US3305753A - Magnetron having magnetic bias of such strength as to make cyclotron frequency equal to twice pi frequency, useful for cold cathode operation - Google Patents

Description

3,305,753 MAGNETRON HAVING MAGNETIC BIAS 0E SUCH STRENGTH As To MAKE USEFUL Feb. 2l, 1967 R. A. WHITE CYCLOTRON FREQUENCY EQUAL TO TWICE Pi FREQUENCY FOR COLD CATHODE OPERATION 5 Sheets-Sheet 2 Filed May 5, 1965 Fig.2.

PULSE SOURCE 3,305,753 MAGNETRON HAVING MAGNETIC BIAS oF SUCH STRENGTH AS To MAKE Feb. 21, 1967 R. A. WHITE CYCLOTRON FREQUENCY EQUAL TO TWICE Pi FREQUENCY, USEFUL FOR COLD CATHODE OPERATION 3 Sheets-Sheet 5 Filed May 5 1963 CYCLOTRO N FREQUENCY Pl-MODE FREQUENCY CURRENT PULSE ANODE VOLTAGE HARTREE LINE MAGNETIC FIELD-'- Figeac.

.overall etliciency of the device.

United States Patent O MAGNETRON HAVING MAGNETIC BIAS OF SUCH STRENGTH AS TO MAKE CYCLO- TRON FREQUENCY EQUAL TO TWICE PI FREQUENCY, USEFUL FOR COLD CATH- ODE OPERATION Richard A. White, Horseheads, N.Y., assigner to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed May 3, 1963, Ser. No. 277,950 9 Claims. (Cl. S15-39.63)

The present invention relates generally to electron discharge devices and more particularly to devices of the magnetron type.

In the normal operation of devices such as magnetrons,- electron emission is initiated and sustained through the thermal emission from an emissive surface cathode. A heating element disposed within the cathode is customarily utilized to bring the cathode to emission temperature.

VIn certain magnetrons, once the device has reached the operation-al state, it is possible to greatly reduce the heater current and hence the thermionically emitted electrons. In this instance, the bulk of the electrons necessary for sustaining ope-ration are secondary emission electrons cau-sed by electron back-bombardment of the cathode. Even `with this secondary emission, the presence of some means for initially bringing the emissive surface of the cathode to operating temperature has always been required. The desirability of having a completely cold cathode, that is, one without a heater, is apparent from at least three standpoints. The lirst of these pertains to the overall eiciency of the device as it is obvious that in the supplyin-g of current to the Acathode heater, considerable power is consumed with a resultant lessening of the A second, although perhaps less obvious, rea-son for desiring a completely cold cathode device is from the Istandpoint of tube failure. One of the common causes of tube failure is the cathode heater. Much eifort and money has been expended in the field of magnetrons, as well as in other types of discharge devices, to provide a cathode heater which will operate satisfactorily and which has suitable life. A third point to be considered is that of production cost. I t is obvious that if the heating element of the cathode is eliminated without substantial increase in cost of production of the rest of the tube, that a more economically manufactured device will be obtained.

It is, therefore, an object of the present invention to provide an improved electron discharge device.

A further object is to provide an improved magnetron.

A stil-l further object is to provide an improved electron discharge device of the cold cathode type.

Still another object is to provide an improved magnetron which is capable of cold cathode starting and operation.

An additional object is t-o provide an improved method of starting and operating a cold cathode magnetron.

Stated briey, the present invention provides a cold cathode magnetron through -synchronous secondary electron emission from the cathode. This emission is achieved by the proper selection of the magnetic field to provide that the number of epicycloidal scallops per anode segment is approximately one, or at least a small integral number, and that the cyclotron frequency is approximately an even multiple of the magnetron operating frequency.

Further objects and advantages of the invention will become apparent .as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this speciication.

For a better understanding of the invention,l reference may be had to the accompanying drawings, in which:

FIGURE 1 is a schematic illustration, partially in Section, of a magnetron embodying the present invention taken along lline I-I of FIG. 2;

FIG. 2 is a schematic illustration, partially in section, taken along line II-II of FIG. 1 to which certain components have 4been added;

FIG. 3 is a fragmentary view, partially in section, of a portion of the cathode and anode of the device and illustrating electron paths;

FIGS. 4a, 4b and 4c are illustrations of certain electron trajectories prevalent in magnetrons;

FI-G. 5 is a graphical representation of the operational output of a device in accordance with the present invention; and

FIGS. 6a and `6b are graphical representations of certain voltage characteristics useful in the explanation of the present invention.

