WO1992014256A2

WO1992014256A2 - Small-size coaxial magnetron

Info

Publication number: WO1992014256A2
Application number: PCT/FR1992/000089
Authority: WO
Inventors: Henri Desmur; Armel Beunas
Original assignee: Thomson Tubes Electroniques
Priority date: 1991-02-12
Filing date: 1992-01-31
Publication date: 1992-08-20
Also published as: WO1992014256A3; DE69212626D1; FR2672729A1; EP0524295A1; FR2672729B1; EP0524295B1

Abstract

A small-size, coaxial magnetron comprises in concentric arrangement a central cathode (1) surrounded by a ring-shaped anode (2) itself surrounded by a coaxial cavity (20) coupled to the anode. The coaxial cavity (20) contains a closed periodic delay line (21). For a given resonance frequency, the size of the coaxial cavity is less than it would be if no delay line (21) were fitted. Application to coaxial magnetrons of reduced size and weight.

Description

COAXIAL REDUCED SIZE MAGNETRON

The present invention relates to coaxial magnetrons. A coaxial magnetron is a microwave tube which mainly comprises:

- a cylindrical cathode which emits electrons; - a crown-shaped anode which surrounds the cathode;

- a cylindrical coaxial cavity which surrounds the anode. The anode has radial fins which are directed towards the cathode and which are regularly fixed inside an external conductive cylinder. It is thus possible to define elementary cavities delimited by two successive fins and a portion of conductive cylinder. This portion of conductive cylinder forms the bottom of the elementary cavity. Coupling slots are provided in the bottom of one elementary cavity in two. The coaxial cavity is coupled to the elementary cavities by the slots. The electrons emitted by the cathode arrive near the end of the fins and transfer their energy to the elementary cavities. This energy propagates in the coaxial cavity.

The frequency of oscillation of the magnetron corresponds substantially to the resonance frequency of the coaxial cavity.

It is essentially the dimensions of the coaxial cavity which determine the frequency of oscillation of the magnetron.

Coaxial magnetrons are generally bulky and heavy and in many applications it would be desirable to reduce their size.

To reduce this size and this weight, it is known to have inside the coaxial cavity, a certain amount of dielectric material with dielectric constant greater than one. Under vacuum, the dielectric material is covered with conductive deposits mainly coming from the cathode. These deposits cause losses detrimental to the proper functioning of the magnetron. The present invention aims to remedy these drawbacks.

It offers a coaxial magnetron which occupies a reduced volume compared to conventional coaxial magnetrons and which is devoid of the drawbacks of magnetrons whose coaxial cavity contains a quantity of dielectric material.

The present invention provides a magnetron comprising, concentrically, a central cathode surrounded by a crown-shaped anode with radial fins directed towards the cathode. The fins define elementary cavities. One in two elementary cavities is coupled to a coaxial cavity arranged around the anode. The coaxial cavity contains a periodically delayed line, closed on itself. This delay line comprises a succession of resonant elements coupled together, which are substantially identical. Preferably, the coaxial cavity is limited by an external side wall and the delay line is carried by the external side wall.

The resonant elements are preferably secondary cavities. The delay line can operate in zero mode or mode II.

The delay line can be a fin line. Two successive fins and the space separating them contribute to forming a resonant element. The fins can be made of a material having a coefficient of expansion lower than the coefficient of expansion of the material making the external wall, to reduce the frequency drift of the magnetron as a function of the temperature.

The delay line can be of the type in 1î and include a crown ground electrode and a succession of radial arches spanning the ground electrode.

The arches can be made of a material having a coefficient of expansion lower than the coefficient of expansion of the material making the external wall, to reduce the frequency of the magnetron as a function of the temperature. The ground electrode is preferably made of the same material as that of the external wall.

The magnetron can be provided with a frequency tuning device comprising a plurality of tuning elements, each tuning element entering a secondary cavity.

These tuning elements can be fingers or trays.

Other characteristics of the invention will appear on reading the following description, illustrated by the appended figures which represent:

- Figure 1: a view of a conventional coaxial magnetron;

- Figure 2a: a view of a coaxial magnetron according to the invention;

- Figure 2b: a cross section of the magnetron of Figure 2a;

- Figure 3: a view of a variant of a coaxial magnetron according to the invention;

- Figure 4: a view of a coaxial magnetron tunable in frequency according to the invention; - Figure 5: a view of a variant of a coaxial magnetron frequency tunable according to the invention.

In the figures, the corresponding elements are designated with the same references.

