EP0049198B1 - Electrons accelerator, and millimeter and infra-millimeter waves generator including the same - Google Patents

Description

The present invention relates to an electron accelerator which can be used in a millimeter and infra-millimeter wave generator. It also relates to generators comprising such accelerators.

Infra-millimeter wave generators such as free electron lasers are known, in particular by the article by L.R. Elias et al, published in 1976, in the review "Phisical review letters", volume 36, pages 717 et seq.

In these free electron lasers, an electron beam which moves in a direction Oz with a speed V _z close to that c of light is periodically accelerated in the direction transverse to Oz.

These periodic transverse accelerations are generally obtained by establishing either a helical magnetic field, of pitch P and of axis Oz, or two fields transverse to the axis Oz, of opposite directions and periodically distributed in space with the same period P .

• - d'une part, la fréquence _P du rayonnement émis selon l'axe Oz par les électrons, périodiquement accélérés dans une direction transversale à Oz, est inversement proportionnelle à la période P puisqu'elle s'écrit:
  
  On a donc intérêt pour augmenter la fréquence à choisir P aussi faible que possible;
• - d'autre part, la puissance rayonnée par les électrons est proportionnelle au carré des accélérations transversales. Pour avoir des accélérations importantes, il faut disposer de champs magnétiques de grande intensité. Pour créer ces champs magnétiques, la période P doit être importante afin qu'il soit matériellement possible de loger les conducteurs qui créent ces champs magnétiques. On a donc intérêt pour augmenter la puissance rayonnée à choisir P aussi élevée que possible.

The problem which arises with these lasers is that two contradictory requirements control the value of the period P of the transverse accelerations:

• - on the one hand, the frequency _P of the radiation emitted along the Oz axis by the electrons, periodically accelerated in a direction transverse to Oz, is inversely proportional to the period P since it is written:
  
  It is therefore advantageous to increase the frequency to be chosen P as low as possible;
• - on the other hand, the power radiated by the electrons is proportional to the square of the transverse accelerations. To have significant accelerations, it is necessary to have magnetic fields of great intensity. To create these magnetic fields, the period P must be long so that it is physically possible to house the conductors which create these magnetic fields. It is therefore advantageous to increase the radiated power to choose P as high as possible.

These lasers make it possible to simultaneously obtain frequencies of a few tens of gigaherz and an alternating magnetic field of a few amplitude teslas only on condition of considerably increasing their length, which is a drawback. In addition, their efficiency is low and the radiated power remains low.

The present invention relates to an electron accelerator and a millimeter and infra-millimeter wave generator comprising this accelerator which are of different design from what is known in the prior art.

The generator according to the present invention makes it possible to simultaneously obtain a high frequency and a radiated power, while maintaining dimensions similar to those of standard electronic tubes.

Thus with a tube of 1 to 2 m according to Oz, one can reach frequencies of approximately 300 GHz with a beam of 2 to 3 MeV.

The efficiency of this generator is high, of the order of 50%, and with a current of about 10 mA in the cathode-anode bias circuit, a radiated power of 7.5 KW is obtained.

In addition, another advantage of this generator is that it does not require too high direct voltage (around 200 KV between the anode and the cathode) and that the value of this direct voltage can vary over a wide range.

The present invention relates to an electron accelerator comprising in a vacuum enclosure, an electron gun which produces an electron beam propagating along an Oz axis, said electron accelerator further comprising a coil which surrounds the enclosure and produces a magnetic field oriented along the axis, characterized in that it includes inside the enclosure, a delay line, supplied by a high frequency generator, this delay line allowing the establishment of a field longitudinal high frequency electric oriented along the Oz axis, in that the intensity of the magnetic field increases at the level of the delay line and in that the electron gun produces electrons whose speeds have a first component oriented according to the axis and corresponding to the Newtonian regime, as well as a non-zero transverse component.

The present invention also relates to a millimeter and infra-millimeter wave generator which comprises an electron accelerator according to the invention.

. ° aIn this generator, the electron beam enters a resonant cavity tuned to the frequency F _M corresponding to a slightly higher pulsation _MM

. ° a

The coil which surrounds the vacuum enclosure at the level of the resonant cavity creates a uniform magnetic field along the Oz axis.

The electron accelerator according to the present invention can be used as will be seen in the following description in the millimeter and infra-millimeter wave generators.

It can also be used in devices other than these generators.

As regards the generator according to the invention, it has the same applications as the generators of the prior art for millimeter and infra-millimeter waves, namely radar emission, measurement in plasma installations, isotipic separation ...

