Patent Application
20180132342
The exemplary embodiments of the present disclosure relate 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 paths. An accelerator, commercially available from IBA Industrial and Sterilization Solutions, is an example of such electron accelerator. An electron accelerator according to the present embodiments can be more compact and may have lower power requirements than conventional accelerator. This allows for the first time to provide a mobile electron accelerator. The element constituting the electron accelerator may be designed to provide a more efficient and versatile fabrication.
Electron accelerators having a resonant cavity are well known in the art. For example, EP0359774 describes an electron accelerator comprising:
As shown on
An acceleratorcan be combined to external equipment such as a beam line and a beam scanning system. An accelerator can be used for sterilization, polymer modification, pulp processing, cold pasteurization of food, detection and security purposes, etc.
Today, the known accelerators are of large size, have a high production cost, and require a high power source of energy to use them. They are designed for sitting at a fixed location and with predetermined configuration. Application of an electron beam at different locations requires drawing an additional beam line, with all additional costs and technical problems associated with.
There is a demand in the industry for smaller, more compact, versatile and lower cost accelerators consuming less energy and which are preferably mobile units. Smaller diameter resonant cavities, however, require a higher power for accelerating electrons over shorter distances which is detrimental to the energy consumption of such compact accelerators. Independent of the size of a accelerator, energy consumption can be reduced by alimenting the RF source and by accelerating electrons during a fraction only of the duty cycle of the rhodotron as described in EP2804451. Even thus, however, energy consumption is higher with smaller resonant cavities.
A resonant cavity with smaller diameter also has a smaller outer circumference which reduces the space available for connecting the electron source and all the electromagnets of the magnet system to the resonant cavity. The production of small compact accelerator is more complex and more expensive than conventional accelerators.
The exemplary embodiments of the present disclosure relate to a compact accelerator requiring low energy, which is mobile, and which is cost-effective to produce. These advantages are described in more details in the following sections.
In particular, the exemplary embodiments of the present disclosure concern an electron accelerator comprising a resonant cavity, an electron source, an RF system, and at least one magnet unit.
The resonant cavity may consist of a hollow closed conductor comprising:
The electron source can be adapted for radially injecting a beam of electrons into the resonant cavity, from an introduction inlet opening on the outer conductor section to the central axis, Zc, along the mid-plane, Pm.
The RF system can be coupled to the resonant cavity and can be adapted for generating an electric field, E, between the outer conductor section and the inner conductor section, oscillating at a frequency (fRF), to accelerate the electrons of the electron beam along radial trajectories in the mid-plane, Pm, extending from the outer conductor section towards the inner conductor section and from the inner conductor section towards the outer conductor section.
The at least one magnet unit may comprise a deflecting magnet composed of first and second permanent magnets positioned on either side of the mid-plane, Pm, and may be adapted for generating a magnetic field in a deflecting chamber in fluid communication with the resonant cavity by at least one deflecting window, the magnetic field being adapted for deflecting an electron beam emerging out of the resonant cavity through the at least one deflecting window along a first radial trajectory in the mid-plane, Pm, and to redirect the electron beam into the resonant cavity through the at least one deflecting window or through a second deflecting window towards the central axis along a second radial trajectory in the mid-plane, Pm, the second radial trajectory being different from the first radial trajectory.
Each of the first and second permanent magnets may preferably be formed by a number of discrete magnet elements, arranged side by side in an array parallel to the mid-plane, Pm, comprising one or more rows of discrete magnet elements and disposed on either side of the deflecting chamber with respect to the mid-plane, Pm. This may allow fine tuning of the magnetic field by addition or removal of one or several of such discrete magnet elements. Preferably, the discrete magnet elements may be in the shape of prisms such as rectangular cuboids, cubes or cylinders.
The magnet unit can further comprise first and second support elements each comprising a magnet surface supporting the discrete magnet elements, and a chamber surface separated from the magnet surface by a thickness of the support element. The chamber surface may form or be contiguous to a wall of the deflecting chamber. Preferably, the chamber surface and magnet surface of each of the first and second support elements may be planar and parallel to the mid-plane, Pm. Depending on the number of discrete elements required for creating a magnetic field of desired magnitude, the chamber surface of each of the first and second support elements may have a surface area smaller than the surface area of the magnet surface. In this case, each of the first and second support elements may preferably comprise a tapered surface remote from the resonant cavity and joining the magnet surface to the chamber surface.
The electron accelerator of the present disclosure may also comprise a tool for adding or removing discrete magnet elements to or from the magnet surfaces of the first and second support elements. The tool may comprise an elongated profile, preferably a L-profile or a C profile, for receiving a number of discrete magnet elements desired in a given row of the array, and an elongated pusher, slidingly mounted on the elongated profile, for pushing the discrete magnet elements along the elongated profile.