Figure 1 shows a classic coaxial magnetron. It comprises a cylindrical cathode 1 centered on an axis XX '. It emits electrons. An anode 2 in the form of a crown surrounds the cathode 1. It comprises a conductive cylinder 8, the interior of which is provided with radial fins 3 directed towards the cathode 1. The fins 3 are regularly distributed inside the conductive cylinder 8. It is possible to define a chain of elementary cavities 4 each delimited by two successive fins 3 and a portion of cylinder 8. The portion of cylinder 8 forms a bottom for the elementary cavities 4. The number of elementary cavities 4 is even. The elementary cavities, which are substantially identical, are coupled together and resonate on the same frequency.

A cylindrical coaxial cavity 5 surrounds the anode 2. The coaxial cavity 5 is limited towards the outside by an external cylindrical side wall 9 and towards the inside by the anode 2. Two flanges transverse to the axis XX ', not shown, close the coaxial cavity. The coaxial cavity 5 is coupled to the elementary cavities 3 by slots 6 arranged in the bottom of an elementary cavity 4 out of two. A transverse output waveguide 10 is connected to the external wall 9 and communicates with the coaxial cavity.

An oscillation frequency of the coaxial magnetron corresponds substantially to the resonance frequency of the coaxial cavity 5.

This frequency can however be adjusted using a tuning device 7. FIG. 1 shows a tuning device 7 with a sliding plate in the coaxial cavity 5.

The coaxial cavity 5 of a coaxial magnetron generally operates in TE _{n 1 1} mode.

All of the elementary cavities 4 operate in mode 11, that is to say that there is a phase shift of II between two successive elementary cavities 4. In mode H, the anode 2 and the coaxial cavity 5 are easily coupled by virtue of the slots 6, in the TE „ _{1 1} mode of the coaxial cavity. Indeed, the slots 6 are excited in phase, which corresponds well to the non-variation of the electric field of the mode indicated in the azi direction utale. FIG. 2a represents a view of a coaxial magnetron according to the invention. Figure 2b shows a cross section of the magnetron of Figure 2a.

The main difference between this magnetron and that of Figure 1 is located at the coaxial cavity 20. There is no change at the anode 2 and the cathode 1. The coaxial cavity 20 is always limited , towards the outside by a cylindrical external side wall 23, towards the inside by the anode 2 and transversely by the two not shown flanges. It is coupled to the anode 2 by the slots 6. The dimensions of the coaxial cavity 20 are smaller than those of the coaxial cavity of the magnetron of FIG. 1, for the same resonant frequency.

Now the coaxial cavity contains a periodic delay line 21, closed on itself. The delay line 21 comprises a succession of substantially identical resonant elements, coupled together. In FIGS. 2a, 2b the delay line 21 is carried by the external side wall 23 of the coaxial cavity.

In FIGS. 2a and 2b, the delay line 21 is a fin line ("vane line" in Anglo-Saxon literature). It comprises radial fins 22 directed towards the anode 2 and connected to the cylindrical external side wall 23. These fins 22 are arranged regularly along the outer wall 23. They are full and a space is provided between the end of the fins 22 and the anode 2. Two successive fins 22 and the space separating them contribute to forming a cavity 24 secondary. The resonant elements of the delay line 21 are the secondary cavities 24. An outer wall portion 23 located between two successive fins provides a bottom to the secondary cavity 24. The delay line 21 comprises a plurality of resonant secondary cavities 24, coupled between them . These secondary cavities 24 are substantially identical. In FIGS. 2a and 2b, six secondary cavities 24 are shown. This number is only an example, there could be less or more. In a plane transverse to the axis XX ', the presence of the fins 22 introduces a reduction in the speed of propagation

- ^• electromagnetic wave to the outer periphery of the coaxial cavity 20 relative to the velocity of propagation between the end fins 22 and the anode 2. The presence of the fins 22 produces substantially the same effect as the introduction in a coaxial cavity, a certain quantity of dielectric material with relative dielectric constant greater than one. The speed of propagation of an electromagnetic wave in a dielectric material (dielectric constant greater than one) is less than the speed of propagation of the same wave in vacuum.

For the same resonant frequency, a coaxial cavity provided with fins with a diameter smaller than that of a coaxial cavity without a fin. The coaxial cavity 20 provided with fins is less bulky and less heavy than a coaxial cavity of a conventional coaxial magnetron resonating at the same frequency.

