US2574562A - Electron discharge device and circuit - Google Patents

Description

N0 13, 1951 c. w. HANsx-:LL

ELECTRON DISCHARGE DEVICE AND CIRCUIT Filed Feb. 27, 1946 000000 0 000000000. 0 000000 0000000000000000000 000000 ,00u0000vvv%%%%%%% 0 000 ATTORNEY Patented Nov. 13, 1951 ELECTRON DISCHARGE DEVICE AND CIRCUIT i Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 27, 1946, Serial No. 650,713

17 Claims. (Cl. Z50- 36) This invention relates to electron discharge devices and circuits therefor, and particularly to magnetrons employing secondary emissive cold cathodes. The invention is especially applicable to amplifiers for high power use, such as may be employed for induction heating purposes, though not limited thereto.

The present invention contemplates the use of a magnetron employing secondary electron emission from cold cathodes produced by electron bombardment of the cold cathodes under the influence of electromagnetic fields oscillating at a frequency much greater than the input and output frequency.

In order to initiate electron emission, I provide in a magnetron a relatively small emission hot cathode 'for producing a priming current, and I control this electron priming current by means of a. grid, thereby making the priming current almost independent of electron emission from the hot cathode, so long as the emission is equal to or greater than the priming current. This small control emission is made to impinge upon a cold cathode in order toproduce secondary electrons. Secondary electrons (under the influence of the magnetic field, a direct current electric eld, and very high frequency electromagnetic fields) are made to circulate out from the cold cathode and back to it with increased energy, so as to produce more secondary electrons and this process is repeated until a relatively very large eletro emission is made available to supply current to an output anode. During the process of secondary emission multiplication, the electrons (in transit from the cold cathode out and back) are given a component of motion parallel to the axis of the vacuum tube but away from the region of the hot cathode toward the region of the output anode. When this motion in an axial direction is great enough, but not too great, the production of secondary emission can continue only so long as the priming emission from the hot cathode is allowed to fall upon the cold cathode. Thus, the very large emission from the cold cathode may be started and stopped by starting and stopping the very small current fro-m the hot cathode. If the current from the hot cathode is started and stopped at a rate of, let us say, 50,000 current pulses per second, then the amplier will supply 50,000 cycles per second output power and the output power can be made to be enormously greater than the input control power.

A cold cathode has many practical advantages for the production of secondary emission. By making it the coldest part of the internal surface of the evacuated magnetron tube, it may be kept continuously activated by an alkali metal such as caesium. The caesium in the vacuum tends to migrate continuously to the coldest surface in a vacuum vessel. If the cathode provides the coldest surface, then the caesium will accumulate there and provide a surface of large secondary emission ratio which is continuously healing itself by caesium condensation as the caesium is sputtered off by ion bombardment. In addition, electron bombardment will not be able to remove the caesium due to a rise in temperature and the consequent rapid re-evaporation, if the cathode is adequately cooled as is contemplated in the present invention.

In general the vapor pressure of the activating material in the main body of the tube will be that corresponding to the cathode temperature and the lower the cathode temperature the lower will be the vapor pressure. In practice, the vapor pressure may be kept low enough to make the effects of ionization of vapor upon the tube characteristics quite small.

When using relatively large tubes, I propose to cool the secondary emission cathode by a continuous ow of water, or any other suitable cooling fluid, in a manner which will assure that the secondary emissive cold cathode is the coldest surface in the tube. In small tubes, air cooling properly applied can accomplish a like result. Since a cold cathode is used in the present invention in order to produce the emission which appears as the anode current, I avoid completely the diicult problem of trying to produce large thermionic cathode emission, such as is encountered in conventional amplifier tubes.

rIhe following is a more detailed description of the invention, in conjunction with the drawing whose single figure illustrates in cross-section a. magnetron tube constructed in accordance with the principles of the invention, and suitable circuits therefor.

Referring to the drawing, there is shown a magnetron oscillation generator used as an amplifier tube, and having a small hot cathode l, a control electrode or grid 2, so to speak, surrounding the hot cathode, and spaced therefrom in an axial direction there is provided a nonthermionic or cold cathode 3 suitably coated with secondary emissive material such as caesium. It should be noted that the hot cathode, control electrode and the cold cathode are arranged along the axis of the tube. Surrounding these elements `there is provided a multiple cavity anode resonant structure 3.3 having a plurality of inwardly projecting target portions. This multiple cavity anode structure may follow the principles de scribed in my United States Patent 2,217,745, granted October 15, 1940. spaced from and surrounding the cold cathode 3 in axial relation thereto is an anode l! for collecting the secondary emissive electrons emitted from the cold cathode. The anode structures 33 may constitute the envelope per se and this envelope is suitably sealed by ymeans of glass seals 6. The interior of the envelope is suitably evacuated. A coil 5 surrounds the anode structure for producing a magnetic field which is parallel to the axis of the tube along which lies the hotI cathoderand-the cold-cathode, and this eld acts transversely ofthe cathode-toanode structure path. Obviously a permanent magnet may be used in place offthe coilffto produce the magnetic field, as is well known in the magnetron art.

