US2438194A - Magnetron - Google Patents

Description

March 23, 1948. H. L. STEELE, JR., ETAL '2,438,194

MAGNETRON Filed June 1a,l 194e Inl/1,111

INVENTORS. L STEELE, 72 h. /Vc/VZ www Patented Mar. 23, 1948 MAGNETRON Howard L. Steele, Jr., Bloomfield, and John W.

McNall, East Orange, N. J., assignors to Westinghouse Electric Corporation,

East Pittsburgh,

Pa., a corporation of Pennsylvania Application June 18, 1946, Serial No. 677,463 11 claims. (c1. 25o-27.5)

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 electricaloscillations of very high frequency. Such devices usually contain lan anode of generally cylindrical shape with an elongated electro-n 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 normalv temperature to its own detriment. Basically, since present-day magnetrons are only of the order of 50% efciency, the cause for the increase lin cathode temperature may be traced to the inefciency 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 cathode and anode.

.But during their sojourn' in the electromagnetic iield, 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 diiierences between the electrons and alternating current field between cathode and anode is permissible because such electrons usually get in step after a few convolutions. than slight phase diiierences the electron usually returns to the cathode, and contributes to the ineii'lciency 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 elciency 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 vtherefor and shortening its life correspondingly.

The gravity of the heat exchange for eiiiciency 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 vpower may be reduced sometimes even 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 suilicient length of time, the bombardment of the cathode and emission stop and then to re-start the operation the cathode heater lament has to be used to again operate the cathode at the proper 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 But with more y 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. 'I'his 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 efIicency of the magnetron.

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

More specifically, an object of the invention is to provide improved means for limiting and regulating the eiective temperature of the cathode derived from electron bombardment.

A further object of the invention is to utilize the main cathode exclusively as a secondary emitter of electrons.

In conjunction with the last preceding object, the invention contemplates oscillation initiation from a source other than the main cathode.

The present concept includes provision of an auxiliary cathode as a primary emitter.

Important amongst the objects of the invention is inclusion of a primary emitter protected from runaway eiect that would cause it to draw larger currents due to temperature increase.

Still further objects of the invention will appear to those skilled in ,the art to which the invention appertains as the description progresses,

tron in accordance with the present invention;

Figure 2 is a cross-sectional view on line II-II of Fig. 1;

Figure 3 is a sectional view of the cathode similar to the showingsthereof in Fig. 1, but onalarger scale; and

Figure 4 is a cross section on line IV-IV of Figure 3.

In the specific embodiment of the invention illustrated in said drawing, the reference numeral I0 designates a cylindrical magnetron body having end plates or covers I I sealed thereto whereby the interior may be evacuated. 'I'he interior of said body is formed to comprise an anode I2 of generally cylindrical shape, shorter than the outside portion of the body which has `end flanges I3 for spacing the covers II from' vthe anode so as to open into both end spaces 14, Y,

Amongst other details ofthe anode common to the prior art is provision therein of aplurality of resonant cavities It which radiate fromthe central or cathode cavity l5 and which likewise are open at their ends through the end faces`Y Intervening y or partition walls l1 between the resonant cavities separate those cavities from each other and K of the anode into said end spaces.

largely determine the size and shape of said cavities. The said radiating cavities constitute cavity resonators and electrical coupling thereof is obtained by straps i8 here the ends of the anode and each of which connects different alternate walls l'l thereof.

