glass envelope portion 36 as well as by the outer wall of copper cylinder 2|. By reason of the location of the cathode II and the electron gun 23 in separate evacuated spaces, the filament of the electron gun is not poisoned by any gases given ofi" by the cathode l l during processing or operation thereof.

Referring to Figure 2, a power source 28 feeds primary winding 21 of a step-down transformer,

the secondary winding 26 of which supplies cursupport enables much The evacuated space Surrounding rent to the filamentary coil 50 of electron gun 22. In order to provide high voltage to effect bombardment of the target 29, a source 35 of high voltage direct current has its negative pole connected to gun 22 and its positive pole connected to the metallic anode [0. The anode It! is in turn connected electrically to the target 29 through the secondary Winding 32 of a pulse transformer 3| which will hereinafter be more fully described. It is obvious that the structure thus far described is a complete cathode heating arrangement for by properly adjusting the voltage on shield 23 and the voltage of source 35, the electrons can be caused to bombard the target 29 to any desired extent. During processing of the tube, the voltages can be raised so as to effect large quantities of heat. When the tube is in operation, whereby some selfheating of the cathode occurs, the voltages can be reduced or stopped altogether and the heavy metallic cone 2!] relied upon to conduct excess heat away.

In Figure 2, there is shown a pulse generator 36 feeding the primary 33 of pulse transformer 3|. Pulse generator 30 may be any form of modulator known to the art, but is here referred to particularly as a generator of very short pulses, each pulse being of duration of the order of a microsecond in duration. The pulse generator 39 has a repetition rate between several hundred and several thousand pulses per second, depending on the particular requirements of the system. The short duration of the pulses tends to create high frequencies which will readily pass, due to capacitive effects, between adjacent objects. With prior art magnetrons, large quantities of energy were lost due to transfer of energy by capacity from the cathode to the filamentary heater, and from that heater to ground through the capacitive effectsbetween the primary and secondary of the filament transformer. With ourconstruction, the filament 40 of the electron gun 22 is located at a position so removed from the cathode Ii that the capacity between the cathode H and the filament 40 is negligible.

It will be seen that the electromotive force of the secondary 32, during the delivery of a pulse, opposes the high voltage existing across source 35 and thereby stops the bombardment of target 29 for the duration of the pulse. This is not a disadvantage since the heavy copper member stores suificient heat as to iron out these very slight irregularities, and moreover the time duration of the pulses is often short as compared with the spaces between pulses.

It is apparent that the invention has its greatest advantages and novelty when applied to magnetrons, but some of the features of this invention have utility in connection with other types of electron discharge devices. We desire it understood that the claims are not limited to magnetrons except as they are by their terms so limited.

We emphasise that it is important, to achieve successful operation of the electron gun heating of a cathode, that the intense magnetic field necessary for operation of the magnetron shall be symmetrical around the cathode and shall not be so disposed as to unwantedly deflect or inhibit the electron stream intended to strike the cathode or the target connected thereto. We have found that it is desirable to analyse the magnetic field around, and in particular rearwardly of, the cathode or its target, and to adaeeam support for said cathode, and means adjacent said support for heating the cantilever "support to thereby heat the cathode.

2. A magnetron "comprising, an anode block defining a cavity, a cathode within said cavity, a support for the cathodeln heat 't'rans'ferrelation therewith, and means "adj acent "said support for bombarding said support with eiectrons "to thereby heat the cathode.

3. A cathode element comprising an electron emitting member, a "cantilever supporting said member in good heat transfer relation with said member, and means adjacent said cantilever for bombarding said cantilever with electrons to thereby heat said electron emitting member. 7

4. A cathode element for electron tubes comprising an electron emitting member, a cantilever metallic support forsaid'niember in good heat transfer relation therewith, means adjacent said support for heating-said support to thereby heat the emitting member, -and metallic heat dissipating means of large area and in good metallic heat transfer c'on'tact'with the support to dissipate excessive heat.

5. An electron discharge devicecomprising an evacuated envelope having a cathode therein, a metallic cantilever support extending through the envelope and constituting-the sole support for said cathode, saidsupport having a large solid cross-section to effect good heat conduction, and means located'o'utside'of said envelope "for applying heat to a limited portion of said support to thereby heat the cathode by-con'duction of heat from said limited portion, said means constituting the sole means for initiating heating of the cathode.

6. A magnetron comprising an anode block having a central cavity, a cathode located in said cavity, a metallic heat transfer member attached to said cathode, said heat transfer member including an electron target, an electron gun aligned with said target for bombarding said target, a modulator having its negative pole connected to said cathode and its positive pole to said anode, and a high voltage power supply for the electron gun having its negative pole connected to the electron gun and the positive pole connected to said anode.

