US2411601A - Electronic discharge device - Google Patents

Description

Nov. 26, 1946. P. L. SPENCER 2,411,601

ELECTRONIC DISCHARGE DEVICE Filed Sept. 50, 1941 2 Sheets-Sheet 1 INvr-:N'l-ol. PERCY L. SPENCER,

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Nov. 26, 1946. P. L. SPENCER ELECTRONC DILLHARGE UEVICE Filed Sept. 30, 1941 2 Sheets-Sheet 2 2 3 L T ,a H e am M T 2 O E A T P N5 m 9 .vLLQ .m YW o l I C R F- I P. Y B AMJ S ..M/ M 2|\| e 7 ////7/ Patented Nov. 26, 1946 y 2,411,601 ELECTRONIC DISCHARGE DEVICE Percy L. Spencer, West Newton, Mass., assignor to Raytheon Manufacturing Company, Newton,

Mass., a corporation of Delaware Application September 30, 1941, Serial No. 412,993

Claims. (C1. Z50- 275) This invention relates to an electronic discharge device, particularly of the magnetron type, and to a cathode for such a device capable of supplying large peak values of current.

In electronic discharge devices, particularly of the magnetron type which are called upon to supply relatively large peak values oi current, various diiliculties have heretofore existed. The cathodes of such devices have been heated to relatively high temperatures in an attempt to supply sumcient thermionic emission to carry such peak values of current. Such cathodes y have had an unusually short life due to the fact that the emissive coating with which the cathodes are normally coated was rapidly driven of! from the cathode. 'Ihis eiect was increased by the fact that the load current through the tube tended to overheat and burnout the cathode.A

The art has resorted to the use of complicated regulating and protective devices in order to protect the cathodes of such magnetrons from being burned out. However, such protective and regulating devices did not substantially aiect the loss of coating due to high operating temperatures of the cathode which resulted in short lite for such cathodes.

An object of this invention is toproduce an electron discharge device of the type which supplies high peak values o! current with its cathode normally operating at a temperature sub1 stantially below that necessary to cause such peak values oi' current to be emitted thermionically.

Another object is to accomplish the above in a A still further object is to devise such a mag.

netron which is capable of supplying much larger amounts of power than have heretofore been possible. f

The foregoing and other objects of this invention will be best understood from the following description of exempliflcations thereof, reference being had to the accompanying drawings, wherein:

Figs. 1 and 2 are diagrammatic representations of a magnetron illustrating certain principles of operation of my invention;

Fig. 3 is a cross-sectional view of one embodiment of my novel cathode;

Fig. 4 is a fragmentary view partly in section of another embodiment of my novel cathode;

Fig. 5 is an illustration of one type of magnetron incorporating my invention, the view in Fig. 5 being taken along line 5 5 in Fig. 6; and

Fig. 6 is a cross-section o1' the magnetron in Fig. 5 taken along line 6-8 of Fig. 5, together with a diagrammatic representation of a circuit with which said magnetron may be used.

In Fig. 1 A1 and Az represent two anodes of a split anode magnetron. C is the centrally located cathode thereof. As is usual in this type of device, a longitudinal magnetic iield is im,- pressed thereon in a direction at right angles to the plane of illustration in Fig. 1. yThe cathode C is connected to a negative potential while the anodes A1 and Aa are connectedv together to a positive potential. Devices of this kind are evacuated to high vacuum conditions in which the gaseous atmosphere plays substantially no part in the discharge. This type of magnetron when energized sets up high frequency oscillations, creatingv an oscillating electrostatic field between the anodes A1 and Az. At one instant of time the anode A1 may be more positive than the anode Aa. Under these conditions an electron e emitted from the cathode C is accelerated toward the anode A1 by the potential thereof. However, the magnetic eld causes the electron e to travel in a curved path which deects the electron to such an extent that it misses the anode A1 and falls upon the anode Az. This imparts a negative characteristic to the device, and causes it to operate as an oscillator.

Under the conditions of` operation which 'I contemplate in my invention, in addition to the action described in connection with'Fig. 1, another action, as exemplied by Fig. 2, also takes place. The electron emission from C causes a swarm S of electrons in the space surrounding C. An electron e'-, which otherwise might followl the path as described in Fig. 1, however encounters interference from the other electrons in the swarm S, and thus never reaches the anodes A1 or Az, but falls back onto the cathode. 'I'he interaction between the electrons in the swarm S may impart considerable energy to the electron e' by collision or otherwise by the time it reaches the cathode C. Assuming the tube to be oscillating, the electrons e' also may receive a considerable amount of energy directly from the oscillating eld 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 substantally equal to the period of oscillation of the voltage appearing between the anodes A1 and A2. This condition permits said oscillating field to exert its accelerating force upon vthe 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 thermionic emission 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 increased supply 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 prefer to 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 nitrocellulose-amylacetate 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 adiacent turns. In order to retain the winding upon the cathode and to insure good electrical contact with the underlying sleeve, the ends 5 of the wire 3 may be welded directly to the sleeve I. After the wire 3 has been wound upon the cathode, the cathode4 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 off the outside of the cathode structure, leaving the top surfaces 4 of the wire 3 bare. The baking of the carbonate coatings ln air not only drives off the binder material. but also largely converts the carbonates into the oxides. A heater coil 6, preferably of tungsten, the turns of which are coated with insulating material, is inserted within the sleeve I for the purpose of enabling the cathode to be raised to a temperature of thermionic emission. The ends of the sleeve I are closed by insulating plugs 1--1, preferably of 4 alumina. The ends 8 of the heater coil 6 extend through said plugs so that heating current may be supplied thereto. An electrical connecter 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 gure, instead of using round Wire, the sleeve I is Wound with a ilat ribbon 3', also preferably of tantalum. This ribbon,l for example, may be .0005 inch thick and .050 to .100 inch wide. Such a ribbon may be initially coated with emitting materials, as 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 II which is preferably made of a block of conducting material, such as copper. This block forms the anode structure of the magnetron. Said block has hollow end sections which are covered by end caps I2 and I3, likewise of conducting material, such as copper. Between the hollow end sections of the block II is a central bridging portion I4. The portion I 4 is provided with a central bore I5 within which is supported substantially at the center thereof a cathode I0 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 vcent1-al bore I5 to within a short distance of the outer wall of the block II.

