EP0359774B1 - Electron accelerator with co-axial cavity - Google Patents

Abstract

A co-axial cavity (CC) resonates at the fundamental frequency and electrons are injected in the median plane perpendicular to the axis. The beam can be accelerated several times along different diameters (d1, d2) by reinjection into the cavity, by means of electron deflecters (D1, D2). Used in the irradiation of various substances.

Description

The present invention relates to an electron accelerator. It finds application in the irradiation of various substances such as agro-food products, either directly by electrons, or by X-rays obtained by conversion on a heavy metal target.

An electron accelerator is known which generally comprises a resonant cavity supplied by a high frequency field source, and an electron source capable of injecting electrons into the cavity. If certain phase and speed conditions are satisfied, the electrons are accelerated by the electric field throughout their passage through the cavity.

In certain types of accelerators, according to this principle, the electron beam crosses the cavity several times. The device then comprises an electron deflector receiving the accelerated beam for the first time, deflecting it by approximately 180 ° and reinjecting it into the cavity for a new acceleration. A second deflector can again deflect the beam which has undergone two accelerations, to make it cross a third time the cavity and thus obtain a third acceleration, and so on.

Such a device is described, for example, in French patent No. 1,555,723 entitled "Electron accelerator of 100 MeV in steady state".

This type of accelerator has the following disadvantage. During the first injection into the cavity, the electron beam follows a path coincident with the axis of the latter. Along this path, the electric field has only one component which is directed along the axis. There is therefore indeed an acceleration of the electrons and there is no deflection of the beam since there is no transverse component of the field magnetic. However, during the second crossing of the cavity, the electron beam follows a path which is no longer directed along the axis. A magnetic component perpendicular to the axial component of the electric field can act on the electron beam. This action will result in a deflection of the electrons. This deviation will depend on the phase of the electromagnetic field, which will produce a scattering of the beam, some of which will consequently be lost on the walls of the cavity. Furthermore, this parasitic phenomenon is amplified during multiple crossings.

However, multi-pass accelerators are known which avoid this pitfall thanks to a particular structure of the deflectors. According to a first variant, described for example in American patent 3,349,335 the electrons make a complete loop outside the cavity and are reinjected in the axis of the latter.

According to another variant, described in FR-A-1,136,936, the acceleration takes place in a resonant cavity and after each crossing the electrons are deflected outside the cavity so that they go around it ci and are fed back into the acceleration axis.

According to yet another variant, sometimes called "Duotron", the electron beam is returned on itself and thus goes back and forth along the axis of the cavity.

In these improved variants, the electron beam always follows, during its multiple traverses, a path for which the deflecting fields are zero (the electric field is parallel to the velocity vector of the electrons, and in opposite directions).

However, these devices are complex of implementation: in the first two, the various trajectories of the electrons do have a common branch merged with the axis of the cavity, but the other branches are external to the cavity, which increases the complexity and the bulk of the device . In the latter, we are limited to a round trip of the beam and the problem of returning the electrons to themselves is not easy to solve.

The object of the present invention is precisely to remedy these drawbacks. To this end, it offers an electron accelerator which benefits from the effects of multiple crossings while retaining the condition set out above on the absence of deflecting fields along the paths taken by the electrons, and which simplifies the problems linked to the deflection and reinjection of electrons into the accelerating cavity.

Specifically, the present invention relates to an electron accelerator of the multiple acceleration type mentioned above and described in particular in FR-A-1 136 936 and which is characterized by the fact that the conductors outside and inside the cavity are cylindrical and the electron beam is injected in a plane perpendicular to the axis of the cavity where the radial component of the electric field is maximum and by the fact that the deflection means comprise a first electron deflector having a inlet facing a first outlet opening pierced in the outer conductor and diametrically opposite to the first inlet opening along a first diameter, this first deflector having an outlet facing a second inlet opening pierced in the outer conductor, a second electron deflector having an inlet facing a second outlet opening pierced in the outer conductor and diametrically opposite to the second inlet opening according to a second diameter distinct from the first, this second deflector having an outlet facing a third inlet opening pierced in the outer conductor and possibly other deflectors associated in the same way with other diameters of the external conductor all distinct from each other but all located in said plane.

