US2713653A

US2713653A - High frequency magnetron

Info

Publication number: US2713653A
Application number: US206611A
Authority: US
Inventors: Elmer D Mcarthur
Original assignee: Individual
Current assignee: Individual
Priority date: 1951-01-18
Filing date: 1951-01-18
Publication date: 1955-07-19
Anticipated expiration: 1972-07-19

Description

July 19, 1955 E. D. MCARTHUR 1 ,6

HIGH FREQUENCY MAGNETRON Filed Jan. 18, 1951 Inventor:

Elmer- D. McAT-thur,

b3 2?! 4. fillwb His Attorney.

United States Patent 715,653 incn FREQUENCY MAGNETRON Elmer D. McArthur, Scotia, N. Y., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application January 18, 1951, Serial No. 206,611 Claims. (Cl. 315-39-53) My invention relates to and, more particularly, known as magnetrons.

In the past few years, applications in the electronics art have made more and more use of ultra-high and super-high frequency oscillations, and the magnetron has been developed and used as one of the most important means for generating such high frequency oscillations from direct current electrical energy. Essentially, all magnetrons in use at the present time employ a cylindrical cathode within a concentric anode, a radial electric field set up between these electrodes by a difference of potential applied to them, and an axial magnetic field established in the space between the electrodes by a permanent or electromagnet. Well known classifications of magnetrons, according to principle of operation,

electron discharge devices to such vacuum tube devices are the negative resistance, electron transit-time, and car;

traveling wave types, which depend mainly upon differences in anode construction and resonating means for their diiferences in principle of opeartion. It has become well known that the traveling wave type magnetron, commonly known also as the multicavity magnetron, employing an anode block having several resonant cavitiesand an output coupling loop or other coupling means is the best suited to generate super-high frequencies with a reasonable degree of efliciency. Thus, the multicavity magnetron its theory of operation is well known to the art.

However, at super-high frequencies the amount of power and length of tube life that may be obtained from a given multicavity magnetron are still limited, especially at the upper super-high frequencies, by possible destruction of the cathode, since the emitting surface is relatively small, resulting in high current density. Further, a considerable cathode back heating effect is encountered, due to electrons returning to and striking the cathode. been increased in order to enlarge the cathode emitting surface, but, for the upper super-high frequencies where the size of resonant cavities is necessarily small due to the short wave lengths, the number of resonant cavities has come into wide usageand space near the free ends of these pole pieces by flaring The diameter of the cathode and anode has L required around the inner surface of the anode became so great that transient and spurious modes of oscillation, that is, instability of frequency, resulted. Like wise, the axial length of the electrodes has been increased in order to enlarge the cathode emitting surface, but this also resulted in instability of frequency, due to transient and spurious modes of oscillation of space charge along the axial dimension. Thus, at the present time, multicavity magnetrons having a reasonable life are limited in power output at the upper super-high frequencies.

It is, therefore, an object of my invention to provide a magnetron for operation at higher frequencies.

It is a further object of my invention to provide a magnetron having longer life and higher power output.

In carrying out my invention, I provide a vacuum tube electron discharge device having opposed parallel pling loop, are positioned 2,713,653 Patented July 19, 1955 2 disk electrodes, the anode having in one surface radial slots of proper depth and shape to form resonant cavities at a desired frequency, and a magnet to establish a radial magnetic field in the space between the electrodes. Suitable output means, such as an output couproperly within one of the resonant cavities in a well known manner. When a difference of potential is applied to the electrodes, an axial electric field is set up in the space between the electrodes, and the device generates high frequency oscillations. Since the cathode emitting area is essentially as large as the anode area, as opposed to a smaller emitting area in conventional magnetrons, more current may be drawn from the cathode at the same current density and any back heating is distributed over a ice larger area.

For a better understanding of my invention, together with further objects and advantages thereof, reference should now be had to the following description referring to the accompanying drawing, in which:

Fig. 1 illustrates, by a partial sectional view, an electron discharge device embodying my invention; Fig. 2 is a side elevation of the permanent magnet shown by sectional view in Fig. 1; Fig. 3 is a perspective view of the anode shown in Fig. 1; and Fig. 4 is a perspective view of another embodiment of the anode which may be used in my invention.

Referring to Fig. 1 of the drawing, an electron discharge device of the magnetron type is shown comprising an outer envelope 1, made of metal for the preferred embodiment illustrated, defining an evacuated chamber in which are mounted a permanent magnet 2, a metallic annular disk forming a cathode 3, and a metallic block forming an anode 4.

The permanent magnet 2 is further illustrated by an end view in Fig. 2 and comprises a shaft-like center pole piece 2a made integral with or secured to a cylindrical outer pole piece 2b through a disk portion 20. The radial magnetic field between inner pole piece 2a and the outer pole piece 2b may be concentrated in the the free end of pole piece 2a, as shown in Fig. 1.

The cathode 3 is supported from an annular disk 5, made of suitable insulating material, which is secured, as shown, to the pole pieces 20: and 2b. The cathode 3, as shown in a preferred form, is made of a suitable metal, such as nickel, to form a hollow annular disk containing a heater element 6 terminated by conductors 7 external tothe envelope 1. A conductor 8 is connected to the cathode 3 and also terminatedexternal to the envelope 1. The surface 3a is made of, or coated with, a suitable material, such as an oxide impregnated nickel matrix, to emit electrons when thermally excited by heater element 6. Concentric cathode shields 3b are metallic extensions of cathode 3 provided to prevent electrons emitted by the cathode surface 3a from reaching the permanent magnet 2 or the envelope 1, thereby limiting the destination of emitted electrons either to the anode 4 or the cathode 3. Shields 3b correspond to the familiar end shields commonly employedin conventional magnetrons.

