Patent Number: 
Section: claims

1. An electron accelerator comprising:a resonant cavity comprising a hollow closed conductor, wherein:the conductor comprises an outer wall having an outer cylindrical portion of central axis, and an inner surface forming an outer conductor section;the conductor comprises an inner wall enclosed within the outer wall and having an inner cylindrical portion of central axis and an outer surface forming an inner conductor section andthe resonant cavity is symmetrical with respect to a mid-plane normal to the central axis and intersects the outer cylindrical portion and inner cylindrical portion;an electron source configured to radially inject a first beam of electrons into the resonant cavity from an introduction inlet opening on the outer conductor section to the central axis along the mid-plane;an RF system coupled to the resonant cavity and configured to generate an electric field between the outer conductor section and the inner conductor section, the electric field oscillating at a frequency to accelerate electrons of the first beam of electrons along radial trajectories in the mid-plane extending from the outer conductor section towards the inner conductor section and from the inner conductor section towards the outer conductor section;a magnet unit comprising a deflecting magnet comprising first and second permanent magnets positioned on either side of the mid-plane and configured to generate a magnetic field in a deflecting chamber in fluid communication with the resonant cavity by a first deflecting window, the magnetic field being configured to deflect a second electron beam emerging out of the resonant cavity through the first deflecting window along a first radial trajectory in the mid-plane and to redirect the second electron beam into the resonant cavity through one of the first deflecting window or a second deflecting window towards the central axis along a second radial trajectory in the mid-plane, the second radial trajectory being different from the first radial trajectory,wherein:the resonant cavity further comprises a first half shell, a second half shell, and a central ring element;the first half shell has a cylindrical outer wall having an inner radius and a central axis,the second half shell has a cylindrical outer wall having an inner radius and a central axis;the central ring element has an inner radius sandwiched at the level of the mid-plane between the first and second half shells; andthe surface forming the outer conductor section is 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. 2. The electron accelerator of claim 1, wherein a portion of the central ring element extends radially beyond an outer surface of the outer wall of the first and second half shells, and wherein the magnet unit is fitted onto the portion of the central ring element. 3. The electron accelerator of claim 2, wherein:the deflecting chamber of the magnet unit is formed by a hollowed cavity in a thickness of the central ring element; andthe first and second deflecting windows are formed in the inner edge of the central ring element, facing the central axis. 4. The electron accelerator of claim 3, further comprising N magnet units, with N>1, wherein the deflecting chambers of the magnet units are formed by individual hollowed cavities in the thickness of the central ring element, with N deflecting windows being formed in the inner edge of the central ring element, facing the central axis. 5. The electron accelerator of claim 3, wherein:the central ring element is made of a ring shaped plate comprising first and second main surfaces separated by a thickness of the ring shaped plate; andeach cavity is formed by a recess open at the first main surface and at the inner edge of the ring shaped plate, the central ring element having a cover plate coupled to the first main surface to seal the recess and to form a cavity opened only at the inner edge to form the first and second deflecting windows. 6. The electron accelerator of claim 1, wherein the first and second half shells have an identical geometry and are coupled to the central ring element with a seal providing tightness of the resonant cavity. 7. The electron accelerator of claim 6, wherein:each of the first and second half shells comprises the cylindrical outer wall, a bottom lid, and a central pillar jutting out of the bottom lid; andan outer surfaces of the central pillars of the first and second half shells forms a portion of the inner conductor section. 8. The electron accelerator of claim 7, further comprising:a central chamber sandwiched between the central pillars of the first and second half shells, the central chamber comprising a cylindrical peripheral wall of the central axis, with openings radially aligned with corresponding first and second windows and the introduction inlet opening;wherein the surface forming the inner conductor section is formed by outer surfaces of the central pillars and by the peripheral wall of the central chamber. 9. The electron accelerator of claim 1, wherein the RF system is coupled to the first half shell, and wherein the central ring and central chamber are mounted onto the first half shell with different angular orientations about the central axis so as to vary the orientation of an electron beam outlet for discharging the electron beam out of the resonant cavity at a desired energy. 