• la figure 1 montre une cavité coaxiale résonnant selon le mode fondamental,
• la figure 2 permet d'illustrer une propriété de la cavité coaxiale relative à l'absence de champ magnétique dans le plan médian de la cavité,
• la figure 3 montre, en coupe, un accélérateur d'électrons selon l'invention,
• la figure 4 illustre des caractéristiques géométriques du dispositif de l'invention, et
• la figure 5 montre une variante de réalisation de l'invention, destinée à diminuer les pertes ohmiques.

In any case, the characteristics of the invention will appear better in the light of the description which follows. This description refers to attached drawings in which:

• FIG. 1 shows a coaxial cavity resonating according to the fundamental mode,
• FIG. 2 illustrates a property of the coaxial cavity relating to the absence of magnetic field in the median plane of the cavity,
• FIG. 3 shows, in section, an electron accelerator according to the invention,
• FIG. 4 illustrates geometric characteristics of the device of the invention, and
• Figure 5 shows an alternative embodiment of the invention, intended to reduce ohmic losses.

In FIG. 1, we can see a coaxial cavity CC constituted by an external cylindrical conductor 10, an internal cylindrical conductor 20 and two flanges 31 and 32. Such a cavity has an axis A and a median plane Pm, perpendicular to the axis. Among all the possible resonance modes of such a cavity, there is one, said to be fundamental, of electric transverse type, for which the electric field E is purely radial in the median plane and decreases on both sides of this plane. to cancel on the flanges 31, 32. Conversely, the magnetic field is maximum along the flanges and is canceled in the median plane by changing direction.

Such a mode can be designated, according to conventional conventions, by TE₀₀₁, the initials TE recalling that it is a mode where the electric field is transverse, where the first index "0" indicates that the field has symmetry of revolution, the second index "0" indicates that there is no field cancellation along a radius of the cavity, and the third index of value 1 indicates that there is a half-wave of the field in a direction parallel to the axis.

Such a cavity can be supplied by a high frequency source SHF coupled to the cavity by a loop 34.

According to the invention, the electron beam is injected into the coaxial cavity in the median plane thereof. It is indeed in this plane that there is no parasitic field capable of deflecting the beam. As this point is essential we can stop there. In part a of FIG. 2, the cavity is seen in cross section in the median plane. The electric fields E1 and E2 are equal along two separate radii. A contour 17 is defined by these two radii and by two arcs of a circle along which the electric field is radial. The circulation of the electric field (that is to say the integral of this field) is zero along this contour. Consequently, the flux of magnetic induction through a surface resting on this contour is also zero. In other words, there is no magnetic component perpendicular to the median plane.

On part b of this same figure 2, we can see the cavity in longitudinal section. The electric field being symmetrical with respect to the median plane, the fields E3 and E4 the along two infinitely close rays located on either side of this plane, are equal. The circulation of the electric field along a contour 18 constituted by these two rays and by two longitudinal branches, is zero. Consequently, the flux of induction through a surface resting on this contour is also zero. In other words, there is no magnetic component in the median plane.

Thus, there is no magnetic component in the median plane Pm (which amounts to saying, pictorially, that the median plane of the cavity is a purely capacitive area). The electron beam will therefore not be subjected to any deflecting force.

Figure 3 shows, schematically, a complete accelerator according to the invention. The device comprises an electron source S, a coaxial cavity CC, formed of an outer cylindrical conductor 10 and an inner cylindrical conductor 20, two electron deflectors D1 and D2.

