Velocity Modulation Tube With Frequency Multiplication For The Continuous Generation Of High Power Outputs

Abstract

The present invention relates to velocity modulation tubes operating by frequency multiplication. In order to avoid the drawbacks of the prior art, where focusing means are provided in order to give the electron beam a constant cross-section throughout the drift space defined between the resonators of the tube, the invention provides for the utilisation of that part of the beam, 170, in zone arranged between the cathode 101 and the plane of minimum section(plane p). One of the resonators 111, located adjacent the anode 102 receives the wave of frequency f.sub.1 for multiplication, and the second 112, located near the point of convergence of the beam, collects the wave at the multiplied frequency nf.sub.1 . Application to the generation of high power waves at frequencies located near the top end of the microwave spectrum.

Description

The present invention relates to velocity modulation tubes operating by frequency multiplication.

The object of the invention is constituted by high-power velocity modulation tubes operating by frequency multiplication.

The tubes in accordance with the invention are furthermore capable of continuous operation in the sense which will be defined hereinafter.

Finally, the tubes in accordance with the invention produce radio waves of a frequency located at the top end of the microwave range, that is to say some tens to some hundreds of gigacycles per second.

Velocity modulation tubes, also known as klystrons, have been used for many years now to generate very high frequency radio waves, in other words micro waves, within a frequency range within which the conventional valves, with electrodes, used up to that time could not operate.

Since their first appearance on the scene, a vast amount of literature dealing with these tubes has appeared, to which useful reference can be made. For this reason, in the present description only a brief recapitulation of the principle of their operation, in the simplest form of circuit, will be given, namely that of a two-cavity amplifier using drift grouping. An amplifier of this kind essentially comprises two electromagnetic cavities or resonators, consisting of hollow volumes delimited by a wall possessing good electrical conductivity, through both of which volumes there passes an electron beam propagating through a drift space without any electric field or equipotential between the two resonators. When a high frequency wave is injected into the cavity first to be traversed by the beam, this being the modulator or grouping cavity, a high frequency electromagnetic field develops within the volume of this cavity. Under the effect of this field, the electrons of the beam experience a modification or modulation of their velocity, which depends upon the intensity of the high frequency field at the instant at which the electrons appear at the modulator input, that is to say that there is a modification in velocity varying from one electron to the next, at the frequency of the high frequency field in the modulator. This modulation is generally of small amplitude in relation to the high velocity of the electrons at input to the grouping cavity. The electrons follow a trajectory through the drift space between the two cavities, where they drift with a substantially uniform motion, the faster ones catching up with the slower ones. The velocity modulation experienced by the electrons in the modulator is thus converted into a density modulation: at the end of the drift space, the beam exhibits a density of electrons, practically all of the same velocity varying between that of "packets" or "bunches," where said density is greater than that of the initial beam, uniform in density and velocity, and that of zones located between the "packets" or the like, where said density is less than that of the initial beam. The initially uniform beam thus, at the end of the drift space, has a component alternating at the frequency which was responsible for the formation of these "packets," that is to say the frequency of the wave injected into the modulator. On passage through the second cavity or collector, catcher etcetera, the beam induces within the volume thereof an electromagnetic field at the same frequency as said wave. The amplified high frequency wave is picked up in an element coupled to the collector.

The same tubes can equally well operate in a frequency multiplication mode, meaning that it is possible to pick up in the collector a wave of frequency nf.sub.1 produced from a wave of frequency f.sub.1 injected into the modulator, n being a whole number greater than unity. The facility to achieve multiplication stems from the fact that at the end of the drift space the electron beam in a velocity modulation tube not only has an alternating component at the frequency of the wave injected into the modulator, but also afternating components at harmonics of this frequency, as a more detailed analysis of the operation of these tubes shows.

It will be remembered that an electron-gun, whose function is that of generating the electron beam, is generally made up of several electrodes chief among which are the cathode which is the electron source and an anode which, at a positive potential in relation to the cathode, creates an electric field under the effect of which the electrons are extracted from the cathode and then accelerated through the tube. The majority of the electron guns utilised in velocity modulation tubes, produce a beam whose diameter decreases from the cathode towards a point known as the maximum convergence or point of minimum cross-section, often located beyond the anode, and then increases beyond this point in the absence of any means outside the electron-gun to make it retain its minimum cross-sectional area.

