GB2134318A - Magnetrons - Google Patents

Abstract

The RF energy from a magnetron's anode resonant circuits is coupled out to a rectangular waveguide 21, excited in the TE11 mode, by a rod 2a of high dielectric constant, low RF loss, material clad, except at its ends, with a conductive coating and sealingly mounted through the magnetron's vacuum envelope, the envelope- internal end face of the rod being positioned adjacent to, and possibly touching, an end face of the anode, with a part of the rod end face aligned with one of the anode cavities. Rod 20 may be of alumina, magnesia, or beryllia, and may be brazed to the pale piece 12b through which it extends. <IMAGE>

Description

SPECIFICATION Magnetrons This invention concerns magnetrons, and relates in particular to means for extracting power from a magnetron anode.

A magnetron is a type of thermionic valve suited for use as an oscillator device in the generation of signals of extremely short wavelength (having frequencies of the order of tens of Megahertz up to, say, the low hundreds of Gigahertz). The frequencies output by any particular magnetron depend in the main upon its physical dimensions and the exact details of its construction. In general, a magnetron may be said to comprise a vacuum envelope containing a cylindrical cathode emitter surrounded by coaxial annular anode shell having inwardlypointing radial vanes the tips of which form the anode cylinder, there being means for providing an axially-parallel magnetic field in the anode cavities between adjacent vanes and in the interaction space between the vane tips and the cathode.The axial field causes electrons emitted from the cathode to spiral outwards through the interaction space to the anode, and the energy in the currents and magnetic and electric fields induced respectively in the anode around each cavity and in each cavity itself as this happens may be passed out of the magnetron via an integral coupling device to produce the desired high frequency signal. More specifically, the anode cavities (and the spaces at the end faces) form a resonant circuit that causes radio frequency (RF) voltages to appear across adjacent vane tips; the emitted electrons interact with these RF voltages to deliver RF power to the resonant circuits.In the usual (s) mode the voltages between adjacent cavities are equal in magnitude but 180 (n radians) out of phase; the RF magnetic field (90 out of phase with the electric field, and at a maximum near to the anode shell, as are the associated RF currents) is alternately up and down between adjacent cavities. Other modes exist, but are usually unwanted, and can be separated either by connecting alternate vane tips together by wires ("strapping"), or by making alternate cavities large and small (the "rising sun" configuration).

The RF energy from the anode resonant circuits must be coupled out to an external transmission line, normally a rectangular waveguide excited in the TE11 mode. The coupling device must produce the correct loading (or Q) to the resonators, and must incorporate a vacuum tight dielectric window as it passes through the magnetron vacuum envelope.

Many systems are known. In one, for example, there is used a wire loop, in or above one cavity, in which a voltage is induced by the alternating magnetic field in another there is employed a slot, in the anode shell in one cavity, which interrupts the circulating current, so forcing it to flow into an adjacent waveguide circuit. Dielectric windows for all these types are known, each window incorporating a number of precisely made components comprising microwave circuits matched at the desired frequency. In each type it is necessary to make allowances for mechanical stresses due to atmospheric pressure or differential expansion.Unfortunately, these designs become increasingly expensive and difficult to manufacture as the.operating wavelength is decreased (as the frequency increases) because their dimensions have to be proportional to the wavelength.

In essence the present invention provides a coupling device for use with magnetrons employable to generate the highest frequencies (the high tens of Gigahertz and above). The device itself is little more than a short waveguide segment consisting of a rod of high dielectric constant, low RF loss, material clad with conductive metal (except at its ends). In use, it is sealingly mounted through the magnetron envelope with one end projecting externally (for further coupling to an external waveguide) and the other projecting internally and positioned adjacent at least one anode cavity. The device can be made with considerable accuracy, and greatly simplifies anode construction by not necessitating the boring of holes or cutting of slots in the anode shell.

In one aspect, therefore, this invention provides a magnetron of the type defined hereinafter, wherein the coupling device whereby output power is withdrawn therefrom comprises a rod of high dielectric constant, low RF loss, material clad, except at its ends, with a conductive coating and sealingly mounted through the magnetron's vacuum envelope, the envelope-internal end face of the rod being positioned adjacent to an end face of the anode such that the two are in a face-to-face relationship and one half of the rod end face is aligned with one of the anode cavities.

For the purposes of thins invention, a magnetron is in essence a vacuum envelope containing a cylindrical cathode emitter surrounded by a coaxial annular anode shell having inwardly-pointing radial vanes the tips of which form the anode cylinder and the spaces between which (the anode cavities) form energy storing resonators, there being means for providing an axially-parallel magnetic field and means for coupling energy developed in the resonators to an external transmission line.