The operation of this device is as follows. The electron source S emits a beam of electrons Fe directed in the median plane of the coaxial cavity CC shown in section (the plane of the figure being the median plane). The beam enters the cavity through an opening 11. It passes through the cavity along a first diameter d1 of the external conductor. The inner conductor 20 is pierced with two diametrically opposite openings 21 and 22. The electron beam is accelerated by the electric field if the phase and frequency conditions are satisfied (the electric field must remain in the opposite direction to the speed of the electrons).

The accelerated beam leaves the cavity through an opening 12 diametrically opposite the opening 11. It is then deflected by a deflector D1.

The beam is reintroduced into the coaxial cavity through an opening 13. It then borrows a second diameter d2 and undergoes a second acceleration in the cavity. It comes out through the opening 14. When it exits, the beam is deflected again by a deflector D2 then reintroduced into the cavity by an opening 15. It borrows a third diameter d3 and undergoes a third acceleration, etc.

The principle of the accelerator of the invention having been exposed, some practical considerations of implementation will now be developed with regard in particular to the condition of synchronism to be respected and the shunt impedance.

1. Condition of synchronism .

The coaxial nature of the acceleration structure means that the electric field does not have the same direction in the first and in the second half of the path taken by the electrons in the cavity, in other words along the radius which goes from the external conductor. to the inner conductor, then along the radius from the inner conductor to the outer conductor. The spatial variation of the field is accompanied by a temporal variation since the field has a high frequency (a few hundred megahertz). These two variations are taken advantage of by injecting the beam in such a way that the electric field is canceled out when the electrons pass through the central conductor. The time taken by the electrons to pass from one conductor to another must therefore be less than the half-period of the field; the time taken by the electrons to cross the entire cavity is therefore less than the field period. As these electrons are almost relativistic we can consider that their speed is close to the speed of light c. We therefore have: d2 / c <T, provided that we can write d2 ≦ λ where λ is the wavelength of the electromagnetic field.

où k est un entier.If we denote by l the length of the path taken by the electrons outside the cavity, in particular in the deflector, we must have an additional condition which is:
$\text{d2 + l = k λ}$

where k is an integer.

To reduce the size of the device, it is desirable to take k = 1. But in certain particular cases, one could have to choose k = 2 (for example, to accommodate more easily a focusing system between the magnets of deflection and cavity, or to have a larger radius of curvature, in order to use a weaker induction).

est satisfaite.We will assume later that the condition $\text{d2 + l = λ}$

is satisfied.

d'où

   Par exemple, pour n=6 et n=8 on aura respectivement :

   Pour une longueur d'onde de 3m, ce qui correspond à une fréquence de 100 MHz, on aura respectivement :
Ra = 101 cm   Rc = 27 cm
Ra = 111 cm   Rc = 22,1 cm
   Le rayon extérieur R2 délimitant le champ de la cavité doit évidemment être inférieur à Rc pour tenir compte de l'épaisseur de la paroi et permettre éventuellement de loger entre celle-ci et le déflecteur des dispositifs de focalisation auxiliaires. Les dimensions calculées ci-dessus sont compatibles avec ces exigences pratiques.The radius of curvature in one of the deflectors is designated by Rc and by Ra the distance between the axis of the cavity and the inlet eD or the outlet sD of this deflector. These quantities are illustrated in FIG. 4. Furthermore, the angle between two paths is equal to π / 2n. So we have the following relationships:

from where

For example, for n = 6 and n = 8 we will have respectively:

For a wavelength of 3m, which corresponds to a frequency of 100 MHz, we will have respectively:
Ra = 101 cm Rc = 27 cm
Ra = 111 cm Rc = 22.1 cm
The external radius R2 delimiting the field of the cavity must obviously be less than Rc to take account of the thickness of the wall and possibly allow housing between it and the deflector of the auxiliary focusing devices. The dimensions calculated above are compatible with these practical requirements.

2. Impedance-shunt

The electrical quality of an accelerating cavity is classically characterized by its effective impedance-shunt, Zs _eff , ratio of the square of the energy gained by the electron during a crossing of the cavity (expressed in electron-volts) at the power dissipated by the Joule effect.

