FR2695755A1 - Electronic tube with radial structure. - Google Patents

Abstract

The present invention relates to an electron vacuum tube comprising a cathode (1) emitting electrons towards a collector (10), focusing means (7) of the electrons to concentrate them in a beam (13) and an output resonant cavity ( 8) coupled to the beam to take energy from the focused beam. The cathode has a symmetry of revolution about an axis. The beam is radial and is focused in a plane normal to the axis of symmetry of the cathode. Application to power electronic tubes with improved service life.

Description

ELECTRONIC TUBE WITH RADIAL STRUCTURE

  The present invention relates to vacuum electronic tubes used in particular as power amplifiers

in the UHF band.

  Vacuum electronic tubes commonly used as power amplifiers are of two types: the first type comprises electron beam amplitude modulation tubes and the second type comprises electron tubes.

  speed modulation of the electron beam.

  Amplitude modulated tubes are for example triodes or tetrodes while electron beam velocity modulation tubes are klystrons or tubes

traveling wave.

  In television, in the UHF band, the tetrode type grating tubes operate at the upper limit of their frequency band with a gain of the order of 15 dB and a yield of the order of 50% in common amplification. considered is the ratio of the output power during the synchronization pulse on the average power supplied to the transmitter in standard G. The klystron type tubes, for example, are characterized by a high gain of the order of 40 d B and by a low efficiency of the order of 25% by taking the same criteria. Work begun in the 1930s has been reactivated in recent years to develop an amplifier tube that would theoretically achieve a gain between that of klystrons and that of tetrodes and a similar performance as that of tetrodes through operation in class B This type of tube is known under the name of klystrode and described in the book "Microwave tubes" of AS Gilmour, Jr page 196 or under the name of IOT (of the English name Inductive Output Tube), see in particular in the THIOMSON-CSF Technical Review, Volume 23, N 04, December

  1991, page 810 In the rest of the description, this type of tube

  is called IOT An IOT has an axial electron beam.

  It uses as input the principle of amplitude modulation and output the axial structure of the speed modulation tubes. This tube comprises an electron gun with a cathode, an anode and a modulation grid. A modulation voltage is applied between the gate and the cathode thanks to an input resonant cavity tuned to a desired frequency. The electrons generated by the emerging cathode of the grid in packets and converge on the axis of the beam The beam then passes through a resonant cavity of exit Les

  Beam electrons yield their energy to the output cavity.

  This energy is extracted by coupling and directed to a user device such as an antenna. The electrons are

  collected in a collector downstream of the outlet cavity.

  This tube has an axial structure like the klystron and not a radial structure like the tetrode. This axial structure as well as the nature of the materials used limit

  important way VIOT performances.

  The cathode used in IO Ts as in klystrons is generally porous tungsten impregnated with aluminates barium This cathode operates around 1020 OCA this temperature, the barium evaporates and is deposited on the grid which in turn becomes emissive The emitted electron beam is disturbed and the life of the tube is greatly reduced This lifetime can be of the order of 600 hours while one would expect a

  lifetime of around 25,000 hours.

  To try to overcome these disadvantages related to the emissivity of the gate, we can reduce the operating temperature of the cathode but then limit the current density emitted and consequently the power of the IOT If we want to increase the current density emitted it would increase the surface of the cathode The cathode and the grid are hemispherical When the grid is large, then we observe a non-uniform temperature of its bars: they are much hotter in the center than in the periphery because they cool by conduction The hot part of the gate emits, the electron beam is then disturbed and the service life is reduced All proposed solutions each have their disadvantages and they lead

  in any case to a limitation of the power of the IOT.

  The present invention proposes to make a vacuum electronic tube which can operate with a lifetime

  satisfactory at high power.

  This tube instead of having an axial structure has a radial structure The electron beam emitted is no longer

  linear but in the form of a radial plane layer.

  The tube according to the invention comprises a cathode emitting electrons to a collector, means for focusing the electrons and an output cavity coupled to the

  beam to collect electromagnetic energy.

  The cathode is generally rotationally symmetrical about an axis, the electron beam is radial and is focused by the focusing means in a plane substantially normal to the axis of symmetry of the cathode.

  output is coaxial with the cathode.

  The cathode may be cylindrical or in portion of torus It is advantageously in thoriated tungsten A grid surrounds the cathode to modulate the emission of electrons The cathode-grid space is part of a resonant cavity modulation

  wherein a modulation voltage is injected.

  The focusing means may be poloidal coils or permanent magnets They are located on either side of the plane of the beam The collector is mounted coaxially around the axis of the cathode It can be depressed A series of normal radial fins at the plane of the beam can be mounted between the cathode and the first cavity

  resonant reached by the beam.

