US3488550A - High power resonant cavity tube - Google Patents

Description

HIGH POWER RESONANT CAVITY TUBE Filed July 11, 1967 2 Sheets-Sheet 1 Fig! Henry G. Olfmonfln INVENTOR.

AGENT Jan. 6, 1970 H. G. OLTMAN, JR

HIGH POWER RESONANT CAVITY TUBE 2 Sheets-Sheet 2,

Filed July ll, 1967 mac: 5 mom Sel m; S

Henry G. Olrmon,drt

INVENTOR.

United States Patent US. Cl. 315-544 4 Claims ABSTRACT OF THE DISCLOSURE A cavity resonator is operated in the TE mode and is associated with an electron stream so as to enable the electromagnetic field of the cavity resonator to act up n the electron stream for a longer period of time than heretofore available in prior art devices.

BACKGROUND OF THE INVENTION This invention pertains to the field of cavity resonators and, more particularly, the invention is cencerned with an improved cavity resonator for increasing the interaction of the resonator with an electron stream. Various devices exist in the prior art for coupling cavity resonators to an electron stream so as to induce a current or density variation in the electron stream by accelerating and decelerating, which results in bunchin g the electrons as a function of the resonant frequency of the cavity resonator. One such device is disclosed in US. Patent No. 2,379,818, entitled Coupling Between Resonators and Electron Streams by Warren B. Mason. In that patent, there is disclosed a twocavity resonator klystron in which the impedance of the cavity is matched to the electron beam in order to achieve improved operation. A limitation exists for this type of device when operated at millimeter wave lengths in that the spacing beween the cones is so close that the probability of arcing is increased, and this in turn limits the power level at which the device can be operated. Machining becomes critical for re-entrant cones in millimeter wavelength devices because of the small size of components. It would therefore be advantageous to have a device wherein a larger physical cavity resonator achieves the same or greater energy interaction with the electron stream.

SUMMARY OF THE INVENTION In a preferred embodiment of the invention, it is possible to increase the interaction energy between the resonant cavity and the'electron stream by increasing the physical length of the cavity and allowing the cavity to operate in the TE mode for a rectangular or square cavity and a TM mode for a circular cavity. The modes of resonant cavities and waveguides designate the distribution of the electric and magnetic fields. For example, the symbol TE indicates that the electric field is everywhere transverse to the axis of the transmission line or resonator. For the designation TE the subscript m denotes the number of maxima of electric field along the wide dimension of a rectangular waveguide, or in a resonator its equal to its length. The subscript n denotes the number of maxima of electric field along the narrow dimension. The mode TE therefore has one maxima along the length and another along the width. In the time domain this mode, for a period of one cycle, generates an electric field along the long dimension of the cavity which increases from zero to a maximum, then back to zero, increasing to a maximum in the reverse direction and ends the cycle by decreasing to zero. An electron entering the resonant cavity along the axial center at a time near the beginning of the cycle and exiting the cavity at a time near the end of the cycle will be positioned at all times Patented! Jan. 6, 1970 during the cycle to do work on the field. The interaction time of this device will be one cycle instead of one-half cycle or less, obtainable in prior art devices. The power generated by this improved cavity can be four times m re than that available from conventionally operated cavities fabricated, using the same materials. An indirect advantage is gained by the use of a large cavity in that fabrication problems are simplified, which is particularly important at millimeter wavelengths.

Accordingly, it is an object of the Present invention to provide an improved resonant cavity for interaction with an electron stream.

It is a further object of the present invention to provide an apparatus capable of interacting with an electron stream for a longer period of time.

It is another object of the present invention to provide a resonant cavity which can minimize fabrication problems.

The aforementioned and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings, throughout which like characters indicate like parts, and which drawings form a part of this application.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 illustrates diagrammatic-ally a microwave oscillator of the dual resonance chamber type utilizing a rectangular and square type resonant cavity in accordance with the invention;

FIGURE 2 illustrates the distributed electric field in the square resonant cavity illustrated in FIGURE 1; and

