US3390301A - Cavity resonator having alternate apertured drift tubes connected to opposite end walls - Google Patents

Description

June 25, 1968 RYOKA SAWADA ET AL 3,390,301

RESONATOR HAVING ALTERNATE APERTURED DRIFT CAVITY TUBES CONNECTED TO OPPOSITE END WALL 2 Sheets-Sheet 1 Filed Dec.

F/G. M PRIOR ART INVENTORS rQYd/ffl Snwnon ATTORNEY June 25,- 1968 RYOKA SAWADA ET AL 3,390,301

CAVITY RESONATOR HAVING ALTERNATE APERTURED DRIFT TUBES CONNECTED TO OPPOSITE END WALLS Filed Dec. 9, 1965 2 Sheets-Sheet 2 FIG 4 Y INVENTORS RYO/(A 3/; W000 Yon: m KIM/5K0 ORNEY United States Patent 3,390,301 CAVITY RESONATOR HAVING ALTERNATE APERTURED DRIFT TUBES CONNECTED TO OPPOSZTE END WALLS Ryoka Sawada, Kokuhunji-shi, and Yoichi Kaneko, Hachioji-shi, Japan, assignors to Hitachi, Ltd., Tokyo, Japan, a corporation of Japan Filed Dec. 9, 1965, Ser. No. 512,673 Claims priority, application Japan, Dec. 18, 1964, 39/ 71,061 2 Claims. (Cl. SIS-5.39)

ABSTRACT OF THE DESCLOSURE A cavity resonator in the form of a hollow conductor casing having end walls including aligned apertures positioned on the axis of the casing and along which a beam of charged particles is propagated, a plurality of aligned apertured members arranged along the path of the particles Within the casing defining a plurality of gaps therebetween, and supporting conductors interconnecting alternate ones of said apertured members and connected to respective opposite end walls of the casing.

This invention relates to cavity resonators.

Cavity resonators previously used in klystron amplifiers for microwave frequency bands and particularly for relatively low frequencies have generally been very large in size, having a considerably large outside diameter even in cases of klystrons of relatively low power level. This naturally has resulted in increase in size of the focusing coil to economical disadvantage and, even with high power tubes, it has been extremely difficult to minimize their external dimensions without detracting from their efficiency. Mere reduction in size of cavity resonators may possibly be realized by the method of increasing the electrostatic capacitance of the gaps, where the electron beam and the electric field of the cavity resonator interact, while reducing their inductance. Cavity resonators so designed, however, would be impractical because of reduction in characteristic impedance and increase in power loss.

Besides single-cavity resonators referred to above, multicavity resonators have also been in use which correspond to a combination of a multitude of single-cavity resonators and have advantages such as increase in frequency bandwidth and reduction in power loss. The multicavity type of resonator, however, provides modes of operation corresponding in number to the cavities included in the resonator and this requires separation of any unwanted modes other than the principal one and thus causes a difficult design problem.

Description Will next be made of the structure of a conventional form of cavity resonator illustrated in FIG- URES laand 1b.

This form of cavity resonator includes two posts 2 formed with aligned axial holes 3 for passage therethrough of an electron beam, and carried on opposite conductor walls, which are integral with a hollow conductor casing 1 enclosing the posts. With this device, reduction in size of the cavity results in proportional reduction in inductance L and in capacitance C of the resonator. In this case, however, since the inductance L of the resonator apparently is determined by its size, the capacitance C must be increased for the resonator to maintain the same basic resonant frequency. To meet this requirement, it would be necessary to increasethe cross-sectional area of the posts 2 or to reduce the gap 4 therebetween with the result that the electric field is unnecessarily collected in areas other than those where it is intended to interact "ice with an electron beam, disadvantageously increasing the power loss of the resonator. In the case of a multicavity resonator, which is actually a combination of a multitude of single-cavity resonators, it is possible to divide the exciting RF voltage into parts corresponding in number to the resonant cavities and to apply such partial voltages to the respective cavities. By doing this, the power loss of such multicavity ressonator can be reduced while increasing its frequency bandwidth. On the other hand, the multicavity resonator inherently involves a disadvantage in that it provides unwanted modes of operation besides the basic resonant frequency.

The present invention has for its primary object to provide a cavity resonator reduced in outside diameter.

Another object of the present invention is to provide a cavity resonator which provides no unwanted modes of operation in the vicinity of its basic resonant frequency.

A further object of the present invention is to provide a cavity resonator which is relatively limited in loss and gives a high gain when employed, for example, in velocity modulation tubes.

