EP0295981B1 - Electron curtain accelerator - Google Patents

Description

The present invention relates to a sheet electron accelerator. It finds an application in the irradiation of various substances and in particular in the polymerization or crosslinking of materials in the form of sheets or strips.

We already know about accelerators of tablecloth electrons. Figure 1 recalls the principle. A vacuum bell 2 comprises, at its lower part, an outlet window 3, grounded, and at its upper part, a cathode K supported by an insulator 4. The cathode is connected to the negative pole of a HT source high voltage, a few hundred thousand volts. An electric field E, directed towards the cathode accelerates the electrons emitted by this one until the exit window. An electron sheet is thus formed and accelerated. It bombs a band 5 which passes under the bell.

Such a device has many drawbacks. The use of a voltage of a few hundred kilovolts means that the cathode has to be supported by high quality insulators. It is also necessary to provide a sealed crossing supporting such a voltage.

If the cathode is made emissive by heating, it is necessary that the source of heating current is itself at high voltage which is not very convenient. This drawback can possibly be remedied by making the cathode emissive by ion bombardment using an ion source located at the window. But the complexity of the device is increasing.

To avoid these drawbacks, we can think of accelerating the electrons by a high frequency field and no longer electrostatic. This field can be established in a resonant conductive enclosure. The cathode can then be brought to a potential very close to that of the enclosure and there is no longer high voltage generator and insulator needed. In addition, the cathode can be placed outside the enclosure, which facilitates maintenance of the device, which is generally delicate on electrostatic devices.

To implement this concept of accelerator we can think of using a parallelepiped resonant cavity. The interesting vibration mode is such that the electric field, directed along the smallest dimension of the cavity, is constant along this direction, but distributed sinusoidally along the other two dimensions: it is maximum in the center and zero at the edges. By having an electron source elongated along the greatest length, one could accelerate the electrons by the resonant field thus established.

• 1) le champ électrique n'est plus uniforme, mais présente un maximum au centre. Il en résulte que l'énergie atteinte finalement par les électrons est fonction de la trajectoire des électrons : l'énergie n'est plus uniforme dans une section droite de la nappe.
• 2) Dans le plan médian de la cavité, au voisinage duquel se trouvent les trajectoires électroniques, il existe une composante transversale du champ magnétique. Ce champ dévie la trajectoire des électrons et cette déviation est fonction de la phase de l'onde. Le faisceau obtenu ne permet plus d'assurer une irradiation homogène.

This simple arrangement would however have two drawbacks:

• 1) the electric field is no longer uniform, but has a maximum in the center. It follows that the energy finally reached by the electrons is a function of the trajectory of the electrons: the energy is no longer uniform in a cross section of the sheet.
• 2) In the median plane of the cavity, in the vicinity of which the electronic trajectories are, there is a transverse component of the magnetic field. This field deviates the trajectory of the electrons and this deviation is a function of the phase of the wave. The beam obtained no longer ensures uniform irradiation.

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 obtain the same uniform accelerating field without deviating magnetic field. This result is obtained by the use of a general structure of coaxial shape.

Specifically, the subject of the present invention is a sheet electron accelerator, comprising a linear cathode emitting a sheet-like electron beam, a accelerating structure in which there is an electric field directed in the plane of the sheet, characterized in that the accelerating structure is a resonant coaxial cavity having a fundamental resonance mode consisting of an outer cylindrical conductor and an inner cylindrical conductor having the same axis, this cavity being closed by two flanges perpendicular to the axis, this cavity being excited by a high frequency source at the resonance frequency of the fundamental mode, and by the fact that the cathode is contained in the median plane of the perpendicular cavity at the axis, the electron beam emitted by the cathode being injected into the cavity in this median plane, the outer and inner cylinders being pierced with openings, in the form of arcs of a circle centered on the axis of the cavity for the passage of the beam.