When such a magnetron is placed between suitable magnetlc 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 ilow between able circuit, one of which is shownV 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 30 is adapted to be connected to a suitable source of alternating current. Interposed in the circuit of a secondary winding 28 is a switch 3| 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 Il, constituting the anode, and the leadf in wire I6 for the cathode I0. Interposed in the circuit for the source 33 is an interrupter or chopper 34 which interrupts the circuit so that the magnetron generate-s 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. 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 wave length 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 circult.` 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 thermionio emission occurs. This thermionic emission may emanate largely from the oxide coating which is exposed to the discharge area 5 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 I by having the ends 5 welded thereto. In this way the current can flow through a direct low 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 eilicient.

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 in- 45vention 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 50 other devices which are called upon to supply high peak values of current, particularly in connection with micro-wave generators. It is accordingly desired that the appended claims be given a broad interpretation.

What is claimed is:

1. The method of operating an electron discharge device of the type for supplying a predetermined peak current, comprising a thermionic and secondary emissive cathode, an anode, and means for deflecting electrons emitted from said cathode back to said cathode, said method comprising starting said discharge device with said cathode at a temperature producing substantial thermionic emission, and then maintaining said cathode at a temperature producing thermionic emission substantially less than that required to carry said peak current, so that electrons emitted from said cathode return to said cathode under the inuence of said means and liberate secondary electrons in suilicient numbers to produce a total electron emission constituting said peak current.

2. The method of operating an electron discharge device of the magnetron type for supply- 75 ing a predetermined peak current, comprising a thermionic and secondary emissive cathode, an

anode, and means for setting up a magnetic field transverse to the discharge path between said cathode and anode, said method comprising starting said discharge device with said cathode at a temperature producing substantial thermionic emission, and then maintaining said cathode at a temperature producing thermionic emission substantially less than' that required to carry said peak current, so that electrons emitted from said cathode return to said cathode under the inuence or said magnetic eld and liberate secondary electrons in sulcient numbers to produce a total electron emission constituting said peak current.

3. The method of operating an electron discharge device of the magnetron type for supplying a predetermined peak current, comprising a thermionic and secondary emissive cathode, an anode of the plural cavity resonator type having a plurality of anode elements symmetrically disposed around said -cathode, and means for setting up a magnetic iield transverse to the discharge path between said cathode and anode elements,

said method comprising starting said discharge device with said cathode at a temperature producing substantial thermionic emission, and then maintaining said cathode at a temperature producing thermionic emission substantially less than that required to carry said peak current, so that electrons emitted from said cathode return to said cathode under the influence of said magnetic eld and the oscillating electrostatic fields of said cavity resonators and liberate secondary electrons in suflicient numbers to produce a total electron emission constituting said peak current.

4. The method of operating an electron discharge device of the magnetron type for supplying a predetermined peak current, comprising a thermionic and secondary emissive cathode, an anode, and means for setting up a magnetic field transverse to the discharge path between said cathode and anode, said method comprising starting said discharge device with said cathode at a temperature producing substantial thermionic emission, then maintaining said cathode at a temperature producing thermionic emission substantially less than that required to carry said peak current, so that electrons emitted from said cathode return to said cathode under the in- Iluence of said magnetic eld and liberate secondary electrons in suiiicient numbers to produce a total electron emission constituting said peak current, impressing a voltage between said cathode and anode of a sufllcient value to cause said peak current to ow, and periodically interrupting said voltage so that said device is caused to passsaid peak current discontinuously;

5. The method of operating an electron discharge device of the type for supplying a predetermined peak current, comprising a thermionic and secondary emissive cathode, an anode, said method comprising starting said discharge device with said cathode at a temperature producing substantial thermionic emission. then lowering the cathode temperature and maintaining said cathode at a temperature producing thermionic emission substantially less than that required to carry said peak current, so that electrons emitted from said cathode return tu said cathode under the influence of said means and liberate secondary electrons in sumcient numbers to produce a total electron emission ccnstituting said peak current. i

PERCY L. SPENCER.

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