The magnet unit can also comprise a yoke holding the first and second support elements at their desired position. Preferably, the yoke may allow fine tuning the position of the first and second support elements.
In a preferred embodiment, the resonant cavity of the electron accelerator of the present disclosure is formed by:
Each of the first and second half shells may comprise the cylindrical outer wall, a bottom lid, and a central pillar jutting out of the bottom lid. The electron accelerator can further comprise a central chamber sandwiched between the central pillars of the first and second half shells. The central chamber may further comprise a cylindrical peripheral wall of central axis, Zc, with openings radially aligned with corresponding deflecting windows and the introduction inlet opening. In a preferred embodiment, the surface forming the inner conductor section may be formed by an outer surface of the central pillars and by the peripheral wall of the central chamber sandwiched therebetween.
A portion of the central ring element may extend radially beyond an outer surface of the outer wall of both first and second half shells. This is advantageous in that the at least one magnet unit can thus be fitted onto said portion of the central ring element.
The deflecting chamber of the at least one magnet unit can be formed by a hollowed cavity in a thickness of the central ring element, with the deflecting window being formed at the inner edge of the central ring element, facing the centre of the central ring element.
In a preferred embodiment, an electron accelerator according to the present disclosure, may comprise N magnet units, with N>1, and the deflecting magnets of n magnet units are composed of first and second permanent magnets, with 1≤n≤N.
Preferably, the at least one magnet unit may form a magnetic field in the deflecting chamber comprised between 0.05 T and 1.3 T, preferably 0.1 T to 0.7 T.
These and further embodiments of the present disclosure 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) may comprise:
The resonant cavity (1) may be 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 can ignore 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 may form a hollow closed conductor in the shape of a toroidal volume.
The mid-plane, Pm, can be vertical, horizontal or have any suitable orientations with respect to the ground on which the rhodotron rests. In another embodiment, it is vertical.
The resonant cavity (1) may further comprise openings for connecting the RF system (70), and the vacuum system (not shown). These openings may preferably be made in at least one of the two bottom lids (11b, 12b).
The outer wall may further comprise openings intersected by the mid-plane, Pm. For example, the outer wall may comprise an introduction inlet opening for introducing an electron beam (40) in the resonant cavity (1). It may further comprise an electron beam outlet (50) for discharging out of the resonant cavity the electron beam (40) accelerated to a desired energy. Additionally, the outer wall may further comprise deflecting windows (31w), bringing in fluid communication the resonant cavity with corresponding deflecting chamber (31, see below). Generally, an accelerator may comprise several magnet units and several deflecting windows.
An accelerator can generally accelerate the electrons of an electron beam to energies which can be comprised between 1 and 50 MeV, preferably between 3 and MeV, more preferably between 5 and 10 MeV.
The inner wall may comprise openings radially aligned with corresponding deflecting windows (31w) permitting the passage of an electron beam through the inner cylindrical portion along a rectilinear radial trajectory.
The surface of the resonant cavity (1) comprising a hollow closed conductor can be 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 1.2 m, more preferably between 0.5 m and 0.7 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 diameter of an accelerator including a resonant cavity (1), an electron source (20), a vacuum system, a RF system (70), and one or more magnet units, 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 accelerator 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.
The electron source (20) can be 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 can be an electrical component that produces a narrow, collimated electron beam that has a precise kinetic energy.
The vacuum system may comprise a vacuum pump for pumping air out of the resonant cavity (1) and creating a vacuum therein.
The RF system (70) may be coupled to the resonant cavity (1) via a coupler and typically may comprise an oscillator designed for oscillating at a resonant frequency, fRF, for generating an RF signal, 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 may generate a resonant radial electric field, E, in the resonant cavity. The resonant radial electric field, E, may oscillate 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 deflecting window (31w). The resonant radial electric field, E, may generally be of the “TE001” type, for which the electric field may be transverse (“TE”), may have a symmetry of revolution (first “0”), may not be cancelled out along one radius of the cavity (second “0”), and may be a half-cycle of said field in a direction parallel to the central axis Z.
The magnet system may comprise at least one magnet unit (301) comprising a deflecting magnet composed of first and second permanent magnets (32) positioned on either side of the mid-plane, Pm, and adapted for generating a magnetic field in a deflecting chamber (31). The deflecting chamber may be in fluid communication with the resonant cavity (1) by at least one deflecting window (31w).