The cold cathode 3 is hollow in its interior and iszcooledby means of cooling water supplied to l lan;inlet pipe 'I. The outlet pipe/is designated 8.

The. direction of Water now is `indicated in the drawing 'by means of the arrows. Reference is madeeto my co-pendingapplications, Serial No. 534,066, led May 4, 1944, now Patent No. 2,420,- 744 issued May 20, 1947, and Serial No. 553,138, l'edSeptemberB, 1944, now'APatent No. 2,448,527, -issued September `'7, 1948, for descriptions of cathode cooling systems.

i `The. field coil 5 is energized by a source of ldirect-current 9 over leads lli. The variable resistor--I-I provides a means for varying the amount of Ymagnetizing current fed to the ,eld coil. The thermionic cathode I is heatedvby `means of a source Aof alternating current power fed to the primary winding of iron core transformer I2. The secondary winding of the transformer I2 is connectedvia leads I3 to the terminals of the hotcathode-I. The anode 4 is maintained at a -relatively high positive potential-relative to both the hot-cathode I land the control electrode 2 by means of a direct current power source I4. Itshouldbe noted that the positive terminal of '-thev direct current power source is connected tothe# output anode A through ya tuned output circuit I5. Thenegative terminal of the high voltage power source vI4 'isconnected by means 01E-lead I6-to`a tap on a resistor Iig-one terminal ofwhich is connected to a tuned input circuit I8 'andfthence to the control electrode 2 over a lead I9; and the other terminal of whichis connected to'fthe hot cathode `I by means of lead 29 and one of theflament lheating leads I3. A suitable source of low direct current potential 2! is f-'- connected to these terminals of the resistor il. It will thus be seenV that the control electrode 2 is'maintained at a low negative direct current potential relative to the cathode by means of source 2|, while the small hot cathode I is at la direct current potential which `is more positive than the potential of the cold cathode 3. This is because'the Ycold cathode 3 is connected -by way -of lead 22 to the negative terminal of the `high voltage direct ycurrent power source Id. f" An input source of alternating current power Y23 such as power of low radio frequency) is magnetically coupled to the parallel tuned input circuit Iwlfiich is connected between the control electrode 2 and the hot cathode I. Element 24 designates a by-pass condenser for energy voffthe'flow Yradio frequency supplied by source `23y andfthis condenser is connected between one cathode lead I3 and the tuned circuit i8. Element 25 designates another by-pass condenser the-resonant frequency of the multiple cavity anode structure.

which is connected across the high voltage direct current power source I4 for by-passing energy of the operating frequency.

In order to set up a very high frequency electromagnetic field in the multiple cavity resonator anode structure 33, there is provided a source 23 which is coupled via a `coaxial transmission line 27 to a coupling loop 28 located in the interior of the anode structure. Source 2'6 furnishes very high frequency power to the resonator at In this manner the system relies upon the energy from source Z to build up a field-in the 4resonator rather than on any self-oscillating characteristic of the magnetron, although some self-regeneration may take place afteremission .is` built up. In practice, source 25 is ofsuch magnitude as to set up a voltage of several thousand volts `between adjacent target portions of the cavity resonator anode structure The frequency of source y'26 may be of the order of hundreds or thousands of megacycles and, of course, will be matched by the resonant `frequency of the anode structure.

In the operation of the system of the invention, the source 2S of low radio frequency potential serves to control the flow of priming current or'initial electrons from the hot cathode I without which priming electrons secondary emission multiplication cannot readily take place from thecold caesium coated cathode 3. It'will thus be seen that the input `radio frequency from source 23 can be used to determine whether or not oscillations take place and, if they do take place, to control the frequency of oscillations. 1n practice at present, source. `23 Vcan have a frequency anywhere in the range of audio or moderate radio frequencies from Zero up to about one megacycle. The tuned output circuit I5, on the other hand, can havethe same frequency as that of. source 23 or a frequency harmonically related to the frequency of source 23.

