An electron accelerator comprising: (a) a resonant cavity consisting of a hollow closed conductor comprising: • an outer wall comprising an outer cylindrical portion having a central axis, Zc, and having an inner surface forming an outer conductor section, and, • an inner wall enclosed within the outer wall and comprising an inner cylindrical portion of central axis, Zc, and having an outer surface forming an inner conductor section, the resonant cavity being symmetrical with respect to a mid-plane, Pm, normal to the central axis, Zc, and intersecting the outer cylindrical portion and inner cylindrical portion, (b) an electron source 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, (c) an RF system coupled to the resonant cavity and adapted for generating an electric field, E, between the outer conductor section and the inner conductor section, oscillating at a frequency (f RF), 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, (d) at least one magnet unit comprising a deflecting magnet 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, said second radial trajectory being different from the first radial trajectory, characterized in that, the deflecting magnet is composed of first and second permanent magnets positioned on either side of the mid-plane, Pm. (a) a resonant cavity consisting of a hollow closed conductor comprising: • an outer wall comprising an outer cylindrical portion having a central axis, Zc, and having an inner surface forming an outer conductor section, and, • an inner wall enclosed within the outer wall and comprising an inner cylindrical portion of central axis, Zc, and having an outer surface forming an inner conductor section, the resonant cavity being symmetrical with respect to a mid-plane, Pm, normal to the central axis, Zc, and intersecting the outer cylindrical portion and inner cylindrical portion, • an outer wall comprising an outer cylindrical portion having a central axis, Zc, and having an inner surface forming an outer conductor section, and, • an inner wall enclosed within the outer wall and comprising an inner cylindrical portion of central axis, Zc, and having an outer surface forming an inner conductor section, (b) an electron source 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, (c) an RF system coupled to the resonant cavity and adapted for generating an electric field, E, between the outer conductor section and the inner conductor section, oscillating at a frequency (f RF), 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, (d) at least one magnet unit comprising a deflecting magnet 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, said second radial trajectory being different from the first radial trajectory,