Whatever the purpose for which the tubes are to be used, and two examples of such purposes have been quoted hereinbefore, it is the generally accepted procedure in velocity modulation tube techniques to utilise the electron beam over a substantial length beyond its point of maximum convergence at the exit from the electron gun, in a zone where the cross-sectional area of the beam is approximately constant.

To maintain that minimum cross-sectional area it becomes necesssary to focus it, that is to say provide arrangements suitable for imparting to it the said quasi-constant cross-section throughout the operating zone. This focusing, generally produced in tubes whose operating frequencies are in excess of sone 10 s of gigacycles at the very least, by permanent magnets or by coils carrying a direct current in order to produce a magnetic field parallel to the acceleration field of the beam, substantially increases the weight of the tube and, for this reason, constitutes a serious drawback.

Of course, there are velocity modulation tubes in existance which operate without any focusing device. However, it has to be borne in mind that this kind of focusing becomes inevitable beyond a certain power level because, since the power of the beam and also its intensity increase in parallel with the high frequency power which is to be generated, it is impossible to prevent the electron beam in high power velocity modulation tubes from diverging rapidly beyond its point of convergence, without having recourse to focusing arrangements.

Frequency multiplication tubes cannot escape this necessity either, although generally speaking, for a variety of reasons which will not be dealt with in detail here, multipliers will be shorter than amplifiers operating at the same input frequency f.sub.1. In reality, despite this reduction in length, the difficulty is just as great because of the very small cross-sectional area which the beam must possess on passage through the extraction cavity, that is to say the collector of the foregoing examples, which, resonating at the frequency nf.sub.1, has smaller dimensions than those of the collector of an amplifier which resonates, like the grouping cavity, at the frequency f.sub.1.

It can be said in effect, that this difficulty becomes the more severe, other things being equal, the higher the beam power and the greater the beam current density, that is to say the current per unit area of the beam cross-section. However, at 100 gigacycles, the beam diameter at the level of the collector can hardly be more than 0.2 to 0.3 mm. Considering a beam the power of which is to be in the order of several hundreds of kilowatts, if it is desired, despite the mediocre efficiency of frequency multiplier tubes, to achieve a high frequency power of some kilowatts from a collector, then a power density of the order of several megawatts per square mm and a current density possibly having a magnitude measurable in thousands of amperes per cm.sup.2, are the result.

The smallness of these sizes furthermore aggravates the consequences of collision between the electrons of the beam and the walls of the tube in the case of maladjustment of the focusing, and makes the conditions imposed upon focusing even more stringent than in amplifiers. This kind of collision is dangerous by reason of the increase in temperature which it would produce in that part of the tube struck by the electrons. However, the consequences may not be so serious if the tube is one which operates with widely spaced-short-duration pulses, the total quantity of heat produced by electron input during one of these pulses being small and easily dissipated during the time interval elapsing between two consecutive pulses. On the other hand, it may have the most serious consequences, even to the point of destruction of the tube, if the latter is operated continuously or at any rate with long pulses having durations in the order of one milli-second and more. This is the kind of order which defines the aforedescribed continuous operating condition. and this may be the condition under which the tubes forming the subject of the invention operate.

In accordance with the invention, by contrast the electron beam is only utilised at its convergent portion, that is to say under conditions which require no focusing, this being so even in the case of a high power beam. In talking of operation without any focusing device, it is intended to convey that no means associated with the tube are provided in the path of the electrons, beyond said minimum cross-sectional area of the beam, in order to modify their trajectories as produced by the geometry of the electron-gun in which the electron beam is generated.

Nethertheless, it is not excluded that some focusing devices should be provided in the path of the electron beam between the cathode and the said minimum cross-sectional area towards which it converges.

By way of reminder, it is perhaps worth pointing out that these means generally employ either a magnetic field directed substantially in accordance with the direction of propagation of the beam, or electric field gradients localised at a certain number of points distributed along the length of the trajectory of the beam. The fact that it is possible to make use of the beam satisfactorily in this converging zone of the beam is due to observations made by the Applicants which have shown that the minimum beam diameters achievable at the end of this zone are perfectly compatible with good coupling between the beam and the small-sized collector resonating at the multiplied frequency nf.sub.1 without grid on said collector. This point is of course essential for the operation of the multiplier because it is ultimately the conditions of extractions of the available power at the collector, which determine the tube performance.