• la figure 1, déjà décrite, montre un dispositif de l'art antérieur,
• la figure 2 représente une cavité coaxiale résonnant selon le mode fondamental,
• la figure 3 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 4 montre un accélérateur d'électrons selon l'invention,
• La figure 5 illustre une variante de l'invention utilisant des moyens de déflexion magnétique,
• La figure 6 illustre un premier mode de réalisation de moyens de déflexion magnétique,
• la figure 7 illustre un deuxième mode de réalisation de moyens de déflexion magnétique, et
• la figure 8 illustre une variante de réalisation à faibles pertes.

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, already described, shows a device of the prior art,
• FIG. 2 represents a coaxial cavity resonating according to the fundamental mode,
• FIG. 3 illustrates a property of the coaxial cavity relating to the absence of magnetic field in the median plane of the cavity,
• FIG. 4 shows an electron accelerator according to the invention,
• FIG. 5 illustrates a variant of the invention using magnetic deflection means,
• FIG. 6 illustrates a first embodiment of magnetic deflection means,
• FIG. 7 illustrates a second embodiment of magnetic deflection means, and
• FIG. 8 illustrates an alternative embodiment with low losses.

In FIG. 2, we 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 resonance modes possible of such a cavity, there is one, said to be fundamental, of transverse electric type, for which the electric field E is purely radial in the median plane. This field decreases on either side of this plane to come 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 -alternation 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 sheet 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 along two infinitely close rays and 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. In this plane, the electron beam will therefore not be subjected to any deflecting force.

Figure 4 shows, schematically, a sheet accelerator according to the invention. Part a is a longitudinal section and part b is a section in the median plane of the cavity. The device comprises a cathode K, a coaxial cavity CC, formed of an external cylindrical conductor 10 and an internal cylindrical conductor 20 having the same axis A. The cathode is in the form of an arc of a circle centered on the axis A of the cavity and it is located in the median plane Pm thereof.

The operation of this device is as follows. The cathode K emits a beam of electrons Fe directed in the median plane Pm of the coaxial cavity CC. The beam enters the cavity through an opening 11, in the form of a slit, pierced in the external conductor. The inner conductor 20 is pierced with two openings 21 and 22, which are also in the form of slots and symmetrical with respect to the axis. The electron beam is accelerated by the electric field prevailing in the coaxial cavity, if certain phase conditions are satisfied. The beam leaves the cavity through an opening 12, in the form of a slot, pierced in the external conductor and diametrically opposite to the opening 11. The accelerated beam irradiates the strip 5 which passes under the cavity.

The coaxial nature of the acceleration structure means that, at each instant, 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 other words along rays which go from the outer conductor to inner conductor, then along the spokes from the inner conductor to the conductor outside. But the field is at high frequency (a few hundred MHz). It reverses periodically. The electron beam is therefore injected in such a way that the electric field is canceled out and changes direction 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.

It will be observed that the use of a coaxial structure makes it possible to obtain a remarkable property which is the uniformity of the acceleration in the electronic ribbon. In fact, each electron emitted by the cathode in the direction of the axis of the cavity will follow a radial trajectory in this cavity and be subjected to an accelerating field directed along this trajectory. Furthermore, this field is the same regardless of the radius used in the angular sector defined by the sheet. Thus, at the exit of the cavity, all the electrons will have undergone the same acceleration and will therefore have the same energy.

If one wishes to irradiate a material in the form of a strip, the dose is strictly speaking homogeneous only if the strip is arranged so as to follow the shape of a cylinder coaxial with the cavity (by a system of rollers for example).

If a flat band is used, on the one hand the intensity of the radiation decreases as the cosine of the angle of incidence and on the other hand the direction of the radiation is no longer perpendicular to the surface and the penetration is smaller. But these effects partially offset each other: at the surface, the dose received is roughly the same. On the other hand, the depth treated remains proportional to the cosine of the angle of incidence.

To obtain uniform penetration, it is possible to make, outside the cavity, all the electronic trajectories substantially parallel to the median trajectory and this by two magnets, as shown in FIG. 5. In this figure, we see a deflection assembly M1 ( or M2) consisting of two pairs of magnets whose shapes are shown in Figures 6 and 7.

