GB2100919A - Slow-wave structure phase velocity tapering in coupled cavity travelling wave tubes - Google Patents

Abstract

A coupled cavity travelling wave tube is provided in which, to maintain synchronism with the electron beam, slot length tapering is provided together with at least one other form of tapering of which examples, in a cloverleaf tube, are slot width tapering, cavity height tapering and nose diameter tapering.

Description

SPECIFICATION Improvements in or relating to coupled cavity travel ling wave tubes This invention relates to coupled cavity travelling wave tubes and in particular to such tubes of the cloverleaf type.

As is known, reducing the phase velocity of the circuit wave in the output section of a travelling wave tube is an established method of increasing efficiency by maintaining interaction with the slowing beam. Known examples of 6 nose, cloverleaf travelling wave tubes operating at S-band and C-band use a velocity taper which varies in its effect as a function of frequency, that is to say the taper is a differential taper. This is achieved by increasing the lengths of the coupling slots over a number of cavities to change the dispersion characteristic.

There has been found to be a limit to the extent to which differential tapering can be achieved as described above since as slot length is increased further, so problems of radio frequency match or reflection and higher order cavity mode overlap tend to arise.

One object of the present invention is to provide an improved coupled cavity travelling wave tube, and in particular such a tube of the cloverleaf type, in which the above difficulty is mitigated.

Acoupled cavity travelling wave tube is provided wherein there is provided slot length tapering and at least one other form of tapering whereby to reduce the phase velocity of the circuit wave of said travelling wave tube towards the output of said tube.

In one example of a tube in accordance with the present invention said at least one other form of tapering comprises cavity height tapering, the cavity height being reduced from cavity to cavity in the direction of beam travel towards the output of said tube.

Normally the height of the final cavity of said tube is no less than 90% of and preferably approximately 93% of, the height of the first cavity thereof or the height of the cavities thereof which are not subject to tapering.

In another example of tu be in accordance with the present invention said at least one other form of tapering comprising nose diameter tapering, said nose diameter being reduced from cavity to cavity over the last few cavities with the last cavity having the smallest nose diameter.

In another example of tube in accordance with the present invention said at least one other form of tapering comprises slot width tapering, the slot width of the slots in the coupling plates of said cavities being increased from cavity to cavity over the last few cavities towards the output of said tube.

Slot width tapering may be applied with combination of other forms of tapering as described above. In one particular embodiment of the invention in which this is the case, cavity height tapering and nose diametertapering are provided together with slot length tapering.

In order to explain the present invention in greater detail it will be useful to refer to the accompanying drawings in which Figure 1 is a cross section through the cloverleaf structure of a coupled cavity travelling wave tube in accordance with the present invention.

Figure 2 is a section along the line X..X of Figure 1 showing a length of the coupled cavity structure including two adjacent cavities, and Figures 3 to 8 are explanatory graphical diagrams.

Referring to Figures 1 and 2, the cloverleaf structure illustrated therein comprises a slotted coupling plate 1 in which there are twelve coupling slots, such as that referenced 2, all of the same slot lengths Las indicated by the double-headed arrow. The coupling plate 1 also has a central aperture 3 for the passage of the beam along the axis of the tube.

The coupling plate 1 is mounted within an annular shell 4 from which extend, on either side of the plate 1, noses 5 to 10 in front (as viewed in Figure 2) of the plate and noses 11 to 16 (shown in dotted outline) behind the coupling plate 1. It will be noted that the noses 5 to 10 are staggered or rotated relative to noses 11 to 16.

The cavity of which noses 5 to 10 form a part is completed by a further slotted coupling plate 17 which, if slot length tapering is ignored, is similar two slotted coupling plate 1.

Further slotted coupling plate 17 is not shown in Figure 1 of course, being removed in order to show detail of the coupling plate 1 and the noses 5 to 10.

Coupling plates 1 and 17 with noses Sto 10 between them form one cavity of which the height or cavity axial length is H as shown in Figure 2.

The cloverleaf travelling wave tube as a whole includes a plurality of successive cavities each as described above except for any tapering features which may be present as hereinafter described. It will be noted that coupling plate 1 also forms part of the cavity including noses 11 to 16 behind (as viewed in Figure 1) plate 1 and similarly slotted coupling plate 17 forms part of the next cavity, referenced 18 in Figure 2.

