SLOW WAVE CONDUCTOR FOR TRAVELING TUBE

Description

[0001] The present invention relates to a delay line or slow wave guide for a traveling wave tube with the acronym TOP.

[0002] In most microwave tubes the interaction between the wave and the beam is broken down into two stages: a first step: obtaining a grouping of electrons into packets, that is to say carrying out a modulation of the beam current density at the rhythm of the hyperfrequency signal; and a second step: placing the packets of electrons thus obtained in a phase where they are slowed down by the field in order to give up their energy to the wave.

[0003] In the case of TWTs, the grouping of electrons into a bunch is obtained by placing the beam in the field of a traveling wave whose phase velocity is equal to the velocity of the electrons. In a moving frame, the electrons see the field of a standing wave. The electrons are slowed down on one alternation and accelerated on the next. An electron bunch forms around the phase for which we pass from an accelerating field to a decelerating field.

[0004] A conventional waveguide, with a rectangular or cylindrical section, is not suitable for interaction because the phase velocity of the wave propagating in this guide is greater than the speed of light, while the speed of the electrons is less than the speed of light. In addition, an electric field is required that is parallel to the movement of the electrons, whereas the fundamental mode of rectilinear guides with a rectangular or cylindrical section is perpendicular to the axis of the guide. To obtain a phase velocity lower than that of light, a special guide called a slow wave guide or delay line is required. Most often, the delay line is a periodic line obtained by translating a basic cell. This is the case for the helix, the coupled cavity line, the interdigital line, etc.

[0005] In the field of TWTs operating at millimeter wavelengths, a so-called folded guide delay line is often used. This line is obtained by periodically positioning rectangular waveguide sections perpendicular to the beam axis, and by alternately connecting the straight guide sections by 180° E-plane bends. The side view of the folded guide has the shape of a serpentine. The beam slip hole is located in the middle of the rectangular guide straight section. The electric field in the guide is perpendicular to the long side of the guide, and therefore parallel to the movement of the electrons, which allows the beam to be modulated. The electron therefore moves into the slip hole, emerges into the straight guide section where it is subjected to the action of the electric field (interaction space), crosses back through the slip hole and emerges into the next interaction space.The electron therefore sees successive interaction spaces with a period equal to the pitch of the line while the geometric period of the line is equal to twice the pitch. The length of the folded waveguide (straight part and elbows) is determined so that the phase shift of the wave in the guide corresponds to the phase variation linked to the movement of the electrons from one interaction space to the next.

[0006] This folded guide line presents an analogy with the line with cavities coupled by alternating irises if we assimilate the rectangular guide cross-section to a cavity where the wave-beam interaction occurs, and the plane E bends to the coupling irises (see figure 11a ). The particularity of this line is to impose the same dimension for the width of the cavity and the width of the iris (the large side of the rectangular guide), which does not allow the bandwidth to be adjusted.

[0007] US4129803A, US2003 / 030390A1 and US 2012 / 081003A1 describe slow waveguides for TWTs. However, they do not disclose a central plate with a series of through-slots. FR2510814A1 describes iris slow waveguides for TWTs. However, irises are not present in successive blades of the central plate alternately on the lower and upper faces of the central plate.

[0008] It is known to make delay lines as illustrated in the figures 1 à 5 , which schematically represent the central plate construction which is then placed between a lower plate and an upper plate allowing the waveguide to be closed.

[0009] There figure 1 represents a central plate 1, in which a sliding hole 2 of the electron beam is drilled in the length direction of the central plate 1. The central plate 1 has the shape of a rectangular parallelepiped whose faces are parallel to the axis of the sliding hole 2 and symmetrical with respect to the axis of the sliding hole 2.

[0010] As shown in the figure 2 , an opening slot 3, having a serpentine shape, is made in the central plate 1, or in other words over the entire thickness of the plate 1, over most of the length of the central plate 1, having its folds or meanders in the direction of the width of the central plate 1.

[0011] The machined central plate 1 is equivalent to two nested combs 4, 5, as illustrated in the figure 3 , connected at the ends (different hatching). This is also an alternative technology for making this line (using two combs and two rulers to position the combs). The pitch of slot 3 is the distance between successive portions of slot 3 (or successive holes) along the longitudinal axis. The geometric period of slot 3 is equal to twice the pitch.

[0012] The removal of material which accompanies the machining of the slot 3 of the central plate 1 releases the stresses which can result in deformations of the central plate 1. Thus, in particular, a longitudinal displacement or a transverse displacement of one comb relative to the other may appear, as illustrated respectively on the figures 4 et 5 .