These and other objects, features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

FIGURES la and 1b provide longitudinal and transverse sectional views, respectively, of a known cavity resonator;

FIGURES 2a and 2b illustrate one form of multiplegap cavity resonator embodying the present invention;

FIGURES 3a and 3b illustrate another form of multiple-gap cavity resonator embodying the invention;

FIGURE 4 illustrates a further form of cavity resonator embodying the present invention;

FIGURES 5a, 5b, and 5c illustrate various forms of equivalent circuits of the multiple-gap cavity resonator embodying the present invention;

FIGURES 6 and 7 represent diagrams illustrating the operation of the inventive cavity resonator; and

FIGURE 8 illustrates a velocity modulation tube for amplification use incorporating a number of cavity resonators embodying the present invention.

In 2a and 2b parts indicated by reference numerals 1, 3 and 4 correspond to those of the conventional resonator shown in la and 1b which bear the same numerals. Reference numeral 5 indicates apertured members formed each with an axial aperture for passage therethrough of an electron beam. These apertured members 5 constitute a modification for multiple-gaps formation of the posts 2 used in the conventional single-gap cavity resonator shown in FIGURES la and 1b and are alternately interconnected by a pair of supporting conductors 6, with a working gap 4 formed between each two adjacent opposing ones of the ape'rtured members. Though the embodiment shown in FIG. 2a has three gaps, it is to be understood that cavity resonators including a greater number of working gaps can also be readily realized.

The equivalent circuit of the cavity resonator shown in FIG. 2a is illustrated in FIG. 5a. Reference characters C C and C indicate the electro-static capacitance of the respective gaps of the resonator, including any additional electro-static capacitance appearing in regions adjacent to the respective gaps. Characters L and L represent the inductances of the respective supporting conductors 6; L represents the inductance of the cavity; and r a series resistance corresponding to the cavity loss. In cases where the inductances L and L of the supporting conductors are relatively limited, the equivalent circuit is approximately illustrated as in FIGS. 5b and 50. Assuming that the three gaps have the same capacitance, the entire capacitance apparently is three times as large as the one for any single gap and, therefore, the cavity inductance L, can be reduced to one-third of that of an equivalent singlegap resonator. This means that this form of cavity resonator can be greatly reduced in cavity size. In addition, with the resonator, constructed as described above, any unwanted modes other than the basic resonant frequency can be avoided owing to the single-cavity structure of the resonator.

Reference will next be made to FIG. 6, which illustrates the electric field distribution in the working gaps 4 of the inventive cavity resonator at an instant during operation, in the case where the resonator has three gaps, as illustrated in FIG. 2a. As observed, the distribution of the electric field E is alternately reversed in phase for the successive gaps and the high-frequency exciting fields taking place in the respective gaps have substantially the same amplitude since the apertured members interconnected by either of the supporting conductors are held at substantially the same potential. In one ex periment, however, the magnitude of the voltage appearing in the intermediate gap has been approximately twice as large as those appearing in the remaining gaps.

Next, the operation of the cavity resonator when used for velocity modulation of an electron beam will be described, assuming that the resonator includes an odd number 11 of gaps identical in voltage amplitude and alternately reversed in phase of the electric field distributed therein and that the electron transit angle from gap to gap is identical throughout the resonator.

Assuming further that electrons entering the resonator at an initial velocity of V has an exit velocity of V after passage through the multiple-gap cavity of the resonator, the following relation is generally obtained for minute signals:

where V represents the beam voltage, Ve represents the highfrequency exciting voltage for each gap, and [i represents the beam coupling coefficient. [3 can also be expressed as follows:

i Cos 2 (3) It follows, therefore, that a coupling coefiicient 11 times as large as that for a single-gap resonator can be obtained when the electron transit angle is selected, for example, at 1r radians. Thus, in the event such multiple-gap cavity is employed for beam modulation, the overall gap capacity C0 obtained will be 11 times that for a single-gap cavity and the cavity inductance L one-11th of that for the latter. In cases where a series resistance r is selected proportional to L0 or in the same order as that for a single-gap cavity, the parallel resonance resistance R (=L0/C01-) will be reduced to between 1/11 and l/11 of that obtainable with a single-gap cavity and the quality factor Q(=W0L0/1-) of the cavity to 1/11. In the case of a multiple-gap cavity, however, because of its high beam coupling coefiicient, the high-frequency exciting voltage for each gap can be reduced to one-11th, providing for a velocity modulation equivalent to that of a single-gap cavity, and the power loss (=V /2Rs/1) of the cavity, at most equal to that of a single-gap cavity,

can be reduced to one-11th of the latter. This means that the power requirement for the multiple-gap cavity, allowing the largest power loss, can be substantially equal to that for a single-gap cavity, moreover, with cavity resonators of such structure, an advantage is obtained that for some applications the operable frequency bandwidth can be increased approximately 11 times owing to the low Q of the cavity.

Reference will next be made to FIGURES 3a and 3b, which illustrate another embodiment of the present invention and in which reference numerals 1, 2, 3, 4 and 6 indicate parts corresponding to those of the embodiment shown in FIGURES 2a and 2b carrying the same reference numerals. In this second embodiment, each set of alternate apertured members are interconnected by two supporting members, as illustrated. Such structure is not only desirable from the standpoint of mechanical strength but is a so effective to reduce the inductance of the circuit including the effective members compared to the previously described structure, in which alternate apertured members are interconnected by a single supporting conductor.