Phase-frequency diagrams, or dispersion characteristics, describe the behaviour of electromagnetic waves propagating through a coupled cavity structure. That which is obtained for the main EO1 mode coupled cavity structure described above is shown in Figure 3.

Angular frequency, w, is plotted against phase change per cavity H, which is the product of the phase change/unit length p and the cavity height or cavity axial length H. The phase velocity of the wavefront is given by: Vp = p The group velocity at which the energy is transfer red is given by: the height of the proceeding cavities.

The shape of the resulting power band, in the case of this last mentioned example, is shown by waveform (b) of Figure 4, peaking at the low frequency end. The fall at the top end results from having equal taper at all frequencies and this is not ideal because of different beam velocity spreads and differences in the shape of the *ssH diagram towards each cut off point.

The presence of a cavity height taper tends to help in the obtaining of better r.f. matches because the structure and waveguide ports are closer in characteristic impedance. When used in conjunction with periodic permanent magnet focussing it enables the magnet stack passband parameters to be kept at the desired values while the peak field can be raised to control radio frequency defocussing.

Differential Tapering Referring again to Figure 4, where no tapering is employed a power output is obtained as shown by waveform (a). Where cavity height tapering is employed a power output is obtained which is typically as shown by waveform (b). The object of differential tapering is to combine the best levels of waveforms (a) and (b).

Slot Length Tapering An increase in coupling slot length SLfrom cavity to cavity in the direction of beam travel, has the effect of reducing the lower cut off frequency shown in Figure 5. f, and f2 are two frequencies at the top and bottom limits of the operating band of the tube.

If slot length tapering is employed with the length of the slots in each successive coupling plate increased over a number of coupling plates towards the output, with the length of the slots in the last plate being the greatest, the value of pH will increase considerably as a wave of frequency f, passes through the tapering sequence of slots.

The corresponding phase velocity Vp = 2 f, p will be reduced typically to 60% of the original value.

ssL remains constant. Four a wave offrequencyf2the change is much less and Vp is only reduced to about 90%.

Nose Diameter Tapering The nose diameter of a cavity is the dimension shown at Din Figure 1. This diameter may be reduced gradually from cavity to cavity over the last few cavities with the last cavity having the smallest nose diameter D.

Slot Width Tapering Slot width tapering is introduced into a tube by gradually increasing the slot width over the last few cavities towards the output end of the tube.

The effect of combining slot length tapering with other forms of tapering will now be described.

Slot length and cavity height The slot length taper used at C-Band can be expected to reduce the mid-band phase velocity up to 72?o of the starting value and give a variation from 550b to 80g with increasing frequency. This is illustrated in Figure 6. At S-Band the cavity proportions are considerably different because of beam changes and less slot length taper can be used before the r.f.

Vg = S = H tan o 6w Beam velocity, Ve, shown by tane Ve H At synchronism tan e = s0 PH Ve = Ve H Ve = Vp In the cloverleaf cavity stack an additional phase change of H between cavities is incorporated by the effect of staggering the noses in adjacent cells as shown in Figure 1. This results in the EO1 mode having positive group velocity in the operating range of O < pH < H where interaction is most efficient. Energy transfer from the beam to the circuit wave will centre around the frequency where the phase velocity, Vp, is equal to that of the beam electrons, Ve.This synchronous point is found by plotting on the dispersion diagram a line through the origin with the slope of VelH.

When large signal conditions are approached in the output and significant energy is extracted from the beam, the effective value of Ve falls and the syn chronous point moves up in frequency. In this way interaction is more effective at the top end of the band and the saturated output power plot has the shape shown by waveform (a) in Figure 4.

As has already been mentioned, this effect can be mitigated by slot length tapering but subject to the limitations already mentioned.

In a tube in accordance with the present invention, slot length tapering and at least one other form of tapering are applied to the same tube.

The individual effects of different forms of tapering which may be used in a tube in accordance with the present invention will now be described in turn.

Thereafterthe effects of various combinations in accordance with the invention will be described in turn.

