US2492313A - Magnetron - Google Patents

Description

Patented Dec. 27,1949

2,492,313 MAGNETRON Ernest C. Okress, Montclair, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application November 2, 1943, Serial No. 508,699

3 Claims.

This invention relates to magnetrons, and more particularly to the cathode structure thereof.

Electron discharge devices of the magnetron type are employed in the art for generating or sustaining electrical oscillations of very high frequency. Such devices usually contain an anode of generally cylindrical shape with an elongated electron emitting cathode axially sustained therein with both elements functioning in a vacuum.

It is found in the operation of magnetrons that after the cathode is raised to its normal or emissive temperature and copious emission prevails, that, without any alteration of the current to the heater filament, the cathode rises above normal temperature to its own detriment. Basically, since present-day magnetrons average less than 50% efiiciency, the cause for the increase in cathode temperature may be traced to the inefiiciency of the magnetron. The explanation for this appears to be that a proportion of electrons emitted from the cathode are out of phase with the electromagnetic field, and are returned to the cathode after a more or less extensive excursion into the region between the oathode and anode. But during their sojourn in the electromagnetic field, those out-of-phase electrons absorb energy and when they return to the cathode, that energy is converted into heat at the cathode. Slight phase diflerences between the electrons and A. 0. field (between cathode and anode) is permissible because such electrons usually get in step after a few convolutions. But with more than slight phase differences the electron usually returns to the cathode, and contributes to the inefficiency to the extent of energy absorbed in its excursion, from the cathode and back again, which is translated into heat at the journeys end on the cathode. From the average low efficiency indicated above for magnetrons, it becomes apparent that a good portion of the energy imparted is converted to heat at the cathode, causing the cathode to operate at a temperature above normal therefor and shortening its life greatly.

The gravity of the heat exchange for efficiency is rendered more evident by the fact that, when the device is in normal operation, the developed heat is so intense that the external cathode heater power may be reduced to zero and the device continues to operate as well as while the power was being applied. If, however, the anode voltage is interrupted for sufficient length of time, the bombardment of the cathode diminishes and emission decreases and stops and then to start the operation again the cathode heater filament has to be used to again bring the cathode to emissive temperature.

Bombardment results in the loss and destruction of the thermionically active material on the surface of the cathode. Also the higher temperature than normal for the cathode results in serious evaporation of the active material, and in either event the active material lost from the cathode is in greater part found to be deposited on the inner concave face of the anode. This deposit on the anode interferes somewhat with the operating characteristics of the magnetron.

From its more general aspects, the invention has an objective of eliminating the defects referred to above and thereby increase the operating life and efficiency of the magnetron.

Another objectis to provide an improved magnetron assembly and combination of parts.

More specifically an object of the invention is to provide an improved cathode.

A further object of the invention is to main tain the cathode at optimum temperature.

Yet another object of the invention is to provide a cathode requiring no heater therein.

Still further objects of the invention will appear to those skilled in the art to which the invention appertains as the description progresses, both by direct recitation thereof and by impli cation from the context.

Referring to the accompanying drawing in which like numerals of reference indicate similar parts throughout the several views;

Figure 1 is a vertical sectional view of a magnetron in accordance with the present invention; and

Figure 2 is a horizontal cross section thereof taken on line II--II of Fig. 1.

In the specific embodiment of the invention illustrated in said drawing, the reference numeral Ill designates a cylindrical magnetron body having end plates or covers H sealed thereto whereby the interior may be evacuated. The interior of said body is formed to comprise an anode l2 of generally cylindrical shape, shorter than the outside portion of the body which has end flanges l3 for spacing the covers II from the ends of the anode and thereby afiord the usual and necessary end spaces I4 Within the magnetron.

The present showing of magnetron provides a cathode cavity l5 coaxial with the magnetron body and anode and extending through the anode so as to open into both end spaces I l. The anode likewise provides a plurality of resonant cavities l6, shown herein as generally cylindrical and symmetrically distributed around the cathode cavity and connected therewith by radiating slots 11. The ends of the cylindrical cavities and of the radiating slots open into the end spaces M.

