A method of bunching a charged particle beam including the step of generating an electric field during a certain time duration, the electric field being generally parallel to a travelling direction of the charged particle beam having a kinetic energy of 1 eV to 1 MeV, and an intensity of the electric field changing as a quadratic function of time. In addition to the waveform changing as the quadratic function, a waveform changing linearly may also be used. The linearly changing waveform may be superposed upon a waveform of a half period from phase &pgr; to 2&pgr; of a sine wave if the linearly changing waveform monotonously increases, or upon a waveform of a half period from phase 0 to &pgr; of a sine wave if the linearly changing waveform monotonously decreases. Furthermore, the linearly changing waveform and the sine waveform may be superposed upon a waveform of a half period from phase 0 to &pgr; of a cosine wave if the linearly changing waveform monotonously increases, or upon a waveform of a half period from phase &pgr; to 2&pgr; of a cosine wave if the linearly changing waveform monotonously decreases. The charged particle beam having a non-relativistic energy can be shaped into an ultra short pulse.
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