During the operation of the magnetron system of the invention, the anode 4 (which is concentrically positioned With respect to and envelopes the cold cathode 3) is maintained at an average large positive direct current output potential, and this anode potential' varies cyclically at the input and output frequency above and below the direct current potential. The yanode potential relative to the cold cathode 3 reaches a relatively low positive value at the time of maximum electron current ow between the anode 4 and the cold cathode 3. Stated otherwise, when the output anode 4 is at a relatively small positive potential (due to the peak vinstantaneous radio frequency voltage in the tuned output circuit opposing the direct current anode potential), thenumber of electron multiplications on the cold coated cathode 3 is large, thus resulting in large totalremission from the Icold cathode and hence large instantaneous output anode-to-cold cathode current. When the anode lis at its highest positive potential, due to the instantaneous value of radio frequency voltage aiding the direct current anode potential, electron multiplications on the cold cathode are few or none and the instantaneous output anode-to-cold cathode current is very small. It Will thus be seen that the higher the value of instantaneous output anodeto-cold cathode voltage, the lower Will be the anode current, and the lower the value of instantaneous output anode-to-cold cathode voltage the higher will be the anode current. This www@ the condition for negative resistance oscillaons. i

.At the time when peak anode-to-cold cathode current is desired, the moderate positive potential on the output anode 4 and the instantaneously negative potential of the hot cathode I and its control electrode 2 with respect to the cold cathode 3 provides a component of electric iield around the cold cathode which causes electrons in transit out from the `cathodes through the magnetron anode structure and back to the cold cathode to -be subject to van acceleration in an axial direction away from the hot cathode and toward the output anode 4. In a tube Asuitably designed and operated, this acceleration toward theoutput anode 4 is an important factor in the amplication process.

WhenI the output anode potential is low, the axial acceleration of the hopping electrons toward the anode will be small. This results in a large number of hops before the eiect of a group of electrons originating at the hot cathode reaches the output anode in the form of. a large pulse or component of anode current. Because of the large number of hops, secondary emission multiplication of the initial current may be enormous and result in an enormous amount of i anode current as compared with the current from the hot cathode.

Depending upon the character of the surface oi the cold cathode 3, the caesium coating can provide up to about ten secondary electrons for each impacting electron. Under practical condi-` tions, in the present type of tube I would expect to obtain about 10 secondary electrons per primary. electron, or ten to one for each twoelectron hops. The multiplication of the initial current can, accordingly, be approximately expressed by the relation:

where Ia, is the anode electron current, Ii is the initial priming current, and h is the number of successive electron hops between the hot cathode and the output anode.

Let it be assumed that Ia is 100 amperes, and

Ii is 0.0001 ampere (0.1 milliampere), thenv Ia/Ii:(l)6 and h:l2 hops. Thus, in practice, there may be ten or more electron hops from the hot cathode to the output anode.

I prefer that, at moments of minimum instantaneous output-anode-to-cold cathode potential, the number of electron hops be considerably greater than the minimum number required so that the anode current will be space charge limited rather than emission limited. This makes it possible to obtain a high power conversion efciency without instability or critical adjustments.

It may be noted that once the priming current has caused the building up of a space charge limited secondary emission current, the priming current may be expected to lose control of the anode current. Therefore, if the anode potential were maintained continuously at a rather low value, turning on the priming current may start the anode current but turning off the kpriming current will not stop the anode current again. This is analogous to the trigger action of Thyratron and Ignitron type gas discharge tubes, and tubes of the present invention may be used as a substitute for Thyratron and Ignitrons. y YHowever, when the potential on anode 4 is oscillatory due to the reaction of the tuned output circuit I5, the anode potential will not remain low but will oscillate upward again from the minimum value. As it does, it automatically reduces the number of hops from one end to the other of cold cathode and reduces the production of secondary emission to a lovv value, or interrupts it altogether if the priming current has, in the meantimabeen cut off by the input control. Thus the controlled priming current from hot cathode I can initiate the anode current at the proper point of the cycle but interruption of the anode current takes place due to the rise in anode potential.

If ythe priming current is made to be small enough, and the maximum secondary emission current multiplication great enough, it is possible to leave the priming current von continuously, in which case the tube and circuit will be able to oscillate at any frequency, over a broad band, to which the output circuit I5 is tuned. This may be a desirable condition oi operation in industrial applications where frequency stability may be less important than iiexibility and automatic adjustment of the oscillator to changing load conditions.

