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A magnetic switching system replacing thyratron of the kicker magnet power supply in the J-PARC 3 GeV rapid cycling synchrotron (RCS) was studied for stable operation. The core of the magnetic switch is required to have a square B-H loop as an ideal switch. Nano-crystalline alloy (Finemet; FT-1H) was selected for the magnetic core. B-H curves at high magnetization rate (dB/dt) were measured. Magnetization rate dependence of magnetic field and permeability were evaluated based on magnetic domain models. We speculate that the magnetization progress is dominated by the bar-domain mode in the range of magnetization rate we measured. Low leakage current of the magnetic core before saturation flows to the kicker magnets. Closed orbit distortion, , in the RCS caused by the leakage current was evaluated. Calculated results show that the can be suppressed sufficiently. A circuit of the magnetic switching system was designed. A Spice model of the magnetic switching system is described and the simulation results are shown.