Preferably, the magnet system may comprise 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. The number N of magnet units may correspond to (N+1) accelerations of the electrons of an electron beam (40) before it exits the accelerator with a given energy. For example,
The electron beam may be injected in the resonant cavity by the electron source (20) through the introduction inlet opening along the mid-plane, Pm. It may follow a radial trajectory in the mid-plane, Pm, said trajectory crossing:
While conventional accelerators use electro-magnets in the magnet units used for deflecting the trajectories of an electron beam back into the resonant cavity, an accelerator according to the present invention differs from such conventional accelerators in that the deflecting magnet of at least one magnet unit (30i) may be composed of first and second permanent magnets (32).
Generally, an accelerator may comprise more than one magnet unit (30i). In a preferred embodiment comprising a total of N magnet units, with N>1, n magnet units may comprise a deflecting magnet composed of first and second permanent magnets (32), with 1≤n≤N. For example, the accelerator illustrated in
An accelerator preferably may comprise at most one electro-magnet. For example, the first magnet unit (301) located opposite the electron source (20) can differ from the other (N−1) magnet units, because the electron beam reaches said first magnet unit at a lower speed than the other magnet units. In order to return the electron beam into the resonant cavity in phase with the oscillating electric field, the deflection path in the first magnet unit may need to be slightly different from the (N−1) remaining magnet units. The first magnet unit (301) can therefore be an electro-magnet, allowing an easy fine tuning of the magnetic field generated in the corresponding deflection chamber (31).
Changing from conventional accelerators with all magnet units being equipped with electro-magnets to an accelerator according to the present disclosure wherein at least one magnet unit and preferably several magnet units may be equipped with permanent magnets may appear with hindsight to be an easy step, but this is not the case and a person of ordinary skill in the art would have a strong prejudice against taking such step for the following reasons. An accelerator is a very sophisticated piece of equipment, requiring accurate fine tuning to ensure that the electron beam follows the flower shaped path illustrated in
The radius of the circular path followed by the electron beam in the deflecting chamber may depend on the magnitude of the magnetic field created between the first and second permanent magnets (32) of the deflecting magnet. Fine tuning of said magnetic field in each and every magnet unit of the accelerator is essential to ensure that the electron beam follows the pre-established flower-shaped path in phase with the oscillating electric field. This can be achieved with an electro-magnet by controlling the current sent into the coils. Any deviation in the deflecting path of the electron beam at one magnet unit may be reproduced and amplified in the other magnet units, to a point that the final radial trajectory of the electron beam may be offset from the electron beam outlet (50) thus rendering the accelerator inoperable and dangerous.
A permanent magnet, by contrast, generates a given magnetic field which is intrinsic to the material used and can only be varied by changing the volume of the permanent magnet. A person of ordinary skill in the art therefore has a strong prejudice against using a permanent magnet for any of the magnet units of an accelerator, since fine tuning of the magnetic field in the deflecting chamber seems impossible, or at least much more difficult than with an electro-magnet. Chopping bits or pieces off a permanent magnet is not a viable option, as it lacks control and reproducibility. For this reason alone, it is not obvious to a person of ordinary skill in the art to replace an accelerator's magnet unit equipped with a deflecting magnet composed of first and second electro-magnets by a magnet unit equipped with a deflecting magnet composed of a first and a second permanent magnets (32), as fine tuning of the magnetic field for ensuring a proper functioning of the accelerator may not be achievable.
In the present invention, the deflecting magnet of at least one magnet unit (30i) may be composed of first and a second permanent magnets (32). The skilled person's prejudice of the absence of fine tuning the magnetic field in the deflecting chamber may be overcome in the present disclosure by the following preferred embodiment. As illustrated in
By varying the number of discrete magnet elements in each array, the magnetic field created in the deflecting chamber can be varied accordingly. For example, 12×12×12 mm cubes of an Nd—Fe—B permanent magnet material can be stacked two by two to form rectangular cuboid discrete magnet elements of dimensions 12×12×24 mm Other magnetic materials can be used instead, such as ferrite or Sm—Co permanent magnets. One such discrete magnet element disposed on opposite sides of the deflecting chamber can create a magnetic field of about 3.9 10−3 Tesla (T) (=38.8 Gauss (G), with 1 G=10−4 T). For a desired magnetic field, Bz, of about 0.6 T (=6060 G), 156 such discrete magnet elements may be needed on either side of the deflecting chamber. They can be arranged in 12×13 array. The magnetic field, Bz, in the deflecting chamber can thus be tuned by discrete steps of 3.9 10−3/6 10−1=0.6%, by adding or removing one by one discrete magnet elements into or from the arrays. The graph in
With the essential fine tuning of the magnetic field in the individual deflecting chambers being made possible using permanent magnets made of arrays of discrete magnet elements, the use of permanent magnets may offer several advantages over the use of electro-magnets. First, the overall energy consumption of the accelerator can be reduced, since permanent magnets need not be powered. This is advantageous for mobile units, which are to be connected to energy sources with limited power capacity. As discussed supra, even by alimenting the RF source during a fraction only of the duty cycle of the accelerator as described in EP2804451, the power needs of an accelerator increases with decreasing diameter, 2R, of the resonant cavity. Using permanent magnets therefore contributes to decreasing the energy consumption of the accelerator.
