GB2111298A - Improvements in magnetrons - Google Patents

Abstract

A coaxial magnetron has an anode wall 2 with vanes extending inwardly from the region 4, and with slots 3 cut in the wall to provide coupling between inner and outer resonance cavities. Undesirable resonances which occur because of the slots 3 can be discouraged by using an attenuator 8, but power at the operating frequency is also thereby reduced. The slots are designed and disposed so that induced currents between A and B and B and C are exactly balanced so that they cancel, and there is no coupling into the attenuator 8 of power at the operating frequency. B is such that if the slot is short circuited there the inner anode frequency equals the operating frequency. By arranging that B lies within a wide section of the slot cancellation occurs over a range of operating frequencies. A shield 10 co- axial with and outside the anode wall 2 may be used to select the range of frequencies over which cancellation occurs. <IMAGE>

Description

SPECIFICATION Improvements in magnetrons This invention relates to magnetrons and is particularly applicable to magnetrons in which a resonant cavity surrounds or forms part of the anode of the magnetron. The operating frequency of the magnetron is largely determined by the electrical properties of the cavity and such a magnetron is commonly termed a coaxial magnetron. In such a co-axial magnetron the cavity is defined by an outer cylinder or shell which surrounds an anode wall, which is also of a cylindrical shape, with the cathode lying along the axis of the anode shell.

The anode wall generally has uniformly spaced radial vanes on its inside surface, the spaces between which form inner resonance cavities. The space between the free ends of the vanes and the cathode is an interaction where electrons interact with a magnetic field maintained along the axis and a DC voltage between the anode and cathode.

An outer cavity, bounded by the outer cylindrical wall, the anode wall and a pair of end plates, resonates in a predetermined mode. Coupling between the outer cavity and the inner resonance cavities is effected by a series of slots cut in the anode wall. The slots are parallel to the axis of the cylinders and correspond to alternate inner resonance cavities.

However, unwanted resonances may occur, leading to the generation of power at undesirable frequencies. These resonances are called "slot modes" since they excite voltages across the slots and currents along edges of the slots. Oscillation in these modes may be largely discouraged by the introduction of an attenuator iri which current is induced by the voltages across the slots so that power is dissipated within it. The attenuator can take the form of a cylinder of resistive material placed inside the anode wall, spaced from it and overlapping the slots at one end.

However, it is found that power at the operating frequency is also dissipated within the attenuator, and the power lost can be very appreciable, especially in magnetrons which operate at high average power.

The present invention seeks to reduce this undesirable power loss.

According to this invention, a co-axial magnetron includes a cylindrical anode wall defining an inner anode cavity and which is positioned between a central cathode and an outer resonance cavity, which surrounds said wall and which determines the operating frequency of the magnetron, the anode wall including a plurality of elongate slots which couple energy at a frequency lower than the natural resonance frequency of the inner anode cavity from the interior of the anode wall to the outer resonance cavity; attenuation means positioned at an end of each slot, and each slot being shaped and dimensioned such that there exists a notional position along its length between a central position and said end thereof at which, in operation, induced current flowing around the slot between said notional position and said central position is balanced by induced current flowing around said slot between said notional position and said end, and at which, if the slot were short circuited at said notional position, the inner anode cavity would resonate at the operating frequency of the magnetron. When a notional position exists which satisfies these conditions, very little energy indeed is coupled into the attenuation means.This is an extremely important consideration if the magnetron is capable of providing a large output power, since if even a modest proportion of it were dissipated as heat within an attenuator, this would result in unacceptable heating of the magnetron.

The invention is further described by way of example with reference to the accompanying drawings in which: Figure 1 shows part of a co-axial magnetron in accordance with the invention; Figure 2 is a cross section view of part of the magnetron; Figure 3 is a diagrammatic view of the magnetron showing a slot formed in the anode wall in accordance with the invention and Figure 4 shows a modified slot.

Referring to figures 1 and 2 of the drawings, a co-axial magnetron has an anode structure comprising an anode wall 2 which is co-axially surrounded by an outer shell 1.

