Patent Application
20200170099
The present invention relates to an electron accelerator having a resonant cavity centred on a central axis, Zc, and creating an oscillating electric field used for accelerating electrons along several radial trajectories forming the petals of a flower. A Rhodotron® is an example of such electron accelerator. An electron accelerator according to the present invention can extract an electron beam of different energies along a single path.
Electron accelerators having a resonant cavity are well known in the art. For example, EP0359774 describes an electron accelerator comprising:
As shown on
A rhodotron can be combined to external equipment such as a beam line and a beam scanning system. Rhodotrons can be used in industrial applications including sterilization (e.g., of medical devices), polymer modification, polymer crosslinking, pulp processing, modification of crystals, improvement of semi-conductors, beam aided chemical reactions, cold pasteurization and preservation of food, detection and security purposes, treatment of waste materials, etc. X-rays can also be produced by running an electron beam of appropriate energy into a metal target. X-rays can be used in different applications such as for example, (medical) radio-isotope production. The energies and intensity of the electron beams required are highly dependent on the application. Generally, electron beams of energy higher than 10 MeV are avoided to prevent induction and activation of nuclear reactions. X-rays are produced from electron beams of energy generally lower than 7.5 MeV. Electron beams of 7 MeV are usually well suited for sterilization of medical devices, surface sterilization, crosslinking of polymers, and the like. Food processing applications by electron beams can be broadly divided into,
It can be appreciated from the foregoing that it would be advantageous if a given electron accelerator allowed the energy of the extracted electron beam to be varied depending on the desired application. This is the case with rhodotrons. Referring to
The problem with changing energies of the extracted electron beam with current accelerators is that the extraction path changes direction with each energy, depending on the number and positions of deflecting chambers which are added or removed. As shown in
EP3319403 proposes a rhodotron mounted on a rack such that its angular orientation can be varied, to maintain the same orientation of the extracted electron beam, whilst the number of deflecting chambers is varied. Although this design represents a great breakthrough compared with the previous accelerators, changing the orientation of the accelerator relative to the rack is, however, a substantial work and is not adapted for changing from a first application at 7 MeV in the morning to a second application at 5 MeV in the afternoon.
The present invention proposes a rhodotron capable of extracting electron beams of different energies along a single extraction path. The change of extraction energy is easy, quick and reliable and it can be discrete or continuous. This solution can be implemented to rhodotrons of any size, energy, and power and can also be implemented to existing rhodotron units by a simple modification. These advantages are described in more details in the following sections.
The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns an electron accelerator comprising:
The second length (L2) is preferably such that when the electron beam is re-introduced into the resonant cavity, the RF system is synchronized for applying an electric field for decelerating the electron beam along the second radial trajectory.
In a first embodiment, the at least one vario-magnet unit is a discrete vario-magnet dual unit comprising,
The first and second set of magnets are preferably adapted for generating a magnetic field,
In a second embodiment, the at least one vario-magnet unit is a moving vario-magnet unit comprising moving means for discretely or continuously moving radially the at least one vario-magnet units back and forth along a bisecting direction parallel to a bisector of the angle formed by the first and second radial trajectories at the central axis, Zc, and thus discretely or continuously varying the energy, W, of the accelerated electron beam extracted from the outlet. The moving means can comprise a motor for displacing back and forth the at least one moving vario-magnet unit along the corresponding bisecting direction.
In a preferred embodiment, the rhodotron can further comprise deflectors,
The use of deflectors has the advantage that the gyroradius of the electron beam, and therefore the magnitude of the magnetic field, needs not be varied with the radial distance of neither first and second sets of magnets, nor of the moving vario-magnet.
The second length (L2) is preferably equal to the sum of the adding length (L+) and one or more halves of the wavelength, λ, of the electric field, E, i.e., L2=(L+)+nλ/2, with n∈, and n is preferably equal to 1.
A preferred example of rhodotron comprises a single vario-magnet unit, which is positioned directly upstream of the outlet. The rhodotron is characterized by an energy gain or loss by an electron beam upon one pass across the resonant cavity to an ith magnet unit or from an (i−1)th magnet unit, defined as follows:
The number N of magnet units is preferably equal to 6, wi is preferably equal to 1 MeV/pass±0.2 MeV/pass for i=1 to 6 and comprised between −1 and 1 MeV/pass±0.2 MeV/pass for the last (7th) pass, and wherein the extracted electron beam is preferably comprised between 5 MeV±0.2 MeV and 7 MeV±0.2 MeV.