It is to be noted that, in practice, some conditions at the cold cathode might result in production of an effect which consists of a persistence of some emission after impacting electrons have been stopped. This phenomenon could result in the cold cathode supplying its own priming current in a manner to cause loss of control by means of the input power. This would not be objectionable in industrial applications but might be very objectionable in some forms of communications.

It is also probable that the initial priming current source can be omitted entirely in tubes for industrial application and reliance had upon cosmic rays, photo-emission, cold emission accompanying evaporation and condensation, or other causes to provide all the priming current needed to start the emission. In this connection, it may be noted that a single electron leaving the cold cathode at a proper time is all that is needed to initiate growth of emission up to space charge limiting, brought about by the very high frequency electric fields in the device.

It is of some interest to note that the cathode power (electron bombardment power) required to produce emission from the cold cathode is automatically applied and out 01T (as the multiplication occurs or ceases) as needed, so that the average power required to produce emission may be much less than would be required to produce the same peak emission in a system employing merely a hot cathode, where the emission must be produced or made available continuously rather than in pulses as needed.

A factor to be considered in connection with the application of very high frequency power to the anode structure is that the length of transmission line 21 and loop 28 coupling source 26 to the anode structure 33 should be short and of such a length that, as the secondary emission from the cold cathode waxes and wanes, the resulting change in load impedance and tuning of the anode structure will automatically vary the load resistance presented to the source of very high frequency power in a direction tending to hold more or less constant amplitude electric elds. This is accomplished by making the coupling line with its coupling loops effectively an integral number of half Waves long and adjusting the relative frequency of the source 26 and suchfa way as' tok provide a maximum loading of theusource 'iwhenspace charge due tosecondary emission 'from the vcoldv cathode is present and a minimum loading'when it is not. That is, the effectiveA loading should appear at source 26 as though the length of the line 21 between source` 26jand cavityresonator 33 is zero.

'1f the design of the system is such that self? oscillation occursat' the frequency of source 26 after the circuit has been started to operate, then it may be advantageous to change the adjusvtment of the transmission line 2l from the valuesspecified above. In such a case the coupling'V and' tuning ofthe line may better be adjusted'for a maximum of power transferwhen thereis'no' space'charge built up.

Only experience with particular tubes and circuit assemblies can'provide a reliable guide to th'e'bestcombinations'and adjustments for use in eachlcase.

Inorder to start the magnetron of the invention operating, it may be advisable to first apply a low anode direct current potential and to raise this potential until oscillations begin, .then to continueraismg the anode potential up to the normal operating value.v

,Whatis claimed is:

le'. Anjfelectron dischargedevice comprising a thermionic cathode, a 'control electrode inthe path V`of 4the electronsemitted by said thermionic cathode, `a secondary emissive cold cathode spaced fromsaid thermionic cathode and having effective electronv emission surface appreciably vgreater' than theelectron emission surface of sad ,thermionic cathode, a resonant structur'es'ur'roundingsaid cathodes, means coupled to said resonant structure for setting up a very high frequency electromagnetic.fieldV in said resonant structu1"'e,"anoutput anode adjacent said cold f.'

cathode foi-"collecting electrons emanating from said-cold cathode, means coupled between said control electrodeand thermionic cathode for supplying'alternating current energy thereto, and an outputitun'ed circuitcoupled to said output anode.

2. An" electron discharge device comprising a thermionic cathode, a control electrode in the path of the electrons emittedby said thermionic cathode, a secondary emissivecold cathode spaced fromjsai'd thermicnic cathode and having an effectivev electron emission vsurface appreciably greater than the electronemission surface of said thermionic cathode, a cavity resonator surrounding 'said 'cathodes`, a source of high frequency power coupled'to said cavity resonator foi` setting up 'a high frequency neldY therein, an output electrons emanating from said cold cathode, a

source of alternating currentV energy coupled between said' control electrode and i thermionic cathodega parallel tuned output circuit coupled to'said output anode, and means for supplying said output anodewith a relatively high direct current positive voltage relative'to said thermionic cathode and control electrode,