10. The electron accelerator of claim 1, wherein the first and second magnets are permanent magnets. 11. The electron accelerator of claim 10, wherein:the first and second permanent magnets are formed by a plurality of discrete magnet elements; andthe magnet elements are in the shape of prisms, arranged side by side in an array parallel to the mid-plane to form rows of discrete magnet elements, the magnet elements being disposed on either side of the deflecting chamber with respect to the mid-plane. 12. The electron accelerator of claim 10, further comprising N magnet units and N-n first and second deflecting magnets, wherein:N is greater than 1;the first and second deflecting magnets are permanent magnets; andn is between 0 and N−1. 13. The electron accelerator of claim 10, wherein:the magnet unit forms a magnetic field in the deflecting chamber; andthe magnetic field is between 0.05 T and 1.3 T. 14. The electron accelerator of claim 13, wherein the magnetic field is between 0.1 T and 0.7 T. 15. A method of accelerating electrons, the method comprising:providing a resonant cavity comprising a hollow closed conductor, wherein:the conductor further comprises an outer wall having an outer cylindrical portion of central axis, and an inner surface forming an outer conductor section;the conductor further comprises an inner wall enclosed within the outer wall and having an inner cylindrical portion of central axis, and an outer surface forming an inner conductor section; andthe resonant cavity is symmetrical with respect to a mid-plane normal to the central axis and intersects the outer cylindrical portion and inner cylindrical portion;radially injecting, by an electron source, a first beam of electrons into the resonant cavity from an introduction inlet opening on the outer conductor section to the central axis along the mid-plane;generating, by an RF system coupled to the resonant cavity, an electric field between the outer conductor section and the inner conductor section, the electric field oscillating at a frequency so as to accelerate electrons of the first beam of electrons along radial trajectories in the mid-plane, the trajectories extending from the outer conductor section towards the inner conductor section and from the inner conductor section towards the outer conductor section; andgenerating, by a magnetic unit comprising a deflecting magnet composed of first and second permanent magnets positioned on either side of the mid-plane, a magnetic field in a deflecting chamber, the deflecting chamber being in fluid communication with the resonant cavity via a first deflecting window, the magnetic field being configured to:deflect a second electron beam emerging out of the resonant cavity through the first deflecting window along a first radial trajectory in the mid-plane, andredirect the second electron beam into the resonant cavity through one of the first deflecting window or a second deflecting window towards the central axis along a second radial trajectory in the mid-plane the second radial trajectory being different from the first radial trajectory; wherein:the resonant cavity further comprises a first half shell, a second half shell, and a central ring element;the first half shell has a cylindrical outer wall, having an inner radius and a central axis;the second half shell has a cylindrical outer wall, having an inner radius and a central axis;the central ring element has an inner radius, sandwiched at the level of the mid-plane, between the first and second half shells; andthe surface forming the outer conductor section is formed by:an inner surface of the cylindrical outer wall of the first and second half shells; andan inner edge of the central ring element. 16. The method of claim 15, further comprising coupling the RF system to the first half shell;wherein the central ring and central chamber are mounted onto the first half shell with different angular orientations about the central axis so as to vary the orientation of an electron beam outlet to discharge the electron beam out of the resonant cavity at a desired energy. 17. The method of claim 15, further comprisingradially extending a portion of the central ring element beyond an outer surface of the outer wall of the first and second half shells; andfitting the magnet unit onto the portion of the central ring element. 18. The method of claim 17, wherein:the deflecting chamber of the magnet unit comprises a hollowed cavity in a thickness of the central ring element; andthe first and second deflecting windows are formed in the inner edge of the central ring element, facing the central axis. 19. The method of claim 15, wherein:each of the first and second half shells further comprises the cylindrical outer wall, a bottom lid, and a central pillar jutting out of the bottom lid; andan outer surface of the central pillars of the first and second half shells form a portion of the inner conductor section. 20. The method of claim 19, further comprising providing a central chamber sandwiched between the central pillars of the first and second half shells, wherein:the central chamber comprises a cylindrical peripheral wall of central axis, with openings radially aligned with corresponding first and second deflecting windows and the introduction inlet opening, andthe surface forming the inner conductor section is formed by an outer surface of the central pillars and by the peripheral wall of the central chamber sandwiched therebetween.