A series of Identical slots 3 is cut in the anode wall 2 parallel to the axis X of the magnetron.

The anode wall 2 includes vanes 4 which are regularly spaced and extend radially from the inside surface of the anode wall 2 towards a cathode 5 which lies along the axis X.

The vanes 4 form inner resonance cavities, the slots 3 being cut in alternative inner resonance cavities. There is also an interaction space 6 between the free ends of the vanes 4 and the cathode 5, which together with the vanes 4 constitute an anode cavity.

An outer cavity 7 is formed by the anode wall 2, the outer shell and annular end plates (not shown) between them and normal to the axis X.

An attenuator 8 comprising a ring of resistive material is located within the anode wall 2 and at a distance from it. It is placed at one end of the anode wall 2 so that it overlaps the slots 3 at one end.

The slots 3 extend for substantially the whole of the axial length of the outer cavity 7, the vanes 4 of the anode wall being much shorter and located mid-way along the anode wall 2.

The slots 3 provide coupling between the inner anode cavity and the outer resonance cavity 7.

However, as well as the desired resonance there are numerous other resonances which may generate power at frequencies other than the operating frequency. These undesirable resonances are called "slot mode" resonances since they excite voltages across the slots 3 and currents along the edges of the slots 3.

The slot mode resonances are effectively damped by the action of the attenuator 8.

Currents induced by voltages across the slots 3 flow in the attenuator 8 and therefore power is dissipated.

A major problem which arises from the use of the attenuator 8, particularly in magnetron which operate at high average power, is that power at the operating frequency is also dissipated.

The present invention provides a structure for which the power loss into the attenuator 8 is greatly reduced over a desired range of operating frequencies as compared with previous magnetrons. In contrast to the usually provided slots, the invention provides the slots which are shaped and dimensioned so as to enable this power loss to be greatly reduced. The effect of such a slot is explained with reference to Figure 3, which shows, in much simplified form, a sectional diagrammatic view of the magnetron. For the sake of clarity only a single slot 9 is illustrated.

Each of the series of slots 3 in the anode wall 2 is identical with the slot 9 which is of constant width and is aligned with the axis X. In operation, currents flow circumferentially around the anode wall 2 and these currents are intercepted by the slots, Each element of these currents divides and flows along the edge of the slot, part towards the point C at the inner anode, and part towards the closed end A of the slot and the attenuating means. This last current is dissipated as heat which is very undesirable in magnetrons of high average power.When the dimensions of the inner anode are correctly chosen in relation to the operating frequency of the magnetron; and the slot 9 correctly placed in relation to the two end plates 10. 11 of the outer resonance cavity 7, a point B (between the end A and the centre points C) occurs such that the total current at A caused by currents intercepted between A and B, is exactly equal in magnitude but opposite in sense to that caused by the circulating currents intercepted between B and C. The total current at A is thus zero and no power is dissipated in the attenuating means placed at this point.The point B is theoretically such that if the slot were short circuited at this point, the inner anode together with the portion BC of the slot would itself resonate at the same frequency as the outer resonance cavity 7.The magnitude of the circulating currents varies along the length of the anode wall - its variation is shown in Figure 3 as a plot of current i, against distance 1.

In the case shown in the drawings, where the structure is symmetrical about the mid-point C, (i.e. the vanes are at the centre of the slots, and the slots are equally spaced between the end plates of the outer resonance cavity) the same action occurs at the corresponding point on the other half of the slot 9.

The operating frequency of the magnetron is usuaily changed by moving one end plate 10 of the outer resonance cavity. This changed the conditions required for zero current at the slot end for two reasons. Firstly the sinusoidal pattern of the exciting currents round the anode wall 2 changes, and secondly the response of the slot and inner anode to those currents changes, as exemplified by movement of the notional point B.

These two effects operate in opposite sense if the attenuating means is placed at the end A of the slot nearest to the end plate 10 which is moved, and hence zero loss into the attenuating means can be maintained over a wide range of operating frequencies.