Coaxially within the aforementioned cathode cavity.` vl5 is a cylindrical. secondary emission cathode I9. As shown, it is preferable that this cathode have its upper end lower than the plane of the upper end of the anode. Heating of this cathode, which for brevity willbe distinguished as the main cathode, is derived from electron bombardment, and means are provided for limiting and regulating temperature of said cathode of optimum intensity for maximum efciency as a secondary emitter but of insumcient intensity to cause any substantial primary emission. This heat control is shown accomplished by provision of a hollow within the cathode to and from which a fluid cooling medium is circulated by means exterior to the magnetron. Air, aswell as water, oil and other liquids, are examples of cooling mediums which may be used. Circulation is obtained by provisionof an entry or in-iiow pipe coaxially within an out-flow or discharge pipe 21.' The discharge pipe extends Well Vwithin the cathode coaxially thereof whereas the in-iiow pipe 20fis shorter so as to accommodate fluid passage Vfrom the end thereof into the vregion exterior to the in-flow pipe but interior of the out-flow pipe. As a manufacturing expedient, the pipes are preferably assembled in proper terminal relation and the end of the outer pipe is then lled with a plug 22A after which the body material for the cathode is cast directly around the pipe. The portion of the pipewithin the cast material,` as well as plug 22, become' uniedwith the cast material during the casting operation which affords a permanentrigid mounting for the cathode as well as high thermal conductivity for heat exchange purposes. ,Y The secondary emitter or main cathode I9 maybe shown as rin-gs at j and has been an undesired source of electronY is exemplified by the structure at the upper end of theranode in upper end space I4. As there shown, an auxiliary cathode 24 is provided oppo- `site the upper end of the main cathode I9V in coaxial alinement therewith but spacedk therefrom. Preferably the two cathodes are of approximately the same diameter, but it is desirable to have thel auxiliary cathode of slightly smaller diameter than the main cathode in order to avoid evaporation diiculties. Furthermore, the auxiliary cathode is of a character which is copiously emissive of thermionic electrons. By preference,

and as illustrated, the auxiliary cathode 24 is a coil of wire which is emissive material throughout or some metal coated with emissive material such as strontium or barium oxide, vor `which is otherwise rendered copiously emissive. The coil constitutes a thermionic cathode and accordingly is constructed with appropriate electrical resistance so it can be heated by flow of current therethrough, for which purpose it is in electrical lcontinuity with lead-in wires 25 whichare introduced from the exterior radially through the end anges i3 of the magnetron body into the end space lll. Appropriate seal and insulation for these lead-in wires 25 is provided, one form ofV which may be. seen in Fig. 2 as comprising a hollow nipple 2B sealed to the magnetron body and having a sleeve 21 sealed at its outer end and which in turn is sealed to a glass cap 28 through which the wire passes in sealed contact. Y

vAuxiliary emissive cathode 24 Vis mounted within a recess in hat 29 which isrof inverted cup shape so that its end wall is most remote from andopposite the end of the secondary emissive or main cathode. Said end wall of hat 29 has holes therein for passage of the ends of the auxiliary cathode therethrough, insulators 30 of suitable material for thepurpose, of which Y silver-magnesium is preferred; beryllium-copper alloy is .another suitable material;y and otheriexamples may be given of copper `bases onwhich a coating is applied, for instance a coating of sprayed aluminum.

It is to be particularly noted that the coaxial pipes 20, 2l, project from what will be termed the lower end of the main cathode and into the end space It thereat, and that the said lower end of the cathode is substantially in a common plane with the lower end of the anode. -Below said plane, coaxial to said pipes and securely held on the outer of said pipes by solder or otherwise, is a heavy metal disc 23 general term for which in the art, is hat Said Yhat is of greater diameter than the main cathode and preferably at least equal to the diameter of the cathode cavity of the anode. The function of the hat in the prior art has been to conne the electrons from escape at the end of the cathode cavity. In practice, however, especially when the magnetron is used for continuous wave operation, the hat has suffered .from electron bomdepends from the end wall .of the hat toward the being provided in the holes for electrically isolating the said ends from the hat. This hat 29, by virtue of its cup shape, is furthermore provided `with a peripheral wall or skirt 3l which anode and girdles the auxiliary cathode, said skirt preferably being as deep as the auxiliary cathode so the latter is contained entirely within the recess of the hat. The end'of the hat 29 toward the ymain cathode and anode preferably has an outer diameter substantially as great as the diameter of the cathode cavity l5 and thus,

like the cap 23 at the opposite end of theanode,

functions to deter escape of electrons into the end space Hl. Enclosure of the Vauxiliary cathode 2li within hat 29 in conjunction with potentials employed and magnetic field from the usual magnets, not shown, for the magnetron, virtually constitutes the hat 29 and Yauxiliary cathode 24 an electron gun from which emitted electrons pass into theY cathode cavity i5. Saidhat V29 is preferably non-magnetic and a good heat conductor of high melting point, such'as molybdenum.

