EP0013242A1 - Generator for very high frequency electromagnetic waves - Google Patents

Abstract

An electron beam (1) is subjected to a magnetic field B constant in time directed along the axis XX, along which it propagates, and to the electromagnetic field of resonant volumes placed on its path. These volumes are excited at the cyclotronic frequency fc of the electrons in the B field by a source coupled by the antenna (5). A power on a frequency close to a multiple nfc of the cyclotronic frequency is collected in the load (8) coupled by (6). In the example, the volumes consist of a single guide (20) whose circular section (2) has been deformed according to (3) and (4); the electron beam comes from the barrel (19, 12, 14). Application: generators of radio waves of power over a few tens of gigahertz, for radars in particular.

Description

The invention relates to a radio wave generator for the microwave domain. It relates more particularly to a generator operating at the top of this field, namely over a few tens of gigahertz, that is to say in millimeter and submillimetric waves.

Various types of electronic tubes intended for this use are known from the prior art and, in particular, those in which the electron beam progresses along an axis along which it is subjected to the action of a uniform magnetic field, constant in time, directed along this axis, and that of a high frequency electric field directed transversely to the previous one and whose frequency is equal to the cyclotron frequency of the electrons in the magnetic field in question.

In tubes of this type, the electrons are produced by a device which imparts to them a speed component directed transversely to this axis. This device is generally an electron gun whose cathode has the shape of a ring and produces a hollow cylindrical beam.

As for the high frequency electric field, it consists of the electric component of the electromagnetic field prevailing inside resonant volumes placed on the beam path, all along it, and coupled to the latter.

Under these conditions, the electrons progress along the axis on spiral trajectories and its capable on the last part of their path to give up radioelectric energy on the frequency of the electromagnetic field or on a multiple of it thanks to the high frequency alternating components formed within the beam in the first part of the path. The radioelectric energy produced on this frequency is collected in one or more charges coupled to the last resonant volume.

In tubes of this type of the prior art, energy is supplied to the electrons exclusively by the continuous source which accelerates them; no other energy source is present in the system.

This type of tube has been the subject in recent years of developments on which we will find information in the communication by V.A. FLYAGIN, A.V. GAPONOV, M.I. PETELIN, V.K. JULPATOV "The Gyrotron" Second International Conference and Winter School on Submillimeter Waves and their Applications Dec. 6-11, 1976 - Puerto-Rico.

, e et m représentant respectivement la charge et la masse de l'électron au repos.These tubes are therefore characterized by high values of the continuous acceleration voltage applied to the beam to give it a high level of desired energy. They are also characterized by high intensity magnetic fields. We know that the intensity of the magnetic field B and the cyclotronic frequency f (ω _c = 2 πf _c ) are two proportional quantities: ω _c =

, e and m respectively representing the charge and the mass of the electron at rest.

However, the application of high DC voltages comes up against isolation difficulties in the case of these wavelengths, where the circuits are very small. As for the application of the necessary magnetic fields, it generally leads , if we want to be able to reach the necessary values, which exceed the maximum values sands under normal conditions, to use superconductive circuits, which we know that the realization and implementation are difficult.

The subject of the invention is a millimeter wave generator of the type to which reference has been made above, using a longitudinal magnetic field and a high frequency electric field whose lines of force are arranged transversely to it. ci, to reduce the difficulties reported.

For this purpose, the generator of the invention is divided into two successive sections along the axis. In the first, that by which between the beam, the resonant volumes have a resonant frequency equal to the cyclotronic frequency of the electrons in the magnetic field B. In addition, these volumes are fed at high frequency, by a wave at the cyclotronic frequency f. In the second section, which resonates at a multiple or harmonic frequency nf of the latter (n being the rank of the harmonic), has leu the energy sampling. By this arrangement, part of their energy is communicated to the electrons by the high frequency electric field prevailing in the first section. It is thus possible, all other things being equal, to reduce the DC voltage applied to the beam and to reduce the difficulties resulting from this application.