The invention concerns the device used to couple energy in the resonators (the anode cavities) to an external transmission line (a suitable waveguide system), and apart from its use of this device the magnetron perse may be any such apparatus used or suggested for use in the art, and so needs no further detailed description at this point. Nevertheless, it is useful to point out that a typical magnetron of the type to which the invention relates is that available commercially as the English Electric Valve MG5200.

The coupling device of the invention uses a conductively-clad dielectric rod mounted in the vacuum envelope. The clad rod - a longitudinallyextending member - is itself a waveguide, and thus may have whatever sectional shape and dimensions are appropriate to the waves it is to guide. However, because the external transmission line will commonly be a rectangular waveguide propagating microwave radiation in the Transverse Electric11 - TE11 mode it is preferable that the coupling device be so shaped and dimensioned as to support this mode preferentially. Accordingly, while a rod of circular cross-section is possible, the preferred sectional shape is rectangular, the dimensions being less than those of the external guide by a factor of about W (where e is the dielectric constant of the rod material - about 9 for those specified below).Typical section dimensions are 30 x 15 thou (.75 x .37 millimetres), the external waveguide having a section of 100 x 50 thou (2.2 x 1.1 millimetres). Such a rod may conveniently be prepared to the necessary di mensional tolerances by drawing and grinding a bar of the requisite material.

Although it is perhaps most common that the whole of the coupling device will extend parallel to the anode/cathode axis, and will project out of the vacuum envelope at its top (or bottom) end face, nevertheless it is possible to employ a coupling device having a "bent" waveguide, the minor portion near the anode end face being axially parallel with the anode/cathode axis and the major portion from then on (extending through the vacuum envelope to the outside) being axially at right angles thereto. This latter device can easily be prepared using a dielectric rod having at one end thereof a short section at right angles to the rest and "connected" thereto by an E-plane (or H-plane) mitred bend - "bent" waveguide devices are in general well known in the art, and need not be discussed further herein.

The coupling device includes a rod of a material having a high dielectric constant and a low RF loss.

Examples of such materials are alumina (and its crystalline forms, such as sapphire), magnesia, a highly pure form of spinel (a crystalline oxide of both aluminium and magnesium) and beryllia. As appropriate, these materials may be employed in any suitable way for example, as a sintered powder or as a synthesized "single crystal".

Covering the dielectric rod, except at its ends, is a conductive coating. This coating is very preferably one of a highly conductive metal, such as gold, silver or copper, conveniently evaporated into place and about 2 thou (50 micrometres) thick. This conductive coating, or cladding, itself forms the waveguide constituted by the coupling device.

The conductive coating covers the device's dielectric rod except at its ends so that RF energy may indeed be fed into and out of the device. The coating will normally coverthe rod everywhere except at least the end faces of the rod, and preferably, in order primarily to avoid any significant possibility of shorting to the anode member, but also to optimise the transfer of RF power, the coating will in addition be absent from a short length of the rod immediately adjacent the envelope-internal end.

The coupling device is sealingly mounted through the magnetron's vacuum envelope - that is, it projects through a suitably shaped aperture in the envelope with one end inside and the other outside, and is sealed to the envelope (to the edges of the aperture) in order to retain the desired vacuum-tight nature of the envelope. The sealing may be effected in any convenient manner, but one preferred way, suitable for use when both the conductive coating and the envelope are made of metal (as will usually be the case), is simply to braze the coating to the envelope.

The envelope-internal end of the coupling device's rod is positioned adjacent, and preferably abutting, an end face of the anode, the arrangement being such that the two are in a face-to-face relationship (the internal end face of the rod facing the anode end face) and one half of the rod end face is aligned with one of the anode cavities. The purpose of this is to have the magnetic fields produced in the chosen anode cavity extend into the coupling device in a manner corresponding to the desired (TE11) mode.

Accordingly, the field in any one cavity - and preferably the larger cavity of the "rising sun" type of anode - should be aligned with one lateral half of the waveguide end face. Naturally, however, it is possible - and the dimensions of both anode and coupling device will normally allow this, whether the anode cavities are unequal in size, as in the "rising sun" design, or not -forthe device's internal end face to be aligned with two adjacent anode cavities (in which the magnetic fields are in opposite directions) with one half of the device end face aligned with one cavity and the other with the second. This increases the efficiency with which RF energy is passed into the device.

The device's internal end face is conveniently in contact with the anode end face, aiding proper alignment of the two.

The envelope-external end of the coupling device will in use be coupled to the waveguide transmission system into which the magnetron's output is to be fed. The concept of feeding power from one guide into another is well known, and needs no detailed description here. Nevertheless, it is useful to point out that the device can abut or intrude into the external guide either coaxially or transversely, and that the exact positioning of the intrusion and the external guide end stops, and the length of the intrusion, will be arranged to maximise power transfer with minimum reflection.