VA further feature of the present invention is provisionof means for cooling the auxiliary cathode hat or gun This is accomplished by means quite similar to that described above for cooling the main cathode. In greater detail, said means comprises inner and outer coaxial pipes 32, 33 which are respectively employed as inlet and outlet for a cooling medium or fluid such as air, water or oil. The outer or outlet pipe 33 has its inner end secured in a radially disposed hole in said cap 29 preferably in the region of the end wall of said cap. Since the cap is preferably molybdenum, a copper solder or braze may be employed to secure the pipe tightly in the cap. The said outlet pipe 33 projects radially through the ilange I3 of the magnetron body and through a nipple 34 sealed to said flange but with the pipe out of contact from either the flange or nipple. At the outer end of the nipple is a sleeve 35 sealed thereto and of metal capable of sealing to one end of a glass collar 36 the other end of which is sealed to a similar sleeve 31 which in turn is sealed to a metallic head 38. Pipe 33 extends from the cap 29 through the nipple 34, sleeves 35, 31 and collar 36 to said head 38 and opens at its end into a lateral passage 39 from which a discharge pipe lll projects for carrying off fluid from the outlet pipe 33. A suitable choke el is-provided on outlet pipe 33 to prevent ow of radio frequency current from the magnetron by that path.

. Inner or inlet pipe 32 has its inner end or end toward the cap 29 inset from the end of the outer pipe 33 thereby permitting fiuid'flow from the end of the inner pipe into the end region of the outlet pipe and then back to the discharge outlet through the space between the outer surface of the inner pipe and the inner surface of the outer pipe. The other or outer end of the inner pipe 32 passes through the aforementioned lateral passage 39 beyond the end of the outer pipe and is sealed in said head and in communication with a supply pipe 42. The structure thus described, therefore provides for cooling medium ilow to the cap 29, prevents radio frequency loss, `and yet provides for applying desired direct current potential to the hat. The electrical connection for applying the direct current potential may be made to head 33 by virtue of metallic pipe connection 33 from the head to the hat which will act as an electrical conductor and support insulated from the magnetron body. Use of a more positive potential on the hat will increase primary emission from auxiliary cathode 24 and permit utilization of a lower emitter temperature and heating power. As it is desired to utilize the electrons in the cathode cavity, and as positive potential on the hat tends to attract the electrons thereto, the potential on the hat is kept as least positive as possible, or even negative.

Without repeating the description in detail,-

but by duplicating essential reference numerals on the drawing, it will be clear that the inlet and outlet insulated mounting for the cooling medium pipes 20, 2| for the main cathode is like that for the gun and that the outlet pipe for the cathode constitutes an electrical connection for application of desired potential to the said cathode.

In review of the advantageous results obtained by the present invention, it must be emphasized that magnetron operation on direct current rather than on pulsed voltage has the difficult problem of cathode overheating, and that even at 1000:1 duty cycle, it is common experience for pulsed thermionic cathodes to maintain an emission temperature, or higher, from back bombardment alone, but on continuous wave operation such heating becomes definitely prohibitive ex- .cept at very much lower powers. Maintenance of high power, however, with continuous wave operation is essential for a commercially practical magnetron. Solution of the problem therefore not only includes cooling of the main cathode, but also provides for that cathode to be copiously emissive of secondary electrons. However, a cathode of that characteristic necessitates introduction of means for starting it with a source of thermionic current, in other Words, requires inclusion of an emitter of primary electrons. The invention, therefore, also solves this problem and overcomes shortening of the life of both the primary and secondary emitters and provides a magnetron wherein stable operation can be maintained.

It will be understood that the problems to be solved are more severe for magnetrons used on continuous wave operation than on pulsed operation. However, in either use, it appears quite preferable to utilize a main cathode and an auxiliary cathode which are substantially both of the same general shape, cylindrical as shown, and which are of approximately the same diameters. For either use, it'has been found satisfactory for production of radio frequency, to situate the cathode emissive of primary electrons about an eighth inch from the plane of the end of the anode for a magnetron having approximately a 10 cm. wavelength output. However, with such close spacing, the auxiliary cathode is insufficiently remote from the anode to prevent excessive heating on continuous wave operation from back bombardment. The problem thereby introduced has been solved by the gun construction of auxiliary cathode with cooling of the hat forming part thereof. Electrical isolation of the cathodes enables desired potential to be applied to each for regulation of eifective emission.

We claim:

1. 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.

2. 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 from 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.