Each of the N magnet units generates a magnetic field in the deflecting chamber preferably comprised between 0.01 T and 1.3 T, more preferably from 0.02 T to 0.7 T. The electron beam can have an average power comprised between 30 and 700 kW, preferably between 150 and 650 kW.
In a preferred embodiment, the resonant cavity is formed by:
These and further aspects of the invention will be explained in greater detail by way of example and with reference to the accompanying drawings.
The figures are not drawn to scale.
The resonant cavity (1) comprises:
The resonant cavity (1) is divided into two symmetrical parts with respect to the mid-plane, Pm. This symmetry of the resonant cavity with respect to the mid-plane concerns the geometry of the resonant cavity and ignores the presence of any openings, e.g., for connecting the RF system (70) or the vacuum system. The inner surface of the resonant cavity thus forms a hollow closed conductor in the shape of a toroidal volume. The height of the resonant cavity measured along the central axis, Zc is generally ½λ, where λ is the RF wavelength. The diameter of the resonant cavity, measured normal to the central axis, Zc, can be 0.72 λ to allow transit in the deflecting chambers.
The mid-plane, Pm, can be vertical, horizontal or have any suitable orientations with respect to the ground on which the rhodotron rests. Preferably, it is horizontal or vertical.
The resonant cavity (1) may comprise openings for connecting the RF system and the vacuum system (not shown). These openings are preferably made in at least one of the two bottom lids (11b, 12b).
The outer wall also comprises apertures intersected by the mid-plane, Pm. For example, the outer wall comprises an introduction inlet opening for introducing an electron beam (40) in the resonant cavity (1). It also comprises an electron beam outlet (50) for discharging out of the resonant cavity the electron beam (40-5 to 40-7) accelerated to a desired energy. It also comprises cavity outlet/inlet apertures (31w), bringing in fluid communication the resonant cavity with corresponding deflecting chamber (31, see below). Generally, a rhodotron comprises several magnet units and several cavity outlet/inlet apertures.
A rhodotron generally accelerates the electrons of an electron beam to energies which can be comprised between 1 and 50 MeV, preferably between 3 and 20 MeV, more preferably between 5 and 10 MeV. As discussed supra, to avoid nuclear reactions, energies of not more than 10 MeV are applied in most industrial applications. Electrons are relativistic and at 50 keV they reach 0.4 c (wherein c is the light speed), at 1 MeV, they reach about 0.94 c and at 10 MeV they reach 0.9988 c. After one passage across the resonant cavity, the velocity of the electrons at an energy of typically 1 MeV can safely be approximated as being substantially constant.
Rhodotrons have a high average power, which can be comprised between 30 to 700 kW, preferably between 150 and 650 kW, more preferably between 160 and 190 kW. For example, IBA's rhodotron model TT50 can extract a beam of energy of up to 10 MeVaverage power comprised between 1 and 10 kW. The TT50 has a resonant cavity of 0.6 m diameter and accelerates the electron beam by an energy gain, wi, per passage of 1 MeV/pass. With a resonant cavity of 1 m diameter, IBA' s rhodotron model TT100 can extract electron beams of energy comprised between 3 and 10 MeV, with an energy gain, wi, per passage of 0.83 MeV/pass at a power of up to 40 kW. With a 2 m diameter resonant cavity, TT200 extracts 3 to 10 MeV electron beams at a rate wi=1 MeV/pass and at a power of up to 190 kW. The TT1000 has a resonant cavity of same diameter of 2 m as the TT200 but extracts beams of 3 to 7 MeV at a rate wi=1.2 MeV/pass at a power of up to 630 kW.
The inner wall comprises openings radially aligned with corresponding cavity outlet/inlet apertures (31w) permitting the passage of an electron beam through the inner cylindrical portion along a rectilinear radial trajectory (intersecting the central axis, Zc).
The surface of the resonant cavity (1) consisting of a hollow closed conductor is made of a conductive material. For example, the conductive material can be one of gold, silver, platinum, aluminium, preferably copper. The outer and inner walls and bottom lids can be made of steel coated with a layer of conductive material.
The resonant cavity (1) may have a diameter, 2R, comprised between 0.3 m and 4 m, preferably between 0.4 m and 3 m, more preferably between 0.5 m and 2 m.
The height of the resonant cavity (1), measured parallel to the central axis, Zc, can be comprised between 0.3 m and 4 m, preferably between 0.4 m and 1.2 m, more preferably between 0.5 m and 0.7 m.
