FR2658950A1 - FREQUENCY TUNABLE MICROWAVE TUBE. - Google Patents

Abstract

The present invention relates to a microwave tube constructed around an axis XX 'comprising at least one longitudinal electron beam (31) passing through at least one cavity (30) provided with a frequency tuning device. The tuning device comprises at least one rod (35), substantially parallel to the axis XX 'plunging into the cavity (30). The rod (35) is movable along the axis XX 'and is actuated from outside the tube. Applications to single or multibeam klystrons.

Description

TUNABLE MICROWAVE TUBE

FREQUENCY

  The present invention relates to microwave tubes with longitudinal electron beams passing through at least one frequency tunable cavity. It applies more particularly to klystrons whether they are single-beam or multi-beam. A single-beam klystron is built around an axis It mainly comprises an electron gun which produces a longitudinal electron beam This beam crosses successive cavities and sliding tubes A sliding tube connects two successive cavities The cavities are used to modulate the speed of the electrons The electron beam is collected in a collector which is arranged in the extension of the last cavity A focusing device surrounds the cavities It prevents the beam

of electrons to diverge.

  The frequency matching of the cavities of a klystron is necessary to optimize the performance of the tube at the time of the tests. Indeed, a certain number of parameters of the tube depend on the frequency shifts of the cavities by

  compared to others and the operating frequency of the tube.

  These parameters are for example the gain of the tube, its

  yield or its instantaneous bandwidth.

  Between the time when the cavities are assembled and the time when the tube is being tested, numerous mechanical and thermal operations have taken place and the frequency agreements may have changed. These operations are, for example, mechanical resumptions, baking, brazing, etc. On the other hand, the electron beam, the microwave energy output device etc. are not strictly identical to those initially planned, and a variation of the resonance frequencies of the cavities allows a

some catching up.

  In certain uses, it is necessary to be able to change the operating frequency of the tube; this is particularly the case in television transmitters, air traffic control radars or in telecommunications systems. It is then desirable for the tube to be fitted with a system for adjusting, on site, the frequency of

resonance of its cavities.

  Multibeam klystrons are well known in the prior art and French patents N '992 853, N 2 596 198 and No

2,596,199 describe them.

  A multibeam klystron can be produced by placing several electron guns on a ring centered on an axis These guns produce elementary beams parallel to this axis The elementary beams pass through successive cavities, separated by sliding tubes Each cavity is crossed by all elementary beams The beams can be collected in a common collector The focusing device can also be common to all the beams. Multibeam klystrons are developing more and more because they make it possible to obtain a compact, high-performance tube

  while using a low accelerating voltage.

  In single-beam klystrons, these three requirements are contradictory. Indeed, a high efficiency can only be obtained with a beam of low pervency, that is to say with a high accelerating voltage. In addition, the length of the

  tube believes as the square root of the acceleration voltage.

  The cavities of the klystrons whether they are single-beam or multi-beam are generally of simple shape They are often cylindrical or parallelipiped They have on two opposite walls, orifices also opposite, to allow the beams to pass of electrons The sliding tubes which connect two successive cavities have their ends which penetrate into the cavities by these orifices They then create protuberances inside the cavities The space between two protuberances opposite

the interaction space.

  The resonant frequency of a cavity is proportional to the product: (L C) L and C represent respectively the self and the equivalent capacity of the cavity These parameters are a function of the

geometry of the cavity.

  To vary the resonance frequency of a cavity, you can either modify L and make a selfic tuning device, or modify C and make a tuning device

capacitive, or combine the two.

  A self-tuning device modifies the position of one or more walls of the cavity In general, a piston and membrane device is used which deforms a side wall of the cavity, substantially parallel to the axis of the tube. from the outside of the tube thanks to a

  control mechanism and this mechanism is quite bulky.

  A focusing device surrounds the cavities It is composed of a set of coils In order not to increase either the volume of the tube or its weight, we arrange so that the focusing device is very close to the side wall of the cavities L space is reduced to insert the control mechanism of the piston and diaphragm tuning device, between

  the cavities and the focusing device.