The generator of the invention therefore appears as a system with two sections, one, accelerator, in which a high frequency field on the frequency f communicates energy to the electrons, and the other, collector, from which is taken a part of the energy of these electrons. The device of the invention is, in other words, as a generator on the frequency nf to which a low frequency accelerator f has been incorporated. The advantage of transferring energy to the electron beam at this low frequency lies in the fact that these transfers generally have a higher efficiency at low frequency.

The applied magnetic field has an intensity corresponding to the cyclotronic frequency f _c and, therefore, is also reduced compared to that which the frequency nf would require.

Finally, it is no longer necessary in the generators of the invention to provide, as in the prior art, an arrangement of the electron gun making it possible to confer on the electrons a transverse speed, namely, as has been said, a ring cathode barrel. This transverse speed is given to them, in the generator of the invention, by the high frequency field prevailing in the first section.

These possibilities constitute advantages of the invention compared to the prior art.

As will be seen, the resonant volumes of the two sections can, in the context of the invention, be integral parts of a single resonant enclosure.

• - figure 1 : des exemples de sections de guides d'ondes utilisées dans les générateurs de l'invention ;
• - figure 2 : une vue générale schématique d'un générateur de l'invention ;
• - figure 3 : un schéma d'une autre variante du générateur de l'invention.

The invention will be better understood on reading the description which follows and the attached figures which represent:

• - Figure 1: examples of waveguide sections used in the generators of the invention;
• - Figure 2: a schematic general view of a generator of the invention;
• - Figure 3: a diagram of another variant of the generator of the invention.

It appears that in generators of this type, using a magnetic field and an electric field of high frequency crossed, the operation is effectively possible on a harmonic of the cyclotronic frequency of the particles, of pulsation w _c , according to the previous notations. For this operation to have sufficient efficiency, it is however necessary that the high frequency field has, as has been said, a large amplitude on the harmonic in question.

In a variant of the invention, the single resonant volume is chosen so as to present space harmonics of large amplitude on the desired operating frequency. To this end, a resonant volume is used, for example, a waveguide of the type known at microwave, resonating on the cyclotron frequency, and the cross section of which has been deformed so as to favor the presence of these harmonics in the configuration. of the electromagnetic field that prevails there.

Such a guide is therefore of the type of one of those used in microwave; it has a regular section whose dimensions are large relative to the wavelength of the wave to be generated.

It allows the use of a cylindrical beam easy to produce, propagating along its axis, along and in the vicinity of which, given the dimensions of the guide, it should be noted that the fields are of small amplitude.

But thanks to the deformations of this section of the guide used in the invention, it is possible to locate the lines of force of the space harmonics so that their amplitude in the region of the beam has a sufficient value for a efficient interaction between the electron beam and these harmonics.

An example of these deformations is given below in the case of originally circular guides. This example is given without limitation, to fix ideas, it being understood that other forms of guides, and in particular guides with rectangular section, would offer similar possibilities.

Figure 1 (a, b, c,) shows some of the shapes with high amplitude space harmonics on the frequency nf in the case of a circular guide: the lines with the arrows represent the lines of force of the field electric with a high value component on harmonics 3 and 5 in TE ₁₀ mode.

The beam propagates in this guide, under the action of a high continuous voltage applied between the cathode by which it is produced and an anode placed in front. As we said, this high voltage provides it with part of its energy, longitudinal, the other, transverse, being supplied to it by the high frequency electric field prevailing in the waveguide in which it propagates beyond. of this anode, a guide which is itself at the voltage of this anode, with which it forms an equipotential space into which the beam is introduced by various means known in the art, and which will not be mentioned. He describes there, in the operating conditions, a spiral trajectory whose radius increases as the beam progresses and it acquires energy. This trajectory follows a generally conical surface, of revolution around the axis of the system, the direction of which coincides with that of the magnetic field. tick. It can be likened to a series of successive circular turns, whose radius increases, roughly, linearly as a function of the abscissa on the axis, and each described in a time equal to the cyclotronic period in field B.

This trajectory must remain entirely within the waveguide.