The outer diameter of a rhodotron including a resonant cavity (1), an electron source (20), a vacuum system, a RF system, and one or more magnet units (30i), measured parallel to the mid-plane, Pm, may be comprised between 1 and 5 m, preferably between 1.2 and 2.8 m, more preferably between 1.4 and 1.8 m. The height of the rhodotron measured parallel to the central axis, Zc, may be comprised between 0.5 and 5 m, preferably between 0.6 and 1.5 m, more preferably between 0.7 and 1.4 m.
Electron Source, Vacuum System, and RF system
The electron source (20) is adapted for generating and for introducing an electron beam (40) into the resonant cavity along the mid-plane, Pm, towards the central axis, Zc, through an introduction inlet opening. For example, the electron source may be an electron gun. As well known by a person of ordinary skill in the art, an electron gun is an electrical component that produces a narrow, collimated electron beam that has a precise kinetic energy.
The vacuum system comprises a vacuum pump for pumping air out of the resonant cavity (1) and creating a vacuum therein.
The RF system is coupled to the resonant cavity (1) via a coupler and typically comprises an oscillator designed for oscillating at a resonant frequency, fRF, for generating an RF signal of wavelength, λ, followed by an amplifier or a chain of amplifiers for achieving a desired output power at the end of the chain. The RF system thus generates a resonant radial electric field, E, in the resonant cavity. Absent any measure to the contrary, the resonant radial electric field, E, oscillates such as to accelerate the electrons of the electron beam (40) along a trajectory lying in the mid-plane, Pm, from the outer conductor section towards the inner conductor section, and, subsequently, from the inner conductor section towards a cavity outlet aperture (31w). The resonant radial electric field, E, is generally of the “TE001” type, which defines that the electric field is transverse (“TE”), has a symmetry of revolution (first “0”), is not cancelled out along one radius of the cavity (second “0”), and is a half-cycle of said field in a direction parallel to the central axis Z.
N magnet units (30i) are distributed around an external circumference of the outer wall, and centred on the mid-plane Pm, with N>1 and N∈. Each one of the N magnet units comprises a set of deflecting magnets adapted for generating a magnetic field in a deflecting chamber (31). The deflecting chamber is in fluid communication with the resonant cavity (1) by a cavity outlet aperture and a cavity inlet aperture, which can be separate apertures or merge in a single aperture, all referred to by the numeral (31w). All the deflecting chamber enclose a portion of the mid-plane Pm.
Preferably, the magnet system comprises several magnet units (30i with i=1, 2, . . . N). N is equal to the total number of magnet units and is comprised between 1 and 15, preferably between 4 and 12, more preferably between 5 and 10. In conventional rhodotrons, the number N of magnet units yield (N+1) accelerations of the electrons of an electron beam (40) before it exits the rhodotron with a given energy (N+1)*wi, wherein wi is the energy gained or lost by an electron beam upon one pass across the resonant cavity to a magnet unit (30i) or from a magnet unit (30(i−1)). For example,
The magnetic field generated in each deflecting chamber by the corresponding magnetic units is adapted for deflecting an electron beam entering into the deflecting chamber through the cavity outlet aperture at the end of a first radial trajectory in the resonant cavity along the mid-plane, Pm, over a first deflecting trajectory having an adding length (L+). The first deflecting trajectory extends from the cavity outlet aperture to the cavity inlet aperture, which can be the same as or different from the cavity outlet aperture, through which the electron beam is re-introduced into the resonant cavity towards the central axis along a second radial trajectory in the mid-plane, Pm. The second radial trajectory is different from the first radial trajectory and intersects the latter at the central axis, Zc. The adding length (L+) is such that when the electron beam is re-introduced into the resonant cavity, the RF system is synchronized for applying an electric field for accelerating the electron beam along the second radial trajectory between the cavity inlet aperture and the central axis, Zc (cf.
The electron beam is injected in the resonant cavity by the electron source (20) through the introduction inlet opening along the mid-plane, Pm. It follows a first radial trajectory in the mid-plane, Pm, said trajectory sequentially crossing:
As illustrated in
The magnetic field required in the deflecting chambers must be sufficient for bending the trajectory of an electron beam exiting the resonant chamber along a radial trajectory through a cavity outlet aperture (31w) in an arc of circle of angle greater than 180° to drive it back into the resonant chamber along a second radial trajectory. For example, in a rhodotron comprising nine (9) magnet units (30i), the angle can be equal to 198°. The radius of the arc of circle (=gyroradius) can be of the order of 40 to 250 mm, preferably between 50 and 180 mm The chamber surface must therefore have a length in a radial direction of the order of 65 to 260 mm The magnetic field required for bending an electron beam to such arcs of circle is of the order of between 0.01 T and 1.3 T, preferably 0.02 T to 0.7 T, for example 0.2 or 0.3 T, depending on the desired gyroradius.