3. A magnetron comprising an evacuated enclosing housing having an anode therein, a main cathode in said anode, an auxiliary cathode emissive of primary electrons opposite an end of said main cathode, means around said auxiliary cathode for confining the region perlpherally around said auxiliary cathode, and means connected with and for conducting a cooling medium both to and from said conlining means.

4. 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 emission from which is substantially secondary electrons only, an auxiliary cathode at an end of said main cathode, said auxiliary cathode being emissive of primary electrons,

and'means proximate to and forcooling hothof saide-cathodes.

5; A magnetron comprising an evacuatedV en V closing housing having ananode'thereimsaid Vanode providing a cathode cavity'longitudinally means around saidv auxiliary cathoderforconfin-v ing the region peripherally around said auxiliary cathode, and means connected with and for conducting cooling medium both to and from' said confining means and to and from the main cathode. s 6; A magnetron comprising an evacuated enclosing housing having an* anode'the'rein, said anode providing a Vcathode cavity longitudinally central thereof, a cathode coaxial of andwithin said cavity emissionv from which is substantially secondary electrons only, and means proximate to and for conducting acooiing means to and from said cathode, said conducting means comprising pipes one within another of which the inner pipe opens at its end within the outer pipe and of which the outer diameter of the inner pipe is less 'than the inner diameter ofthe outer pipe for eiecting ow passage from one pipe to the other and between said pipes. i

7. A magnetron comprising an evacuated enclosing housing having an anode therein, a main cathode in said anode, an. auxiliary cathode emissive 'of primary electrons opposite an end of said main cathode, means around ysaid auxiliary cathode for confining the regiony `peripheralli around said auxiliary cathode, and means proximate to and for conducting a cooling mediuml both to and from said coniining means, said econducting means comprising'pipes one within another of Which'the inner pipe opens vat its end next said confining means Within the outer'pipe and of which the outer diameter of theinner pipe is less than the inner diameter of the outer pipe for eiecting iiow passage from one pipe to the other and between said pipes.

8. A magnetron comprising an evacuated enclosing housing having an anode therein, said anode providing a cathode cavity longitudinally central of the anode opening at opposite ends thereof, a cathode in said cathode cavity, hats opposite and in proximity to the open ends of said cathode cavity, and means connected with and forY conducting cooling fluid to and effective upon each of said hats for cooling the same. 9. A magnetron comprising an evacuated enclosing housing having an anode therein, said anode providing a cathode cavity longitudinally central of the anode opening at oppositcends thereof, 'a main cathode in said'cathode cavity, hats opposite andinproximity to the open vends of said cathode cavity, one of said hats having a recess directed toward and on a commonaxis Iwith the cathode cavity,` and an'auxiliarycath'- ode in said recess, said cathodes both' being cylindrical on a common axis and substantially equal in diameter.

10. A magnetron comprising an evacuatedfen.- closing housing having an'anode therein, said anode providing a cathode cavity longitudinally central of the anode opening at oppositeends thereof, a main cathode in rsaid cathode cavity, hats opposite 'and in proximity to the operi ends of said cathode cavity, one ofY said hats having aY recess directed toward and on a common axis Y withthe cathode cavity, an auxiliary cathode in said recess, Vand means connected with and for conducting cooling fiuid to said recessed hat.

11. A magnetron comprising an evacuated enclosingr housing having an anode therein, said anode providing a cathode Vcavity Alongitudinally central ofr the` anode opening at yopposite ends thereof, a main cathode in saidcathode cavity, hats opposite'and in proximity to the ends oi said cathode cavity, one ofrsaid hats having a recess directed toward and on a common axis with the cathode cavity, an auxiliary cathodev in said recess, means* connected with andfcr conductingy cooling fluid to said recessed hat, and means connected with and for conducting cooling fluid intosaid main cathode at the end thereof remotev from said recessed hat andvr through the other of said hats. i Y Y,

HOWARD L. STEELE, Je. .JOHN MCNALL.

REFERENCES CITED e The following references are of record in-the iiie of this patent; Y Y

LUNTTED STATES PATENTS r Date Y VNumber 'Name 2,163,157 Samuel'` s June 20,' 1939 2,409,033 Hansei1 oct. 8,1946 2,410,396 Spencer Oct. 29, 1946 2,411,601

.Spencer Nov. 26, 1946

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