A cathode I 8 extends axially through the oathode cavity, projectin'g at its ends into said end spaces. The ends of the cathode have metallic headers I9 engaging within the same, and on outer shoulders of the headers are mounted me tallic discs 20 which overlie the ends of the oath ode cavity to act as shields, but which are spaced from the anode to be out of electrical contact therewith. The body of the cathode is metallic and cylindrical and has its ends spun, pinched or otherwise caused to grip the headers and held vacuum tight thereon by solder or other suitable attachment. The major portion of the outer cylindrical surface of the cathode body is coated or otherwise rendered electron emissive when heated. However, peripherally around the cathode cy1- inder is a band 2| of radio active material which gives off beta rays, so as to emit negative electrical charges without need of heat, there being numerous such materials known which occur in nature or are artificially manufactured, examples being radium B, C, D, E, Y, thorium 1, 2, B, I, X, uranium X1 and X2, actinium B, C" or D, and many others. This ring of material is preferably at a part of the cathode subject to the least amount of bombardment, and is here shown next one end of the cathode. With positive potential applied to the anode, the electrons emitted by the band 2| of radio active material are attracted thereto and in traveling in the magnetic field established by the usual magnet 22, the electrons spiral and the device begins to function. Some electrons, as explained above return to the oathode and develop heat therein, thereby rendering the cathode emissive by virtue of its emissive coating. Emission increases as the cathode temperature rises.

In order to maintain the cathode at an optimum temperature, means are provided for passing a cooling fluid through the cathode longitudinally thereof. The means here shown comprises a tube 23 coaxial with the cathode and carried at its ends in said headers l9 to which it is soldered or otherwise made vacuum tight. Said headers are hollow and next their outer ends have lateral openings within which are soldered or otherwise secured metallic tubes 24 which extend radially of the end spaces 14 and out through openings in the flanges 13 without contact therewith and so as to constitute the center conductor of a coaxial lead-in structure.

For convenience in manufacture the lateral tubes are shown fabricated as short lengths 25 carried by the headers and intermediate lengths 26 extending through the flanges and outer lengths 21 therebeyond preferably formed from material to which glass may be securely sealed. Such material may well be one sold in the trade as Kovar which is an alloy comprised essentially of iron, nickel and cobalt as described in Patent No. 2,062,335. One lead constitutes a fluid inlet the other an outlet ',for passing the fluid through the tube 23 within the cathode.

Secured in the openings through the flanges out of which said leads project, are open-ended nipples 28 which have large enough passages therethrough for said leads so the leads will not make contact therewith. In the outer portions 4 of these nipples are situated high frequency chokes 29 and projecting from the outer ends of the said nipples are "Kovar or other suitable rings 30 sealed vacuum tight to said nipples. The outer ends of said rings have glass tubes 3| sealed thereto, the outer ends of said tubes being sealed to the Kovar length 21 of the lead-in tubes.

The structure accordingly permits a controlled flow of fluid to the cathode cooling tube. That tube, however, is out of direct contact with the cathode, thereby allowing the cathode to come quickly toemitting temperature, but will carry off heat radiating inwardly thereto from the cathode in adequate amount to hold the cathode from overheating. Flow of the cooling medium may be variedin amount thereby maintaining the cathode at a reasonably steady and normal temperature when once that temperature is attained. Only the two conventional terminal assemblies are required from the magnetron body and cathode potential is applied therethrough asusual.

I claim:

1. A magnetron comprising an anode and a cathode coaxially therein and in part protruding therefrom, said anode having cavity resonators opening toward the cathode, said cathode having a greater part of its surface area thermo-electron emissive and having a radioactive lesser area circumferential of the cathode substantially at the part thereof protruding from the anode.

2. A magnetron comprising an anode and a cathode coaxially therein and in part protruding therefrom, said anode having cavity resonators opening toward the cathode, said cathode having a greater part of its surface area Within the anode thermo-electron emissive and having a radioac tiVe lesser area circumferential of the cathode substantially at the part thereof protruding from the anode, and means in proximity to the thermoelectron emissive area of the cathode for cooling said cathode.

3. A magnetron comprising an anode and a cathode coaxially therein and in part protruding therefrom, said anode having cavity resonators opening toward the cathode, said cathode having a greater part of its surface area within the anode thermo-electron emissive and having a radioactive lesser area circumferential of the; cathode substantially at the part thereof protruding from the anode, and means in proximity to both said areas for cooling said cathode throughout its entire emissi've length.

ERNEST C. OKRESS.

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

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