In these figures, part a is a section perpendicular to the median plane and part b is a section in the median plane, therefore in the plane of the electron beam.

For type M magnets shown in Figure 6, the pole pieces 41-42, on the one hand, and 43-44, on the other hand, define two air gaps with parallel edges. The magnets have a thickness which increases as one moves away from the median plane Pm (part b). The path traveled by the electrons between the pole pieces is therefore longer for the electrons which follow trajectories distant from the median plane than for the trajectories close to this plane. The action of the magnetic field is therefore prolonged for the former and abridged for the latter. The strongly inclined paths are therefore more curved than the others, which gives the beam parallel paths.

FIG. 7 illustrates another variant of these deflection means. The pole pieces 51-52, on the one hand, and 53-54, on the other hand, define air gaps which tighten when moving away from the axis. The magnetic field between such pole pieces increases as one moves outward. We thus find a greater deflection for the trajectories located far from the median plane than for the trajectories close to this one.

The principles of the invention having been exposed, some numerical data will now be specified, with regard to the dimensions and the value of the effective impedance-shunt. We know that this last quantity, a significant characteristic of the quality of the energy transfer to accelerated electrons, is equal to the ratio of the square of the energy transferable to an electron (expressed in electron volts) to the power lost by the Joule effect in the cavity.

The calculation shows that, for a 400 keV machine, the optimum is obtained for R2≃ 0.265 λ and R1≃ (R2 / 5), λ being the wavelength. The length of the cavity is L = λ / 2.

Under these conditions, we find for example for a frequency of 180 MHz λ = 1.67m, R2 = 0.44m, Zs _eff ≃ 6.25, MΩ, L = 0.835m.

The shunt impedances obtained in practice are somewhat lower (typically 30%) than the calculated values.

As a result, the power dissipated in the cavity would be close to 33 kW for a 400 keV machine.

The dimensions and power lost are very modest.

We see that the optimum is obtained for (R2 / R1) ≃5.

It can also be observed that it is possible to reduce the ohmic losses due to the currents flowing in the flanges of the cavity by modifying the shape of the internal conductor, as illustrated in FIG. 8. The internal conductor 20 ends in two frustoconical parts 33 and 35. The inductance of the cavity is thereby reduced. To keep the same resonant frequency, you have to 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 area (2 to 4 times less than with the coaxial cavity) which can be advantageous for facilitating cooling and reducing annoying effects (deflection, tensions internal, etc.) due to the thermal gradient in the walls.

Claims (3)

1. Array electron accelerator, incorporating a linear cathode (K) emitting an electron beam (Fe) in the form of an array, an accelerating structure where there is an electric field (E) directed into the plane of the array, characterized in that the accelerating structure is a coaxial cavity CC constituted by an external cylindrical conductor (10) and an internal cylindrical conductor (20) having the same axis (A), said cavity being closed by two flanges (31, 32) perpendicular to the axis, said cavity being excited by a high frequency source (SHF) to the resonant frequency of the fundamental mode and in that the cathode (K) is contained in the median plane (Pm) of the cavity perpendicular to the axis, the electron beam (Fe) emitted by the cathode (K) heing injected into the cavity in said median plane, the external (10) and internal (20) cylinders having circular arc-like openings (11, 12, 21, 22) centred on the axis of the cavity for the passage of the beam (Fe).
2. Electron accelerator according to claim 1, characterized in that the internal conductor (20) has truncated cone-shaped ends (33, 35) having a larger diameter at the ends of the internal conductor (20).
3. Accelerator according to claim 1, characterized in that it also comprises, at the beam exit, a magnetic deflection means (M1, M2) for the electrons constituted by two magnetic circuits symmetrical to the air gap (41, 42, 43, 44 and 51, 52, 53, 54), the lateral edges of the beam respectively traversing these two air gaps, said air gaps heing such that the action of the magnetic field on the electrons increases as the electrons are further from the centre of the beam.

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