US2430309A - Electronic discharge device - Google Patents

Description

Nov. 4, 1947. I P. L. SPENCEIR 2,430,309

ELECTRONIC DISCHARGE DEVICE Original Filed Sept. 50, 194i 2 Sheets-Sheet 1 1 QI 8 I: 8

/NVEN7UI2 PERCY L. SPENcER I Nov. 4, 1947. P. L. SPENCER 2,430,309

' -ELECTRONIC DISCHARGE DEVICE Original Filed Sept. 30-, 1941 2 Sheets-Sheet 2 5' CHOPPER I INVENTUI? PERCY L. SP NCER B 5 Patented Nov. 4,

430,309 mmecmc DISCHARGE Device fi rjcvll S en Wes w o iMas' -l isi nof w Raytheon Manufa oturing qompany; Newton? Mass acorporatio'ri of Delaware- OIigiHM fli n e ion. S i mb e .30 .lQA LS tia N o. 412,993.- Divided and ,this application June 5, 1946; Serial No. 674,453

the magnetron type which are called upon tosupQ- ply're'latively large peak 'va-lues of current; various difiiculties have heretofore existed. The cathodes of suchdevice's-have-heen heated to relatively high temperatures unanattempt to supplysuifi cient thermionic eiiiission to carry snch'peak values of current} Such cathodes *have had 'an" unusually short life due'to the factthat-the'emisi' sive coating with" which the cathodesare nor: m'ally coated wasrapidljdriven ofi'iroi'n' the cathode". This efiectf'wa's in'creased 103 th -face that the load current throt'lgiitfie tubetendedrto overheat andbunfoiit-thecathodef' The arthasf resorted to the use 'of coinplicated regulating and protective devices in orderto protect the cathodes of such Inag rietions from being burned out; However, such protective andfregulating devices did 'n'ot substantially affect thlossof coating'due' to highoperatingwernperatures of, the cathode which resulted inshort life for such'catho'des.

An object of-th is" invention is"to produce an" electron discharge device of the'type which sup? plies high-peak-values "of current with its cathode I normally operating at a; temperature" snbstana" tially belowthat necessarytocause' such peak" values of current to be emitted thermionicallyl' Another'objectis to accomplish the above in a magnetron type of discharge device.

Another object is tocause such peak" values 'of current to be supplied largely by secondarv ems:

sion.

A further ob'j'ctis to devise'a cathode insuch a device which will emit large" numbers of sec-i ondary electrons without substantial time delay,- and which will have a long life. I

A further object is to devise a magnetron which does not need special regulating and protecting" devices to prevent the cathode from being burned' out.

A still further object is to devise such a ma netron which is capable'of 'supplying'much larger amounts of power than have heretofor" been possible.

The foregoing and other objects of this-inven"- tion will be best understood-1m nthe following description of exemplific'ationsthereof;reference 2; r being held to" the accompanying drawings, whereiii ne 1 ge e-g are diagrammatic representations; of; a magnetron illustrating" certain principles "of' operation offmyinvention; d

Fig. 3 is g' "cross'-sectiona1 view" of one embcdi ment of my novel cathode;

of" another embodiment pfln'y novel cathode; K

Fig. 5 is' an illustrationof'onetype' of mag'hee tron" inc'orp'oratingflmy invention, the view inf Fig: 5 being takenalong line E -5 Fi .6; and

Flg. 6 is a cross-section of 'th'ev magnetron in" N Fig; 5"talgen along" line 69- 6"ofFig1. 5,' tog'etli'e r 1,5,

with a diagrammaticrep esentatiori' of afcircuit withwhich" said magnetrl oninay be used;