When a couple closes on themselves several identical cavities therefore resonating at the same frequency, the group of cavities has as many resonance frequencies as cavities. These resonant frequencies are offset from each other and correspond to the phase difference existing between two successive cavities. The resonance condition being that the phase shift across the group is an integer of times 11.

This phase shift depends on the frequency and therefore a discrete set of resonant frequencies is observed.

For two determined frequencies, the phase shift between two successive cavities is 0 or 1 The group of cavities operates in If mode or in zero mode.

In a coaxial magnetron, the anode generally operates in "II" mode. The coupling slots 6 are excited in phase. They can easily induce a zero mode in the delay line 21, which determines an operating frequency.

The angular position of the fins 22 of the delay line 21, relative to that of the slots 6 is indifferent. The number of resonant secondary cavities 24 is also indifferent. This construction is very easily achievable. But to favor in the coaxial cavity 20 the establishment of a TE mode n, ± ₁ , ₁ , it will be advantageous to have 2n fins 22 in the coaxial cavity 20. (n is an integer greater than zero).

In order for the delay line 21 to operate in mode II, the number of secondary cavities 24 must be even. This determines another operating frequency. The position angular of the fins 22 of the delay line 21 relative to that of the slots 6 is important. The number of fins 22 of the delay line 21 is equal to the number of fins 3 of the anode 2. The fins 22 of the delay line 21 are arranged in the extension of the fins 3 of the anode 2.

If the delay line has 2n fins, in a plane transverse to the axis XX ′, the mode which is established in the coaxial cavity 21 comprises electric field lines similar to a series of 2n bridge arches, (n is an integer greater than zero). A maximum of electric field is found at the end of a fin 22 towards the anode 2. This variant is not shown but it would suffice to add fins 22 and to arrange them in the extension of the fins 3 of anode 2.

The fins 22 are metallic. They can be made of the same material as that of the external wall 23. The latter is generally made of copper.

The delay line 21 may include means for reducing the frequency drift of the magnetron as a function of the temperature. The means are located at the level of the resonant elements. For this, the fins 22 can be made of a material having a lower coefficient of expansion than that of the material making the external side wall 23 of the coaxial cavity 20. The fins 22 can for example be made of molybdenum and the external side wall 23 in copper . The coaxial cavity 20 is coupled to a transmission line 10. This line is not shown in FIG. 2b.

Instead of using a fin line, other types of delay lines can be used. For example, FIG. 3 represents a magnetron according to the invention whose external wall

34 is equipped with a delay line 30 with a U-shaped bar. The magnetron conforms to that of FIGS. 2a, 2b except at the level of the delay line 30 which is of another type. This type of line comprises, at least one ground electrode 31 which extends longitudinally and arches 32, in the form of * ïï, placed transversely with respect to the ground electrode 31. The arches 32 straddle the ground electrode 31. The arches 32 are regularly distributed along the electrode ground 31. The ground electrode may optionally be placed on a base 37. The pillars 33 of the arches 32 are fixed on the base 37, on either side of the ground electrode 31. In FIG. 3, only one ground electrode 31 has been shown. The delay line 30 is direct dispersion. The ground electrode 31 has the form of a ring coaxial with the axis XX 'of the magnetron. The delay line 30 is integral with the internal face of the external lateral wall 34 of the coaxial cavity 35.

The arches 32 are radial, they are directed towards the anode

2. They are without contact with the anode 2. Two arches - successive and the space separating them form a resonant element.

The resonant elements can be assimilated to secondary cavities 36. The external side wall 34 can serve as a base for the line 30. The ground electrode 31 can have a rectangular cross section. Preferably, for reasons of cost and * - * technical implementation, the ground electrode 31 and the base 37

(if there is one) are an integral part of the outer wall 34.

They are then made of the same material as the external side wall 34, in copper, for example. In FIG. 3 the ground electrode 31 and the base 37 are integrated into the external side wall 34. The arches 32 are fixed to the external side wall

34 or to base 37 (if there is one) by soldering or any other known means.

The arches 32 can be made of the same material as the external side wall 34. The delay line 30 can include means for reducing the frequency drift of the magnetron as a function of the temperature. These means are located at the level of the resonant elements. For this, the arches 32 are made of a material having a coefficient of expansion lower than the coefficient of expansion of the material of the outer side wall 34 of the magnetron. For example, they can be made of molybdenum if the outer wall 34 is made of copper.

The delay lines shown in Figures 2a, 2b and 3 are direct dispersed. When operating in zero mode their resonant frequency is the lowest of the possible frequencies. One could also use reverse dispersion lines, in which case their resonant frequencies would be higher.