  Another series of fins may be provided before the collector The fins of a series are preferably in odd number Means are provided to avoid a collision between the electrons and the fins At least one of the resonant cavities may be coupled to a cavity auxiliary. One or more resonant cavities can be

tunable in frequency.

  Other features and benefits of this

  invention will appear on reading the description made

  Hereinafter, illustrated by the accompanying drawings which show: Figure 1, a schematic view of a tube according to the invention; Figure 2, a variant of a tube according to the invention In these figures, the same elements have the same references For reasons of clarity, the ribs are not respected. Figure 1 schematically shows a tube

electronic device according to the invention.

  This tube has a symmetry of revolution about an axis XX 'It is seen, mounted coaxially around the axis XX', a

  cathode 1 which emits electrons and around a grid 2.

  The space between the cathode 1 and the gate 2 is part of a resonant modulation cavity 3 A modulation voltage 5 is applied in the resonant modulation cavity 3 In general, the resonant modulation cavity 3 is dimensioned in X / 4 or in 3 X / 4 (X represents the resonant wavelength of the cavity) A tuning piston 4 can make it possible to tune the resonant modulation cavity 3 to a desired frequency. This piston 4 is located globally at a node circuit voltage

resonant thus formed.

  The cathode 1 can be made with a thoriated tungsten mesh heated directly or indirectly. The grid 2 can be in pyrolytic graphite. These elements are comparable to those used in conventional tetrodes. The pollution of the grid observed in the IO Ts is

thus eliminated.

  The gate 2 and the cathode 1 may be conventionally cylindrical. To facilitate the focusing of the emitted electron beam, it is possible to envisage that the cathode 1 and the gate 2 have a concave toroidal portion shape. This variant is represented in FIG. to use a cylindrical cathode or toroidal portion to achieve a cathode

  large area and therefore produce a power tube.

  Cathode 1 and gate 2 are generally worn

at a high negative voltage.

  The cathode 1 emits electrons radially relative to the axis XX 'At the output of the gate 2, the electrons grouped into packets form a radial beam 13 This beam 13 is attracted to an anode 6 brought to a less negative potential than the The anode 6 is formed of two rings located on either side of the plane of the electron beam Focusing means are provided so that the beam is concentrated in a plane normal to the axis XX 'of the

cathode.

  These focusing means are firstly the electrostatic optics of the grid. To improve the concentration of the electrons, it is possible to provide poloidal coils or permanent magnets on either side of the plane of the electron beam. The thickness of the beam measured along the axis XX 'is smaller than the emissive height of the

cathode 1.

  The electron beam 13 then passes through a resonant Coaxial cavity 8 output The two rings of the anode 6

  form a sliding space 14 which enters the cavity 8.

  The interior of the cavity 8 is coupled to the electron beam 13 through annular coupling openings 9. These openings, two in number, are located on either side of the plane of the electron beam 13. the output cavity are decelerated and are collected in a collector 10 coaxial with the axis XX ', in the form of a torus, for example, this collector 10 will preferably be cooled, for example by

  forced ventilation or by circulation of a fluid.

  The output cavity 8 resonates on a frequency that can be adjusted by means of a tuning device 11. In FIG. 1, there are two mobile pistons 11 parallel to the plane of the beam located on either side of the plane of the plane. electron beam The exit cavity 8 is preferably dimensioned in X / 2, that is to say that the pistons are spaced 1/2 and are located on a voltage node The electron beam is located there to a belly of tension There is then a coupling as good as possible between the cavity and the beam. The electron beam transfers energy to the outlet cavity 8 and this energy is extracted by appropriate means. It can be a pallet 12 as in FIG. 1. This energy is transmitted to a user device such as

than an antenna for example.

  It can be envisaged, with a view to increasing the bandwidth of the tube, that the output cavity 8 is coupled to an auxiliary cavity 20. The coupling between the two cavities can be capacitive, as in FIG.

  2, the energy is extracted at the auxiliary cavity 20.

  The auxiliary cavity will preferably be dimensioned in X / 2 and the extraction of the energy will be done near a belly of

  voltage if the output coupling is capacitive.

  It may be desirable to stiffen the mechanical structure of the tube and to limit the birth of parasitic oscillations in guided mode to divide the space between the cathode 1 and the outlet cavity 9 by radial fins 22 substantially normal to the plane of the beam Their number will preferably be odd, for example three, five or more. In order for these fins 22 not to be bombarded by the electrons of the beam 13, it will be arranged for the cathode 1 to have non-emissive zones in front of the radial fins. it is advantageous to also place radial fins 23 between the outlet cavity 8 and the collector 10 These fins 23 are preferably aligned with those between the cathode 1 and the outlet cavity 8 These fins 22, 23 are advantageously made of metal. It is possible to provide, as in FIG. 2, an electrically insulating part 24 for mechanically holding the anode 6 and the gate 2 while electrically insulating them. The anode 6 and the outlet cavity 8 are generally brought to a ground. is here conical and can be ceramic.