v In Figjd A1 andA represen t two anodes of fa" split-f anode magnetron. C 'isth'ef centrally lo-'1 cated-"cathode thereof'j usual 'in' thistype" of device, a longitudinal magnetic field is ,iIn-j pressed thereon in direction 'at right angles to: the plane' of' illustration in Fig, l. The cathode Q connected" tof ja negative'potential while the: anodes Ai arrd A'are] connected to ether to a positive peter tial; Devices 0f this kind are evacfu f atedj 'to high vacuum 7 conditions in which the; gaseous atmosphere; plays substantially no part f irr-t leci chargei jI'his type of magnetron when energized sets up" high frequency oscillations, creating'an oscillating electrostatic field between'j the anode iii and A2. At one'in stant of time the: anode A; may be morefpositive than the anode; A12 Under-theseconditionsan electron e emitted from the cathode C is acceleratedgtoward the an;

odeA by the 'potential'thereof; However, the

magnetic field" causes the ele'ctronfeto' travelin a curved'path which deflects the electron to such I anextent thatfitmiss'esjtheanode Ai and falls; u'pon the;anode #2; This imparts a' negative characteristic to the'device, and'causes it to operate as an oscillator.

Under'the' condftionsofope'ration which I con;- teinplateiinimy i vention m addition to the 'acj tion described in connection with Fig. 1, another aetion; as: exemplified by; 2 also takes place; Theelectron' einiss'ionfrom C causes a'. swarm. S"

of electronsinthe'space surrounding C; An nee;

tr n e; whicho therwisemight ronowjth'e path as described ln Rig: 1 however" encounters 'intere "fei ence'irom "the'otherelectrons in the swarm S,

andflthu's nev'ef'" reaches "the anodes A1 or but falls back onto thefc'athodei The interaction e tween the electrons "in theswarm" s may impart Assuming the tube to be oscillating, the electrons 6' also may receive a considerable amount of energy directly from the oscillating field between the two anodes A1 and A2. The magnetic field tends to give to the electron e a. definite orbital period in its travel around C, which period is substantially equal to the period of oscillation of the voltage appearing between the anodes A1 and A2. This condition permits said oscillating field to ex ert its accelerating force upon the electron e in the proper phase and at the proper time to successively impart energy to said electron.

Due to the above effects, electrons of the e type can be made to fall upon the cathode C with considerable speed and energy, which may be substantially above 100 volts. If the cathode C is made so as to be a good secondary electron emitter, then such impinging electron may give rise to the emission of several additional electrons. The current due to secondary emission may be made several times the current due to simple thermionioemission at the operating temperature of the cathode. In addition, when the tube is called upon to supply greatly increased peaks of current, then the secondary electron emission can be made to increase enormously to carry such peak currents without substantial time delay. In other words, such a device can be made to operate as an electron multiplying arrangement in which the normal thermionically-emitted electrons are multiplied to give an increasedsupply of electrons which in turn are again multiplied by a similar process.

In accordance with my invention I utilize such secondary emission to supply a large part of the peak currents which such a device may be called upon to supply. For this purpose I preferto construct the cathode of the discharge device in a special form, as shown for example in Fig. 3. The cathode illustrated consists of a sleeve I made of some suitable material, such as tantalum or nickel. In one example of this cathode the cylinder was about six millimeters in diameter and about fifteen millimeters long. The sleeve is coated, except for the end portions thereof, with a layer 2 of a mixture of barium and strontium carbonates in a nitrocelluloseamylacetate binder. In some instances I prefer to add from one to one and one-half per cent. of borax in order to decrease the evaporation rate of the coating material during operation. The sleeve so coated is baked in air at a temperature of about 400 F. Thereupon a winding 3, preferably of tantalum wire, is Wound over said coating. In the embodiment mentioned above, this wire has consisted of tantalum .004 inch in diameter, spaced .003 inch between adjacent turns. In order to retain the winding upon the cathode and to insure good electrical contact with the underlying sleeve, the ends of the wire 3 may be welded directly to the sleeve I. After the wire 3 has been wound upon the cathode, the cathode is again coated with the coating material described above and again baked in air at a temperature of about 400 F. Thereupon the coating is scraped oif the outside of the cathode of the sleeve I are closed by insulating plugs 'I--'I, preferably of alumina. The ends of the heater coil 6 extend through said plugs so that heating current may be supplied thereto. An electrical connector tab 9 has one end thereof welded to the sleeve I and the other end welded to one of the heater ends 8, so that electrical connection may be established to the emitting surface of the cathode.