• La figure 1, la répartition des champs magnétiques et une trajectoire électronique dans les lasers à électrons libres selon l'art antérieur;
• Les figures 2 et 3, la trajectoire suivie par un électron en deux points du générateur selon l'invention;
• Les figures 4 et 5, une vue longitudinale suivant l'axe Oz et une vue transversale selon le plan F de la figure 4 d'un mode de réalisation du générateur selon l'invention.

Other objects, characteristics and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures which represent:

• Figure 1, the distribution of magnetic fields and an electronic trajectory in free electron lasers according to the prior art;
• Figures 2 and 3, the path followed by an electron at two points of the generator according to the invention;
• Figures 4 and 5, a longitudinal view along the axis Oz and a transverse view along the plane F of Figure 4 of an embodiment of the generator according to the invention.

In the different figures, the same references designate the same elements, but, for reasons of clarity, the dimensions and proportions of the various elements are not observed.

FIG. 1 represents the distribution of the magnetic fields and an electronic trajectory in the lasers with free electrons according to the prior art which has been previously discussed.

It will be recalled that in these lasers an electron beam 1 which moves in a direction Oz with a speed V _z close to that of light is periodically accelerated in the direction transverse to Oz.

For this, one can establish, over a certain length L, two fields B ₁ and 8 ₂ , transverse to the axis Oz and of opposite directions. These fields B ₁ and B ₂ are distributed periodically over the length L with the same period P.

Because of the fields 8 ₁ and 8 ₂ the electron beam 1 rises and falls as shown in the figure and therefore undergoes transverse accelerations. The electrons radiate a power which is proportional to the square of the transverse accelerations.

FIG. 4 represents a longitudinal view, along the axis Oz of an embodiment of the generator according to the invention.

• - une première partie 3 dans laquelle s'effectue l'accélération du faisceau d'électrons 1;
• - une deuxième partie 4 dans laquelle s'effectue le prélèvement d'énergie sous forme d'ondes millimétriques et infra-millimétriques.

This generator 2 has two parts:

• - A first part 3 in which the acceleration of the electron beam 1 takes place;
• - A second part 4 in which the energy is taken in the form of millimeter and infra-millimeter waves.

We will first describe the electron accelerator 3.

This accelerator comprises, in a vacuum enclosure 5, an electron gun which produces an electron beam with a non-zero speed in a direction transverse to Oz and with a speed along the axis Oz, V _z , substantially lower than that of light, V _z = o, 1. c for example.

Figure 2 shows the helical path followed by an electron at the exit of the electron gun.

This electron gun generally comprises a cathode 7 in the form of a ring which produces a hollow cylindrical beam.

Such electron guns are known in particular from the thesis defended on 12.07.79 at the Polytechnic Institute of Grenoble by J.L. ALIROT and entitled "Injector for high frequency wave generator tube of the gyrotron tube with central injection".

The accelerator according to the invention can also operate without using a hollow cylindrical beam but by using a thin off-center beam.

In FIG. 4, only the cathode 7 of this gun has been shown diagrammatically, and the anode in two parts, 6 and 8.

The high continuous voltage applied between the cathode and the anode is chosen so as to impart the longitudinal speed V _z to the electron beam.

A focusing coil 9 surrounds the vacuum chamber at the level of the electron gun. At this level, the vacuum enclosure is made of an insulating material, glass or ceramic, because it receives the high DC voltage.

This coil 9 creates a magnetic field in the opposite direction to that which is created in the rest of the accelerator. This is necessary so that the electron beam has in the rest of the accelerator a spiral path centered on the axis.

After the electron gun, the vacuum enclosure 5 includes a delay line 10 arranged along the axis Oz and supplied by a high frequency generator 11.

This delay line must allow the establishment of a longitudinal high frequency electric field, along the axis Oz. This delay line is generally formed by an iris guide, as shown in FIG. 4. Other types of delay line could be used such as a helical delay line for example.

The frequency delivered by this generator 11 is independent of that delivered by the generator according to the invention; the frequency delivered by the generator 11 is generally much lower than that delivered by the generator according to the invention, and between 1 GHz and 10 GHz.

As soon as it enters the iris guide 10, the electron beam 1 is subjected to a magnetic field which increases along the axis Oz and which is produced by a coil 12.

As soon as it enters the iris guide 10, each electron describes a spiral trajectory which is closer to the Oz axis.

In Figure 3, there is shown in thin lines the lines of force of the magnetic field increasing along the axis; these drill lines are getting closer and closer to the Oz axis.

In Figure 3, there is also shown in strong lines the spiral path of an electron which coils approximately on a magnetic field tube and approaches the Oz axis.

The increasing magnetic field makes it possible to increase the speed of rotation of the electrons around the Oz axis. The longitudinal energy supplied by the H.F. generator 11 is transformed into transverse energy, and the electrons therefore receive significant transverse accelerations.