The position of point B can be altered by varying the width of the slot 9 since the inductance between the vanes 4 and the slot 9 must remain constant for a particular operating frequency, and the inductance is given by the slot area between point B and point C.

The range of frequencies over which current cancellation may occur can be increased by increasing the width of the slot 9, since in this case point B need only change position slightly in order to change the slot area between point B and point C, and hence the inductance, by an appreciable amount.

The rate of movement of point B is controlled by the relative dimension so the slot width and anode shell thickness to the dimensions of the inner anode; and movement of the circulating currents by the proportions of the outer cavity and position of the slot reiative to the end plates of the cavity. The rate at which B moves with frequency is inversely proportional to the slot width at B; but B has to be in the correct position relative to the currents in the outer cavity.

The total area of the slot 9 is chosen so that it provides optimum coupling between the outer cavity 7 and the inner resonance cavities. If the slot area is too large the difference in frequency between the inner anode resonance and the operating frequency is too great for efficient operation. Thus in order to increase the range of operating frequencies available the width of the slot 9 rnay be increased but a centre section of the slot 9 must be correspondingly decreased so that the total slot area remains constant. The centre narrow section extends over the axial length of the vanes 4 and the dimensions are chosen so that point B lies outside the centre section and within the width part of the slot 9.A mcdfiedd slot of this shape is illustrated in Figure 4, in which wide sections are positioned on each side of a narrower central section.

There are limits on the width of the slot 9. If the wide section of the slot 9 is too wide current may flow round the inside of the anode wall 2 from one slot to the next instead of round the slot 9. If the narrow centre section is too narrow its capacity will shunt the cavity impedance.

Further control of the conditions for cancellation of the induced currents can be obtained by adjusting the slot length and also by using the shield 10.

The shield 10 is located in the outer cavity 7 concentric with the anode wall 2 and at a distance from it. If the attenuator excitation due to currents between point A and point B exceeds that due to currents between point B and point C for a desirable resonance spectrum it may be reduced by using the shield 10, which is mounted at the end of the anode wall 2 at which the attenuator 8 is located. Then the currents flow on the shield 10 instead of the anode wall 2, enabling a balance of currents to be found for a different range of operating frequencies.

The shield is a conducting cylinder concentric with but of greater than the anode shell; but not in electrical contact. The fields in the outer resonance cavity induce currents round this shell rather than the anode shell and these currents are not, of course, intercepted by the slots. This shield will normally be over one end of the inner anode.

The shield modifies the rate of change of intercepted currents with movement of the end plate 10 and can be used to make the condition for zero loss occur over a wider range of frequencies than would otherwise be possible.

Claims (8)

1. A co-axial magnetron including a cylindrical anode wall defining an inner anode cavity and which is positioned between a central cathode and an outer resonance cavity, which surrounds said wall and which determines the operating frequency of the magnetron, the anode wall including a plurality of elongate slots which couple energy at a frequency lower than the natural resonance frequency of the inner anode cavity from the interior of the anode wall to the outer resonance cavity; attenuation means positioned at an end of each slot, and each slot being shaped and dimensioned such that there exists a notional position along its length between a central position and said end thereof at which, in operation, induced current flowing around the slot between said notional position and said central position is balanced by induced current flowing around said slot between said notional position and said end, and at which, if the slots were short circuited at said notional position, the inner anode cavity would resonate at the operating frequency of the magnetron.

2. A magnetron as claimed in claim 1 and wherein each slot is of uniform width.

3. A magnetron as claimed in claim 1 and wherein each slot has a centre section which is narrower than sections on each side of the centre section.

4. A magnetron as claimed in claim 2 or 3 and wherein each slot has the ame dimensions as the other slots.

5. A magnetron as claimed in any of the preceding claims and wherein the plurality of slots are parallel to the axis of the anode wall.

6. A magnetron as claimed in any of the preceding claims and wherein the plurality of slots are regularly spaced apart from each other.

7. A magnetron as claimed in any preceding claim and wherein a shield co-axially surrounds the anode wall in the region of the attenuation means.

8. A co-axial magnetron substantially as illustrated in and described with reference to the accompanying drawings.