  In the case of multibeam klystrons another disadvantage appears The cavities of multibeam klystrons are generally cylindrical and their diameter is much greater than their height The beams are arranged on a circle of small diameter compared to that of the cavity, to create in the region central cavity, a strong electric field If you do not want to use a too bulky tuning device, it only acts on a small portion of the side wall and then has little influence on the

cavity volume.

  In the capacitive tuning devices, the interaction space between two sliding tubes is varied opposite or else a capacitance is created by means of a pallet that is approached or that the 'we move away from a sliding tube. The pallet is actuated from the outside of the tube and it is moved transversely to the axis of the tube. We find the same drawback as before because of the reduced space between

  the outside of the cavity and the focusing device.

  In multibeam klystrons, a palette system causes asymmetry of the electric fields in the interaction spaces. This asymmetry causes defocusing,

  oscillations and the appearance of parasitic modes.

  The present invention aims to remedy these drawbacks by proposing a microwave tube with at least one frequency tunable cavity. The frequency tuning device does not disturb the operation of the tube and neither increases nor

volume or weight of the tube.

  The present invention provides a microwave tube constructed around an axis XX 'comprising at least one longitudinal electron beam passing through at least one cavity provided with a frequency tuning device characterized in that: the tuning device comprises at least one rod substantially parallel to the axis XX 'plunging into the cavity, the rod being

  movable along the axis and being actuated from the outside of the tube.

  The rod of the tuning device can be actuated by a control mechanism comprising: a shaft external to the tube, substantially parallel to the axis XX ', actuated in rotation, causing in its movement a first pinion integral with a transmission device penetrating inside the tube, at least a second pinion, inside the tube, driven in rotation by the transmission device, a threaded rod, substantially parallel to the axis XX ', integral with the second pinion, screwed to the inside the stem of the tuning device, the stem of the tuning device moving in translation by sliding in a guide piece. The tuning device may include one or more rods. Preferably, the number of rods is equal to the number of electron beams or either to a submultiple or to a multiple of the number of electron beams Preferably, the end of a rod, inside of the cavity, will be rounded Preferably, when there are several rods they will be distributed regularly on a circle centered on the axis

  XX ', this circle surrounding the electron beam (s).

  According to a variant, when there are several rods, their end inside the cavity will be fixed on a single crown, the rods then being actuated simultaneously The rods and / or the crown may be made of metal, in

copper, for example.

  The rods and / or the crown may be made of a dielectric material, of alumina or of beryllium oxide, by

example.

  Other characteristics and advantages of the invention

  will emerge on reading the following description, given to

  by way of nonlimiting examples and illustrated by the appended figures which represent: FIG. 1, in longitudinal section, a cavity of a single-beam klystron, provided with a selfic tuning device; Figure 2, in cross section, a cavity of a multibeam klystron, provided with a capacitive tuning device; Figure 3, in longitudinal section, a cavity provided with a tuning device of a multibeam klystron according to the invention; Figure 4, a cross section along the axis AA 'of the cavity of Figure 3; FIG. 5, a graph showing the variation of the frequency of a cavity, as a function of the sinking of one or more rods; Figure 6, in longitudinal section, a variant of a cavity provided with a tuning device of a multibeam klystron according to the invention; Figure 7, a cross section along the axis BB 'of the

Figure 6 cavity.

  In these figures the same elements bear the same references. Figure 1 shows a cavity with a self ique tuning device This cavity belongs to a single-beam klystron built around an axis XX 'It is assumed that the cavity has the shape of a rectangular parallelepiped with two opposite transverse walls 1 to the axis XX 'and four walls 2 opposite two by two, parallel to the axis XX' The electron beam 5 passes right through the cavity The two transverse walls 1 are each provided with a sliding tube 3 The sliding tubes face each other and they

  each form a protuberance inside the cavity.