For example, for a cavity operating at 100 MHz, taking R2 = 0.8m, a fairly flat maximum of Zs _eff is obtained in the vicinity of (R1 / R2) = 1/4.

Under these conditions, the calculation gives Zs _eff ≃ 10 MΩ and to obtain an energy gain of 10 MeV with six passages, the power dissipated would be 278 kW.

The shunt impedances obtained in practice are somewhat lower than the theoretical values, and in fact the dissipated power will be close to 350 kW.

The impedance-shunt, for homothetic cavities, is proportional to the root of the wavelength. A cavity operating at 700 MHz increasing the energy of electrons by 5 MeV would therefore consume around 125 kW.

For a different number of passages, the radii of the cavity would differ somewhat, but the shunt impedance would vary little, and as a first approximation the dissipated power would vary in a manner inversely proportional to the number of passages.

It is therefore advantageous to use a large number of passages. We are in practice limited in this way by the correlative reduction in the radii of curvature of the beam in the deflection magnets, which on the one hand results in a reduction in the cross-section offered to the beam and on the other hand requires an increase in l 'induction.

The powers required are compatible with a continuous operation, and in any case do not require the use of relatively complex and expensive pulse generators.

The ohmic losses due to the currents flowing in the flanges of the cavity can be reduced by modifying the shape of the inner conductor, as illustrated in FIG. 5. The inner conductor 20 ends in two frustoconical parts 33 and 35. The inductance of the cavity is reduced. To keep the same frequency, you need to increase the capacitance, so lengthen the cavity a little.

The benefit from such a provision with regard to impedance-shunt is not very significant (of the order of 10%). However, this arrangement has the advantage of greatly reducing the maximum power dissipated per unit of area (2 to 4 times less than with the coaxial cavity), which may be advantageous for facilitating cooling and reducing annoying effects (arrow, internal tensions, etc.) due to the thermal gradient in the walls.

On the other hand, the inventors have demonstrated a considerable reduction in the transverse dimensions of the beam and a less great sensitivity to misadjustments by using deflection magnets whose faces, at the entrance and at the exit of the beam, are tangent to a corner dihedral at the apex close to π (1- (1 / 2n)) if n is the number of beam crossings of the cavity.

Claims (3)

1. Electron accelerator of the type comprising a resonant cavity with an external conductor (10) and an internal conductor (20) having the same axis of revolution (A), a high frequency source (SHF) coupled to the cavity and supplying an electromagnetic field at a resonance frequency of the cavity, an electron source (S) able to inject into said cavity an electron beam (Fe) across a first inlet opening (11) in the external conductor (10), the beam being injected along an electric field line (E) of the resonant cavity, electron deflection means positioned outside the cavity, said accelerator being characterized in that the external and internal conductors of the cavity are cylindrical and that the electron beam is injected into a plane perpendicular to the cavity axis, where the radial component of the electric field is at a maximum and in that the deflection means comprise a first electron deflector (D1) having an inlet facing a first outlet opening (12) in the external conductor (10) and diametrically opposite to the first inlet opening (11) along a first diameter (d1), said first deflector having an outlet facing a second inlet opening (13) in the external conductor (10), a second electron deflector (D2) having an inlet facing a second outlet opening (14) in the external conductor (10) and diametrically opposite to the second inlet opening (13) along a second diameter (d2) separate from the first (d1), said second deflector (D2) having an outlet facing a third inlet opening (15) in the external conductor (10) and optionally further deflectors associated in the same way with other diameters of the external conductor (10), which are all separate from one another, but all located in said plane.
2. Accelerator according to claim 1, characterized in that the internal conductor (20) has truncated cone-shaped ends (33, 35).
3. Accelerator according to at least one of the claims 1 and 2, having n passages of the cavity by the beam, characterized in that use is made of electron deflectors with magnets, whose faces, at the entrance and exit of the beam, are tangential to a dihedron with an apex angle close to π(1-1/2n).

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