The magnet units may comprise electro-magnets which allow an easy control of the magnitude of the magnetic field created in the magnet unit. In a preferred embodiment, one or more magnet units, preferably N magnet units, may comprise a first and second permanent magnets instead of or additionally to a first and second electromagnets. Permanent magnets and electro-magnets are discussed below.
In the present document, a radial trajectory is defined as a rectilinear trajectory comprised in the mid-plane, Pm, and intersecting perpendicularly the central axis, Zc.
When a change of the target energy of an electron beam extracted from a rhodotron of the prior art is accompanied by a change of orientation of the extraction path of said electron beam requiring a re-orientation thereof towards a target (100) as illustrated in
Like in conventional rhodotrons, rhodotrons according to the present invention, provided with N magnet units (301-305), including at least one vario-magnet unit (306-5, 306-7, 306v), can generate (N+1) accelerations of the electrons of an electron beam (40) before it exits the rhodotron with a given energy (N+1)*wi. This is illustrated in
The vario-magnet units (306-5, 306-7, 306v) are suitable for varying the deflecting trajectory of the electron beam in the deflecting chamber from the first deflecting trajectory of length, L+, to a second deflecting trajectory of length, L2, different from, preferably higher than, the adding length, L+, of the first deflecting trajectory. This has the effect of changing the synchronization of the penetration into the resonant cavity of the electron beam through the cavity inlet cavity (31w) with respect to the frequency of the RF electric field E.
In a preferred embodiment, the second length (L2) is such that when the electron beam is re-introduced into the resonant cavity, the RF system is synchronized for applying an electric field for decelerating the electron beam along the second radial trajectory, thus reducing the energy W of the electron beam. For example, in the embodiment illustrated in
Referring to
The terms “accelerated” and “decelerated” are used herein to refer to an change of energy, although the relativistic electron beam rapidly approaches the speed of light and its velocity can be approximated to be substantially constant, though not exactly constant. Irrespectively of the relativistic behaviour of the electrons, the energy of the electron beam increases at each passage through the resonant cavity by exposure to the electric field (W=q E d).
If the radial distance to the central axis, Zc, of a vario-magnet unit (306v, 306-5) is increased, the corresponding first and second radial trajectories are prolonged and since they are divergent, the radius of the deflecting trajectory (called “gyroradius”) required to join the free ends of the first and second radial trajectories must also be increased. Since the gyroradius is inversely proportional to the magnetic field, absent any other measure for preventing such increase of the gyroradius, the magnetic field of a vario-magnetic unit must decrease with increasing radial distance to the central axis, Zc.
The increase of the gyroradius with increasing radial distance to the central axis, Zc, of a vario-magnet unit is clearly visible in
In a preferred embodiment illustrated in
In a preferred embodiment, the rhodotron comprises a single vario-magnet unit (306-5, 306-7, 306v)), which is positioned directly upstream of the outlet (50). The energy wi gained or lost by an electron beam upon one pass across the resonant cavity to a magnet unit (30i) or from a magnet unit (30(i−1)), is constant for i=1 to N, and varies between (−wi) and (+wi) for the last ((N+1)th) pass of the electron beam across the resonant cavity to the outlet (50). With N=6, and wi=1 MeV/pass for i=1 to 6 and comprised between −1 and 1 MeV/pass for the last (7th) pass, as in the embodiment of
The use of at least one vario-magnet unit (306v, 306-5, 306-7) in a rhodotron elegantly solves the problem of extracting along a single path electron beams (40-5 to 40-7) of different energies, W. Different types of vario-magnet units can be implemented in the present invention, including discrete vario-magnet dual units (306-5, 306-7) and moving vario-magnet units (306v).
In a first embodiment illustrated in
The first and second set of magnets (306.7, 306-5) can be adapted for generating a magnetic field either in a single deflecting chamber (31) common to both sets of magnets, or to a first and second deflecting chambers (31), respectively, the first deflecting chamber being in fluid communication with the second deflecting chamber by one or two windows. The two-chamber option of the present invention can be implemented very easily on existing conventional rhodotrons.
The foregoing vario-magnet unit configuration permits toggling between two predefined and discrete values of energies, W. For this reason, this embodiment can be referred to as “discrete vario-magnet dual unit.” Toggling from the first set (306-7) to the second set of magnets (306-5) can be done very easily by activating and deactivating the first set of magnets (306-7). Deactivating the first set of magnets can be easily performed with electro-magnets by feeding or not electrical current. If permanent magnets are used instead, they must be removed far enough from the deflecting chamber to drop the magnetic field at the level of the mid-plane Pm. Preferably, the first set of magnets comprises electro-magnets.