The modulator is arranged in the path of the beam, between the cathode and the point of convergence: the cross-sectional area of passage offered to the beam by this kind of modulator, which resonates at the frequency f.sub.1, makes it possible, without difficulty to arrange this modulator at the desired distance from the collector in order to achieve there a current density on the part of that alternating component of the beam which has a frequency nf.sub.1, corresponding to the optimum achievable in tubes of this kind.

The operation of the beam in this zone renders it unnecessary to use a focusing device and therefore avoids the aforestated drawbacks of these devices. This factor is a substantial advantage of the tubes in accordance with the invention.

As we shall see later on, in exceptional situations the electron beam can also be operated within the tubes in accordance with the invention, over a short part of its length beyond said zone, still without any focusing.

To provide a better understanding of the invention, the following description and the attached figures are used, the figures schematically illustrating a prior art velocity modulation tube, and various variant embodiments of velocity modulation tubes using frequency multiplication, in accordance with the invention.

FIG. 1 illustrates schematically and in section, a prior art velocity modulation tube. One of the objects of this figure is in particular to show the general appearance of the electron beam in a prior art velocity modulation tube.

The tube shown in the figure is a two-cavity klystron amplifier all of the elements of which are solids of revolution about the axis XX.

In this figure, there can be seen inside an evacuated enclosure 10 illustration of which has been limited to a portion thereof only, without prejudice to the manner in which this envelope is actually located in relation to the other constituent parts of the tube, a cathode 1 of spherical cap form made of an electron-emissive material or covered with a layer of electron-emissive substance and an anode 2 placed at a positive voltage V in relation to the cathode 1 by a voltage source 3.

In this figure there can also be seen a component 5 located in the neighbourhood of the cathode 1, the function of which is to cause the beam to converge at exit from the cathode, towards the axis XX. This component, which may be a focusing electrode, is placed at a negative direct voltage in relation to the cathode, by the source 6. The cathode 1, the focusing electrode 5 and the anode 2, constitute what is conventionally refered to as the electron-gun of the tube, which may also comprise other electrodes which have not been illustrated here since they are not essential to the considerations which follow.

Under the joint action of the anode 2 and the focusing electrode 5, the cathode, when subjected to the conditions required for emission, produces a beam of electrons 7 (area covered with thin lines) converging towards the axis XX of the tube and directed towards the right, considering the figure. The focusing device 8 located beyond the anode and producing a magnetic field which is constant with time and directed towards the axis XX, gives the beam a virtually constant cross-sectional area over a large part of its trajectory towards an electron collector which is at a positive potential in relation to the cathode and marked 4 in the figure. This electron collector is connected, like the anode 2, to the positive terminal of the source 3 in the example shown in the figure.

T and T', in FIG. 1, represent the trajectories of two electrons located at the periphery of the beam; the trajectories of all the other electrons of the beam, some of which have been partially illustrated, are comprised within the volume whose section through the plane of the figure, is limited by the cathode and these two trajectories.

Thus, in the trajectory of the beam between the cathode 1 and the electron collector 4, three zones can be distinguished, namely the zone 70 roughly delimited to the right of the cathode by the anode 2 in the example of the figure, where the beam issuing from the cathode converges to a minimum section, followed by a zone 71 along which the focusing elements 8 are located and where the beam has a cylindrical form with a section roughly equal to the preceding section, followed in turn by a zone 72 in which the beam diverges towards the electron collector 4.

As the figure shows, in the zone 70 the electron beam does not converge towards a "point" although in the foregoing description mention has been made of a point of convergence. As those skilled in the art will appreciate, in other words, in electron-gun design a beam never converges strictly to a point instead its section simply diminishing to a minimum value which, in the case of the figure, is located at the end of the zone 70.

In prior art velocity modulation tubes, it is in the zone 71 that, by a mechanism of drift through the field-free space 9, conversion of the velocity modulation imparted to the electrons of the beam by the modulator 11, into density modulation at the collector 12 takes place. The two resonators 11 and 12, each arranged at one of the ends of the drift space 9, have been represented in the figure in the form which they generally exhibit, that is to say cylinders with a re-entrant profile, fitted with grids 15 and 17 which provides the coupling between the beam and the said resonators. References 14 and 16 respectively represent, within the amplifier klystron shown in the figure, the means used to couple the modulator 11 to the source supplying the wave which is to be amplified, and the means used to couple the collector 12 with the load picking up the amplified wave.

