A high‐voltage pulse output modulator using a step‐up transformer with two stages of magnetic pulse compression circuits has been developed as a spiker for the purpose of obtaining the breakdown of the gas mixture when using the spiker‐sustainer excitation technique for pumping XeCl discharge lasers. When the transformer input is fed by a current pulse with a peak value and a full width of 1.16 kA and 1.6 &mgr;s, respectively, formed by a discharge of an initially charged 19‐kV, 57‐nF capacitance in series in the transformer primary loop, the 2.7‐nF output capacitor bank (Cp) of the spiker can be charged up to 67 kV with a (10%–90%) rise time of 59 ns. The modulator has been tested at a repetition rate of up to 600 Hz in burst mode using a resistor in parallel withCpas the load.