The electrons can thus reach for example an energy equal to 4 W _o , where W _o = 511 KeV is the energy of the electrons at rest.

The magnetic field created by the coil 12 along the axis Oz is growing slowly. For example, each electron describes around ten orbits in the iris guide 10.

où e est la charge électrique de l'électron, m_o, la masse de l'électron au repos, W_o et W, l'énergie de l'électron respectivement au repos et en mouvement.When an electron is placed in a magnetic field of intensity B, its speed of rotation in the plane perpendicular to the electric field is written:

where e is the electrical charge of the electron, m _o , the mass of the electron at rest, W _o and W, the energy of the electron respectively at rest and in motion.

où C est une constante du mouvement qui s'écrit:

avec r le rayon de l'orbite décrite par les électrons.Placed in a slowly growing magnetic field with Oz and in an electric amplifier field study E, according to Oz, which is produced by the HF 11 generator, each electron describes a spiral. The component along the Oz axis of the force acting on the electrons has the expression:

where C is a constant of the movement which is written:

with r the radius of the orbit described by the electrons.

The energy transmitted to the electrons comes from the electric field of amplitude E, according to Oz, which is produced by the generator H.F. 11.

The energy variation in electron-Volts of the electron beam from the input to the output of the accelerator is given by the equation:

It can be seen from the above equation that the energy variation of the beam from the input to the output of the accelerator does not depend on the variation of the speed V _z of the electrons along the axis OZ.

The speed V _z can therefore be constant on the axis Oz. The following relation must then be verified between the value of the electric field according to Oz, E, and the variations of the magnetic field according to Oz:

To obtain a constant speed V _z , it is necessary to act on the increasing magnetic field created by the coil 12.

In the following, we give a numerical example corresponding to the case where we want to obtain a final energy of 4 W _o .

la vitesse totale des électrons: V = 0,9682 . c.From the equality W = 4 W _o , we obtain per relation:

the total speed of the electrons: V = 0.9682. vs.

• - champ magnétique initial: 0,1436T
• - champ magnétique final: 3T
• ― fréquence cyclotronique: (e/2πm₀) . B = 84 GHz
• ― fréquence synchrotronique: (e/2πm) . B = 21 GHz
• - rayon d'orbite initial: 10-^zm
• ― rayon d'orbite final: 0,219 . 10^-2m
• - constante du mouvement C: 1,149 . 10-²⁴ (unités S.I.)
• ― énergie initiale: W₁ = 1,314 . W₀, où W₁ représente l'énergie de l'électron à l'entrée de la ligne à retard
• - accélération totale en énergie: 1372 keV
• - haute tension continue: 160 kV
• ― longueur de l'accélérateur: > 13. 10^-2m.

By choosing the characteristics of the electron gun and in particular of the direct voltage between the anode and the cathode, we choose to take a constant longitudinal speed equal to: V _z = 0.1. c and a transverse speed equal to: 0.9631. vs. The following quantities are then determined:

• - initial magnetic field: 0.1436T
• - final magnetic field: 3T
• - cyclotronic frequency: (e / 2πm ₀ ). B = 84 GHz
• - synchrotron frequency: (e / 2πm). B = 21 GHz
• - initial orbit radius: 10- ^z m
• - final orbit radius: 0.219. 10 ^-2 m
• - constant of movement C: 1,149. 10- ²⁴ (SI units)
• - initial energy: W ₁ = 1.314. W ₀ , where W ₁ represents the energy of the electron at the input of the delay line
• - total energy acceleration: 1372 keV
• - continuous high voltage: 160 kV
• - length of the accelerator:> 13. 10 ^-2 m.

After describing the first part 3 of the generator 2 according to the invention in which the acceleration of the electron beam 1 takes place, we will now describe an embodiment of the second part 4 of this generator in which the the sampling of millimeter and infra-millimeter waves.

In this second part, the vacuum enclosure 5 of the generator 2 has a diameter smaller than that which it presents at the level of the accelerator 3.

Thus, one can slide between this enclosure and the coil 12 two inclined mirrors 13, metallic for example, which receive the coherent radiation emitted by the electrons and reflect it in a direction parallel to Oz so that it can be used next.

In FIG. 5 which is a transverse view along the plane F of FIG. 4 of the generator according to the invention, we see the rectangular section of the mirrors 13.

At the second part 4 of the generator, the coil 12 creates a uniform magnetic field along the axis Oz.

So that the radiation emitted by the accelerated electrons is made coherent, the electron beam passes, after passing through the accelerator, between two parallel reflectors 14.

, où N est un nombre entier et λ_M la longueur d'onde du rayonnement cohérent que l'on va obtenir et qui sera précisée par la suite.These two reflectors are separated by a distance equal to N.