With reference now to FIGS. 1 and 2, there is shown a magnetron which includes a centrally disposed cathode 10 which consists of la sleeve member 12, of a suitable material such as molybdenum having on the outer surface thereof an electron emissive coating 14. The emissive coating 14 is one which is a good secondary electron emitter, "for example thorium oxide. Symrnetrically disposed around and spaced from the cathode 10 so as to provide a region 16 therebetween is an anode indicated generally `at 18, The anode 18 is comprised of an outer, substantial-ly hollow cylindrical member 20 and an inner Washer or apertured disc member 22. The two portions 20 and 22, made of a suitable material such as copper, may be formed from a single block of material by appropriate means such as by Imachining or may be separately formed and secured together by suitable means such as brazing. The inner portion 22 of the anode20 is provided with a plurality of cavity resonators 24 of the well known hole and slot type. In the illustrated embodiment, eight such resonator-s equally spaced about the anode are shown.

As seen in FIG. 1, an output means 26, in communication with one of Vthe cavities 24 by means of a slot 28 extending through the anode to the cavity, is provided to remove energy from the device.

The remaining basic components of the device are best seen with respect to FIG. 2 and include a pair of end plates 30 which may be vacuum sealed to the outer portion 20 of the anode 18 to maintain a vacuum within the interior of the device. Also provided are a pair of substantially U-shaped magnets 32 and a pair of substantially identical pole pieces 34, of satisfactory magnetic material such as soft iron, which extend from the magnets into the vacuum portion of the device, terminating in close proximity to the region 16 between the cathode and the anode. The magnets 32 and the pole pieces 34 collectively comprise the magnetic circuit of the device. l

Also schematically illustrated in FIG. 2 is a potential means 36 which is connected, by means 38 and 40, respectively to the cathode 10 and the anode 18. While in the illustrated embodiment, the potential means 36 is shown to be a battery with its positive terminal connected to the cathode and its positive terminal connected to the anode, it is to be understood tha-t, as is common practice in the ait, the anode would in all probability be maintained at ground potential while the cathode would be connected to a suitable source of negative potential. Also, an electronic switch 37 might be used to rapidly turn on and off, that is, pulse the magnetron.

In normal magnetron operation, electrons are thermionically emitted from the cathode and, by virtue of an electrical field extending substantially radially between the cathode and the anode and a magnetic field extending substantially perpendicular to the electric field and parallel to the axis of symmetry, follow what are generally described as cycloidal or, more correctly, epicycloidal paths. lnasmuch as each of the cavities is a resonant circuit, those electrons which are properly phased give up energy to the cavities to produce usable power which may be removed from the magnetron by some suitable means. One usual way to improve efiiciency is to have many epicycloidal scallops per resonator. lnasmuch as the electrons, after having given up their energy to the cavities are collected on the anode, the plurality of scallops insures only a small amount of residual rotational energy associated with the electron when it strikes the anode. This action is one which is well known in the art and further discussion thereof is not deemed warranted here.

In the present invention, the efliciency of the magnetron is enhanced through the utilization of a completely cold cathode, which is made possible by the enhancement and synchronization of secondary electron emission. Throughv the proper selection of magnetic and electrical field parameters, preferably only one epicycloidal scallop per anode segment, and at most a very small integral number of scallops per anode segment is utilized. The single scallop per anode segment is illustrated by line 40 in FIG. 3.

The present invention provides such synchronous secondary electron emission by utilizing a magnetic field which is of a value such that the cyclotron frequency is twice (or an integral even multiple) of the operating frequency of the magnetron. The customary operating frequency of a magnetron is the pi-mode and magnetron types operating in this mode are well known in the art, for example, strapped magnetrons. The cyclotron frequency of a magnetron is known to be calculable by the following formula: fc=eB/21rm, which is equal to approximately 28B (where fc=the cyclotron frequency in gigacycles; B=the magnetic flux density in Webers per square meter; e=electronic charge, a fundamental constant of nature with a value of 1.602X-19 coulomb; m=electron rest mass, a fundamental constant of nature with the value 9.108 X1031 kilograms; and 21r= 6.283185, a dimensionless geometrical constant).

FIGS. 4a, b and c will help clarify the present invention. As is well known, the trajectories of electrons within the magnetic field of a circular type magnetron, in the absence of a space charge, are epicycloids as illustrated in FIG. 4a. The diameter of the generating circle of the epicycloid is determined by both the electrical and magnetic fields. The rate of rotation of the generating circle is determined only by the magnetic field and is the cyclotron frequency as is set forth above, i.e., fc=28B. FIG. 4b, the curve of which may be designated a prolate epicycloid, illustrates the effect of initial electron velocity from the cathode.