The field-free space 9 in the figure is generally the internal space within a tube 13 made of an electrically conductive material placed at a positive potential in relation to the cathode, in this case connected to the positive terminal of the source 3. Frequently of course, in the technology of velocity modulation tubes, the anode 2, the electron collector 4, the tube 13 and the resonators 11 and 12 are an integral part of a single mechanical component so that a single connection suffices to establish the potential conditions defined hereinbefore.

FIG. 2 illustrates in schematic section a velocity modulation tube of frequency multiplication kind, in accordance with the invention. As in the case of FIG. 1, this is a tube all the constituent elements of which are solids of revolution about the axis XX, with the exception of the coupling wave guides associated with the resonators. In FIG. 2, as in the following figures, only that half of the tube located above the axis XX has been shown.

In FIG. 2, within the evacuated closure 10, there can again be seen, albeit with slightly different shapes, a certain number of the elements of FIG. 1 including the cathode 101, the anode 102, the focusing electrode 105, the resonators 111 and 112 having no grids, the tube 113 and the trajectory T of a peripheral electron of the beam. From this figure, it will be seen that the electron beam 107, which is not subjected to the action of any focusing element but controlled simply by the electrodes of the tube, has a different shape to that encountered in the prior art tubes, of which FIG. 1 provides an example. In the beam, there are now only two zones to distinguish, the first 170 in which the beam converges until it reaches a minimum section of radius b, in the plane p perpendicular to the plane of the figure, and the second 172 where it diverges rapidly on its way towards the electron collector which has not been shown.

The resonators 111 and 112 are located in the trajectory of the electron beam as indicated in the figure, that is to say the former at the level of the anode 2 of which, in the particular example chosen, it is an integral part, and the latter at the level of the point where the beam has the minimum section of radius b.

From a source which has not been illustrated, the resonator 111 receives the wave of frequency f.sub.1 through the medium of the coupling element 114, coupling the resonator 111 with said source, and velocity modulates the electrons of the beam. The conversion of the velocity modulation to density modulation takes place along the drift space inside the tube 113. At the level of the plane p, the electron beam current has an alternating component of frequency nf.sub.1 which induces in the resonator 112 a wave of frequency nf.sub.1.

The coupling element 116 directs this wave to a load (not shown) in the manner indicated by the arrow.

The coupling elements 114 and 116, in the example in question, are waveguide sections, the cross-section through which has been illustrated in the plane of the figure.

As already pointed out, the applicants have been able to show that this component occurs under conditions which make possible satisfactory coupling thereof with the resonator 112 in the manner explained hereinafter.

This coupling can be satisfactorily ensured using resonators having no grids to limit high frequency electric field. In contradistinction with the prior art, shown in FIG. 1, where these grids are referenced in 15 and 17, in the high power frequency multiplication tube in accordance with the invention, such grids are at variance with the density of the electron beam due to the overheating that could occur.

The efficiency with which high frequency power of frequency nf.sub.1 is picked off in the collector 112, can be described roughly by the expression:

.eta. .noteq. M.sup.. (In/2Io) .sup.. .alpha. (1)

where M indicates the coupling factor between the electron beam and the collector, (In/2Io) the standarised alternating component of the beam current at frequency nf.sub.1, the first degree n.sup.th order Bessel function of which Jn (nr) represents a good approximation, and .alpha. is a coefficient expressing the condition of non-reflection of the electrons on passage into the collector, that is to say the limit which must not be exceeded by the voltage induced by the beam across the collector terminals, if the slowest electrons of the beam are not to be reflected back in the opposite direction to the direction of propagation of the beam. At very high frequencies, (beyond 10 or so gigacycles per second) this limit is further reduced because of the need to avoid the risk of arcing between the opposite walls of the collector in the re-entrant zone thereof, that traversed by the beam, under the effect of the high frequency electromagnetic field prevailing there. This explains the low value of .alpha. in the following example. In this formula, r represents the degree of grouping which is the product of the modulation depth and the length of the drift tube measured in terms of transit angle.

