EP0359774A1

EP0359774A1 - Electron accelerator with co-axial cavity.

Info

Publication number: EP0359774A1
Application number: EP88904976A
Authority: EP
Inventors: Guyen Annick N; Jacques Pottier
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1987-05-26
Filing date: 1988-05-25
Publication date: 1990-03-28
Anticipated expiration: 2008-05-25
Also published as: DE3880681T2; DE3880681D1; KR960014439B1; IL86448A; IL86448A0; WO1988009597A1; EP0359774B1; ES2007889A6; AU613381B2; US5107221A; JP2587281B2; JPH02503609A; AU1943788A; FR2616032B1; KR890702416A; CA1306075C; FR2616032A1

Abstract

Accélérateur d'électrons. Selon l'invention, on utilise une cavité coaxiale (CC) résonnant selon le mode fondamental et on injecte les électrons dans le plan médian perpendiculaire à l'axe. Le faisceau peut être accéléré plusieurs fois le long de diamètres différents (d1, d2) par injection dans la cavité, grâce à des déflecteurs d'électrons (D1, D2). Application à l'irradiation de substances diverses.Electron accelerator. According to the invention, a coaxial cavity (CC) resonating according to the fundamental mode is used and the electrons are injected in the median plane perpendicular to the axis. The beam can be accelerated several times along different diameters (d1, d2) by injection into the cavity, thanks to electron deflectors (D1, D2). Application to the irradiation of various substances.

Description

COAXIAL CAVITY ELECTRON ACCELERATOR

DESCRIPTION

The present invention relates to an electron accelerator. It finds an 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 includes an electron deflector receiving the accelerated beam for the first time, deflecting it by around 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 "100 MeV electron accelerator in steady state".

This type of accelerator has the following disadvantage. During the first injection into the cavity, the electron beam follows a path and coincides 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 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 this 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 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 two improved variants, the electron beam always follows, during its multiple crossings, a path for which the deflecting fields are zero (the electric field is parallel to the speed vector of the electrons, and in opposite directions). However, these two devices are complex to implement: in the first, 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 size of the device. In the second, 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 uses a cavity whose original shape, in this application makes it possible to benefit from the effects of multiple crossings while retaining the condition stated above on the absence of deflecting fields along the paths taken by the electrons. Specifically, the present invention relates to an electron accelerator of the multiple acceleration type mentioned above and which is characterized in that the resonant cavity is a coaxial cavity resonating according to the fundamental mode, with an external conductor and a inner conductor having the same axis, the electron beam being injected into this cavity in the median plane perpendicular to the axis and along a first diameter of the outer conductor, the electron deflector keeping the electrons in the median plane and reinjecting the beam in the cavity always in the median plane and along a second diameter of the external conductor, etc. the outer conductor and the inner conductor being pierced with diametrically opposite openings taken by the beam during its successive crossings of the cavity.

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,

- Figure 2 illustrates a property of the coaxial cavity relating to the absence of magnetic field in the median plane of the cavity, Figure 3 shows, in section, an electron accelerator according to the invention, Figure 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, a coaxial cavity CC is seen, 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, called 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 TE ₀₀₁ , 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 the 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 alternation of the field in a direction parallel to the axis.

Such a cavity can be supplied by a high frequency SHF source 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 distinct 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 represented in section (the plane of the figure being the median plane). The beam enters the cavity through an opening 11. It crosses 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 to 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. On its exit, the beam is again deflected 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 ≤ A where λ is the wavelength of the electromagnetic field.

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: 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).

We will assume later that the condition d2 + l = λ is satisfied.

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 it to be housed 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 conventionally characterized by its efficient shunt impedance,

Zs _eff , ratio of the square of the energy gained by the electron during a crossing of the cavity (expressed in electron-volts) to the power dissipated by the Joule effect.

For example, for a cavity operating at 100

MHz, taking R2 = 0.8m, we get a fairly flat maximum of

Zs _eff 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 dissipated power 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 the electrons

5 MeV would therefore consume around 125 kW.

For a different number of passages, the radii of the cavity would differ somewhat, but the impedance-shunt 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 decrease 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 internal conductor, as illustrated in FIG. 5. The internal 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 respect to impedance-shunt is not very significant (in the order of 10%). However, this arrangement has the advantage of greatly reducing the maximum power dissipated per unit area (2 to 4 times less than with the coaxial cavity), which can be useful to facilitate cooling and reduce 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

1. An electron accelerator of the type comprising a source (S) emitting an electron beam (Fe), a resonant cavity, a high frequency source (SHF) supplying the cavity in an electromagnetic field at a resonance frequency of the cavity, the electron beam (Fe) being injected into the cavity and crossing it along an electric field line (E) of the resonant field, the beam being thus accelerated a first time, at least one electron deflector (D1) placed outside the cavity, this deflector receiving the beam having passed through the cavity, deflecting it and reinjecting it into the cavity, the beam passing back through the cavity and undergoing a second acceleration, this accelerator being characterized by the fact that the resonant cavity is a coaxial cavity (CC) with an outer cylindrical conductor (10) and an inner cylindrical conductor (20) having the same axis (A), this cavity being ex ited at the frequency of the fundamental mode, the electron beam being injected into this cavity in the median plane (Pm) perpendicular to the axis (A) and along a first diameter (d1) of the external conductor (10), the deflector of electrons (D1) keeping the electrons in the median plane (Pm) and reinjecting the beam into the cavity still in the median plane (Pm) and according to a second diameter (d2) of the outer conductor (10), the outer conductor ( 10) and the inner conductor (20) being pierced with diametrically opposite openings (11, 12, ... 21, 22) and taken by the beam during its successive crossings of the cavity.

2. Accelerator according to claim 1, characterized in that the central conductor (20) has frustoconical ends (33, 35).

3. Accelerator according to at least one of claims 1 and 2, comprising n beam crossings of the cavity, characterized in that electron deflectors with magnets are used, the faces of which, at the inlet and at lo beam exit, are tangent to a corner dihedral at the vertex close to π (1- (1 / 2n)).