3, An electron discharge device comprising a thermionic cathode, a control electrode inthe pathof the electrons emitted by said thermionic cathode, a secondary emissive cold cathode spaced from'said thermionic cathode and having an effective lelectron emission surface appreciably greater than the electron emission surface of saidr thermionic cathode, a multiple cavity resonant structure surrounding said cathodes,"a source of hi'g'lrfrequency power coupled-to said cavity res- @naar 'strueturef'fo'settmgup therein d high "ir-f quency field, an output anode adjacent said cold cathode vfor collecting' 'electrons emanating `from said cold"cathode`, a source ofaltern'ating current '-ienergy coupled between said control electrode and therniionic cathode, a parallel tuned output cir` cuit tuned to'the frequency of said source of alternating current or, a frequency harmonically related thereto coupled to said output anode, and means for'supplying said output anode with a direct current potential which is highly positive relative vto said thermionic cathode, control electrode and coldcathode.

a fl; fAn electronv discharge device comprising a. l ithermioniccathode, a control electrode in' the pathA of the electrons emitted by said thermionic cathode, la secondary emissive f cold cathode spaced fromlsaid thermionic cathode and having `ariV eifectiveeflectron emission surface appreciably 1 greater'than the electron emission surface of said thermionic cathode, a resonant structure surrounding said cathodes, a source of high'fre-l quency power having a frequency the same as'the resonant frequency of said resonant structure coupled' to said resonant structure for setting up an electromagnetic field in said resonant struc-q` ture, an output anode adjacent said cold cathode for collecting electrons emanating from'` said cold cathode, a source of alternating current energy coupled between said control electrode and thermioniccathode, andan output tuned 'circuit coupledto said output anode.

5 A magnetron electron vdischarge device com prisng a controllable source of priming electrons,

a"co1d cathode spaced from said source and adapted to be bombarded by the'electrons from said; source of priming electrons, said cold cathode y the sameas'the resonant frequency of said cavity structure coupled to said structure, an output anodefadjacent' to saidcold cathode for collecting the secondary electrons'emitted by saidcold cathode,a source of power of relatively low radio frequency coupled to said controllable source of priming electrons-g and an output tuned circuitcoupled to saidoutput anode, said output tuned circuit being resonant to a frequency nf, where n is an 'integer-and f is the frequency of the said source of low radio frequency power.

6. tAn-electron discharge device comprising a i thermionic-cathode, a control electrodev inA the path `rof the electrons emitted by said thermionic cathode, a secondary emissive cold cathode spaced from said thermionic cathode and having an effective" electron# emission surface appreciably vgreater than the electron emission surface of said thermionic cathode, 'a resonant structure surrounding said cathodes, means for producing'a magnetic'eld acting transversely to the path between'said coldfcathode and the surrounding surface 'of vsaid resonant structure, an output anode adjacentl said' cold' cathode for collecting electrns'femanatingfrom said cold cathode, means for'setting up an electromagnetic field in said rescnant structure,V a source of alternating current-coupledbetweensaid control electrode and thermionic cathode, an output tuned circuit coupled to said output anode, andmeans in series with "said output tuned circuit for supplying said output Aanode Y'with a direct current polarizing potential Vwhich v'is highly positive relative to said thermionic "cathode,g control electrode and cold cathode.

*i153 7. A ,magnetron amplifier comprising an en- .'Velope the form -of a cavity resonant struc- 'diurea thermionic cathode and a, spaced tubular Secondary emissive cold cathode both arranged Y .along ythe axis of said envelope, a control elec- 'trade .for controlling the iiow of electrons from 'said thermioniccathode, means for supplying a l-fmagnetic eld actingparallel to said axis, an output anode adjacent to said cold cathode, a source of high frequency power coupled to said cavity resonant structure for setting up therein a high frequency field, means for supplying said output anode with a direct current potential which is positive relative to said cathodes and said control electrode, a source of alternating current coupled between said thermionic cathode and said cold cathode, an output tuned circuit coupled to said output anode.

8. A magnetron tube and circuit therefor comprising a relatively small thermionic cathode, an elongated secondary emissive cold rcathode spaced from said thermionic cathode, both said cathodes being arranged along the axis of said tube, a control electrode for controlling the electron emission from said thermionic cathode, a tubular output anode in coaxial relation to and enveloping a portion of said cold cathode whi-ch is furthest removed from said thermionic cathode, a resonant structure surrounding said cathodes and said output anode, means for setting up an electromagnetic field within said resonant structure, a source of alternating current coupled between said thermionic cathode and said control electrode, a tuned output circuit coupled to said output anode, and means for supplying said output anode in series with said tuned circuit with a direct current potential which is positive relative to said cathodes and said control electrode.