We compared below the minimum radius that must present, in TE ₁₀ mode, the guide used to be able to operate at the cyclotronic pulsation ω _c , that is to say the value of this radius corresponding to the cut-off at this frequency, and the radius r of the trajectory of the electrons at their maximum energy.

, Wo représentant l'énergie de l'électron au repos etIt was admitted, in this evaluation, that all the energy of the electrons is conferred on them by the high frequency electric field, which is an ideal hypothesis made for the sole purpose of allowing this evaluation; an energy transmitted to the electrons by the high frequency field of 50 keV above their energy at rest, that is to say an increase of about one tenth of the latter, has also been assumed. We have under these conditions

, Wo representing the energy of the electron at rest and

de la vitesse, ici entièrement transversale, de l'électron, à celle de la lumière . On a

.W its energy gain on its trajectory; t represents the relativistic factor, equal to the ratio

of the speed, here entirely transverse, of the electron, to that of light. We have

.

We have related these radii a and r to the wavelength λ _o corresponding to this harmonic.

est égal à 1,238, alors que celui correspondant au rayon a, c'est-à-dire 2π

vaut 1,841.As an example for n = 3, the ratio 2 π

is equal to 1.238, while that corresponding to radius a, i.e. 2π

is worth 1,841.

The radius of the guide is therefore much larger than the maximum radius of the path. The guide is then deformed to obtain the space harmonics on the pulsation nω _c in it.

The wave generator of this variant of the invention is presented according to the general diagram of FIG. 2.

An electron beam 1 is directed along the axis XX of a waveguide 20 whose section 2, circular in the example has the two extensions, of rectangular section, 3 and 4, diametrically opposite. These lateral volumes preferentially guide a harmonic of the frequency of the guide in TE ₁₀ mode; the field lines of the electrical component on the mode in question are represented by the arrows.

A magnetic field B (arrow) is directed longitudinally along the axis XX of the guide. An oscillator excites the guide at the pulsation ω _c , will equal the cyclotronic pulsation of the electrons of the frisceau in the magnetic field B. This oscillator 7 is coupled to the guide by the antenna 5, which has been schematized by its loop. A second antenna, shown diagrammatically at 6, makes it possible to collect the power generated in the guide at the frequency nω _c . In the figure, the start of the path of the beam 1 has been shown within the limits of the drawing, showing the first turns thereof; antenna 6, placed at the level of the last of them, should be placed further away, as will be seen in a numerical example. In the example, the electron beam is produced by a gun which comprises a cathode 10, circular, a Pierce electrode 12, and an anode 14 accelerating the beam.

Under these conditions, the electrons yield high frequency energy to a load 8 coupled to the output antenna 6. The energy which they receive in continuous and high frequency form places them in relativistic conditions, that is to say ie such that their variation in mass following the increase in their energy in the accelerating section causes a variation in their phase with respect to the electromagnetic field; at these speeds it is found that the moving electron is capable of yielding energy to a high frequency electromagnetic field. This is so for the values of the pulsation, or angular velocity, w _s , of the electrons included in a certain range around the pulsation of the electromagnetic field with which they are interacting. This can lead, in the generators of the invention, to using a magnetic field whose intensity varies with the abscissa along the axis XX.

The generator of the invention appears as a high power frequency multiplier.

This leads to underline, to be rigorous, that there is not exactly multiplication of the cyclotronic pulsation w _c by the preceding factor n, but multiplication by a factor slightly different from n, because the condition w _s = nω _it is not strictly completed.

Below are given three examples of operating characteristics of the device of the invention.

A first example concerns the pulse operation of the generator of the invention. This is shown as shown in Figure 2. The cylindrical waveguide has in its central part a radius of about 5 mm and two diametric extensions trally opposite, rectangular, and proportioned as in the example in this figure. An ordinary type gun provides a beam of 1 amp, accelerated under 10 kilovolts by the anode 14.

