GB2098390A

GB2098390A - Buffer section for microwave amplifier

Info

Publication number: GB2098390A
Application number: GB8114606A
Authority: GB
Original assignee: EMI Varian Ltd
Current assignee: EMI Varian Ltd
Priority date: 1981-05-13
Filing date: 1981-05-13
Publication date: 1982-11-17
Also published as: GB2098390B

Abstract

A circuit for the output section of a microwave amplifier, e.g. a kolystron, includes a first resonant cavity (C1) defining a portion of a beam path (1), a second resonant cavity (C2) defining a successive portion of the beam path and a third resonant cavity (C3) coaxial with the first and second cavities. The third cavity communicates respectively with the first and second cavities through slots (9, 10) and (9', 10') provided in the common wall between cavities (C1, C2) and (C3). To ensure one dominant mode exists within the third cavity a pair of annular flanges 11, 12 are provided which define an annular gap (13). The two cavities (C1, C2) have a common wall (6) provided with three coupling slots (8) spaced regularly around the beam path (1). <IMAGE>

Description

SPECIFICATION Buffer section for microwave amplifier This invention relates to a buffer section for a microwave amplifier.

Published European Patent Application 0008896 discloses a klystron amplifier having an output section having a high bandwidth/efficiency product. In order to take full advantage of the output section when if operates at, for example, high bandwidth, it is necessary to provide a buffer or buncher section of correspondingly high bandwidth. The published application discloses such a section, which is schematically illustrated in Figure 5 of the published application and modifications of it are described in the application.

The disclosed section comprises first and second klystron cavities on the beam path directly coupled only via the beam path, and a third cavity (which may be coaxial) which is coupled only to the first and second cavities via slot couplings, or possibly by loop couplings.

It has been found that in practice, although such a section is useful with the output section, it is difficult to avoid spurious modes in the third cavity, and difficult to place the poles of response at the required frequencies.

According to the present invention there is provided a circuit for use in a microwave amplifier for operating with a predetermined mid band frequency and having a linear beam path for a beam of predetermined beam voltage and perveance, the circuit comprising a first resonant cavity defining a portion of the beam path and adapted to interact with the beam, a second resonant cavity defining a successive portion of the beam path and adapted to interact with the beam, a third resonant cavity coaxial with the first and second cavities and which is arranged to not interact with the beam, overlapping slot coupling the third cavity to the first and second cavities, and a pair of annular flanges coaxial with the beam path in the third cavity and defining an annular gap between the flanges for defining a single dominant mode for the third cavity.

In an embodiment, the first and second cavities are also directly coupled via further slot coupling means. This further coupling means allows an increase in bandwidth although it may worsen stability.

The third cavity provides a coupling between the first and second cavities which varies with frequency to maintain stable operation despite the further slot coupling means.

For a better understanding of the present invention reference will now be made by way of example to the accompanying drawings in which Figure 1 is a cross-section through a section in accordance with the invention, and Figure 2 shows coupling slots between the cavities on the beam path of the section of Figure 1.

The section of Figures 1 and 2 may be used in each buncher section or in each buffer section of the klystron amplifier described with reference to Figures 7 and 8 of published European Patent Application 008896 (7930167 7-EMI-Varian), replacing the section of Figure 5 of that application.

As shown in Figures 7 and 8 of that application, in practice two such sections are used in series along the beam path.

Referring to Figure 1, the section comprises first and second resonant klystron cavities C1 and C2 forming successive portions of the beam path 1 and a third cavity C3 which is coaxial with the cavities C1 and C2. Thus the third cavity encircles the cavities C1 and C2 as shown in Figure 1.

The cavities C1 and C2 have end walls 2 and 3 with drift tube portions 4 and 5 the beam path 1 providing the only coupling to preceding and succeeding sections (not shown). The cavities C1 and C2 have a common wall 6 between them with a drift tube 7.

As best shown in Figure 2, the wall 6 has three coupling slots 8 regularly distributed in it to couple cavities C1 and C2 directly in addition to the coupling via the beam path. (Only one of the coupling slots 8 is apparent in the section at view of Figure 1).

Coupling between the cavities C1 and C2 and the coaxial cavity C3 is provided by three pairs of overlapping coupling slots of which only two pairs (9,9') and (10,10') are apparent in Figure 1. These coupling slots (9,9') etc. are provided in the common wall between cavities C1 and C2 and cavity C3. As best shown at (10,10') in Figure 1 the slots of each pair of overlapping slots merge into one another.

Within cavity C3 are a pair of annular flanges 11 and 12 encircling the cavities C1 and C2 and defining an annular gap 13. These flanges ensure that there is only one dominant mode associated with a single resonant frequency 11 for the cavity.

Other, non-dominant modes, may still exist, however. Because of the coupling slots 8 there is a feedback path between cavities C1 and C2, which may cause instability. However, these slots are necessary if a large bandwidth is to be achieved.

Stable operation of the section is provided by the coaxial cavity C3 which varies the coupling between the cavities C1 and C2 with frequency.

The dimensions of the cavities C1, C2, C3, and the values of the couplings between them must also be chosen for stability.

In practice, to obtain a large bandwidth with a reasonably level gain in the band, two of the sections shown in Figure 1 in series are required.

The two sections provide the same bandwidth, but within the band have complementary gain variations.

