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It remains a challenge to generate cold air plasma at atmospheric pressure. In this paper, a room-temperature atmospheric-pressure air plasma plume is generated by a dc power supply. Current-voltage measurements show that the discharge actually appears periodically with a frequency of about 25 kHz. The discharge current has a pulsewidth that is less than 100 ns while its peak value reaches about 17 mA. There is no risk of glow-arc transition. The plasma can be touched by a bare hand without any feeling of electrical shock or warmth. The maximum length of the plasma is about 2 cm. The whole device, including the power supply, is less than 1 kg. The preliminary inactivation experiment results show that the plasma can effectively kill Enterococcus faecalis, one of the main types of bacterium causing the failure of root canal treatment.