  A focusing device 8, composed of a set of coils, surrounds the cavities of the klystron The coils are

  placed as close as possible to the walls 2 of the cavity.

  The tuning device 4 is a piston 6 and membrane tuning device 7 The membrane 7 partially or completely replaces at least one of the walls 2 parallel to the axis XX 'The piston 6 is actuated from the outside of the tube, substantially transverse to the axis XX ', thanks to an appropriate control mechanism This mechanism is bulky and is placed outside the cavity, between the latter and the focusing device 8 The piston drives in its movement the membrane 7 The displacement of the membrane 7 makes it possible to increase or decrease the volume of the cavity Consequently, the frequency of the cavity varies A bellows device is provided to maintain the vacuum tightness of the cavity relative to the outside the tube This device is not

not shown.

  To house the control mechanism, a larger and heavier Locating device 8 should be used than that which would have been used in the absence of the tuning device. This tuning device is ill-suited to the cavities of multibeam klystrons Indeed, the cavities of a multibeam klystron have large dimensions and the electron beams are grouped in their central part The deformation of part of the wall of the cavity has little influence on the frequency For the deformation to be effective, it would be necessary to use a control mechanism still

more cumbersome.

  FIG. 2 represents, in cross section, a cavity provided with a tuning device 25 of a multibeam klystron The cavity is cylindrical The klystron comprises six electron beams 21 distributed on a circle centered on the axis of the cylinder Les bundles 21 emerge from a sliding tube 22 entering the cavity 20 and penetrate into another sliding tube 22 leaving the cavity 20 Inside the cavity, for each beam 21, two sliding tubes 22 are face each other They are separated by an interaction space The circle on which the beams are distributed has a diameter much smaller than that of the cylinder The device

  25 is a capacitive pallet tuner 23.

  The pallet 23 is actuated from the outside of the tube It can move away from or approach a sliding tube by moving substantially radially A bellows device 24 makes it possible to maintain the tightness of the interior of the cavity Localization device has not been shown This tuning device is effective for varying the resonance frequency of the cavity On the other hand, it disturbs the electric fields in the interaction spaces which are close to it and little or no fields electric in the interaction spaces which are distant from it This disturbance leads to defocusing, oscillations and the appearance of parasitic modes. FIG. 3 represents a cavity 30 provided with a frequency tuning device, according to the invention This cavity 30 is part of a klystron with six electron beams 31 Only two of the beams 31 have been represented Another cavity 40, not fitted with a tuning device, follows cavity 30 The two cavities 30,40 are separated by a space

  free 39 The cavity 40 is only partially shown.

  The beams 31 are distributed regularly on a circle centered on an axis XX 'They pass right through the cavities 30.40 Sliding tubes 32 connect two successive cavities, 40 They each contain an electron beam 31 A sliding tube penetrates on one side into a cavity and on the other into the next cavity and this forms protuberances 36 inside the cavity. FIG. 3 shows in cavity 30 provided with the tuning device, for each electron beam 31, two protrusions 36 facing each other An interaction space separates two protrusions 36

opposite.

  The cavities 30, 40 are cylindrical and preferably identical. The cavity 30 has a side wall 34 and two

  walls 33 substantially transverse to the axis XX ', opposite.

  The two walls 33 carry the sliding tubes 32 They are traversed by the electron beams 31 A focusing device 42 surrounds the cavities 30.40 It comprises a set of coils 41 producing a magnetic flux used to

  avoid the divergence of the beams.

  The frequency tuning device is a capacitive device It comprises at least one rod 35 substantially parallel to the axis XX 'The rod plunges inside the cavity 30 by passing through one of the walls 33 The rod is movable the

along axis XX '.

  The end 38 of each rod 35, inside the cavity, is preferably rounded to reduce the risk of electric arcs. The rod 35 is actuated from the outside of the tube by means of an appropriate control mechanism. The control mechanism can, for example, transform a rotational movement into a translational movement. A shaft 43 located outside of the tube This shaft 43 is parallel to the axis XX ′ of the tube The shaft 43 is integral with a first pinion 44 which drives in rotation a transmission element 45 such as a Galle chain The transmission element 45 penetrates at

  inside the tube passing between two coils 41.