, where N is an integer and λ _M the wavelength of the coherent radiation that we will obtain and which will be specified below.

Each of the reflectors 14 has a semi-reflecting zone 15 which lets through a fraction of the radiation and reflects the rest and a reflecting zone 16. The reflective area of one reflector faces the semi-reflective area of the other reflector, and vice versa.

Therefore, the radiation is collected through the vacuum enclosure 5 which is made of glass at this location in two opposite directions on each zone which lets the radiation from the two deflectors pass.

The mirrors 13 make it possible to reduce the radiation in the direction Oz because it is not possible to let it propagate perpendicular to the reflectors because of the presence of the coil 12.

In fact, the second part 4 of the generator in which the millimeter and infra-millimeter orders are taken constitutes a resonant cavity tuned to the frequency F _M corresponding to λ _M.

This cavity can be open, that is to say constituted for example by two parallel reflectors as is the case in the embodiment shown in FIG. 4.

This cavity can also be closed and made up, for example, of a waveguide portion.

mais qu'il rayonne au mieux autour des harmoniques de ω_s : ω_m = K . ω_s, avec un maximum pour:

c'est-à-dire autour d'une pul-

We know from the work of J. SCHWINGER. and in particular by the article published in his name in "Physical review", of 15.06.49, volume 75, number 12, pages 1912 to 1925, that an electron of energy W which is placed in a magnetic field of intensity B rotates with the angular speed: W

but that it radiates at best around the harmonics of ω _s : ω _m = K. ω _s , with a maximum for:

that is to say around a pul-

Indeed, it is known that the stimulated synchrotron radiation always occurs at a frequency very close to the resonator frequency and greater than the frequency of the synchrotron harmonic.

In the case of the digital example cited above, the wavelength λ _m and the frequency F _M have substantially the value: λ _M = 222 μu and F _M = 1344 GHz.

The accelerator according to the invention being linear makes it possible to vary, along the linear acceleration path, parameters, such as for example the thickness of the valves, as a function of z in order to adapt to the mass, to the velocity of particles which vary along the z axis.

Along a circular route, the conditions are necessarily periodic.

Claims (8)

1. Electron accelerator comprising in an evacuated chamber (5) an electron gun (6, 7, 8, 9) which produces an electron beam (1) which is propagated along an axis Oz, said electron accelerator comprising moreover a coil (12) which surrounds the chamber (5) and produces a magnetic field oriented along this axis Oz, characterized in that it comprises in the interior of the chamber (5) a delay line (10) fed by a high-frequency generator (11), said delay line permitting the creation of a longitudinal high-frequency electric field oriented along the axis Oz, that the intensity of the magnetic field increases at the level of the delay line, and that the electron gun produces electrons whose velocities have a first component (V_z) which is oriented along the axis (Oz) and corresponding to the Newton range and a transverse component which is not zero.

2. Accelerator according to claim 1, characterized in that the delay line (10) is formed by an iris guide.

3. Accelerator according to claim 1 or 2, characterized in that the increase of the magnetic field along the axis Oz is such that each electron executes 10 orbits in the delay line (10).

4. Accelerator according to one of claims 1, 2 or 3, characterized in that by acting on the coil (12) which produces the magnetic field increasing along Oz the velocity (V z) of the electrons along the axis Oz is rendered constant, the following relationship then holding true between the amplitude E of the electrical field along Oz due to the high-frequency generator (11) and the variations of the magnetic field along Oz:

where C is a movement constant and m the mass of an accelerated electron.

5. Millimeter wave and inframillimeter wave generator (2) comprising an electron accelerator (3) according to one of claims 1 to 4, characterized in that after having traversed the delay line (10) the electron beam (1) penetrates into a resonante cavity tuned to the frequency F_M corresponding to an angular frequency ω_M which is somewhat greater than

and that the coil (12) (5) generates a uniform magnetic field along the axis Oz at the level of the resonance cavity.

6. Generator according to claim 5, characterized in that the resonance cavity is formed by two parallel reflectors (14) between which the electron beam passes, said reflectors being separated by a distance equal to N .

, n being an integer and λ_M the wavelength corresponding to the angular frequency ω_M.

7. Generator according to claim 6, characterized in that each of the reflectors (14) comprises a semi- reflecting zone (15) and a reflecting zone (16), the reflecting zone of one reflector facing the semire- flecting zone of the other reflector, characterized in that two inclined mirrors (13) are disposed between the vacuum chamber (5) of the generator and of the coil (12) which generates the uniform magnetic field at the level of the zone (15) of each reflector (14) which allows the radiation to pass, said two mirrors (13) deflecting the radiation in the direction Oz.

8. Generator according to claim 5, characterized in that the resonance cavity is formed by a portion of a waveguide.

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