FIGURE 3 illustrates diagrammatically a microwave oscillator of the dual resonance chamber ty-pe utilizing cylindrical resonant cavities in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGURE 1 shows a two-cavity oscillator 10 associated with the cavity resonator 16. This showing is typical only as the resonant cavity of this invention may be associated with different types of electron tubes and circuits. The oscillator 10 has an evacuated container 12 of dielectric material enclosing an electron gun 13 and a collector 14. Passing through the wall of the container 12 are the disc like walls of conducting resonant cavities 15 and 16. Apertured openings pass through the center of each of the disclike walls of resonant cavities 15 and 16 and inserted into these openings are conductive grids 23, 24 and 25, 26, respectively. These grids increase the inter-action of the field with the stream. The openings permit an electron stream to pass from the electron gun 13 through to the collector 14 with a minimum amount of hindrance. A coaxial feedback line 17 having a coupling loop 18 at both ends serves to feed back output energy from resonant cavity 16 to resonant cavity 15. Energy of the desired oscillation frequency may be withdrawn from resonant cavity 16 by an output coaxial line 19 having a coupling loop 20 projecting into the cavity. The resonant cavity 16 is approximately twice as long as resonant cavity 15. FIGURE 1 also illustrates the difference between the physical size of a rectangular cavity which is operated in the TE mode (cavity 15) as versus a square cavity operated in the TE mode (cavity 16). The increased length enables a longer interaction time upon the electron stream which passes through the openings in the disc-like walls of the resonant cavity.

A potential source 27 is connected between the electron gun 13 and the electron collector 14 so as to provide the necessary operating potentials. A tap 29 connected to the power source 27 provides a polarizing potential to the resonant cavities 15 and 16. A filament supply 30 provides the electron gun 13 with the necessary potential for heating the filament. In operation, the electron stream flows from the electron gun 13 to the collector 14 and thereby passes through gaps 21 and 22 in resonators 15 and 16, respectively. The electron stream in passing gap 21 is subjected to a velocity variation which, in the course of transit through to the gap 22, becomes bunched so that at gap 22, it may react with the electromagnetic field within resonant cavity 16 to yield energy thereto. Oscillations of a desired frequency are fed back by the coaxial cable 17 to the resonant cavity 15. This enables oscillations of the desired frequency to be sustained. Energy of the desired oscillation frequency may be withdrawn from cavity 16 by use of the output coaxial cable 19 and coupling loop 20. Also, in order to sustain oscillations, it is essential that the groups or bunches of electrons arising at gap 22 reach it at such times that on the whole, they will deliver more energy to the electromagnetic field than they extract from it. The electromagnetic waves in the interior of the cavity resonator periodically establishes a high potential difference between the grids 23 and 24, alternating in polarity at the frequency of the electromagnetic waves. During the portion of the frequency cycle when grid 24 is positive with respect to grid 23, the electromagnetic field produced by the grid is of such polarity as to aid the motion of the electrons from the electron gun 13 toward the collector 14. The electrons situated between grids 23 and 24 are accelerated by the field produced by the cavity resonator 15 and kinetic energy is stored in these electrons. The accelerated electrons begin to gain upon those ahead and thus tend to create a non-uniform density of electrons in the electron stream.

During the portion of the frequency cycle, when grid 24 is negative with respect to grid 23, the field produced by the resonator is in such a direction as to oppose the electron motion, thus slowing them down with the result that the field absorbs energy from the electrons. This cycle of operation has assumed that those electrons that have been accelerated by the difference in potential between the grids have already passed through the cavity area and are in the space between grids 24 and 25. If, for some reason, these electrons have not yet passed completely through the cavity, when the field reverses, work will be done upon these electrons, negating the acceleration imparted to the electrons in the first half cycle. Assuming that the electrons within the grid area (that is, the area beween grids 23 and 24) are those that have had no work done upon them as yet, then when the fields potential reverses, the field will then tend to oppose their motion, thus slowing these electrons down with the result that the field absorbs energy from the electrons. The electrons slowed down are gained upon by the next group of accelerated electrons, resulting in a tendency to group a bunch of the electrons. The energy which the field of the resonator delivers to the electrons by accelerating them during one-half cycle is returned to the field during the succeeding half cycle when other electrons are slowed down by the field. In resonant cavity 16, operated in the TE mode, there exists one complete cycle of electromagnetic waves or, in other words, one complete potential difference, that is, we may think of the resonant cavity as having at approximately the halfway point an additional grid with respect to which if the potential between the grid 25 and the central grid is rising toward the maximum, then the potential between the central grid and the grid 26 is heading or decreasing toward a corresponding negative maximum. As the electron bunch enters this cavity past the grid 25, it generates electromagnetic waves in the cavity or does work in the cavity for the entire transition time between grids 25 and 26. This is accomplished due to the fact that when the electron bunch passes this imaginary central grid the cycle, that is, the potential of polarity between the central grid and grid 26, reverses such that it becomes identical to what was the potential between grids 25 and the central grid. The electron then interacts with the electromagnetic field of the cavity for a period that is approximately twice as long'as that achievable in a TE mode.

