The invention relates to an electrostatic beam blanker for a particle-optical apparatus, in which the blanker is used to generate a train of pulses with a fixed repetition rate. Such pulse trains with a sub-picosecond pulse length are for example used in the study of chemistry in the femtosecond scale.The beam blanker according to the invention uses a resonant structure, as a result of which the voltage is amplified by the quality factor Q of the resonant structure. During each zero-crossing of the signal, thus twice per period of the resonant frequency, the beam is transmitted, and the beam is blanked during the rest of the time. In a preferred embodiment the resonant structure comprises a transmission line. Impedance matching of signal source and resonant structure may be performed by tuning stubs.