EP0124395B1 - Electron gun for microwave generators - Google Patents

Description

The present invention relates to an electron gun for generators of radio waves for microwaves. It relates more particularly to an electron gun providing an electron beam propagating along cycloidal paths intended for use in microwave generators of the masers type with cyclotron resonance.

In generators called cyclotronic resonance masers such as gyrotrons, a beam of electrons coming from an electron gun propagates along helical paths while being guided by a uniform magnetic field directed along the axis of the helix. The beam then crosses an electromagnetic cavity resonating at a frequency f _o close to a multiple of the cyclotronic frequency, cavity in which the transverse speed components of the electrons interact with a transverse electric field component of the wave to give them their energy. . In this case, the beam propagates essentially parallel to the magnetic field. Now the interaction taking place with the transverse velocity component vL of the electrons, the parallel velocity component v // therefore corresponds to an unused energy. We therefore seek to eliminate this parallel speed by proposing a new type of cyclotronic resonance masers using the same interaction between the electrons rotating in a magnetic field and a resonant cavity as that used in the masers of the prior art but characterized by the fact that the speed of the electrons parallel to the magnetic field is zero or substantially zero throughout the maser while there is a drift speed perpendicular to the magnetic field due to a continuous electric field prevailing in the electron gun and the resonant structure.

In this case, however, electron guns of the type comprising a conical cathode, and a conical axial anode subjected to an axial magnetic field, used in cyclotron resonance masers such as gyrotrons, are not suitable.

However, we also know from French patent 984020, a traveling wave tube. In this case, the cathode sends an electron beam between a delay line and an electrode brought to a negative potential compared to the delay line, the electrons yielding their energy to the electromagnetic wave propagating in the delay line. . In this case, the beam follows a cycloidal trajectory but at a turning point with an invariable amplitude. On the other hand, the propeller receives a progressively decreasing pitch in the direction of the beam in order to reduce the speed of the wave as the speed of the beam decreases.

The object of the present invention is to provide a new type of electron gun capable of providing an electron beam propagating along a cycloidal trajectory in a transverse magnetic field under the effect of a drift speed due to a continuous electric field. with specific characteristics.

It therefore relates to an electron gun for generators of radio waves for microwave frequencies of the type comprising a so-called sole electrode being at a negative or zero potential and an anode, produced by plates facing each other and brought to two different potentials of so as to create between them a continuous electric field as well as a cathode positioned in the extension of the hearth and brought to the same potential as the latter, the cathode injecting an electron beam into the space between the hearth and the anode, the assembly being subjected to a magnetic field transverse to the directions of the beam and of the electric field characterized in that, in the plane perpendicular to the magnetic field, at least one of the electrodes has a divergent profile relative to the other electrode so the distance between electrodes is increasing from the catode to the outside of the barrel so as to provide an electron beam propagating along a cyclic path oïdal whose arches tighten under the effect of a speed of drift of the electrons lower than the speed of rotation.

• - la figure 1 est une vue en coupe axiale schématique, dans le plan perpendiculaire au champ magnétique, d'un premier mode de réalisation d'un canon à électrons conforme à la présente invention;
• - la figure 2 est une vue semblable à celle de figure 1 d'un deuxième mode de réalisation d'un canon à électrons conforme à la présente invention;
• - la figure 3 est une vue semblable à celle de figure 1 d'un troisième mode de réalisation d'un canon à électrons conforme à la présente invention;
• - la figure 4 représente schématiquement la trajectoire d'un électron soumis à un champ électrique E_c continu et à un champ magnétique B.

Other characteristics and advantages of the present invention will appear on reading the description of various embodiments made below with reference to the attached drawing in which:

• - Figure 1 is a schematic axial sectional view, in the plane perpendicular to the magnetic field, of a first embodiment of an electron gun according to the present invention;
• - Figure 2 is a view similar to that of Figure 1 of a second embodiment of an electron gun according to the present invention;
• - Figure 3 is a view similar to that of Figure 1 of a third embodiment of an electron gun according to the present invention;
• - Figure 4 shows schematically the trajectory of an electron subjected to a continuous electric field E _c and a magnetic field B.

