Accelerator apparatus, being configured for accelerating charged particles with pulsed radiation, comprising - a horn-shaped coupling device with at least one horn coupler (11, 15) having an input aperture, electrically conductive walls and an output aperture and being configured for receiving the pulsed radiation at the input aperture and focusing the pulsed radiation along a longitudinal beam direction towards the output aperture, and - a waveguide device coupled with the output aperture of the at least one horn coupler (11, 15) being configured for receiving the focused pulsed radiation, wherein - the waveguide device includes an injection section being configured for providing a bunch of charged particles and for subjecting the charged particles to an acceleration by the pulsed radiation in the injection section, and - the waveguide device includes a lateral output port being arranged at the injection section for releasing the accelerated charged particles along a particle acceleration direction, characterized in that - the at least one horn coupler (11, 15) is adapted for receiving linearly polarized single cycle pulses formed by plane waves having a Gaussian shape along at least one direction perpendicular to the longitudinal beam direction, including a broadband frequency spectrum with a centre frequency in the THz range or in the RF range and having a duration equal to or below one oscillation cycle of the electromagnetic field at the centre frequency and being shaped as a linearly polarized plane wave and for focusing the linearly polarized single cycle pulses, wherein the at least one horn coupler (11, 15) has an input aperture size, a longitudinal length, a horn angle and an output aperture size being adapted for focusing the single cycle pulses and wherein the input aperture of the horn coupler (11, 15) has a rectangular shape with side lengths larger than a centre wavelength of the pulsed radiation, the output aperture of the horn coupler (11, 15) has a rectangular shape with side lengths of half the centre wavelength of the pulsed radiation, and the walls have the horn angle close to the total angular spread of the incoming pulsed radiation, - the waveguide device has a non-resonant broadband transmission characteristic, and is arranged such that - the particle acceleration direction is perpendicular to the longitudinal beam direction. - a horn-shaped coupling device with at least one horn coupler (11, 15) having an input aperture, electrically conductive walls and an output aperture and being configured for receiving the pulsed radiation at the input aperture and focusing the pulsed radiation along a longitudinal beam direction towards the output aperture, and - a waveguide device coupled with the output aperture of the at least one horn coupler (11, 15) being configured for receiving the focused pulsed radiation, wherein - the waveguide device includes an injection section being configured for providing a bunch of charged particles and for subjecting the charged particles to an acceleration by the pulsed radiation in the injection section, and - the waveguide device includes a lateral output port being arranged at the injection section for releasing the accelerated charged particles along a particle acceleration direction, - the at least one horn coupler (11, 15) is adapted for receiving linearly polarized single cycle pulses formed by plane waves having a Gaussian shape along at least one direction perpendicular to the longitudinal beam direction, including a broadband frequency spectrum with a centre frequency in the THz range or in the RF range and having a duration equal to or below one oscillation cycle of the electromagnetic field at the centre frequency and being shaped as a linearly polarized plane wave and for focusing the linearly polarized single cycle pulses, wherein the at least one horn coupler (11, 15) has an input aperture size, a longitudinal length, a horn angle and an output aperture size being adapted for focusing the single cycle pulses and wherein the input aperture of the horn coupler (11, 15) has a rectangular shape with side lengths larger than a centre wavelength of the pulsed radiation, the output aperture of the horn coupler (11, 15) has a rectangular shape with side lengths of half the centre wavelength of the pulsed radiation, and the walls have the horn angle close to the total angular spread of the incoming pulsed radiation, - the waveguide device has a non-resonant broadband transmission characteristic, and is arranged such that - the particle acceleration direction is perpendicular to the longitudinal beam direction.