The velocity (Vd) of the generating circle (the drift velocity) is given by the formula Vd=E/B where E is the electric field between the anode and cathode in volts per meter and B is as defined above. Therefore, the radius of the generating circle is determined by mE/eB2. In the customary magnetron, the traveling electromagnetic wave on the anode structure as is illustrated in FIG. 1 is synchronized with E/B (the drift velocity). Electrons which gain energy from the resonators 24 assume a trajectory as is shown in FIG. 4b. Those electrons which give energy to the resonators 24 assume a trajectory such as is illustrated in FIG. 4C (a curtate epicycloid) and gradually drift to the anode after having given up most of their energy. As has been stated, one method of improving efficiency has been to have a large number of such scallops per anode segment so that electrons following the curtate epicycloidal type of path have little energy left when they reach the anode.

The present invention provides that only a very small number, preferably one, of epicycloidal scallops per anode segment is utilized.

The operation of such a single scallop per anode segment device is as follows. Emission of an electron under a positive anode segment will cause the emitted electron to be accelerated and to arrive at the cathode under the next anode segment when that segment has become positive. Secondary electrons will be emitted and function in the same manner on the next half cycle of the magnetron oscillation. If an emitted electron loses energy to the field because it is emitted under a negative segment of the anode, it will continue to do so until collected by the anode. The relative phase of the oscillation of the magnetron is adjusted with respect to the emission time to insure that electrons in both categories are generated. Thus, it is seen that those electrons which are properly phased with the cavity voltages across gaps 25 serve to give energy to the magnetron while those which are improperly phased with respect to this are returned to the cathode in a manner to produce secondary electrons. Part of the secondary electrons will be properly phased and part will be improperly phased to give energy to the cavities. This mode of operation provides that a sufficient number of secondaries are produced and that the tube may be operated with no primary emission.

FIG. 5 is a curve resulting from data taken from a magnetron operated in accordance with the present invention. It is noted that the anode current peaks quite rapidly in a region in which the cyclotron to pi-mode frequency ratio is approximately two. Another peak (not illustrated) was found where this ratio was approximately four but it was noted this latter peak was of lesser height.

Thus, it is seen that by the proper selection of the cyclotron `frequency that the secondary emission of the electrons is synchronized so as to provide an enhancement thereof.

The foregoing discussion has been premised upon the assumption that there are electrons available with which to start the .aforedescribed operation. The problem remains, however, as to the manner in which the first electron `to initiate the abovedescribed operation is produced. It has been found in practice that the action may be initiated by supplying a negative voltage pulse to the cathode of sufficient lmagnitude so that if an electron were emitted yfrom the cathode it would reach the anode. Such a pulse source is shown schematically at 37 in FIG. 2. The above value of voltage is commonly referred to las the Hull cut-off value. This value is represented graphically in FIGS. 6a and 6b. After the above-described pulse has been applied, it was found that if the cathode voltage is then dropped to the normal operating level (t'he Hartree level) 4and if the above relationship of cyclotron frequency to operating frequency are observed, that complete cold cathode operation was initiated :and sustained. By way of example, a device was operated with a Hartree level of 2S kilovolts; the application of voltage near the Hull cut-off value, approximately 61 kilovolts, for a period of .53 microsecond suf'liced to initiate operation.

While it is not certain where lche first electron necessary for operation of t-he device comes from, it has been theorized, although not proven, that the highly secondarily emissive material of the cathode (and this is particularly true in the case of thorium oxide) is slightly radioactive and :as such emits higih velocity beta electrons which have sufiicient energy to escape from the coating. These electrons strike the anode with a velocity which is sufiicient to effect X-radiation from the cathode which, in turn, initiates photoemission from the cathode and hence the electrons necessary for operation are available.

It is thus seen that there has been provided a device of Iche magnetron type capable of complete cold cathode operation which is achievable through the synchronization of secondary electron emission from the cathode in the magnetron -by making the magnetic field of4 such a value that the cyclotron frequency is twice the operating frequency or close thereto or any even multiple of the operating frequency or close thereto. A particular set of values for such a described Adevice would be for a magnetron which is designed to operate in the pi-mode frequency at approximately 1.310' gigacycles. In such a device, the cyclotron Afrequency required would be approximately 2.620 gigacycles and from the above formula, where fc is equal to .approximately 28B, the magnetic field within the region 16 would be approximately 0.0935 weber .per square meter.

