GB2087143A

GB2087143A - Magnetrons

Info

Publication number: GB2087143A
Application number: GB8133759A
Authority: GB
Original assignee: MO Valve Co Ltd
Current assignee: MO Valve Co Ltd
Priority date: 1980-11-10
Filing date: 1981-11-09
Publication date: 1982-05-19
Also published as: GB2087143B

Abstract

In a magnetron employing a re- entrant periodic structure the periodic structure is non-dispersive over such a range of frequencies that the electron stream circulated around the structure in operation can be caused to interact with any selected one of two or more waves of different frequencies and modes supported by the structure. In use the frequency of operation of the magnetron is selected by momentarily injecting a low level priming signal of the relevant frequency into the periodic structure. As described, the periodic structure may be of strapped vane type, with the strap (5, 7) and strap slot (9) dimensions chosen to give the non- dispersive property; or of helical or ring-and-bar structure. (Figures 5 and 6, not shown). <IMAGE>

Description

SPECIFICATION Magnetrons This invention relates to magnetrons, more particularly to magnetrons employing a re-entrant periodic structure.

As is well known, such a magnetron operates as a result of interaction between a circulating stream of electrons and a wave supported by the periodic structure. To ensure stable operation at a desired frequency the periodic structure is carefully designed so that with a given applied voltage and magnetic field the circulating electrons can interact only with a selected mode of wave supported by the periodic structure.

A small degree of variation of the frequency of operation by perturbation of the selected mode can be achieved with conventional magnetrons by phase locking, or by some adjustment of the impedance of one or more of the resonant elements of the periodic structure either mechanically or electronically e.g. by means of plungers, PIN switches or varactors.

Otherwise the frequency of operation can only be changed by operation in different modes which requires a major disruption of the operating voltage and/or the applied magnetic field.

It is an object of the present invention to provide a magnetron employing a re-entrant periodic structure whose frequency of operation can be changed without recourse to adjustment of the applied anode voltage or the magnetic field.

According to the present invention in a magnetron employing a re-entrant periodic structure the periodic structure is non-dispersive over such a range of frequencies that an electron stream circulated around the structure in operation of the magnetron can be caused to interact with any selected one of two or more waves of different frequencies and modes supported by the structure.

The periodic structure of a magnetron according to the invention may take any convenient form e.g. a strapped vane structure, a helix structure or a ring and bar structure.

In use of a magnetron according to the invention the frequency of operation, i.e. the frequency and mode of the wave supported by the periodic structure with which the circulating electrons interact, can be selected by momentarily injecting a low level priming signal of the relevant frequency into the periodic structure.

Thus according to a second aspect of the invention there is provided a magnetron circuit arrangement including a magnetron according to the present invention and means for injecting into the periodic structure of the magnetron a signal at any selected one of two or more of said different frequencies.

Three magnetrons in accordance with the invention and their method of operation will now be described by way of example with reference to the accompanying drawings in which: Figure lisa plan view of the anode structure of the first magnetron; Figure 2 is a sectional view on the line ll-ll in Figure 1; Figures 3 and 4are graphs illustrating the operation of the magnnetron of Figures 1 and 2; Figure 5 is a plan view of the anode structure of the second magnetron; and Figure 6 is a plan view of the anode structure of the third magnetron.

Referring to Figures 1 and 2, the first magnetron to be described employs an anode structure comprising a tubular outer wall 1 with twenty-four inwardly extending equally spaced radial vanes 3. The structure thus comprises a re-entrant periodic structure of twenty-four coupled cavities. The vanes are centrally positioned along the length of the wall to leave spaces at either end of the structure which accom mode the pole pieces (not shown) of a magnet for providing the magnetic field required in operation of the magnetron. The space between the inner ends of the vanes 3 houses a cylindrical cathode (not shown) of conventional form.

The vanes are provided with two strapping rings 5 and 7, one at each end of the vanes, one strap 5 connecting one set of alternate vanes 3 and the other strap 7 connecting the other set of alternate vanes 3, slots 9 being provided in the vanes 3 in conventional manner where a strap 5 or 7 is required to pass without making electrical connection.

