EP2034507A1

EP2034507A1 - Travelling-wave-tube wide band amplifier and corresponding method of fabrication

Info

Publication number: EP2034507A1
Application number: EP07425554A
Authority: EP
Inventors: Agostino Nicosia; Rosario Martorana
Original assignee: Galileo Avionica SpA
Current assignee: Selex Galileo SpA
Priority date: 2007-09-07
Filing date: 2007-09-07
Publication date: 2009-03-11

Abstract

A travelling-wave-tube amplifier includes an amplifier tube (11), having a hollow cylindrical casing (18) made of conductive material, provided with longitudinal projections (18b) projecting for a stretch inwards. The longitudinal projections (18b) are made of a single piece with the casing (18).

Description

The present invention relates to a travelling-wave-tube wide-band amplifier and to a corresponding method of fabrication.
As is known, travelling-wave-tube wide-band (TWT-WB) amplifiers are used in the telecommunications sector for processing large amounts of information at a high frequency (in particular, in the bands C, X, Ku, between 4 and 18 GHz approximately). As illustrated by way of example in Figure 1, a helical TWT amplifier 1 comprises a substantially cylindrical conductive casing 2, in which a vacuum is formed, and a slow-wave helical structure 3, coaxial to the outer casing. The slow-wave structure 3 is kept in position by dielectric supports 5, which extend in a radial direction between the slow-wave structure 3 itself and a cylindrical internal surface of the casing 2. According to a configuration that is prevalently adopted, three dielectric supports 5 are used, spaced at uniform angular distances apart.
By a source not illustrated herein, an electron beam is injected along the axis of the slow-wave structure 3, in which the radiofrequency electromagnetic signals to be amplified are also injected. The slow-wave structure 3 is shaped so that the phase velocity of the electromagnetic signals according to the axis of the amplifier is reduced until it is comparable with the speed of the electron flow, enabling interaction between the electromagnetic signals themselves and the electron beam. Following upon said interaction, the electrons are decelerated and transfer energy to the electromagnetic signals, which are thus amplified.
A problem commonly linked to TWT amplifiers is the marked dependence of the phase velocity upon the frequency, which leads to major gain variation in the bands of use of TWT amplifiers. In fact, the phase velocity can vary also to the extent of 10% and above.
To overcome this problem (variant of Figure 2), it has been proposed to use plane conductive laminae (commonly referred to as "vanes") 7, fixed to the internal surface of the casing 2. The vanes 7 develop mainly in a longitudinal direction and moreover extend radially from the internal surface of the casing 2 towards the axis of the TWT amplifier 1. The presence of the vanes 7 has a prevalent effect on the electrical field of the electromagnetic wave in the lower part of the frequency band of use (capacitive effect). In practice, the vanes 7 reduce significantly the dependence of the phase velocity upon the frequency, thereby determining a reduction in the radiofrequency gain of the TWT amplifier. It is in practice possible to make TWT amplifiers that operate in a satisfactory way on the entire band of use.
However, the fabrication of TWT amplifiers equipped with vanes is complex and problematical. The vanes are in fact long and thin, and fixing thereof to the internal surface of the casing, which is normally performed by brazing, is critical. The length of the vanes may in fact be also hundreds or thousands of times greater than the width (tens of centimetres as against tenths of a millimetre). The precision of the operations of mounting is then hard to control, and, since the vanes are very liable to damage, the risk of producing defective pieces is somewhat high.
In any case, the assembly of the vanes entails long times and is expensive.
Other techniques envisage the metallization of the dielectric supports of the slow-wave structure. In this case, however, it is frequently necessary to use highly toxic materials, such as beryllium oxide, which is carcinogenic. Furthermore, the quality of the process of metallization has a marked effect on the performance of the TWT amplifier.
The aim of the present invention is to provide a travelling-wave-tube amplifier and a method of fabrication of a travelling-wave-tube amplifier that are free from the drawbacks described and, in particular, enable a reduction in the dependence of the phase velocity upon the frequency and, at the same time, are simple to produce.
According to the present invention, a travelling-wave-tube amplifier and a method of fabrication of a travelling-wave-tube amplifier are provided, as defined in Claims 1 and 11, respectively.
For a better understanding of the invention, there will now be described an embodiment thereof, provided purely by way of non-limiting example and with reference to the annexed drawings, wherein:

Figure 1 is a front sectional view of a known first travelling-wave-tube amplifier;
Figure 2 is a front sectional view of a second known travelling-wave-tube amplifier;
Figure 3 is a simplified side view of a travelling-wave-tube amplifier according to one embodiment of the present invention;
Figure 4 is a front view of the amplifier of Figure 3, sectioned according to the plane of trace IV-IV of Figure 3 and enlarged;
Figure 5 is a partial side view of the amplifier of Figure 3, sectioned according to the plane of trace V-V of Figure 4 and enlarged;
Figure 6 is a side view of a conductive body in an initial step of a method of fabrication according to the present invention;
Figure 7 is a front view of the body of Figure 6, sectioned according to the plane of trace VII-VII of Figure 6, in a subsequent fabrication step;
Figure 8 is a schematic front view of pre-assembled components; and
Figure 9 is a side view of the body of Figures 6 and 7, sectioned along the plane of trace IX-IX of Figure 7, in a step of mounting of the pre-assembled components of Figure 8.