Naturally, the invention extends to any apparatus employing a magnetron as described and claimed herein.

Various embodiments ofthe invention are now described, though by way of illustration only, with reference to the accompanying drawings in which: Figure 1 shows an axial sectional view of the key components of a magnetron of the invention; Figures 2A, B and Cshowtop plan views of various arrangements of coupling device and anode, suitable for use in a magnetron of the invention; and Figures 3A and B show, in section, how linear and bent coupling devices respectively may be arranged between a magnetron ofthe invention and an associated external waveguide.

The Figures are diagrammatic, and not to scale.

The magnetron shown in Figure 1 comprises an indirectly heated cathode (10) surrounded by a vaned copper anode (11) itself disposed between the pole pieces (12a,b) of a ferromagnet. The cathode is heated by a heater coil (13) and supported by a cathode holder (14) itself mounted sealinglywithin an axial channel through the upper (as viewed) pole piece 12a. The anode consists of a ring-like anode shell (15) having a multiplicity of inwardly-directed vanes (16), and is sealingly positioned between the pole pieces by two cylindrical ceramic spacers (17a,b), one on either side. The combination of the two pole pieces (and end cap 18), the two spacers and the anode shell forms a vacuum enelope.

The dimensions of the device are such that the distances across and from top to bottom of the pole pieces are respectively about 20 and 40mm.

The lower (as viewed) pole piece 1 2b contains an off-axial channel (19) in which is sealingly mounted (by brazing) a waveguide device (20) in accordance with the invention. This device consists of an inner rectangular-section core of dielectric clad, except at and adjacent its ends, in a thin gold sheath. It is so positioned that the upper (as viewed) end is within the vacuum envelope, and abuts the lower (as viewed) edge of the vanes (various possible configurations of this are shown in Figures 2A, B and C), while the lower (as viewed) end is without the envelope; this latter end is arranged to project into a second waveguide (21), to which in use is transferred the energy fed into the first waveguide 20.

In Figures 2A, B and C are shown different ways that the waveguide device 20 can be positioned relative to the anode vanes 16. In Figures 2A (which shows a "rising sun" anode) and 2C the waveguide is disposed across a vane so that it is aligned with two adjacent anode cavities, one for each lateral half, while in Figure 2B the guide is aligned with one cavity on!y.

The waveguide 20 may (as shown in Figures 1 and 3A) extend axially parallelly out through a pole piece. Alternatively, however, it may extend laterally, and such an arrangement, using a guide with a mitred bend (30), is shown in Figure 3B (where the guide 20 projects out through the ceramic spacer 1 7a and into a second waveguide (31 ) via an end face of the latter).

Claims (10)

1. A magnetron of the type defined hereinbefore, wherein the coupling device whereby output power is withdrawn therefrom comprises a rod of high dielectric constant, low RF loss, material clad, except at its ends, with a conductive coating and sealingly mounted through the magnetron's vacuum envelope, the envelope-internal end face of the rod being positioned adjacent an end face of the anode such that the two are in a face-to-face relationship and one half of the rod end face is aligned with one of the anode cavities.

2. A magnetron as claimed in claim 1, wherein the coupling device is so shaped and dimensioned as to support the TE11 mode preferentially.

3. A magnetron as claimed in either of the preceding claims, wherein the whole of the coupling device extends parallel to the anode/cathode axis, and projects out of the vacuum envelope at its top (or bottom) end face.

4. A magnetron as claimed in any of the preceding claims, wherein the coupling device rod is formed of a highly pure spinel.

5. A magnetron as claimed in any of the preced- ing claims, wherein the conductive coating covering the dielectric rod is formed of goid.

6. A magnetron as claimed in any of the preceding claims, wherein the conductive coating covers the coupling device's dielectric rod except at its ends and a short length immediately adjacent its envelope-internal end.

7. A magnetron as claimed in any of the preceding claims, wherein, where both the coupling device's conductive coating and the envelope are made of metal, the coupling device is sealed to the envelope by brazing its coating to the envelope.

8. A magnetron as claimed in any of the preced- ing claims, wherein the envelope-internal end of the coupling device's rod is positioned adjacent and abutting an end face of the anode.

9. A magnetron as claimed in any of the preceding claims, wherein the alignment of the internal end face of the coupling device's rod with the anode cavities is such that one lateral half of the device end face is aligned with one cavity and the other is aligned with an adjacent cavity.

10. A magnetron as claimed in any of the preceding claims and substantially as described hereinbefore.