The electron processing system (EPS) is widely applied for industrial purposes，such as radiation modification of polymers，medical product sterilization，and so on. In this EPS，a Cockcroft-Walton (CW) type high-voltage DC generator is often used and composes of the capacitors and diodes connected as cascade，and a step-up transformer. We have developed a new resonance type air-core transformer used as the step-up transformer. This transformer is operated in the resonance condition between the exciting inductance and the stray capacitance of CW circuit. The design concept of the resonance type air-core transformer is shown and the optimum structure of the transformer has been proposed. Adapting the air-core transformer to CW circuit，it results in downsizing of the capacitors to operate at the higher frequency than the conventional one. We have successfully demonstrated the excellent performances in the small DC-300kV 100mA model，and shown the approximately 40% reduction of the volume in the typically 1MV 100mA DC generator.

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