Claims (Filed 11.5.82) 1. A circuit for use in a microwave amplifier for operating with a predetermined mid band frequency and having a linear beam path for an

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Buffer section for microwave amplifier This invention relates to a buffer section for a microwave amplifier. Published European Patent Application 0008896 discloses a klystron amplifier having an output section having a high bandwidth/efficiency product. In order to take full advantage of the output section when if operates at, for example, high bandwidth, it is necessary to provide a buffer or buncher section of correspondingly high bandwidth. The published application discloses such a section, which is schematically illustrated in Figure 5 of the published application and modifications of it are described in the application. The disclosed section comprises first and second klystron cavities on the beam path directly coupled only via the beam path, and a third cavity (which may be coaxial) which is coupled only to the first and second cavities via slot couplings, or possibly by loop couplings. It has been found that in practice, although such a section is useful with the output section, it is difficult to avoid spurious modes in the third cavity, and difficult to place the poles of response at the required frequencies. According to the present invention there is provided a circuit for use in a microwave amplifier for operating with a predetermined mid band frequency and having a linear beam path for a beam of predetermined beam voltage and perveance, the circuit comprising a first resonant cavity defining a portion of the beam path and adapted to interact with the beam, a second resonant cavity defining a successive portion of the beam path and adapted to interact with the beam, a third resonant cavity coaxial with the first and second cavities and which is arranged to not interact with the beam, overlapping slot coupling the third cavity to the first and second cavities, and a pair of annular flanges coaxial with the beam path in the third cavity and defining an annular gap between the flanges for defining a single dominant mode for the third cavity. In an embodiment, the first and second cavities are also directly coupled via further slot coupling means. This further coupling means allows an increase in bandwidth although it may worsen stability. The third cavity provides a coupling between the first and second cavities which varies with frequency to maintain stable operation despite the further slot coupling means. For a better understanding of the present invention reference will now be made by way of example to the accompanying drawings in which Figure 1 is a cross-section through a section in accordance with the invention, and Figure 2 shows coupling slots between the cavities on the beam path of the section of Figure 1. The section of Figures 1 and 2 may be used in each buncher section or in each buffer section of the klystron amplifier described with reference to Figures 7 and 8 of published European Patent Application 008896 (7930167 7-EMI-Varian), replacing the section of Figure 5 of that application. As shown in Figures 7 and 8 of that application, in practice two such sections are used in series along the beam path. Referring to Figure 1, the section comprises first and second resonant klystron cavities C1 and C2 forming successive portions of the beam path 1 and a third cavity C3 which is coaxial with the cavities C1 and C2. Thus the third cavity encircles the cavities C1 and C2 as shown in Figure 1. The cavities C1 and C2 have end walls 2 and 3 with drift tube portions 4 and 5 the beam path 1 providing the only coupling to preceding and succeeding sections (not shown). The cavities C1 and C2 have a common wall 6 between them with a drift tube 7. As best shown in Figure 2, the wall 6 has three coupling slots 8 regularly distributed in it to couple cavities C1 and C2 directly in addition to the coupling via the beam path. (Only one of the coupling slots 8 is apparent in the section at view of Figure 1). Coupling between the cavities C1 and C2 and the coaxial cavity C3 is provided by three pairs of overlapping coupling slots of which only two pairs (9,9') and (10,10') are apparent in Figure 1. These coupling slots (9,9') etc. are provided in the common wall between cavities C1 and C2 and cavity C3. As best shown at (10,10') in Figure 1 the slots of each pair of overlapping slots merge into one another. Within cavity C3 are a pair of annular flanges 11 and 12 encircling the cavities C1 and C2 and defining an annular gap 13. These flanges ensure that there is only one dominant mode associated with a single resonant frequency 11 for the cavity. Other, non-dominant modes, may still exist, however. Because of the coupling slots 8 there is a feedback path between cavities C1 and C2, which may cause instability. However, these slots are necessary if a large bandwidth is to be achieved. Stable operation of the section is provided by the coaxial cavity C3 which varies the coupling between the cavities C1 and C2 with frequency. The dimensions of the cavities C1, C2, C3, and the values of the couplings between them must also be chosen for stability. In practice, to obtain a large bandwidth with a reasonably level gain in the band, two of the sections shown in Figure 1 in series are required. The two sections provide the same bandwidth, but within the band have complementary gain variations. Claims (Filed 11.5.82)

1. A circuit for use in a microwave amplifier for operating with a predetermined mid band frequency and having a linear beam path for an electron beam of predetermined beam voltage and perveance, the circuit comprising, a first resonant cavity defining a portion of the linear beam path and adapted to interact with the beam, a second resonant cavity defining a successive portion of the linear beam path and adapted to interact with the beam, a third resonant cavity coaxial with the first and second cavities and which is arranged to not interact with the beam, overlapping slot coupling means coupling the third cavity to the first and second cavities and, a pair of annular flanges, coaxial with the beam path, in the third cavity and defining an annular gap between the flanges for defining a single dominant mode for the third cavity.

2. A circuit according to Claim 1 wherein the said first and second cavities are directly coupled by means of slots communicating between the cavities.

3. A circuit according to Claim 2 wherein the first and second resonant cavities include a common wall provided with a drift tube portion, coaxial with the beam path, and a plurality of slots in the wall which are spaced regularly around the beam path and communicate between the first and second cavities.

4. A circuit according to Claims 1 to 3 wherein the first and second cavities are provided with respective end walls each provided with a drift tube, coaxial with the linear path.

5. A microwave amplifier including a circuit according to any one of Claims 1 to 4.

6. A circuit substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.

7. A microwave amplifier substantially as hereinbefore described.