The anode 4 is secured to one end of envelope 1 by suitable connecting means, as for instance by welding, so that the two are always at the same electric potential. Radial resonant cavities 4a are provided in the face of the anode 4 which faces the cathode 3 and a circular depression 4b permits the pole piece 2a and one shield 3b to extend thereinto as shown in Fig. l. A coupling loop 9, is disposed in one of the cavities 4b and connected by a conductor 10 external to envelope 1 to provide output means for the device. Other well known output means, such as an output wave guide coupled to one of the cavities 4!), may be employed. Suitable insulating seals,

such as glass beads 11, are provided to permit the conductors 7, 8 and to extend from within envelope 1 while a positive vacuum seal is maintained.

Fig. 3, by a perspective view, shows more clearly the construction of anode 4 and the radial resonant cavities 4a formed in one face thereof. The width and depth, as well as the number, of resonant cavities 4a is determined in the design of the anode by the desired frequency of oscillation to be obtained from the magnetron. The cavities 4a, however, are not limited to the slot type shown by Fig. 3 but may be of the slot and hole type shown in Fig. 4 or may be of the familiar rising sun configuration. Strapping means, such as straps 12 in Fig. 4, may be provided around the curved surface of the anode 4 to connect alternate segments and, thus, to stabilize the mode of oscillation.

In operation, which may be pulsed operation, the cathode 3 is maintained at a direct current potential negative with respect to the grounded envelope 1 and anode 4. This establishes an axial electric field between anode 4 and cathode 3. A current is supplied to heater element 6 through conductors 7 to heat surface 3a and, thus, to cause it to emit electrons. Electrons emitted from surface 3a move under the combined influence of the axial direct current electric field and the radial magnetic field to produce electromagnetic fields varying at radio frequency within the resonant cavities 4a. While the paths of electron travel are not the same as those paths in a conventional magnetron, the basic principle of operation, which causes a net transfer of energy from the direct current field to the radio frequency fields is similar in nature to that of the conventional magnetron.

It will be readily apparent that the emitting surface 3a is as large or larger in area than the anode receiving surface. Therefore, the emission current densities for a given power output need not be as high, nor cathode overheating due to electron bombardment as likely, as in the conventional magnetron. Thus, the magnetron of my invention may be constructed to have a reasonable power output and tube life at the upper super-high frequencies, and greater power output than presently available at lower frequencies.

The magnetron of my invention may be provided with water cooling features, as are well known, to further increase its power output capacity and life. It is conceivable that the envelope 1 of my invention may, in some applications, preferably be made of glass. This may be done by providing a separate lead from the anode 4 through the envelope 1 to the exterior thereof. Also by shaping the envelope 1 properly with a deep depression in one end, the permanent magnet 2 may be removably positioned external to the envelope 1 so that it can be interchanged with other magnets providing different magnetic field strengths.

While the present invention has been described by reference to a particular embodiment thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron discharge device of the magnetron type comprising an evacuated envelope, an annular disk cathode therein, means to heat said cathode, a circular anode in spaced relation with said cathode provided with resonating means therein, output coupling means in conjunction with said anode, and means disposed within the evacuated envelope for establishing a radial magnetic field in the space between said cathode and said anode.

2. An electron discharge device of the magnetron type comprising an evacuated metallic envelope, an annular disk cathode therein, means to heat said cathode, a circular block anode connected to said envelope in spaced relation with said cathode and having one face thereof defining a plurality of radially oriented resonant cavities, an output coupling loop positioned in one of said cavities, and a permanent magnet disposed within the evacuated envelope for providing a radial magnetic field in the space between said cathode and said anode.

3. An electron discharge device of the magnetron type comprising an evacuated metallic envelope, an annular disk cathode therein, two concentric and cylindrical shields on said cathode, a heater element positioned within said cathode, a cylindrical anode connected to said envelope in spaced relation with said cathode and having one face thereof containing a plurality of radially oriented resonant slots, an output coupling loop positioned in one of said slots, 21 permanent magnet disposed within the evacuated envelope for providing a radial magnetic field in the space between said cathode and said anode, vacuum seals in said envelope, and conductors extending through said seals from said cathode, said heater element, and said coupling loop.

4. An electronic discharge device of the magnetron type comprising an evacuated envelope, a permanent magnet within said envelope and comprising a disc portion, a central pole piece portion and an outer cylindrical pole piece, said central pole piece and said cylindrical pole piece being substantially coextensive, an annular cathode about said central pole piece and within said outer cylindrical pole piece, an anode axially spaced from said cathode and central pole piece, an insulating disc between said disc portion of said magnet and said cathode, cathode shields extending from said cathode in proximity to said central pole piece, and end shields inside said cylindrical pole piece in proximity thereto.

5. An electron discharge device as defined in claim 4 but further characterized by said insulating disc and said end shields being in contact.

References Cited in the file of this patent UNITED STATES PATENTS 2,270,777 Von Baeyer Jan. 20, 1942 2,411,953 Brown Dec. 3, 1946 2,412,824 McArthur Dec. 17, 1946 2,437,279 Spencer Mar. 9,1948 2,443,179 Beniotf June 15, 1948 2,485,401 McArthur Oct. 18, 1949