The oscillator is a magnetron operating in pulses at a frequency of 16 gigahertz; it excites the guide with a power of 60 kilowatts, in which a field is established whose lines of force in TE ₁₀ mode are those of the arrows in solid line. The value of the magnetic field is 0.6 tesla; the electron beam describes, under these conditions, around the axis XX of the system, a spiral situated on a generally conical surface, widening in the direction of propagation. It is modulated along its trajectory, and the modulated current has components at frequencies nx 16 gigahertz. The lateral extensions preferentially guide one of these frequencies, the frequency of 80 GHz in particular, in the same mode as the fundamental frequency. The maximum energy it reaches is 60 kilovolts after 10 periods. At 16 GHz, the guide length required is approximately 4 centimeters, which corresponds to a consumed power of 3 kw for a guide having an overvoltage of 800, or 5% of the power communicated to the electrons. Bundles are created within the cylindrical electron beam whose diameter is 1.2 mm, while the radius of their orbit is 1.35 mm. The current component at the harmonic. 5 is, without other focusing means, about 0.21 I _o , I _o being the beam current. The output power is 300 kW.

The other two examples relate to continuous operation of the generator of the invention. The oscillator used at high frequency excitation is here a klystron operating at 10 GHz. In the table below are given the characteristics corresponding to two different levels of excitation.

As in the previous example, the generator structure can be that of FIG. 2, using one and the same resonant volume, the waveguide, for the excitation frequency and its harmonic.

However, it is also possible to use two successive cavities aligned along the magnetic field, for excitation and then for drawing energy from the harmonic, these cavities operating in TM ₁₀ mode, with suitable connection, between the two cavities.

This case is shown schematically in Figure 3.

In the example of this figure, the electron beam passes into the first resonant volume, or cavity, 40, supplied at high frequency by a Jclystron, and the energy is drawn from the harmonic frequency in a second cavity 60 , separated from the first, 40, by an adaptation device 70. The beam is produced by the accelerator BO.

It will be noted that the adaptation section, represented at 70 in FIG. 3, could include a device for injecting a signal to be amplified at frequency nω _c In this case, it would include a resonant element coupled to the device for injecting the signal.

We have described in the foregoing a structure of the generator of the invention with a cylindrical waveguide using an electron beam also cylindrical. The generator of the invention can also be produced with a flat beam, having a rectangular section, and a waveguide whose section has the same shape, and whose width can reach up to 1.5 times the length d λo wave. In this case, the beam is thin and wide and allows high applied powers.

Finally, the beam can be supplied by a cathode and accelerated by an anode at the entrance to the microwave part, as in the example in FIG. 2. It can also be produced in a separate installation, before it enters the guide. waves or in the cavities of the generator, that is to say in the microwave part; such an installation is for example a betatron, a storage ring etc. (figure 3).

The generator of the invention has the same applications as the generators of the prior art for millimeter waves, namely measurement in plasma installations, radar transmission, telecommunications, etc.

Claims (5)

1. Generator of radio waves for microwave using an electron beam propagating along an axis, subjected to a magnetic field directed along this axis and to the electromagnetic field of resonant volumes arranged along this axis characterized in that it comprises , coupled to these resonant volumes, a wave source at a frequency equal to the cyclotronic frequency f _c of the beam electrons in the magnetic field, and a charge from which an energy is taken from a frequency close to a multiple nf of this cyclotronic frequency. 2. Radio wave generator according to claim 1, characterized in that these resonant volumes consist of a single cylindrical guide whose section is deformed so as to have two extensions, of rectangular section, diametrically opposite, and coupled to the source at one of its ends and the load at the other. 3. Radio wave generator according to claim 1, characterized in that the resonant volumes consist of two separate resonant chambers, crossed successively by the electron beam, the first, coupled to the source, resonating on the cyclotron frequency, and the second, coupled to the load, resonating on the neighboring frequency in question. 4. Generator of radio waves according to claim 1, characterized in that the resonant volumes consist of three separate resonant chambers, crossed successively by the beam of electrons, the first, coupled to the source, resonating on the cyclotron frequency, the third, coupled to the load, resonating on the neighboring frequency in question, and the second, coupled to a generator of a radioelectric signal to be amplified, on the neighboring frequency in question. 5. Generator of radio waves according to claim 1, characterized in that the magnetic field has a value which varies along the axis of propagation.

Images