With reference to Figures 3-5, a travelling-wave-tube amplifier or TWT amplifier 10 comprises: an amplifier tube 11, which extends along an axis A; an electron gun 12; a collector 13; an input-signal coupler 15; and an output-signal coupler 16.
The electron gun 12 is coupled to one end of the amplifier tube 11 for emitting, in use, a beam of electrons focused substantially along the axis A of the amplifier tube 11, in which the vacuum is created. To guarantee focusing, a permanent-periodic-magnet (PPM) structure is present, (known and not illustrated herein). The collector 13 is located at an opposite end of the amplifier tube 11 for receiving the electrons coming from the electron gun 12. The input-signal coupler 15 and the output-signal coupler 16 are arranged in the proximity of the electron gun 12 and of the collector 13 and enable, respectively, injection of a low-power input signal SIN into the amplifier tube 11 and picking-up of an amplified output signal SOUT.
As illustrated in greater detail in Figures 4 and 5, the amplifier tube 11 comprises a casing 18 and a slow-wave helical structure 20, which is coaxial with, and is aligned to, the axis A.
The casing 18 is made of conductive material and is externally cylindrical and hollow. Internally, the casing 18 has first portions of cylindrical surface 18a, having a first radius R1, and longitudinal projections 18b, which extend parallel to the axis A and are projected radially for a stretch towards the slow-wave structure 20 from the portions of cylindrical surface 18a. In the embodiment herein described, three longitudinal projections 18b are present, spaced at uniform angular distances apart. The longitudinal projections 18b are made of a single piece with the rest of the casing 18, of the same conductive material and in cross section are shaped like the capital of a column. More precisely, the longitudinal projections 18b are delimited radially by respective second portions of cylindrical surface 18c having a second radius R2 smaller than the first radius R1.
The slow-wave structure 20 is defined by a conductor wound in a helix about the axis A with a third pre-determined external radius R3 and is kept in position by supports 22 of dielectric material, which rest directly against respective longitudinal projections 18b (the supports 22 are as many as the longitudinal projections 18b). In the embodiment described herein, the longitudinal projections 18b have larger widths than the supports 22 so as to provide a convenient resting base. For example, the supports are approximately 0.6 mm wide, whilst the width of the longitudinal projections 18b is approximately 1.4 mm. Furthermore, the ratio between a radial dimension of the longitudinal projections 18b and a radial dimension of the supports 22 is preferably approximately 0.5 mm. More precisely, the radial dimension of the longitudinal projections 18b is equal to the difference R1 - R2 between the first radius R1 and the second radius R2. The radial dimension of the supports 22 is substantially equal to the difference R2 - R3 between the second radius R2 and the third radius R3 of the slow-wave structure 20.
The conformation of the longitudinal projections 18b enables excellent results to be achieved as regards the reduction of the dependence of the phase velocity upon the frequency, which can be brought down even below 3% on the entire band of operation (for example, from 5 GHz to 15 GHz). In addition, the fabrication and the assembly are considerably simplified, as will be clarified in what follows, with reference to Figures 6-9.
Initially (Figure 6), a cylindrical ingot 25 of conductive material is bored axially to obtain a through cavity 26, having the second radius R2.
The surface that delimits the through cavity 26 is then subjected to a lapping process, to minimize the roughness. In this way, the thermal coupling with the supports 22, once these have been installed, is optimal.
Subsequently (Figure 7), the longitudinal projections 18b are made by removal of material starting from the bored ingot 25, in particular by electrical-discharge machining (EDM), for example wire-EDM. In this step, in practice, the internal surface of the ingot 25 is machined locally by removing portions of material (dashed line in Figure 7) to form recesses that separate the longitudinal projections 18b. The machining proceeds until the first portions of cylindrical surface 18a having the first radius R1 are defined. The casing 18 is thus obtained starting from the cylindrical ingot 25.
Separately (Figure 8), the slow-wave structure 20 and the supports 22 are pre-assembled in the respective corresponding positions of use.
Then, the slow-wave structure 20 and the pre-assembled supports 22 are introduced within the casing 18, as illustrated in Figure 9, so that the supports 22 slide along the second cylindrical surfaces 18c of the longitudinal projections 18b. Correct position of the slow-wave structure 20 and of the supports 22 within the casing 18 is ensured by a pre-determined interference.
The amplifier tube 11 is then completed in a conventional way with the mounting of the electron gun 12, the collector 13, the input-signal coupler 15, and the output-signal coupler 16.
The amplifier according to the invention and the corresponding method of fabrication have different advantages.
In first place, it is possible to obtain an optimal reduction of the dispersion of the phase velocity, without using the conventional longitudinal vanes. The longitudinal projections 18b, albeit having a different conformation, obtain the same effect as the vanes on the interaction between the electron beam flowing along the axis A and the radio-frequency electromagnetic signals that travel on the slow-wave structure 20. The construction of the longitudinal projections 18b, made of a single piece with the casing 18, is, however, much simpler and does not entail complex assembly operations. It is possible to use extremely accurate and reliable machining techniques, such as for example EDM. The longitudinal projections 18b could in any case be obtained also with other techniques, for example by precision-milling.
Thanks to their conformation, moreover, the longitudinal projections 18b do not suffer from the intrinsic structural brittleness of the vanes, so that the risk of damage during machining is practically inexistent.
The method of fabrication is thus substantially simplified, fast, and has a high yield.
Further advantages derive from the use of short supports 22 for the slow-wave structure 20. On the one hand, in fact, there is a mechanical benefit because the action of support of the slow-wave structure 20 is more stable. On the other, also the dispersion of the heat generated in the slow-wave structure 20 is improved. The supports 22 are in fact made of dielectric material having low thermal conductivity. In the amplifier according to the invention, however, the dispersion of the thermal energy in a radial direction occurs only for a stretch in the dielectric, whereas the remaining path traverses the longitudinal projections 18b of electrically conductive material, normally a metal that has also high thermal conductivity.
The interface between the supports 22 and the longitudinal projections 18b is moreover treated by lapping and thus offers the maximum surface of heat exchange, further favouring the dispersion of heat.
Finally, it is evident that modifications and variations can be made to the amplifier and to the method described herein, without departing from the scope of the annexed claims.
In particular, the longitudinal projections and the dielectric supports could have shapes and proportions different from the ones described and illustrated herein. For example, the longitudinal projections could have the same width as the supports. Also the radial dimensions and their ratio could be different. Also the number of the longitudinal projections and of the supports could be different (for example, there could be present four longitudinal projections and as many supports).