While there have been shown and described what are at present considered to be the preferred embodiment of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit 'and scope of the invention.

I claim as my invention: i

1. An electron discharge device comprising a cathode capable of electron emission, an anode including a plurality of cavity resonators spaced from and circularly arranged about said cathode, said cavity resonators being primarily determinative of the operating frequency of said device, means for applying an electrical potential between said cathode and said anode whereby there is established an electrical field in the region therebetween, and means for the establishment of a magnetic field in the region between said cathode :and said anode which is substantially perpendicular to said electrical field, said electrical and magnetic fields acting to impart Ia substantially epicycloidal motion to electrons emittedV from said cathode, said magnetic field being of a value such that the frequency of said cycloi-dal motion of said electron is equal to approximately twice that of the operating frequency of said device.

2. An electron ,discharge device of the magnetron type comprising a cathode capable of electron emission, an anode including a plurality of cavity resonators spaced from and circularly arranged about said cathode, said cavity resonators being primarily determinative of the operating frequency of said device, means for applying an electrical potential between said cathode and said anode whereby there is established an electrical field in the region therebetween, and :means for the establishment of a magnetic field in the region between said cathode and said anode which is substantially perpendicular to said electrical field, said electrical and magnetic fields acting to impart :a substantially cycloidal type motion to electrons emitted from said cathode, said magnetic field being of a value such that the frequency of said cycloidal motion of said electron is equal to approximately an even integral multiple of the operating frequency of said device.

3. An electron discharge device of the magnetron type comprising a substantially circular cathode capable of electron emission from the outer surface thereof, an anode including a plurality of cavity resonators spaced from :and circularly arrange-d about said cathode, said cavity resonators being primarily determinative of the operating frequency of said device, means for applying an electrical potential Ibetween said cathode and said :anode whereby there is established an electrical field in the region therebetween, and means for the establishment of a magnetic field in the region between said cathode and said anode which is substantially .perpendicular to said electrical field, said electrical and magnetic fields acting to impart a substantially epicycloidal motion to electrons emitted from said cathode, said magnetic field being of a value such that the frequency of said cycloidal motion of said electron is equal to approximately an even integr-al multiple of. the operating frequency of said device.

4. An electron discharge device of the magnetron type comprising a cathode capable of electron emission, an anode including a plurality of cavity resonators spaced .from and circularly arranged about said cathode, said cavi-ty resonators being primarily determinative of the operating frequency of said device, and said device operative in the pi-mode, means for applying an electrical potential between said cathode and said anode whereby there is established an electrical field in the region therebetween, yand means for the establishment of Aa magnetic field in the region between said cathode and said anode which is substantially perpendicular to said electrical field, said electrical and magnetic field acting to impart a substantially cycloidal type motion to the electrons emitted from said cathode, saidi magnetic field being of a value su-ch tha-t the frequency of said cycloidal motion .of said electrons is approximately an even integral multiple lof the operating frequency of said device.

5. An electron discharge device of the magnetron type comprising a cathode capable of electron emission including an electron emissive coating of thorium oxide thereon, an anode including a plurality of cavity resonators spaced from and circularly arranged about said cathode, said cavity resonators being primarily determinative of .the operating frequency of said device, and said device operative in the pi-mode, means for applying an electrical potential between said cathode and said anode whereby there is established an electrical field in the region therebetween, and means for the establishment of a magnetic field in the region between said lcathode and said anode which is substantially perpendicular to said electrical field, said electrical and magnetic field acting to impart a substantially cycloidal lmotion to the electrons emitted from said cathode, said magnetic field being of a valfue such that the frequency of said cycloidal motion of said electrons is approximately an even integr-al multiple of the |operating frequency of said device..

6. An electron discharge device yof the magnetron type comprising 'a cathodeA capable of electron emission including a thorium oxide coating thereon, an anode including a plurality of cavity resonators spaced from and circularly arranged about said cathode, said cavity resonators being primarily determinative of the operating frequency of said device, and said device operative in the pi-mode, means for applying an electrical potential between said cathode and said anode whereby there is established an electrical field in the region therebetween, and means for .the establishment of a magnetic field in the region between said cathode and said anode which is substantially perpendicular to said.I electrical field, said electrical and magnetic field .acting to impart a substantially cycloidal motion to the electrons emitted from said cathode, said magnetic field being of a value such that the frequency of said cycloidal rnotion of said electrons is approximately double the operating frequency of said device.