The present invention may also be embodied in various modified forms. For example, in a modification shown in FIG. 4, in which reference numerals 1, 2, 3, 4 and 6 indicate parts corresponding to those in FIGURES 2a and 2b carrying the same reference numerals, supporting conductors associated with any one of the apertured members have not necessarily the same axial length nor are required to be symmetrical. The supporting conductors may have any appropriate configuration while obtaining substantially the same effect as the previously described cavity structures as long as the apertured members arranged alternately are electrically interconnected. Also it will be apparent to those skilled in the art that in cases where the apertures for passage of an electron beam are relatively large in size, appropriate reticulate grid means may be employed to maintain the entire aperture area in an equipotential surface.

It will be appreciated from the foregoing description that the cavity resonator according to th present invention has various advantageous features including compactness in size, relatively limited power loss, a wide operational frequency range, lack of unwanted modes is the vicinity of the basic resonant frequency and high gain.

FIG. 8 illustrates a velocity modulation tube employing a number of multiple-gap cavity resonators according to the present invention. In this application, an input cavity resonator 9, a bunching cavity resonator 10 and an output cavity resonator Ii are arranged in axial alignment in the order named between an electron gun 7 for emitting an electron beam and a collector 8 for collecting the latter after its passage through the resonators. Each of the cavity resonators is constructed ac cording to the present invention and includes a frequency control mechanism 12 in the form of a plate which is disposed in the cavity in opposing relation to the working gaps. The frequency control plate 12 provides an additional capacitance to the gaps and is adjustable to vary the resonant frequency of the resonator. An electromagnetic wave to be amplified is introduced through the coaxial input terminal 13 formed on the input cavity 9 to excite the cavity resonators, applying to the respective gaps exciting voltages alternately opposite in phase and substantially equal to each other. The space between any two adjacent gaps is so established as to give an electron transit angle of approximately 1r radians. In this instance, since, as described hereinbcfore, the gap coefiicient is higher and the Q of the cavity is lower than those obtainable with a single-gap cavity, velocity modulation can be elfected upon the electron beam with higher efficiency over a frequency range wider than that of a single-gap cavity. The electron hcam is hunched for density modulation before entering the following cavity and,

through excitation of the latter or bunching cavity 10, is subjected to an additional velocity modulation. Thus, in the output cavity 11, the beam contains all the density modulation components caused in the respective preceding cavities. The energy of the electron beam then leaves the coaxial output terminal 14 formed on the output cavity 11 to be supplied to the external load as a highfrequency exciting power. In the case where the elecron transit angle from gap to gap in each civity is slightly la ger than 1r radians, the Q of the cavity is reduced to further extend the amplification bandwidth because of the positive loading. For the output cavity, which itself is heavily loaded with a wider bandwidth, a negative beam loading can be used as long as it causes no self-excitation by selecting an electron transit angle smaller than 1r radians thereby to further improve the output efficiency.

It will be apparent to those skilled in the art that the present invention is applicable not only to amplifier tubes but also has many other applications including microwave oscillators, frequency multipliers, acceleration or deceleration of charged particles and phase focusing, with such advantageous features as compactness in size, high efficiency and wideband characteristics.

It is to be understood that the invention is not restricted to the details set forth but many changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

What We claim is:

1. A cavity resonator comprising a hollow conductor casing including end walls extending at right angles to the axis of said casing, a number of apertured members arranged along the path of charged particles coincident with the axis of said casing so as to define a plurality of gaps therebetween and having aligned apertures for passage therethrough of the charged particles, and supporting conductors interconnecting alternate ones of said apertured members and connected at one end to the respective opposite end walls of said conductor casing in spaced parallel relation to each other to form in said gaps electric fields alternately opposite in phase.

2. A velocity modulation tube of the type including an electron gun for producing a beam of electrons, a collector for collecting said beam of electrons, input, bunching and output cavities arranged between said electron gun and said collector in the order named, a mechanism for introducing an electromagnetic wave into said input cavity, and an output circuit for taking out the electromagnetic Wave amplified, said tube being characterized in that at least one of said cavity comprises a hollow condoctor casing including end walls extending at right angles to the axis of said casing, a number of apertured members arranged along the path of charged particles coincident with the axis of said casing so as to define a plurality of gaps therebetween and having aligned apertures for passage therebetween of the charged particles, and supporting conductors interconnecting alternate ones of said apertured members and connected at one end to the respective opposite end Walls of said conductor casing in spaced and parallel relation to each other to form in said gaps electric fields alternately opposite in phase.

No references cited.

ELI LIEBERMAN, Primary Examiner.

S. CHATMON, JR., Assistant Examiner.

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