For purposes of illustration, only the output cavity 16 was illustrated operating in the TE mode. It will be obvious to persons skilled in the art that the modulating cavity 15 also may be operated inthe TE mode with a substantial increase in efficiency for the oscillator.

It will also be obvious to personsskilled in the art that amplifiers may also utilize-this, improved resonant cavity with success. For example,-in the oscillator illustrated in FIGURE 1, energy is coupled back from the output cavity 16 to the modulating cavity 15. In an amplifier system, this feedback loop can be eliminated and an input from a transmission line can be directly coupled to the resonant cavity 15 and the signalpresent on this transmission line would determine the modulation,characteristics of the electron stream passing through the resonant cavity 15. In this way, within a certain. band width, the input signal could be amplified with more efiiciency than would be possible in cavities operated in the TB mode.

Referring now to FIGURE 3, resonant cavities 15 and 16 have been replaced with corresponding cylindrical cavities 30 and 31, respectively. The circular mode which corresponds to the rectangular mode TE is the TM and cavity 31 is dimensioned to support this mode for the frequency range of interest. The cylindrical cavity has the distinct advantage of being relatively simple to manufacture. 1

While there has been shown what are considered. to be the preferred embodiments of the present invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as fall within the true scope of the invention.

What is claimed is:

1. A high power resonant cavity tube comprising:

(a) a source of an electron stream having a predetermined velocity;

(b) means for modulating said electron stream; and

(c) a cavity resonator following said means for modulating and having a pair of spaced openings, said openings having a spacing related to the velocity of said electron stream so that said electrons pass through said resonator between said opening within one cycle of an electromagnetic wave excited within said resonator, said resonator having such a shape that said electromagnetic wave within said resonator has alternating fields varying in such. a manner that an electron entering said cavity resonator at the beginning of a cycle is capable of delivering energy to said electromagnetic wave during its entire passage through said cavity.

2. A high power resonant cavity tube comprising:

(a) a source of an electron stream including means for accelerating said electrons to a predetermined average velocity;

(b) a first resonant cavity for velocity modulating said electrons; and

(c) a second cavity resonator operable in the TE mode, said second'resonator having a pair of openings with a-spacing sorelated' to said average electron velocity that the electrons pass' between said openings within one cycle of an electromagnetic wave generated in said second resonator, whereby a bunch of electrons entering the first one of said openings at the beginning' of a cycle of said electromagnetic wave'are 'capable'of delivering energy to said second resonator during theen-tire'cycle of said-Wave.

3. 'A'cavi'ty resonator as'defined'in claim 2 wherein said firstandsaid 'secoridres onatorsare electromagnetically coupled, thereby/"to generate microwave oscillations.

4. A'high power resonantcavity tube'comprising:

-(a) a source of an electron stream including means for imparting to said electrons a predetermined average velocityg- I a '(b) a first cavity disposed to be transversed by said References Cited (Sleig stream for velocity-modulating said electron UNITED STATES PATENTS (c) a second cavity resonator having a pair of spaced 2,466,704 4/1949 Harrison 315 5-43 openings capable of operating in the TM mode, 5 2,621,304 12/195?- Q y et 31545-44 said openings being spaced such a distance related 10/1958 f to the average velocity of said electrons that said 3,012,170 12/1961 Hell electrons pass between said openings during one 3,376,524 4/1968 Wang 315-552 X cycle of an electromagnetic wave excited in said second resonator, whereby an electron entering said HERMAN KARL SAALBACH Pnmary Exammer second resonator at the beginning of a cycle of the 10 SAXFIELD CHATMON, 1a., Assistant Examiner electromagnetic wave is capable of delivering energy to said second resonator during the entire cycle U.S. Cl. X.R.

of said electromagnetic wave. 3155.43, 5.51

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