Cavity Height Tapering If the cavity height, H, is reduced from cavity to cavity in the direction of beam travel the ssH diag ram is unchanged, sop is increased and the phase velocity Vp = a'lpfallsto match the slowing beam in the output. Another way of expressing the effect is to say that the original synchronism is maintained because L is reducing as Ve falls, keeping the beam line slopeVe/H constant.

Cavity height tapering is introduced into a tube by gradually reducing the cavity height over the last few cavities towards the output end of the tube to a height. If height tapering were applied alone, the height of the final cavity would typically be 70% of match as the waveguide transition deteriorates. The phase velocity profile is then between 70% and 95% with 82% mid-band. By adding a small amount of cavity height taper to 93% a total profile of 65% to 88% is obtained. This extra taper may be expected to produce increases in power over the lower and centre parts of the frequency range, for example increasing the 1dB band from 7% to 8% and the 3 dB band from 9% to 10.3%. Overtapering at the high frequency end is minimised by working at a high initial synchronous point.The reduced final cavity height may be expected to improve the reflected power levels at the waveguide transition by several dB's resulting in improved gain ripple characteristic.

It should be noted that considerably less height change is employed in an arrangement in accordance with the present invention where height tapering is combined with slot length tapering than is normally the case where height taper is applied above, as considered hereinbefore where height taper as such is discussed.

Slot length, cavity height and nose diameter Although the additional height taper as described above is beneficial over the low and centre parts of the band, it has a negative effect at the top end. The ideal taper would reduce to zero at the upper edge of the band. With the object of minimising the remaining taper at this point the nose diameter is gradually reduced over the last few cavities. The ssH = II point is tuned up so that the dispersion characteristic of the standard and final cavities intersect at frequency f2, the upper band edge (see Figure 7). In this way the differential taper at frequency f2 is reduced substantially to zero with negligible effect at frequency f1, the lower band edge.The only remaining taper at frequency f2 is that resulting from the cavity height reduction.

Slot length and nose diameter If a long slot can be used for maximum low frequency taper without additional cavity height reduction, then nose diameter tapering will reduce the high frequency taper down towards the ideal of zero.

The limit to achieving cancellation is likely to be higher order mode overlap causing instabilities.

Slot length and slot width The effect of adding slot width tapering over the final cavities is to achieve a wider differential taper with the least increase at the upper frequency. This reduces the frequency of all points on the dispersion characteristic but by a greater amount at the lower end, as illustrated by Figure 2. When the values of PH at frequencies f, and f, are considered it is found that the percentage change at the lower end is typically double that at the top. A further differential velocity taper has then been created to add to that from the slot length taper. An advantage of the slot width taper is that higher order modes move down proportionately less which tends to minimise band overlap.

Interaction impedance also tends to be maintained, which is not the case when the slot length is significantly increased, and r.f. reflection levels tend to be reduced.

Claims (7)

1. A coupled cavity travelling wave tube wherein there is provided slot length tapering and at least one other form of tapering whereby to reduce the phase velocity of the circuit wave of said travelling wave tube towards the output of said tube.

2. A tube as claimed in claim 1 and wherein said at least one other form of tapering comprises cavity height tapering the cavity height being reduced from cavity to cavity in the direction of beam travel towards the output of said tube.

3. Atube as claimed in claim 2 and wherein the height of the final cavity of said tube is no less than 90% of the height of the first cavity thereof or the height of the cavities thereof which are not subject to tapering.

4. Atube as claimed in claim 2 orthree and wherein the height of the final cavity of the tube is approximately 93% of the height of the first cavity thereof or the height of the cavities thereof which are not subject to tapering.

5. A tube as claimed in any of the above claims and wherein said at least one other form of tapering comprises nose diameter tapering, said nose diameter being reduced from cavity to cavity overthe last few cavities with the last cavity having the smallest nose diameter.

6. Atube as claimed in any of the above claims and wherein said at least one other form of tapering comprises slot width tapering, the slot width of the slots in the coupling plates of said cavities being increased from cavity to cavity over the last few cavities towards the output of said tube.

7. A tube as claimed in any of the above claims and wherein cavity height tapering and nose diameter tapering are provided together with slot length tapering.