Instead of making the cathode as illustrated in Fig. 3, it can take a variety of other forms, one of which is illustrated in Fig. 4. In this figure,

' instead of using round wire, the sleeve I is wound with a fiat ribbon 3', also preferably of tantalum. This ribbon, for example, may be .0005 inch thick and .050 to .100 inch wide. Such a ribbon may flbe initially coated with emitting materials, as

structure, leaving the top surfaces 4 of the wire described above, and wound upon the sleeve I with about half of each turn of the ribbon overlapping the preceding turn. Here again the coating may be baked in air as described above, and the coating scraped from the outside of the cathode structure, leaving the top surfaces 4' of the ribbon material 3' bare.

The cathode structures as described above possess the property of being excellent secondary electron emitters, particularly from the scraped tantalum surfaces, as well as good thermionic emitters from the exposed oxide surfaces. The tantalum has a tendency to reduce the barium oxide, liberating small amounts of barium on the surface of the coating which tends to give excellent electron emission. Also the barium so liberated tends to coat the bare surfaces of the tantalum, making it an excellent secondary electron emitter. Even without any barium coating, tantalum in itself is a good secondary emitter.

Cathodes of the type as illustrated in Figs. 3 and 4 may be incorporated, for example, in a magnetron of the type as illustrated in Figs. 5 and 6. The magnetron therein illustrated comprises an envelope I I which is preferably made of a block of conducting material, such as copper. This block forms the anode structure of the mag- .-netron. Said block has hollow end sections which are covered by endcaps I 2 and I 3, likewise of conducting material, such a copper. Between the hollow end sections of the block II is a central bridging portion I4. The portion I4 is provided with a central bore I 5 within which is supported substantially at the center thereof a cathode III which, as pointed out above, is preferably of the type as illustrated in Figs. 3 and 4. The cathode I0 is supported by a pair of lead-in conductors I6 and I1 fastened respectively to the ends 8 of the cathode structure, and sealed through glass seals I8 and I9 mounted at the outer ends of pipes 20 and 2| hermetically fastened within the walls of the block II adjacent the upper and lower hollow end sections. A plurality of slots 22 extend radially from the central bore I5 to within a short distance of the outer wall of the block I I.

When such a magnetron is placed between suitable magnetic poles 23 and 24 to create a longitudinal magnetic field and the device is energized, oscillations are set up whose frequency and consequently whose wave length are determined primarily by the dimensions of each of the slots 22. It is also desirable that the value of the magnetic field is such as to impart to the electrons travelling around the cathode an orbital frequency substantially equal to the frequency of said oscillations. Moreover the voltage applied to the anode structure should be of the proper value to cause such oscillations to occur and for the desired peak value of current to flow between the cathode and anode structure. The oscillations produced in the slots 22 reinforce each other and may be led out from the tube by means of a coupling conductor 25 fastened to the central bridge portion I l in the central bore l5 between two of the slots 22. The coupling conductor 25 leads out from the magnetron through a glass seal 2'! at the outer end of a pipe 25 likewise hermetically fastened through the wall of the envelope II adjacent the upper hollow portion thereof.

The magnetron may be connected in any suitable circuit, one of which is shown diagrammatically in Fig. 6. In this circuit the cathode is supplied with heating current from the secondary winding 28 of a heating transformer 29 whose primary winding 3!] is adapted to be connected to a suitable source of alternating current. Interposed in the circuit of a secondary winding 28 is switch 31 and a current-regulating resistance 32. A source of potential 33, which in a practical embodiment may be of the order of 12,500 volts, is connected between the envelope ll, constituting the anode, and the lead-in wire 16 for the cathode l0. Interposed in the circuit for the source 33 is an interrupter or chopper 34 which interrupts the circuit so that the magnetron generates short pulses of high intensity high frequency oscillations. The frequency of interruption may be of the order of two thousand times a second. The duration of each energization of the tube may be of the order of a half a micro-second.