  Inside the tube, the transmission element 45 rotates at least one other pinion 46 The pinion 46 is integral with a threaded rod 47 substantially parallel to the axis XX 'Each threaded rod 47 is associated with a rod 35 of the tuning device The threaded rod 47 is screwed into the rod 35

  of the tuning device, at its other end 48.

  This end 48 is located outside the cavity 30.

  The end 48 comprises a foot 52, in the form of a disc, for example, of diameter greater than that of the rod 35 The foot 52 slides in a hollow guide piece 49 This guide piece 49 may be cylindrical It is integral with a side

of the wall 33 of the cavity 30.

  The foot 52 and the part 49 each comprise a device which prevents the rod 35 from moving in rotation. This device is produced, for example by a stud 50 on the foot 52 and a groove 51-hollowed out in the internal wall of the part. guide 49 The pin 50 slides in the groove 51 The rod 35 of the tuning device can only move in translation when the threaded rod 47 moves in rotation A bellows device 37 seals the interior of the cavity 30 It is located, for example, inside the guide piece 49 and it surrounds the rod 35 It can be fixed in a sealed manner, for example by welding, on the one hand on the wall 33 of the cavity 30 and on the other hand on foot 52 of the

rod 35.

  Figure 4 is a cross section along the axis AA 'of the cavity 30 provided with the frequency tuning device The two figures are not on the same scale Three rods are shown 35 The transmission element 45 transmits the movement

simultaneously with the three rods 35.

  Three gears 46 and three threaded rods 47 are then used.

  Even if the control mechanism is large, we can

  place it in the space 39 separating the two cavities 30.40.

  The focusing device 42 can remain close to the side wall 34 of the cavity 30 The volume and the weight of the tube

are not increased.

  Preferably, the number of rods will be equal to the number of electron beams or either to a submultiple, or to a multiple, of the number of beams The cross section of a rod

  could be circular or have another shape.

  When using several rods, preferably, they

  will distribute regularly on a circle centered on the XX 'axis.

  The purpose of this condition is to disturb the symmetry of the electric fields within the interaction spaces as little as possible. In the case of multibeam klystrons, the rods are preferably arranged in a space between the protrusions 36 and the side wall 34 to facilitate their mounting. We can activate the rods, one after the other or all together, or combine the two possibilities

previous.

  A rod 35 can be made either of metal or of a dielectric material. Copper can be used in one case, in the other case, for example, alumina will be chosen.

  low losses or beryllium oxide.

  It could be envisaged to use one or more rods crossing a transverse wall 33 and one or more rods crossing the other transverse wall 33 vis-à-vis, it suffices

that they do not meet.

  We will now see the variation of the resonant frequency of the cavity as a function of the position of one or more rods 35 The more a rod is pressed the more the frequency decreases The effect of the rod is felt on the frequency

  before the rod reaches the interaction space.

  We gradually push a rod 35 inside the cavity We stop before the end 38 of the rod

  touches the wall 33 opposite that which it crosses.

  The variation of the resonance frequency of the cavity is represented on graph I. The graph It shows the variation of the resonance frequency of the same cavity when a second is pressed

  rod, the first rod remaining pressed down as far as possible.

  Graph III shows the variation of the resonant frequency of the same cavity when a third is pressed

  rod, the first two remaining pressed down to the maximum.

  The dotted graph shows the variation of the resonance frequency of the same cavity when the three rods are pressed simultaneously The frequency variation is faster If the rods are actuated simultaneously, when they are pressed at the maximum the frequency obtained is substantially equal to that obtained when the

stems one after the other.

  These measurements were carried out with a cylindrical cavity of height 57 mm in diameter: 175 mm

and rods with a diameter of 3 mm.

