Slow waveguide for travelling wave tube

Abstract

A slow waveguide for travelling wave tube includes a central plate comprising a beam slip hole, rectilinear in the same direction as the longitudinal axis of the central plate, a bottom plate and a top plate closing the waveguide, respectively arranged on and under the central plate, and a slit folded in the form of a snake having its folds in the direction of the thickness of the guide.

Claims

1. A slow waveguide for travelling wave tube comprising: a central plate comprising a beam slip hole, rectilinear in the same direction as the longitudinal axis of the central plate, a bottom plate and a top plate closing the waveguide, respectively arranged on and under the central plate, and a slit folded in the form of a snake having a plurality of folds folded vertically up and down in the direction of the thickness of the waveguide.

2. The waveguide according to claim 1, wherein the plurality of folds of the slit are produced by irises that are present alternately in at least one of: successive blades of the central plate on one face then the other of the central plate, and the bottom and top plates facing the slit separating the blades.

3. The waveguide according to claim 1, wherein the plurality of folds are notches.

4. The waveguide according to claim 1, wherein the plurality of folds are rounded or of circular form.

5. The waveguide according to claim 1, wherein the central plate is made of copper, of copper alloy, or of molybdenum.

6. The waveguide according to claim 1, wherein the bottom and top plates are made of copper, of copper alloy, or of molybdenum.

7. A method for fabricating a slow waveguide for travelling wave tube comprising steps of: drilling a beam slip hole, rectilinear in the same direction as the longitudinal axis of a central plate; drilling a series of parallel open slits in the central plate, the slits being at right angles to the beam slip hole, forming a series of blades between two parallel consecutive slits; and producing irises forming a plurality of folds of a folded slit folded vertically up and down in the direction of the thickness of the waveguide, by at least one of: alternately machining the successive blades on one face then the other of the central plate, and alternately machining bottom and top plates facing the parallel slits.

8. The method according to claim 7, further comprising a step of closing the waveguide by the bottom plate and the top plate, fixed respectively onto the bottom face and onto the top face of the central plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood on studying a few embodiments described in the way of nonlimiting examples and illustrated by the attached drawings in which:

(2) FIGS. 1 to 7, 11a and 11b schematically illustrate examples of production of folded waveguides, according to the prior art;

(3) FIGS. 8 to 10, 11c, 12a to 12c schematically illustrate various embodiments of a slow waveguide, according to various aspects of the invention.

(4) In all the figures, the elements that have identical references are similar.

DETAILED DESCRIPTION

(5) In the present description, the embodiments described are nonlimiting, and the features and functions well known to the person skilled in the art are not described in detail.

(6) FIGS. 8 and 9 represent a folded waveguide whose folds are in the form of notches.

(7) A beam slip hole 2 is drilled that is rectilinear, in the same direction as the longitudinal axis of a central plate 1, and a series of parallel open slits are drilled in the central plate 1, the slits being at right angles to the slip hole 2, forming a series of blades between two consecutive slits, and irises are produced forming the folds of a folded slit 3, by alternately machining the successive blades on one face then the other of the delay line plate 1, or by alternately machining bottom 6 and top 7 plates facing the slits, or partly both.

(8) Thus, a waveguide is obtained comprising a central plate 1 comprising a beam slip hole 2, rectilinear in the same direction as the longitudinal axis of the central plate 1, and comprising a folded slit 3, the central plate 1 being arranged between a bottom plate 6 and a top plate 7 closing the waveguide, the folded slit 3 having its folds in the direction of the thickness of the central plate 1. In this nonlimiting example, the folds of the folded waveguide 3 are produced by irises machined alternately in successive blades of the central plate 1 on one face then the other of the central plate 1, or machined alternately in the bottom 6 and top 7 plates facing the slits separating the blades, or alternately partially in a blade of the central plate 1 and one of the bottom 6 or top 7 plates.

(9) This example is nonlimiting, because any variant folded slit 3 whose folds or meanders are in the direction of the thickness of the central plate 1 is suitable, for example with irises forming the folds that can be machined wholly or partly in the bottom 6 and top 7 plates. One such example of folds of rounded or circular form is illustrated in FIG. 10, produced alternately in the bottom 6 and top 7 plates.

(10) FIGS. 11a and 11b concern lines according to the prior art, with irises in the form of flat E bends at 180 for FIG. 11a and with straight irises of a length less than the pitch for FIG. 11b. These figures represent a cross-sectional view of the line of the central plate 1, along a plane parallel to the top and bottom faces of the central plate 1, passing through the longitudinal axis of the beam slip hole 2. The irises 9 forming the folds are represented shaded by small dots.

(11) FIG. 11c represents a cross-sectional view of the plates 1, 6 and 7 assembled, along a plane at right angles to the top and bottom faces of the central plate 1, passing through the longitudinal axis of the beam slip hole 2. The irises 9 forming the folds are represented shaded by small dots.

(12) FIGS. 12a, 12b and 12c represent various embodiments of a waveguide according to one aspect of the invention, with folds or irises of the folded slit 3 in the form of notches, i.e. with bends at 90. In these cases, it can be considered that the folds of the folded slit 3 are produced by means of parallel emerging slits in the central plate 1, the slits 10 being at right angles to the slip hole 2, forming a series of blades between two consecutive slits. In FIGS. 12b and 12c, the graphs on the right represent the scatter diagram of the periodic line, also called Brillouin diagram, which shows, on the x axis, the phase shift of the wave for a pitch p (therefore from one interaction space to the next) and, on the y axis, the pulsing =2F, F representing the frequency in Hz and the propagation constant of the wave in rad/m.

(13) In this case, it is possible to consider the folded slit 3 as a series of parallelepipedal cavities 10 coupled by irises 9 that are also parallelepipedal.

(14) In the case of FIG. 12a, the feature of the folded slit 3 is that the width of the cavity is equal to the width of the iris, i.e. the thickness of the central plate 1, when the folded slit 3 is entirely machined in the central plate 1. As a variant, it is possible to choose an iris width that is different from the width of the cavity in order to choose the mode on which the interaction takes place and to adjust the bandwidth of the tube.

(15) It is possible, as a variant, as illustrated in FIG. 12b, to take an iris width smaller than that of the cavity, which means a resonance frequency of the iris greater than that of the cavity: in this case, the lowest frequency mode (that with which the beam interacts) is the cavity mode. Reducing the width of the iris reduces the bandwidth of the mode (and that of the corresponding travelling wave tube), but increases the margin in relation to the oscillation at frequency 2.

(16) It is not possible to machine an iris wider than the rest of the folded slit 3, but it is possible, as illustrated in FIG. 12c, to machine an iris by giving it the form of a ridge guide to obtain a resonance frequency of the iris lower than that of the cavity. The lowest mode is then the iris mode.