[0013] The longitudinal displacement of one comb relative to the other, as illustrated in the figure 4 , modifies the width of slit 3 which is no longer regular. Moving along the beam axis, in slip hole 2, an electron sees a short interaction space followed by a long interaction space (portions of slit 3). The period of the folded waveguide, or in other words the period of slit 3, seen by the electron beam is no longer the pitch of slit 3 but approximately double. We therefore have a biperiodicity which can result in a strong mismatch and risks of oscillations.

[0014] The transverse displacement of one comb relative to the other, as illustrated in the figure 5 , results in a shift of the sliding tunnel from one tooth of one comb to the next tooth of the other comb. There is then biperiodicity and risk of oscillation. In addition, the misalignment reduces the useful section for beam transport, because it induces shifted portions of the sliding hole 2, and results in a greater interception of the electron beam, which limits the average power of the traveling wave tube using such a waveguide.

[0015] Furthermore, a combination of problems induced by longitudinal sliding and transverse sliding of the two combs relative to each other is also possible.

[0016] THE figures 6 And 7 schematically represent a waveguide respectively in exploded view and in sectional view along the longitudinal axis of the central plate 1.

[0017] In the example shown, the waveguide comprises a central plate 1 provided with a beam sliding hole 2, rectilinear in the same direction as the longitudinal axis of the central plate 1, and comprises a slot 3, machined through the central plate 1. A lower plate 6 and an upper plate 7 close the waveguide, the slot 3 having its folds in the direction of the width of the central plate 1. In this example, which is in no way limiting, the folds or meanders of the folded waveguide or slot 3 are in the form of crenellations or rectangular.

[0018] One aim of the invention is to overcome the problems mentioned above.

[0019] The invention is set forth in the attached set of claims, comprising a serpentine-shaped folded slot having its folds in the direction of the thickness of the guide, i.e. in the direction of the thickness of the central plate, i.e. at 90° to the width direction of the state of the art.

[0020] A slow wave guide for traveling wave tube or folded wave guide whose folds or irises are in the direction of the thickness of the central plate, i.e. in the direction of the thickness of the guide, makes it possible to avoid the problems of longitudinal and / or transverse displacement.

[0021] The invention will be better understood by studying a few embodiments described as non-limiting examples and illustrated by the appended drawings in which: there figure 1 à 7 , 10 , 11a et 11b schematically illustrate examples of the production of folded waveguides, according to the state of the art; figures 8 à 9 , 11c , 12a à 12c schematically illustrate various embodiments of a slow waveguide, according to various aspects of the invention.

[0022] Throughout the figures, elements with identical references are similar.

[0023] In the present description, the embodiments described are in no way limiting, and the characteristics and functions well known to those skilled in the art are not described in detail.

[0024] THE figures 8 And 9 represent a folded waveguide whose folds are in the form of crenellations.

[0025] A beam sliding hole 2 is drilled, rectilinear, in the same direction as the longitudinal axis of a central plate 1, and a series of parallel through-slots are drilled in the central plate 1, the slots being perpendicular to the sliding hole 2, forming a series of blades between two consecutive slots, and irises are produced forming the folds of a folded slot 3, by alternately machining the successive blades on one face then the other of the delay line plate 1, or by alternately machining lower 6 and upper 7 plates opposite the slots, or partly both.

[0026] Thus, a waveguide is obtained comprising a central plate 1 comprising a beam sliding hole 2, rectilinear in the same direction as the longitudinal axis of the central plate 1, and comprising a folded slot 3, the central plate 1 being arranged between a lower plate 6 and an upper plate 7 closing the waveguide, the folded slot 3 having its folds in the direction of the thickness of the central plate 1. In this non-limiting 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 lower 6 and upper 7 plates opposite the slots separating the blades, or alternately partially in a blade of the central plate 1 and one of the lower 6 or upper 7 plates.

[0027] The delay line plate can be made of copper, copper alloy (tungsten-copper W-Cu, molybdenum-copper Mo-Cu), molybdenum, or any other material with good thermal conductivity, and non-magnetizable so as not to disturb the beam focusing magnetic field.

[0028] The use of molybdenum or a refractory material allows for a high melting temperature, which is advantageous in the case of bombardment by the electron beam.

[0029] Making the bottom and top plates from the same material as the center plate avoids differential expansion problems during brazing.

[0030] This example is not limiting, because any variant of folded slot 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 of rounded or circular shape.