  The measurements show that it is possible to obtain a frequency variation of the order of 10 to 15% without appearance

parasitic mode.

  The measurements also show that the R / Q factor of the cavity is very little affected by the tuning device according to the invention The R / Q factor of a cavity characterizes the coupling of the cavity with the electron beam (s) and therefore by

  consequently influences the gain and the yield of the tube.

  In the case of a multibeam klystron, the measurements of the factor R / Q made in all the interaction spaces gave values which are substantially constant. electric fields in spaces

of interaction.

  Figures 6 and 7 show respectively in longitudinal and transverse section, a multibeam klystron cavity provided with a variant of the tuning device. These figures have not shown either the control mechanism of the tuning device, nor the focusing device, this

for the sake of clarity.

  The main difference between Figures 3, 4 and 6, 7 is located at the end 68 of the rods 65 inside the cavity 30 Now we have fixed the end 68 of the rods 65 on a crown 61 This crown moves at the same time as the rods 65 and the latter are actuated simultaneously The measurements show that the variation of the frequency of the cavity is substantially the same with or without crown It can be seen that the QO overvoltage coefficient of the cavity is more low It is around 100 while it

  could go up to 600 and even 1000 without crown.

  But the main advantage of this variant is that we have pushed back the limits of appearance of electric arcs at the end of the rods. Indeed, the electric fields are distributed over the whole crown instead of accumulating on

the ends 68 of the rods.

  The advantage of this device is to allow the tube a

  operation at higher peak powers.

  The crown can be made either of metal or of a dielectric material. It is possible, for example, to use copper,

  alumina or beryllium oxide.

  The stems and the crown can be either in the same

  material, either in different materials.

Claims (9)

  1 microwave tube constructed around an axis XX 'comprising at least one longitudinal electron beam (31) passing through at least one cavity (30) provided with a frequency tuning device characterized in that: the agreement comprises at least one rod (35) substantially parallel to the axis XX 'plunging into the cavity (30), the rod (35) being movable along the axis XX' and being operated from outside the tube.   2 microwave tube according to claim 1 characterized in that the rod (35) of the tuning device is actuated by a control mechanism comprising: a shaft (43) external to the tube, substantially parallel to the axis XX ', actuated in rotation, causing in its movement a first pinion (44) integral with a transmission device (45) penetrating inside the tube, at least a second pinion (46), inside the tube, driven in rotation by the device transmission (45), a threaded rod (47) substantially parallel to the axis XX ', integral with the second pinion (46), screwing inside the rod (35) of the tuning device, the rod (35 ) of the tuning device moving in translation, sliding in a guide piece (49).   3 microwave tube according to one of claims 1   or 2 characterized in that the number of rods (35) is equal to the number of electron beams (31) or either to a submultiple or to a multiple of the number of electron beams (31).   4 microwave tube according to one of claims 1   3 characterized in that the rod (35) has one end   (38) rounded inside the cavity (30).   Microwave tube according to one of claims 1   4 characterized in that, when there are several rods (35), they are distributed regularly on a circle centered on the axis XX ', this circle surrounding the beam or beams of electrons (31).   6 microwave tube according to one of claims 1   5 comprising several rods (65) characterized in that all the rods (65) have one end (68) inside the cavity (31) which is fixed to a single crown (61), the   rods (35) being actuated simultaneously.   7 microwave tube according to one of claims 1   to 6 characterized in that the rod (s) (35,65) and / or the crown (61) are made of metal.   8 microwave tube according to claim 7 characterized in that the rod (s) (35,65) and / or the crown (61) are made of copper.   9 microwave tube according to one of claims 1   to 8 characterized in that the rod (s) (35,65) and / or the   crown (61) are made of dielectric material.   Microwave tube according to claim 9 characterized in that the rod (s) (35,65) and / or the crown   (61) are made of alumina or beryllium oxide.   11 Microwave tube according to one of claims 1 to   characterized in that it is a single-beam or multi-beam klystron.