The dimensions of the straps 5 and 7 and the slots 9 are chosen so that the periodic structure is non-dispersive over an appreciable range of frequencies.

A suitable form for the strapping rings may be arrived at empirically by appropriately reducing the strapping of a conventional magnetron.

In Figure 3 line A indicates the variation of phase-shift per cavity with frequency of the structure, while line B indicates the corresponding characteristic of a comparable conventional magnetron with strapping designed to ensure stable operation in the standing wave'joe mode'. In Figure 4 line C indicates the phase velocity ratio/frequency characteristic of the structure of Figures 1 and line D the corresponding characteristic of the abovementioned comparable conventional magnetron.

The linear portion E of line C indicates that the structure of Figures 1 and 2 is substantially nondispersive over the range 10.4 to 14.5 GHz.

The circles and crosses in Figures 3 and 4 indicate the frequencies of the various resonant modes resulting from re-entrancy which the periodic structures will support.

In a conventional magnetron, stable operation is achieved by ensuring that the phase velocity of one chosen mode, normally the 3x mode, differs sufficiently from the phase velocities of the other resonant modes which the magnetron re-entrant periodic structure will support so that synchronous interaction between circulating electrons and waves on the periodic structure of the chosen mode only can be obtained by suitably choosing the applied anode voltage and magnetic field.

With the magnetron of Figures 1 and 2 such synchronous interaction, i.e. magnetron operation, in addition to being obtained in conventional manner in the z mode, can also be obtained, with the same applied voltage and magnetic field, with waves of at least some of the other resonant modes of the structure which line in the non-dispersive range of the structure's phase velocity ratio/frequency characteristic.

In the use of the magnetron, operation in a desired mode, i.e. at a desired frequency, can be initiated by momentarily injecting a low level priming signal of appropriate frequency into the periodic structure.

After priming the saturation behaviour of the magnetron ensures continued operation in the selected mode.

The required priming signal may be injected in any suitable manner, for example via a lead electrically connecting to one of the radial vanes, via a coupling loop in one of the cavities between adjacent vanes or via a waveguide coupled to such a cavity via an aperture in the outer wall of the cavity.

In one particular magnetron of the form shown in Figures 1 and 2 stable operation can be achieved with an applied magnetic field of 1600 gauss and voltage of 395 volts at any selected one of the frequencies 9.5 GHz, 10.4 GHz, 11.2 GHz and 12.1 GHz which correspond to operation in the a, z + 2, z + 3, and z + 4 modes respectively. Operation in the z + 1 mode is found, however, notto be possible, due probably to energy exchange between this mode and the mode resulting from the closeness in frequency of these two modes.

It will be appreciated that in other magnetrons in accordance with the invention non-dispersive reentrant periodic structures of other than strapped vane form may be used.

Figure 5 illustrates one such alternative form comprising a single helix structure 11 of overall circular form supported on a ceramic ring 13. Such a structure may conveniently be manufactured using a stacked lamination technique.

Figure 6 illustrates a second such alternative form comprising a plurality of identical rings 15 secured to a ceramic ring 17 at equally angularly spaced positions, metallized regions (not shown) on the ring 17 providing connections between the rings 15.

The structures of Figures 5 and 6 are of course comparable with the helix and ring and bar slow wave structures used in conventional travelling wave tubes, although of course in a travelling wave tube the slow wave structure is not re-entrant.

Claims

1. A magnetron employing a re-entrant periodic structure wherein the periodic structure is nondispersive over such a range of frequencies that an electron stream circulated around the structure in operation of the magnetron can be caused to interact with any selected one of two or more waves of different frequencies and modes supported by the structure.

2. A magnetron according to Claim 1 wherein the periodic structure comprises a strapped vane structure.

3. A magnetron according to Claim 1 wherein the periodic structure comprises a helix structure.

4. A magnetron according to Claim 1 wherein the periodic structure comprises a ring and bar structure.

5. A magnetron substantially as hereinbefore described with reference to Figures 1 and 2, Figure 5 or Figure 6.

6. A magnetron circuit arrangement comprising a magnetron according to any one of Claims 1 to 5 and means for injecting into the periodic structure of the magnetron a signal at any selected one of two or more of said different frequencies.