7. A magnetron comprising an anode block defining a central cylindrical cavity and a plurality of resonator cavities around the central cavity and opening into the central cavity, an oxide-coated cylindrical cathode member, means constituting the sole support for said cathode member comprising a cantilever metallic supporting member with the suspended end attached to and in good heat transfer relation with the cathode member, said supporting member positioning said. cathode member within and concentric with the central cavity, magnet pole pieces on opposite sides of said anode block for establishing a magnetic field parallel to the axis of the central cavity, one of said pole pieces defining an opening through which said supporting member passes, an evacuated envelope surrounding said central cavity, said envelope init'y, said cantilever support constituting the'o'nly eluding a portion of said supporting member, another portion of said supporting member being outside said evacuated envelope and forming an electron target, an electron gun separated from said target a suificient distance to avoid losses from capacitive effects, means connected to said target and gun for polarizing said target and "gun to efiect bombardment of the target with "consequent heating of said cathode, and-an evacuated envelope surrounding the electron gun "and target.

8. A magnetron comprising an anode block defining a central cylindrical cavity and a plurality of resonator cavities around the 'rnain cavity and opening into the main cavity, magnet pole pieces adjacent opposite endsof the-central cavity for setting up a field in the central cavity parallel to the axis thereof, one of said magnet poles having an opening therethrough "aligned with said central cavity, a cylindrical oxidecoated cathode member, means supportingsaid cathode member in'said central cavity and concentric therewith, said means constituting the sole support for said cathode member and consisting of a tubular member with its axis an'extension of that of the cathode member, said tu' bular member passing through said opening in said pole piece, an evacuated envelope'surrou'n'ding the central cavity and sealed to the outer wall of said tubular member, saidtubular member being sealed at the first end thereof'which supports the cathode member, a sealing member at the second end of said tubular member "to maintain the same evacuated, an electron gun located within the tubular member, pulsing means electrically coupled to said anode and cathode for applying pulse potentials between said cathode and anode, means electrically coupled to said electron gun and-said anode'for supplying high voltage potential'to operate the'electron gun, means adjacent said electron gun to focus the output of the electron gun upon an inner wall of said tubular member nearest the cathode member whereby to heat the cathode, and cooling means associated with the second end of said tubular member.

9. A cathode structure comprising an elongated metallic member a first end of which is an electron emitting surface, said member defining an opening throughout substantially the remainder of the elongated member, said first end sealing the first end of said member, means sealing the second end of said member, an electron gun Within said opening for bombarding the inside of said member at a point near said surface to thereby heat said surface, means sup porting said member only at positions remote from said first end, said supporting means constituting the sole support for said member, an anode adjacent said cathode, and an envelope surrounding said cathode and anode, said envelope being sealed around the outer wall of said tubular member.

10. A magnetron as defined in claim 1 including a first evacuated envelope surrounding the heating means and the portion of said support heated thereby; and a second evacuated envelope surrounding the remainder of said support as well as surrounding the anode block and the cathode.

11. A cathode element comprising an electron emitting member, a cantilever in good heat transfer relation with said member and supporting the latter, electron bombarding means for bombarding said cantilever with electrons to thereby heat said emitting member, said cantilever being tubular and containing the electron bombarding means therein and sealing means for rendering the interior of said cantilever airtight.

12. A magnetron comprising an anode block defining a cavity, a cathode within said cavity, a support for the cathode in heat transfer relation therewith, and an electron gun including an electron emitter for bombarding said support with electrons to thereby heat the cathode, said electron gun having its electron emitter substantially spaced from said support to thereby efiect low capacity between the electron emitter and said cathode.

13. A cathode element as defined in claim 3 in which said bombarding means comprises an electron emitter substantially spaced from said support to thereby efieot low capacity between the last-named electron emitter and the cathode.

14. A cathode structure comprising a unitary member of good thermal conductivity, electron emitting means on a limited portion of said member near a first end thereof, means which constitutes the sole support for said member and is attached to it remote from said first end, and means adjacent said member for heating the member between said limited portion and the second end thereof, said last-named means including a heat generator of suflicient capacity to raise the temperature of the electron emitting means to electron emission temperature.

15. A cathode element comprising an electron emitting member, a support for said member, said support being contiguous with said emitting member and in good heat transfer relation therewith, said support being composed of a material having good thermal conductivity, and means adjacent said support for bombarding said support with electrons to thereby heat said support and in turn heat said electron emitting member, said last named means including a source of electrons and means for accelerating and directing electrons derived from said source upon said support.

16. A cathode element comprising an elongated structural member, an electron emitting surface formed upon said structural member, said structural member being composed of a material having good thermal conductivity and being in good heat transfer relation with said electron emitting surface, and means adjacent said structural member for bombarding said member with electrons to thereby heat said member and in turn heat said electron emitting surface, said last named means including a source of electrons spaced from said elongated structural member and also including means for accelerating and directing electrons derived from said source upon a portion of said structural member.

AMHERST FELIX HOME THOMSON. GEORGE PAUL WRIGHT. ALEXANDER RIZO RANGABE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,309,704 Stoekle July 15, 1919 1,419,547 Ehret June 13, 1922 2,126,894 Knoll et a1 Aug. 16, 1938 2,414,785 Harrison et a1. Jan. 21, 1947 2,435,804 Spooner Feb. 10, 1948 2,438,194 Steele, Jr., et a1. Mar. 23, 1948 2,466,922 Wax Apr. 12, 1949 2,478,534 Kather Aug. 9, 1949 FOREIGN PATENTS Number Country Date 229,019 Great Britain -1 Feb. 19, 1925 453,214 Great Britain 1934 509,102 Great Britain July 11, 1939

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