For a more precise explanation of these quantities, recourse should be had to the theory of velocity modulation tubes, numerous expositions of which are to be found in literature on the subject as already pointed out.

It will simply be recalled here that for n = 5, that is to say for a frequency multiplication by 5, J.sub.5 (5r) is a maximum for r = 1.28, this maximum being 0.37. In fact r is < 1.28 and J.sub.n (nr) may be taken equal to 0.30 for example.

The formula (1), under these circumstances, for frequency multiplication by a factor of 5, becomes:

.eta. .noteq. M.sup.. O.sup.. 3 .alpha.

In an example corresponding to the tube shown in the diagram of FIG. 2, for which the characteristics have also been indicated, M .noteq. 0.6 and .alpha. .noteq. 0.05 were found.

The characteristics of the tube were:

f.sub.1 = 14 GHz; nf.sub.1 = 70 GHz; anode voltage V (in relation to the cathode) = 130 kV; beam current: 4.7 amperes; beam radius (plane p) at the location of minimum section: b = 0.26 mm; radius of resonator orifice (112) a = 0.51 nm; length of the drift tube (113) between the planes P and p (FIG. 2): 20 mm; high frequency voltage across the terminals of the collector (112): 5.2 kV; high frequency power picked off in the load coupled to the collector (112): 5600 W; losses in the collector (112): 225 W; efficiency .about. 90 percent.

FIGS. 3, 4 and 5 illustrate variant embodiments of the velocity modulation tubes in accordance with the invention, in which:

FIG. 3: a third resonator 200 is arranged in the trajectory of the beam, beyond the collector. The resonator 111 receives the wave of frequency f.sub.1 from the resonator 200 to which it is coupled by the device 201. In this case the tube operates as a self excited oscillator.

FIG. 4: a resonator 300 resonating at a frequency intermediate between f.sub.1 and nf.sub.1 is arranged in the trajectory of the beam, between the resonators 111 and 112.

FIG. 5: the electron collector 104, insulated from the anode 102, is "depressed", i.e. placed at a potential in relation to said cathode, which is less than that of the anode 102.

FIG. 6 is a variant embodiment differing from that of FIG. 2 in that the resonator 112 is a dual resonator formed by two identical attached portions 120, 121 along the wall 122, coupled with one another through an opening 123 in the wall.

Of course, the invention is not limited to the embodiment described and shown which was given solely by way of example

Claims

What is claimed is:

1. A high power velocity modulation tube, operating by frequency multiplication, for the production of a high frequency radio wave at a frequency nf.sub.1 from a high frequency radio wave of frequency f.sub.1, n being an integer greater than unity, said tube comprising:

an electron-gun and associated means for producing an electron beam and accelerating it towards a collector,

means provided to make the beam converge at its output from the cathode of the electron gun towards a minimum cross-section,

at least two electromagnetic high-frequency resonators, arranged in the path of the beam, through which the beam passes, separated by a space along which the electrons of the beam propagate with constant velocity, one of said resonators known as the first resonator, resonating at the frequency f.sub.1 and velocity modulating the electrons of the beam, being located close to the cathode of said electron gun whilst one of the other of said resonators known as the second resonator, is arranged adjacent the said miminum cross-section of the beam, the aperture of said second resonator through which the beam passes being of dimensions which are small enough to provide coupling of said second resonator with the beam in the absence of grids in said resonator in the path of the beam, said second resonator which resonates at the frequency nf.sub.1, picking up the high frequency wave generated at frequency nf.sub.1 and directing it towards a load coupled to said resonator.

2. A velocity modulation tube according to claim 1, wherein said said second resonator is a dual resonator constituted by two portions similar one to one another, one of which is coupled to said load and which are attached along a common wall and coupled with one another through an opening in the wall.

3. A velocity modulation tube according to claim 1, wherein said first resonator receives the power at frequency f.sub.1, from a third electromagnetic resonator resonating at this frequency and arranged in the trajectory of the beam beyond the second resonator, and connected furthermore by coupling means to said first resonator.

4. A velocity modulation tube according to claim 1, comprising one or more further electromagnetic resonators arranged between the first and second resonators and traversed by the beam, these resonators resonating at a harmonic of the frequency f.sub.1, which harmonic has a frequency somewhere between the frequencies f.sub.1 and nf.sub.1.