In the figures, the same references designate the same elements but for reasons of clarity, the dimensions and proportions have not been respected.

As shown in FIG. 1, the electron gun according to the present invention is constituted by a cathode 1 and by two electrodes 2, 3 brought to different potentials so as to create between these two electrodes 2, 3 a continuous electric field E _c . More specifically, the electrode 2 called the sole is brought to a negative or zero potential while the electrode 3 called the anode is brought to a positive potential V. On the other hand, the cathode 1 is located in the extension of the sole 2 and is brought to the same potential as this electrode. This cathode 1 comprises for example a filament 4 connected to a voltage so as to obtain, during the heating of the cathode, the emission of electrons.

According to the present invention, one of the electrodes, namely the anode 3 in the embodiment of Figure 1, has a curved profile so that the distance between electrodes 2, 3 is increasing from the cathode 1 towards the outside of the barrel, namely in the direction x. The two electrodes can be produced using copper plates, one of which has been suitably profiled. As explained below in more detail, preferably, the profile of the curved electrode is chosen so that the angle a formed by said profile with the median plane of the electrodes is such that the distance between the two electrodes varies little over a length corresponding to twice the radius of Larmor r _L. On the other hand, the whole of the electron gun is subjected to a uniform magnetic field B perpendicular to the plane of the figure, of which a few lines of force are represented by crosses. This magnetic field is created, for example, from two superconductive coils positioned on either side of the electrodes 2, 3 according to the Helmholz control. For the purpose of simplification, these coils have not been shown in the drawing.

As explained in more detail below, under the combined action of the DC electric field E _vs progressively decreasing from the cathode towards the outside of the barrel as a result of the profile of the electrodes, and of the transverse and uniform magnetic field B, the electrons emitted by cathode 1 are brought to follow a cycloidal trajectory 5 with a drift speed v _d such than

Considérons alors une vitesse v_d telle que

We know indeed that a mobile electron in a magnetic field B and subjected to the action of an electric field E undergoes a force F given by the Lorentz formula, namely the following vector equation:

Then consider a speed v _d such that

So in a new reference system relating to v _d , the electron is only subjected to the magnetic field B and its trajectory is therefore a circle. This reasoning carried out in classical mechanics remains valid in relativistic mechanics, in particular in the case where v ² / _c 2≤1.

• avec e = la charge de l'électron
• m = la masse de l'électron

et d'une vitesse de dérive _Vd donnée par l'équation vectorielle

comme représenté sur la figure 4.Consequently, when an electron is subjected to an electric field E and to a magnetic field B, the trajectory of an electron is substantially equivalent to the superposition of a rotation with an angular speed given by the equation

• with e = the charge of the electron
• m = mass of the electron

and a drift speed _Vd given by the vector equation

as shown in figure 4.

• avec V: la différence de potentiel entre des électrodes 2, 3
• Comme _Vd = v_r

In addition, in the case of the present invention since the electrons come from a cathode 1 without having an injection speed, the speed of rotation is equal to the drift speed and the following equations are obtained:

• with V: the potential difference between electrodes 2, 3
• As _Vd = v _r

Normally, according to this equation if E _c decreases r _L decreases accordingly since B is constant in the invention. However, E _c being a continuous electric field it is shown that if the distance d between the two electrodes varies little over the length corresponding to a rotation, then we are in a regime called "adiabatic" for which the moment of the electron m = k B r _L ² = Cste. In this case, the radius of Larmor r _L remains constant. As the speed of drift v _d decreases at the same time as the electric field E _c , the electrons move on more and more tightened circles as represented on figure 1, equations (1) and (2) remaining valid locally.

We therefore obtain with the electron gun of the present invention, the cycloidal trajectory required by the new type of cyclotronic maser.

In addition, if we compare this system with the axial injection system currently used in gyrotrons, we see that the drift speed _Vd plays an identical role to the parallel speed in said axial system.