[0031] THE figures 11a et 11b concern lines according to the state of the art, with irises in the shape of a 180° E-plane elbow for the figure 11a and with straight irises of less than the length for the figure 11b These figures represent a sectional view of the line of the central plate 1, along a plane parallel to the upper and lower faces of the central plate 1, passing through the longitudinal axis of the beam sliding hole 2. The irises 9 forming the folds are represented in gray by small dots.

[0032] There figure 11c represents a sectional view of the assembled plates 1, 6 and 7, along a plane perpendicular to the upper and lower faces of the central plate 1, passing through the longitudinal axis of the beam sliding hole 2. The irises 9 forming the folds are represented in gray by small dots.

[0033] THE figures 12a , 12b et 12c represent various embodiments of a waveguide according to one aspect of the invention, with folds or irises of the folded slot 3 in the form of crenellations, i.e. with 90° bends. In these cases, it can be considered that the folds of the folded slot 3 are made by means of parallel through-slots in the central plate 1, the slots 10 being perpendicular to the sliding hole 2, forming a series of blades between two consecutive slots. On the figures 12b et 12c , the graphs on the right represent the dispersion diagram of the periodic line, also called the Brillouin diagram, which presents on the abscissa the phase shift of the wave for a step p (therefore from one interaction space to the next) and on the ordinate the pulsation ω = 2πF, F representing the frequency in Hz and β the wave propagation constant in rad / m.

[0034] In this case, it is possible to consider the folded slit 3 as a series of parallelepiped cavities 10 coupled by irises 9 which are also parallelepiped.

[0035] In the case of the figure 12a , the characteristic of the folded slot 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 slot 3 is entirely machined in the central plate 1. Alternatively, it is possible to choose an iris width different from the width of the cavity in order to choose the mode in which the interaction takes place and to adjust the bandwidth of the tube.

[0036] Il is possible, alternatively, as illustrated in the figure 12b , to take an iris width smaller than that of the cavity, which implies a resonance frequency of the iris higher than that of the cavity: in this case the lowest frequency mode (the one with which the beam interacts) is the cavity mode. Reducing the iris width decreases the bandwidth of the mode (and that of the corresponding traveling wave tube), but increases the margin with respect to oscillation at frequency 2π.

[0037] An iris cannot be machined wider than the remainder of the folded slit 3, but it is possible, as illustrated in the figure 12c , to machine an iris by giving it the shape of a ribbed guide (or ridged guide) to obtain a resonant frequency of the iris lower than that of the cavity. The lowest mode is then the iris mode.

Claims

1. Slow waveguide for travelling wave tube, comprising a bottom plate (6) and a top plate (7) which close the waveguide and are respectively arranged on a bottom face and a top face of a central plate (1), wherein: - the central plate (1) comprises: - a beam slip hole (2) which is rectilinear in the same direction as the longitudinal axis of the central plate (1), and - a series of slits opening on the bottom and top faces of the central plate (1), the slits being parallel in the central plate (1), the slits being arranged at right angles to the slip hole (2), and forming a series of blades between two parallel consecutive slits, - irises forming the folds of a folded slit (3), these irises being present in the successive blades of the central plate (1) alternately on the bottom and top faces of the central plate (1), - the folded slit (3) being in the form of a snake having its folds in the direction of the thickness of the central plate (1), the thickness being measured between the bottom face and the top face of the central plate (1), the folds being located alternately on the bottom face side and on the top face side of the plate (1).

2. Guide according to claim 1, wherein a fold is in the form of a notch.

3. Guide according to claim 1, wherein a fold is of rounded or circular form.

4. Guide according to any one of the preceding claims, wherein the central plate (1) is made of copper, of copper alloy or of molybdenum.

5. Guide according to any one of the preceding claims, wherein the bottom (6) and top (7) plates are made of copper, of copper alloy or of molybdenum.

6. Method for fabricating a slow waveguide for travelling wave tube according to any one of the preceding claims, comprising steps of: - drilling a beam slip hole (2) which is rectilinear in the same direction as the longitudinal axis of a central plate (1); - drilling a series of parallel open slits in the central plate (1), the slits being at right angles to the slip hole (2), forming a series of blades between two parallel consecutive slits; - producing irises forming the folds of a folded slit (3), by alternately machining the successive blades on a bottom face then a top face of the central plate (1); and - a step of closing the guide by a bottom plate (6) and a top plate (7), which are fixed respectively onto the bottom face and onto the top face of the central plate (1).