9. A magnetron tube and circuit therefore, comprising a relatively small thermionic cathode, an elongated secondary emissive cold cathode spaced from said thermionic cathode, both said cathodes being arranged along the axis of said tube, a control electrode for controlling the electron emission from said thermionic cathode, a tubular output anode in coaxial relation to and enveloping a portion of said cold cathode which is furthest removed from said thermionic cathode, a multiple cavity resonant structure having a plurality of inwardly projecting electron target portions surrounding said cathodes and said output anode, means for producing a magnetic field acting transversely to the path between the axis of said tube and said resonant structure, a source of high frequency energy coupled to said resonant structure in the interior of one of the cavities thereof for setting up an electromagnetic eld at the frequency of said resonant structure, a source of alternating current coupled between said thermionic cathode and said control electrode, a tuned output circuit coupled to said output anode, and means for supplying said output anode in series with tuned circuit with a direct -current potential which is positive relative to said cathodes and said control electrode.

10. A system in accordance with claim 8, characterized in this that said source of high frequency energy is coupled to said resonant structure by means of a line and coupling probe whose overall electrical length is substantially an integral number of half waves long at the frequency of said source.

11. The method of operating a magnetron havfinga thermionic cathode andan elongated cold secondary emissive cathode, which comprises ini'- tiating a flow of electrons from said thermionic cathode, applying to said magnetron a high'radio frequency electriceld, a unidirectional electric field and a magnetic field acting parallel to said elongated cold cathode,r periodically interrupting the flow oi' electrons from said thermionic cathode, collecting secondary electrons from said cold cathode, resonating said collected electrons at a frequency related to the rate of interruption of the electrons from said thermionic cathode, and cyclically varying said unidirectional electric field at said resonating frequency.

l2. A negative resistance oscillator comprising a vacuum tube having a non-thermionic secondary emissive cathode, an anode surrounding an extended portion of said cathode, a resonant structure surrounding said cathode and anode, means for setting up a high frequency electromagnetic eld in said resonant structure, and a. tuned circuit coupled to said anode, a source of potential coupled to said anode through said tuned circuit for supplying thereto a, potential which is positive relative to said cathode, and circuit components i'or causing secondary emission from said cathode to be large when the anode to cathode potential is small and secondary emission to be small or zero when the anode to cathode potential is high.

13. A device for multiplying secondary electron emission by multiple electron closed impacts upon a cold cathode, in response to very high frequency electromagnetic fields, said device having a non-thermionic secondary emissive cathode, an anode surrounding an extended portion of said cathode, and a tuned circuit coupled to said anode, a source of potential coupled to said anode through said tuned circuit for supplying thereto a potential which is positive relative to said cathode, and circuit components including a resonant structure surrounding said anode and cathode for causing the multiplication from said cathode to be large when the anode-to-cathode potential is low but small or zero when the anode-to-cathode potential is large.

14. A magnetron comprising an evacuated enclosing housing having an anode therein, said anode providing, a cathode cavity longitudinally central thereof, a cathode coaxial of and within said cavity and emission from which is substantially secondary electrons only, and means at one end of said cathode for conducting a cooling medium both to and from said cathode.

15. A magnetron comprising an evacuated enclosing housing having an anode therein, said anode providing a cathode cavity longitudinally central thereof, a main cathode coaxial of and Within said cavity and emission froml which is substantially secondary electrons only, means at one end of said cathode for conducting a cooling medium both to and from said cathode, and an auxiliary cathode at the opposite end of said main cathode from said means.

16. A magnetron comprising an evacuated tube having an anode, said anode providing a cathode cavity longitudinally central thereof, a cathode coaxial of and within said cavity and emission from which is substantially secondary electrons only, and means at one portion of said cathode for conducting a cooling medium both to and from said cathode.

' 11 in said cavity and coated with secondary ernis- UNITED STATES PATENTS sive matrial, an auxiliary cathode adjacent said Number Name Dame main `cathode for` supplying a primary electron .2,104,100 Roberts. 5011.4, 193s current for the main cathode, and moans at one 2,114,114 Roberts Y ADL :12, 1933 end of 'the said main cathode for conducting a 5 2,121,360 Maneret a1 I Y 'June'21,-193 cooling medium t9L and from Said main Cathode- 2,140,832 Farnsworth A 1200,20; 1933 CLARENCE W HANSELL- 2,182,870 Jarvis et a1 Dec. 12, V1939 Y 2,232,158 Banks 2..; Feb. 18;1941 REFERENCES CITED 2,416,303 Y Parker Y Y Feml 25, 1947 The following references are of record in the lo 2,450,763 McNall 1 -Octa'5f1948 file of vthis patent:

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