2 à l'extrémité aval du canon, il faudra réduire v_d dans un rapport 2 le long de la trajectoire du canon, ce que l'on obtiendra en augmentant progressivement la distance entre les électrodes 2, 3 dans un rapport 2.If, for example, we want to obtain the relation

2 at the downstream end of the barrel, it will be necessary to reduce v _d in a ratio 2 along the trajectory of the barrel, which will be obtained by gradually increasing the distance between the electrodes 2, 3 in a ratio 2.

Or l'énergie cinétique continue dans le mouvement de rotation

E_co étant le champ électrique devant la cathode.On the other hand the anode 3 must be placed above the top of the electron trajectory. It follows that the minimum potential Vmin is given by the equation

Now the kinetic energy continues in the rotational movement

E _co being the electric field in front of the cathode.

As a result, the anode 3 must therefore have, relative to the cathode, a potential at least 4 times greater than the energy of rotation of the electrons which is usable in the cyclotronic interaction.

FIG. 2 shows another embodiment of the electron gun of the present invention. In this embodiment the sole 2 'has a curved profile symmetrical to that of the anode 3 with respect to the median plane. This particular shape gives a drift in a constant direction along the median plane.

FIG. 3 represents an alternative embodiment of FIGS. 1 and 2. In this case, the cathode 1 is positioned in the extension of the sole 2 but in a plane forming an angle between -45 ° and -180 ° relative to the plane sole 2. The anode is then extended by a curved profile so as to cover the cathode. In this case, the starting speed of the electrons is in the opposite direction to the drift speed, which makes it possible to reduce the potential difference to be applied between the anode 3 and the cathode 1.

• - On peut modifier la vitesse de dérive des électrons sans modifier le champ magnétique B mais en modifiant seulement le champ électrique E_c.
• - Les électrons ne peuvent pas revenir dans la région du canon s'ils sont réfléchis en aval par le reste du dispositif, car la vitesse de dérive V_d est indépendante en grandeur et en signe de la forme de la trajectoire.
• - La dimension des électrodes et de la cathode selon le champ magnétique B n'est pas limitée. Il en résulte que l'on peut produire des courants électroniques très élevés avec ce type de canon.

The electron guns described above have a number of advantages.

• - One can modify the speed of drift of the electrons without modifying the magnetic field B but by modifying only the electric field E _c .
• - The electrons cannot return to the region of the barrel if they are reflected downstream by the rest of the device, because the speed of drift V _d is independent in magnitude and as a sign of the shape of the trajectory.
• - The size of the electrodes and the cathode according to the magnetic field B is not limited. As a result, very high electronic currents can be produced with this type of gun.

In addition, the electron guns according to the present invention can be used not only in the new types of cyclotron resonance masers mentioned in the introduction but also in microwave tubes requiring injection of an electron beam according to a cycloidal trajectory.

Claims (4)

1. An electron gun for microwave generators, comprising a base elektrode (2) carrying a negative or zero potential, and an anode (3), these electrodes being constituted by plates facing each other and carrying distinct potentials such that a continous electric field is established there-between, and comprising a cathode (1) located in the prolongation of the base electrode (2) and carrying the same potential as the latter, the cathode injecting an electron beam into the interspace between the base electrode and the anode, the unit being submitted to a magnetic field which is transversal to the directions of the beam and of the electric field, characterized in that at least one (3) of the electrodes presents a diverging profile with regard to the other electrode such that the distance between the electrodes increases from the cathode (1) to the outside of the gun, in order to obtain an electron beam which propagates along a cycloid trajectory, the arches thereof becoming narrower under the effect of an electron drift speed which is lower than the rotation speed.

2. An electron gun according to claim 1, characterized in that the base electrode (2) and the anode (3) are symmetrically arranged with respect to their middle plane.

3. An electron gun according to anyone of claims 1 to 2, characterized in that the product between the potential difference AV to which the two electrodes (2,3; 2'(3) are submitted, and the electron charge e is at least four times larger than the rotation energy Wr of the electrons, expressed in electron volts.

4. An electron gun according to anyone of claims 1 and 2, characterized in that the anode (3) is located above the base electrode and the cathode (1) is positioned in prolongation of the base electrode (2) downwards along a plane making an angle of between -45° and -180° with regard to the plane of the base electrode (2) such that an initial speed is communicated to the electrons which is opposed to the drift speed.

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