PURPOSE: To enable stable charge and discharge by mounting a power supply section supplying a coil section magnetically crossing with a saturable reactor with predetermined currents and adding a bias current circuit controlling the state of the operation of charging to a peaking capacitor of the saturable reactor.

CONSTITUTION: A bias current circuit 10 is constituted of a coil section 11 crossing with a saturable reactor 7 and a power supply section 12 supplying the coil section 11 with currents. The ON-OFF states of the saturable reactor 7 are determined by the sum of magnetic flux by currents ib flowing through a conductor 13 in a circuit except the bias current circuit 10 and currents ia. Consequently, when a large quantity of positive bias currents are made to flow previously, the saturable reactor 7 is turned ON even when currents ib are small. When negative bias currents are made to flow, inversely, the reactor 7 is not brought to the ON state even when currents ib are large because reverse magnetic flux is generated, thus freely controlling the ON-OFF states of the saturable reactor 7. Accordingly, desirable magnetic compression is conducted at all times even when breakdown voltage changes, thus remarkably improving discharge characteristics such as excitation efficiency.