7. An electron discharge device of the magnetron type comprising a catho-de capable of electron emission, an anode including a plurality of cavity resonators spaced from and circularly arranged about sai-d cathode, said cavity resonators being primarily determinative olf the operating frequency cf said device, said device being operative in the pi-mode, means for applying an electrical potential between said cathode and said anode whereby there is established an electrical field in lthe Iregion therebetween, and means for the establishment of a magnetic field in the region between said cathode and said anode which is substantially perpendicular to said electrical field, said electrical and magnetic field acting to impart a substantially cycloidal type motion to the electrons emitted from said cathode, said magnetic field being of a value such Ithat t-he frequency of said cycloidal motion of said electrons is approximately twice that of the pi-mode frequency of said device.

l8. An electron discharge device of the magnetron type comprising a cathode capable of electron emission including a coating of thorium oxide, an lanode including a plurality of cavity resonators spaced from and circularly arranged about said cathode, said cavity resonators being primarily determinative of the operating frequency of said device, said device operative in the pimode, means for applying an electrical potential between said cath-ode and said anode whereby there is established an electrical eld `in the region therebetween, and means for the establishment of a magnetic field in the region between said cathode and said anode which is substantially perpendicular to said electrical eld, said electrical -and magnetic eld acting to impart a substantially cycloidal motion to the electrons emitted `from said cathode, said magnetic eld being of a value such that the frequency of said cycloidal motion of said electrons is approximately twice that of the pi-mode frequency of said device.

9. In combination, an electron discharge device of the magnetron type comprising Ia cathode, including a layer of thorium oxide, capable of electron emission, an anode including a plurality of cavity resonators spaced from and circularly arranged about said cathode, said cavity resonators being primarily dete-rminative of the operating frequency of said device, said device operative in the pi-mode, means for sequentially supplying first an electrical voltage pulse between said anode and said cathode of suicient magnitude to draw an electron at the surface of said cathode to said anode and then for applying an electrical potential between said cathode and said anode whereby there is established therebetween an electrical field of sufficient magnitude to sustain operation, means for the establishment of 'a magnetic eld in the region between said cathode and said .anode which is substantially perpendicular to said electrical field, said electrical and magnetic field lacting to impart a substantially cycloidal motion to the electrons emit-ted from said cathode, said magnetic field being of a value such that the frequency of said cycloidal motion of said electrons is approximately twice that of the pi-mode frequency of said device.

References Cited by the Examiner UNITED STATES PATENTS 2,624,863 1/1953 Clogston 315-39.57 X 2,824,231 2/1958 Feinstein et al. 315-3957 X 2,902,653 9/1959 Vaughan 3l5-39.53 X 3,109,123 10/1963 Spencer 313--336 X OTHER REFERENCES Principles of Radar, by members of the Staff of the Radar School, Massachusetts Institute of Technology, Mc- Graw-Hill 1946, pages 5-10 and 5-11.

HERMAN KARL SAALBACH, Primary Examiner.

E. LIEBERMAN, Assistant Examiner.

Claims (1)

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A CATHODE CAPABLE OF ELECTRON EMISSION, AN ANODE INCLUDING A PLURALITY OF CAVITY RESONATORS SPACED FROM AND CIRCULARLY ARRANGED ABOUT SAID CATHODE, SAID CAVITY RESONATORS BEING PRIMARILY DETERMINATIVE OF THE OPERATING FREQUENCY OF SAID DEVICE, MEANS FOR APPLYING AN ELECTRICAL POTENTIAL BETWEEN SAID CATHODE AND SAID ANODE WHEREBY THERE IS ESTABLISHED AN ELECTRICAL FIELD IN THE REGION THEREBETWEEN, AND MEANS FOR THE ESTABLISHMENT OF A MAGNETIC FIELD IN THE REGION BETWEEN SAID CATHODE AND SAID ANODE WHICH IS SUBSTANTIALLY PERPENDICULAR TO SAID ELECTRICAL FIELD, SAID ELECTRICAL AND MAGNETIC FIELDS ACTING TO IMPART A SUBSTANTIALLY EPICYCLOIDAL MOTION TO ELECTRONS EMITTED FROM SAID CATHODE, SAID MAGNETIC FIELD BEING OF A VALUE SUCH THAT THE FREQUENCY OF SAID CYCLOIDAL MOTION OF SAID ELECTRON IS EQUAL TO APPROXIMATELY TWICE THAT OF THE OPERATING FREQUENCY OF SAID DEVICE.

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