In
Of course, the number N of magnets is not necessarily six, the number of non-vario magnet units (301-305) can be different from five, and the number of vario-magnet units can be more than one and is not necessarily located at the last position before the outlet (50). Care should be taken if a vario magnet unit is not at the last position, that the change in synchronization with the RF electric field provoked by a vario-magnet unit is maintained to the following passes including non-vario magnet units. A skilled person can easily design the best arrangement of vario- and non-vario-magnet units to yield the desired energy ranges of extracted electron beams.
A discrete vario-magnet dual unit affords toggling between two predefined second lengths, L2, only. A third magnet unit could be envisaged, but the size of the rhodotron comprising such discrete vario-magnet triple- (or more) units would increase accordingly. If more than two energies (second lengths, L2) are desired, other designs are available, such as a moving vario-magnet unit.
In a second embodiment illustrated in
The moving vario-magnet unit (306v) can move between the closest and furthest positions either continuously or at discrete positions, to vary the second length, L2, between L+ and (L+)+½ λ, so as to obtain an energy gain at the next crossing of the resonant cavity comprised between wi and −wi. In the example of
The energy of the extracted electron beam can thus be set to any value comprised between 5 and 7 MeV in the example illustrated in
A continuous moving is advantageous for a higher flexibility on the control of the energy of the extracted electron beam. On the other hand, a number of predefined discrete positions is easier to use for an operator, with second lengths, L2, strategically predefined as L2=(L+)+(n/m) λ, wherein n/m defines simple fractions with n and m∈N and n≤m≤6.
As illustrated in
Alternatively, deflectors (30d) as discussed supra can be used instead. The deflectors (30d) orient the trajectories of an electron beam between the cavity outlet/inlet apertures and the vario-magnet unit into straight segments parallel to the bisector of the angle formed by the first and second radial trajectories at the central axis, Zc. This embodiment is advantageous as it permits to keep constant the magnetic field generated by the vario-magnet unit regardless of the position of the vario-magnet unit (306v). The second length, L2, of the second deflecting trajectory is therefore simply equal to L2=(L+)+2 r, wherein r is the distance increase of the vario-magnet unit to the central axis, Zc (cf.
The moving means of the moving vario-magnet unit (306v) may comprise a motor for displacing back and forth the the moving vario-magnet unit (306v) along the corresponding bisecting direction.
A rhodotron comprising N magnet units, of which (N−1) are non-vario magnet units (301-305) and one only is a vario-magnet unit (306-5, 306-7, 306v) positioned directly upstream of the outlet (50) can extract an electron beam of energy ranging between wi (N±1). Each time an electron beam crosses the resonant cavity of a rhodotron illustrated in
The magnet units in conventional rhodotrons are generally provided with electro-magnets. It has been discussed in EP3319402 that magnet units provided with permanent magnets could be used instead. A rhodotron according to the present invention may comprise electro-magnets only, permanent magnets only, or a combination of electro-magnets and permanent magnets.
As discussed in EP3319402, permanent magnets have the advantage over electro-magnets of decreasing the energy consumption of the rhodotron since, contrary to electro-magnets, permanent magnets need not be powered. Permanent magnets can be coupled directly against the outer wall of the resonant cavity, whilst the coils of electro-magnets must be positioned at a distance of the outer wall. By allowing the magnet units to be directly adjacent to the outer wall, the construction of the rhodotron is greatly simplified and the production cost reduced accordingly.
One major drawback of permanent magnets is that the magnetic field cannot be varied as easily as with electro-magnets. As illustrated in
By contrast, the magnitude of the magnetic field generated by electro-magnets is very easy to control by controlling the electric current fed to the coils of the electro-magnets. They are, however, bulky and need wiring which complexifies the production of the rhodotron. A combination of electro-magnets and permanent magnets can therefore be used to profit of the advantages and avoid the drawbacks of each type of magnets. In a preferred embodiment, all magnet units comprise permanent magnets, but the ones requiring frequent tuning of the magnetic field. These include, for example,
As described in EP3319403 the rhodotron can have a modular construct as illustrated in the exploded view of
As visible in
In a preferred embodiment illustrated in
With the present invention, it is now possible to extract electron beams of different energies along a single extraction path. This solution is very advantageous to the industry in that a single rhodotron can be used for different applications, such sterilizing medical devices, or treating different foodstuff, by a single tuning of the one or more vario-magnet units.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18208924.3 | Nov 2018 | EP | regional |