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 said outer wall. By allowing the magnet units to be directly adjacent to the outer wall, the construction of the accelerator can be greatly simplified and the production cost reduced accordingly as is described later with reference to
When during use, a conventional accelerator equipped with electromagnets undergoes a power cut, the electromagnets cease to generate a magnetic field, but a remnant magnetic field persists caused by all the ferromagnetic components of a magnet unit. When power is restored, the whole equipment needs calibration in order to produce the desired magnetic fields in each magnet unit. This is a delicate process. Power cuts may not happen very often in fixed installations, but they become recurrent with mobile units, plugged to electric installations of varying capacities and qualities.
As shown in
The chamber surface and magnet surface of each of the first and second support elements may preferably be planar and parallel to the mid-plane, Pm. As shown in
For reasons of stability of the magnetic field, it is preferred to dimension the first and second support elements such as to reach saturation of the magnetic field in the support elements when they are loaded to their maximum capacity of discrete magnet elements.
The magnetic field required in the deflecting chamber may need to be sufficient for bending the trajectory of an electron beam exiting the resonant chamber along a radial trajectory through a deflecting window (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 an accelerator comprising nine (9) magnet units (30i) as illustrated in
The arrays of discrete magnet elements can therefore count a maximum number of rows comprised between 8 and 20 rows, preferably, between 10 and 15 rows, each row counting from 8 to 15 discrete magnet elements, preferably between 10 and 14 discrete magnet elements. With a higher number of discrete elements in each array, a finer tuning of the magnetic field, Bz, in the deflecting chamber can be performed.
Addition to or removal from a magnet surface of discrete magnet units can easily be performed with a tool specifically designed for this purpose. As illustrated in
Removal of a row or of part of a row of discrete magnet elements from an array can be realized very easily with the tool (60) by positioning it at the level of the row to be removed and pushing with the elongated pusher along the row to push the discrete magnet elements out at the other side of the row. With the tool (60) the magnetic field in a deflecting chamber can easily be varied, and even fine-tuned, by removal or addition of individual discrete magnet elements, or of whole rows of discrete magnet elements. This can be done either in plant, by the equipment provider, or in situ by the end user.
In order to hold the elements of the magnet units in place, such as the first and second support elements and, in particular to ensure that the magnetic circuit of a magnet unit is closed, with magnetic lines forming closed loops, the magnet units may comprise a yoke (35), illustrated in
As illustrated in
Conventional accelerators are generally positioned “horizontally,” i.e. with their mid-plane, Pm, being horizontal and parallel to the surface on which the accelerator rests. By rotating the accelerator about the (vertical) central axis, Zc, the electron beam outlet (50) can be directed in any direction along the mid-plane, Pm. It is not possible, however, to direct the electron beam outlet (50) out of the mid-plane (e.g., at 45° or vertically at 90° or 270° with respect to the mid-plane). Accelerators of the present embodiments are preferably positioned “vertically,” i.e., with the central axis, Zc, being horizontal and parallel to the surface on which the accelerator rests and, consequently, the mid-plane, Pm, being vertical. An accelerator unit installed in a vertical orientation can have several advantages. First, it can lead to a decrease of the area on the ground occupied by the accelerator. This can reduce the room required for the installation of a accelerator unit to the point that mobile accelerator units can be installed in the cargo of a lorry. Second, the vertical orientation of an accelerator can allow directing the electron beam outlet (50) in any directions of the space. The accelerator can be rotated about the (horizontal) central axis, Zc, such as illustrated on
To date, two accelerators with different configurations require re-designing individually many parts of the accelerators, said parts having to be tailored and produced individually. As mentioned supra, the present embodiments propose a totally innovative concept, including a set of elements or modules common to accelerators of any configuration. Different configurations of accelerators can be obtained by modifying the assembly of the elements, and not the elements per se. This way, the number of tools and moulds required for the production of accelerators can be reduced substantially, thus reducing the production costs.
