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Summary form only given. 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. The plasma can be touched by bare hand without any feeling of electrical shock or warmth. The maximum length of the plasma is about 2 cm long. The whole device including the power supply is less than 1 kg. The I-V characteristics of the plasma are investigated in detail. It is found that the pulse behavior of the plasma is determined by the ion drift mechanism. The device has no transient spark discharge mode at all. The effects of the applied voltage Vi and gap distance d on repetitive frequency f and peak value Ipeak of the current pulse are also studied. It is found that the applied voltage Vι affects the Ipeak dramatically, but not the repetitive frequency f When the applied voltage is higher than 12 kV, the voltage V2 on the electrode just before breakdown starts to decrease with the increase of the Vι. Detail analysis shows that the residual charges play important role in the initiation of breakdown under this condition. On the other hand, when the applied voltage is lower than 12 kV, V2 just before breakdown keeps constant for various Vι, it indicates that the residual charges play minor role in the initiation of breakdown for this circumstance. As to the gap distance, it affects the repetitive frequency f significantly, but not the Ipeak. It is found that the repetition frequency is determined by the time for the ion drift to the electrode. Further analysis shows that the average electric field keeps constant for various gap distances. The repetition frequency is inversely proportional to gap distance, which is exactly observed in the experiments. Finally, the preliminary inactivation experiment results are presented. It shows that the plasma can effectively kill Enterococcus faecalis, one of the main types of bacterium causing t- e failure of root canal treatment.