  The inside of the tube is conventionally vacuum.

  Sealing can be ensured inside the outlet cavity by two annular windows located on either side of the plane of the electron beam 13.

  let the electromagnetic energy pass but not the air.

  The collector 10 may be of the depressed type. This means that it is brought to an intermediate potential between the potential of the outlet cavity 8 and the potential of the cathode 1. Annular ceramic pieces 26 are provided for

  electrically isolating the collector 10 from the outlet cavity 8.

  By reducing the potential of the collector relative to that of the outlet cavity, the speed of the electrons entering the collector and therefore the density of heat to be discharged by the collector are reduced.

wall of the collector.

  The tube according to the invention can provide a power

  important because the cathode can have a large emitting surface.

  The fact of being able to use a thoriated tungsten cathode makes it possible to eliminate the pollution problems of the

  grid, existing problems with impregnated cathodes.

  The electron beam instead of being long and thin substantially cylindrical as in the klystrons and IO Ts is now radial, substantially disk-shaped Electrons generated by the cathode diverging in the plane of the disk The electron density decreases in s' away from the cathode Because of the low density beam the coupling with the outlet cavity is improved and the collector is subjected to a reduced power density The collector can reliably operate with air cooling IOT type coaxial tubes used in television transmitters have a collector whose surface is limited This collector can be cooled by circulating air The density of thermal power to be dissipated being very important it is

  produces degassing that affects the life of the tube.

  The coaxial collector of the tube according to the invention has a much larger surface and the reliability of the tube is increased even

with air cooling.

Claims (17)

  A vacuum electron tube comprising a cathode (1) emitting electrons to a collector (10), means for focusing (7) the electrons to concentrate them in a beam (13), a resonant cavity (8) coupled to the output beam for taking energy from the focused beam, characterized in that the cathode (1) has a symmetry of revolution about an axis, the beam (13) being radial and being focused by the focusing means (7) in a map   normal to the axis of symmetry of the cathode.   2 electron tube according to claim 1,   characterized in that the cathode (1) is of thoriated tungsten.   3 electron tube according to one of claims 1   or 2, characterized in that the cathode (1) is cylindrical or has a form in portion of torus.   4 electronic tube according to one of claims 1   at 3, characterized in that the resonant cavity (8) is of coaxial structure.   Electronic tube according to one of claims 1   4, characterized in that the cathode (1) is surrounded by a   gate (2) for modulating the emission of electrons.   6 electron tube according to claim 5, characterized in that the space between the cathode (1) and the gate (2) is part of a resonant cavity (3) modulation in   which is injected a voltage (5) of modulation.   Electronic tube according to one of claims 1   to 6, characterized in that the focusing means (7) are   located on both sides of the plane of the beam.   8 electron tube according to one of claims 1   7, characterized in that the focusing means (7) are   poloidal coils or permanent magnets.   9 electronic tube according to one of claims 1   to 8, characterized in that the collector (10) is coaxial with the cathode.   Electronic tube according to one of claims 1   at 9, characterized in that the collector (10) is depressed.   11 electronic tube according to one of claims 1   10, characterized in that a series of radial fins (22), normal to the plane of the electron beam is arranged between the   cathode (1) and the resonant cavity (8) output.   Electronic tube according to one of claims 1   to 11, characterized in that another series of fins (23)   radial, normal to the plane of the beam is disposed between the resonant output cavity (8) and the collector (10).   Electronic tube according to one of the claims   11 or 12, characterized in that the fins of a series are in odd number.   Electronic tube according to one of the claims   11 to 13, characterized in that means are provided for   avoid collisions between the electrons and the fins.   Electronic tube according to one of claims 1   at 14, characterized in that at least one of the resonant cavities   (8) is coupled to an auxiliary resonant cavity (20).   Electronic tube according to one of claims 1   15, characterized in that a frequency tuning device (11) is provided for frequency tuning at least one of the cavities (8) resonating.   Electronic tube according to one of claims 1   at 16, characterized in that the resonant output cavity is sized at X / 2 (where X is the resonance wavelength in the cavity).   Electronic tube according to one of claims 1   at 17, characterized in that the resonant modulation cavity (3) is dimensioned in X / 4 or in 3 X / 4 (X being the length   resonance wave in the cavity).