The modular construction of accelerators according to the exemplary embodiments of the present disclosure is illustrated in the exploded view of
Referring to
As discussed supra, the resonant cavity can have a torus-like geometry of revolution. The whole inner surface of the resonant cavity can be made of a conductor material. In particular, the surface forming the outer conductor section (1o) can be formed by an inner surface of the cylindrical outer wall of the first and second half shells, and by an inner edge of the central ring element, which is preferably flush with the inner surfaces of both first and second half shells. The surface forming the inner conductor section (1i) can be formed by an outer surface of the central pillars and by the peripheral wall of the central chamber sandwiched therebetween.
As visible in
In a preferred embodiment, the deflecting chamber (31) of at least one magnet unit can be formed by a hollowed cavity in the thickness of the central ring element, with the deflecting window (31w) being formed at the inner edge of the central ring element, facing the centre of the central ring element and the central axis, Zc. Preferably, several deflecting chambers, more preferably all the deflecting chambers of the accelerator can be formed by individual hollowed cavities in the thickness of the central ring element, with the corresponding deflecting windows being formed in the inner edge of the central ring element, facing the central axis, Zc. This construction can substantially reduce the production costs of accelerators compared to conventional designs for the following reasons.
Because electro-magnets may comprise coils between which a magnetic field is formed, they cannot be located directly adjacent to the outer wall of the resonant cavity. The deflecting chambers in conventional accelerators, provided with electro-magnets are therefore manufactured as individual components, which are coupled to the resonant cavity by means of two pipes, one aligned with the radial trajectory of the electron beam leaving the resonant cavity, the other aligned with the radial trajectory of the electron beam entering back into the resonant cavity. The two pipes may need to be coupled at one end to the magnet unit and at the other end to the outer wall of the resonant cavity. Coupling of the pipes can be performed by one or more of welding, screwing, riveting, and the like. An sealing O-ring may be used to ensure tightness of the coupling. This coupling operation can only be performed manually by a skilled artisan. It is time consuming, quite expensive, and not devoid or risks of misalignments of the different components (tubes, chamber, etc.).
By using permanent magnets, the magnet units can be located directly adjacent to the outer wall of the resonant cavity. By providing the deflecting chambers as hollowed cavities in the thickness of the central ring element, they can all be machined automatically accurately out of a single ring shaped plate. The magnet units can then be coupled to the central ring over each deflecting chamber thus formed. These operations can be much more accurate, reproducible, quick, and cost effective than coupling each individual magnet unit to the outer resonant cavity by means of two welded pipes, as discussed above.
The deflecting chambers (31) can be formed cost effectively as follows. As discussed supra, the central ring element can be made of a ring shaped plate comprising first and second main surfaces separated by a thickness of the ring shaped plate. As shown in
In an alternative embodiment illustrated in
For further rationalizing the production of an accelerator, it is preferred that the first and second half shells have an identical geometry and are each coupled to the central ring element with sealing means (14) to ensure tightness of the resonant cavity. Half sells can thus be produced in series, regardless of whether they will form a first or a second half shell of the resonant cavity. Beside the cylindrical outer wall already mentioned, each of the first and second half shells can comprise a bottom lid (11b, 12b), and a central pillar (15p) jutting out of the bottom lid. The inner conductor section (1i) can be formed by the first and second pillars contacting when the first and second half shells are coupled on either side of the central ring element. Alternatively, as shown in
With the modules described above, a resonant cavity can be formed by assembling the second half shell (12) to the central ring element (13), by means well known in the art, such as bolts, rivets, welding, soldering. The formed assembly can be assembled to the first half shell with the central chamber sandwiched between the first and second pillars, to complete the resonant cavity provided with an introduction inlet opening, an electron beam outlet (50), and with a number of deflecting windows (31w) in fluid communication with deflecting chambers, and in radial alignment with corresponding openings in the cylindrical wall of the central chamber. With a portion of the central ring element (13) forming a flange extending radially outwards and enclosing the deflecting chambers, the magnet units can be coupled to said flange at the corresponding positions of the deflecting chambers. No electrical wiring may be required in the produced assembly, since the permanent magnets need not be powered. This can considerably reduce the cost of production and the cost of use.
The first half shell may comprise at least one opening for coupling to the RF system (70). If, as shown in
The various accelerator's configurations illustrated in
For a given energy of the electron beam produced by the accelerator (e.g., MeV in the accelerators of
For a given electron beam orientation (e.g., 0° in
All the different configurations illustrated in
It may now be possible to produce mobile accelerators, of relatively small dimensions, requiring fewer power connections. Such mobile accelerator can be loaded in a lorry and transported where it is needed. The lorry can also carry a power generator to be totally autonomous.
| Number | Date | Country | Kind |
|---|---|---|---|
| 16197603.0 | Nov 2016 | EP | regional |