Claims

A travelling-wave-tube amplifier comprising an amplifier tube (11) having a hollow cylindrical casing (18) made of conductive material, provided with longitudinal projections (18b) projecting for a stretch inwards;
characterized in that the longitudinal projections (18b) are made of a single piece with the casing (18).
The amplifier according to Claim 1, wherein the casing (18) is internally defined by first portions of cylindrical surface (18a), having a first radius (R1), and by second portions of cylindrical surface (18c), which delimit the longitudinal projections (18b) radially and have a second radius (R2) smaller than the first radius (R1).
The amplifier according to Claim 1 or Claim 2, wherein the longitudinal projections (18b) are spaced at uniform angular distances apart.
The amplifier according to any one of the preceding claims, comprising three longitudinal projections (18b).
The amplifier according to any one of the preceding claims, comprising a helical slow-wave structure (20), arranged within the casing (18) and coaxial thereto.
The amplifier according to Claim 5, comprising a plurality of dielectric supports (22), arranged in the amplifier tube (11) between the slow-wave structure (20) and the casing (18), for supporting the slow-wave structure (20) in a coaxial position with respect to the casing (18).
The amplifier according to Claim 6, wherein the dielectric supports (22), rest against respective longitudinal projections (18b).
The amplifier according to Claim 5 or Claim 6, comprising as many dielectric supports (22), as the longitudinal projections (18b) are.
The amplifier according to any one of Claims 6-8, wherein a ratio between a radial dimension of the longitudinal projections (18b) and a radial dimension of the supports (22) is approximately 0.5.
The amplifier according to any one of Claims 6-9, wherein the longitudinal projections (18b) have larger widths than the supports (22).
A method for the fabrication of a travelling-wave-tube amplifier comprising the steps of:
making an amplifier tube (11) having a hollow cylindrical casing (18) made of conductive material; and

providing the casing (18) with longitudinal projections (18b) projecting for a stretch inwards;

said method being characterized in that the step of providing the casing (18) with longitudinal projections (18b) comprises making the casing (18) and the longitudinal projections (18b) starting from one and the same conductive body (25).
The method according to Claim 11, wherein the step of providing the casing (18) with longitudinal projections (18b) comprises removing material from the body (25).
The method according to Claim 12, wherein the step of removing material comprises removing by electrical-discharge machining.
The method according to Claim 13, comprising the step of boring the body (25) axially prior to the step of removing material.
The method according to Claim 14, wherein the step of boring comprises making an axial cavity (26).
The method according to Claim 14 or Claim 15, wherein, after the step of boring and prior to the step of removing material, the step of polishing an internal surface of the body (25) is performed.
The method according to Claim 16, wherein the step of polishing comprises carrying out a lapping process.
The method according to any one of Claims 11-17, comprising the steps of:
making a helical slow-wave structure (20); and

housing the slow-wave structure (20) within the casing (18), so that the slow-wave structure (20) and the casing (18) will are coaxial.
The method according to Claim 18, wherein the step of housing comprises arranging a plurality of supports (22) of dielectric material between the slow-wave structure (20) and respective longitudinal projections (18b).
The method according to Claim 19, comprising the steps of:
pre-assembling the slow-wave structure (20) and the supports (22) in respective corresponding positions of use; and

introducing the slow-wave structure (20) and the supports (22) pre-assembled within the casing (18).