An inverted dispersion ff type rod line would have two ground electrodes instead of one. The two electrodes, in the shape of a crown, would be separated from each other by a space facing the middle part of the arch.

This description is only a non-limiting example. Any other type of line used as a delay circuit in a cross-field tube can also, for example, be used. A transmission line coupled to the coaxial cavity 35 has not been shown in FIG. 3, for the sake of clarity.

A coaxial magnetron, according to the invention, can be tuned in frequency by modifying the geometry of its coaxial cavity. Various chord devices can be used.

4 shows a coaxial magnetron according to the invention, provided with a tuning device 41. The magnetron is similar to that shown in Figures 2a and 2b. The tuning device 41 comprises a plurality of fingers 42, a finger 42 plunging into each secondary cavity 24. Preferably, the fingers 42 are integral with the same support 43 so as to be moved along the axis of the magnetron simultaneously. We could consider that they are moved independently of each other, in which case they would not be integral with the same support. The fingers 42 shown are without contact with the fins 22 or the outer side wall 23. This is only one example, they could rub against the fins or the outer wall 23. The fingers are conductive or dielectric.

FIG. 5 represents a coaxial magnetron, according to the invention, provided with another tuning device 51.

The coaxial magnetron shown is similar to that of Figures 2a and 2b.

Instead of having fingers, the tuning device 51 of FIG. 5 comprises a plurality of plates 52, each plate 52 penetrating along the axis of the magnetron, in a secondary cavity 24. The plates 52 are conductive or dielectric.

The area of a plate 52 is substantially equal to that of the cross section of a secondary cavity 24. The plates 52 are fixed on the same support 53 so as to be moved simultaneously. Instead of being integral with the same support, one could envisage that they can move, independently of each other.

Claims

1. Coaxial magnetron comprising concentrically a central cathode (1) surrounded by an anode (2) in the form of a crown with radial fins (3) directed towards the cathode, the fins delimiting elementary cavities (4), an elementary cavity ( 4) on two being coupled to a coaxial cavity (20) arranged around the anode (2), characterized in that the coaxial cavity (20) contains a periodic delay line (21), closed on itself.

10 2. Coaxial magnetron according to claim 1, characterized in that the delay line (21) comprises a succession of resonant elements (24), substantially identical, coupled together.

15 3. Coaxial magnetron according to one of claims 1 or

2, characterized in that the coaxial cavity (20) is limited by an external side wall (23) supporting the delay line (21).

4. Coaxial magnetron according to one of claims 2 or

20 3, characterized in that the resonant elements are secondary cavities (24).

5. Coaxial magnetron according to one of claims 1 to 4, characterized in that the delay line (21) operates on a

6. Coaxial magnetron according to one of claims 1 to 4, characterized in that the delay line (21) operates in an H mode. 0

7. Coaxial magnetron according to one of claims 2 to 6, characterized in that the delay line (21) is a fin line, the fins (22) being directed radially towards the anode (2), two successive fins (22) and the space separating them contributing to form a resonant element.

5

8. Coaxial magnetron according to claim 7, characterized in that the fins (22) are made of a material having a coefficient of expansion lower than the coefficient of expansion of the material making the external wall (23), in order to reduce the drift in frequency of the magnetron.

9. Coaxial magnetron according to one of the claims 2 to 6, characterized in that the delay line (30) is an H-type bar line, comprising at least one ground electrode (31) in a ring and a succession U-shaped arches (32) spanning the ground electrode, the arches (32) being directed radially towards the anode (2), two successive arches (32) and the space separating them contributing to form a resonant element ( 36). 0

10. Coaxial magnetron according to claim 9, characterized in that the ground electrode (31) is made of the same material as the external side wall (34) of the coaxial cavity (35). 5

11. Coaxial magnetron according to one of claims 9 or

10,. characterized in that the arches (32) are made of a material having a coefficient of expansion lower than the coefficient _Q of expansion forming the external side wall (34), in order to reduce the frequency drift of the magnetron.

12. Magnetron according to one of claims 4 to 11, characterized in that the coaxial cavity (20) is provided with a frequency tuning device (41, 51) comprising a plurality of tuning elements (42, 52), each tuning element entering a secondary cavity (24), the tuning elements being moved individually or simultaneously along the axis of the magnetron.

13. Coaxial magnetron according to claim 12, characterized in that the tuning elements are fingers (42).

14. Coaxial magnetron according to claim 12, characterized in that the tuning elements are plates (52) whose area is substantially equal to that of the cross section of a secondary cavity (24).