I have constructed a considerable number of devices substantially as shown in Figs. 5 and 6 and embodying a cathode as illustrated in Fig. 3, as well as the various parameters recited herein. Tubes of this kind were designed to produce oscillations of a wavelength of about three centimeters. In such a tube I have found that during each half micro-second during which the device was energized, the anode current rose substantially instantaneously to a value of about twelve amperes and continued throughout at this value for substantially each period of energization. The average anode current throughout the entire time was of the order of about fourteen milliamperes.

In starting the operation of such a device, the cathode was raised to a temperature at which enough thermionic emission occurred to initiate the operation of the device, such emission being of the order of milliamperes and being much less than that required to supply peak currents of the order of amperes. However, as pointed out above, when operation started, peak currents of the order of amperes were supplied. Furthermore, after the operation of the device had begun, it was possible to open the heating circuit by the switch 3|, and the device continued in operation with no discernible difference, the tube continuing to generate oscillations in the same way and to substantially the same degree as before the opening of said circuit. Also under these conditions, when the pole pieces 23 and 24 were deenergized so as to remove the magnetic field on the device, the current to the anode structure fell to zero and the operation of the device ceased. This is in strong contrast to the usual magnetron device in which if during operation the magnetic field is deenergized, the current between the cathode and the anode structure rises rapidly.

As pointed out above, the heater 6 is supplied with heating energy so as to initially raise the cathode to a temperature at which some thermionic emission occurs. This thermionic emission may emanate largely from the oxide coating which is exposed to the discharge area through the spaces between the coiled winding on the outside of the cathode. Some of this thermionic emission may occur from the surface of the coiled winding itself, particularly if the metal thereof has a thin film of barium coated upon it. However, during operation the electrons which fall upon the cathode largely impinge upon the bare metal surface of the coiled external winding, and liberate the secondary electrons therefrom. The oxide coating between the turns of this winding is largely shielded from such electron bombardment, and thus forms very little, if any, tendency for such bombardment to drive any of the oxide coating from the cathode. However, such coating is always available to supply barium for the initial electron emission as well as barium which tends to increase the secondary electron-emitting qualities of the metal surface of the external winding. An additional advantage of the construction which I have illustrated is that the surfaces from which the secondary electrons are emitted are directly electrically connected to the sleeve l by having the ends 5 welded thereto. In this way the current can flow through a direct 10w resistance metallic path to the very surface at which the electrons are being liberated. This is in contrast to the usual oxide-coated cathode in which the current must flow through the relatively high resistance oxide coating before it reaches the emitting surface. In this way the present cathode structure is much more effective and efficient.

By my present invention I have been enabled to construct practical magnetron devices which have generated enormous peak quantities of micro-wave length power entirely outside of the range of anything which has heretofore been practicable with such devices.

Of course it is to be understood that this invention is not limited to the particular details as described above as many equivalents will suggest themselves to those skilled in the art. For example, it may be possible to incorporate certain fundamental features of this invention in other devices which are called upon to supply high peak values of current, particularly in connection with micro-wave generators.

What is claimed is:

An electron discharge device system comprising'a cathode and an anode, said cathode during normal operation being at an elevated temperature and provided with an active surface which at said temperature is an appreciable but relatively poor thermionic emitter with thermionic emission which is substantially smaller than that required to carry the normal load current of said device and also a good secondary electron emitter, and an oscillatory circuit connected to said cathode and anode for producing a cloud of electrons and for oscillating said cloud of electrons with suflicient energy to cause electrons thereof to collide with said cathode to liberate sufficient secondary electrons therefrom to produce a total electron emission capable of carrying said normal load current, the space between said cloud of electrons and said cathode being unimpeded to cause said collisions to occur freely.

PERCY L. SPENCER.

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