A vario-energy electron accelerator includes a resonant cavity consisting of a closed conductor, an electron source injecting a beam of electrons into the resonant cavity, an RF system coupled to the resonant cavity and generating an electric field in the resonant cavity, magnet units centred on a mid-plane and generating a field in a deflecting chamber in fluid communication with the resonant cavity, the magnetic field deflecting along a first deflecting trajectory of adding length an electron beam exiting the resonant cavity along a first radial trajectory to reintroduce it into the resonant cavity along a second radial trajectory, an outlet for extracting along an extraction path an accelerated electron beam from the resonant cavity towards a target, wherein at least one of the magnet units is adapted for modifying the first deflecting trajectory to a second deflecting trajectory, allowing a variation of the energy of the electron beam.