In this work, we precisely measure the electrical properties of a low-temperature atmospheric-pressure helium plasma jet, such as the rf current and voltage, and analyze in detail its characteristics. The results show that the instantaneous plasma current and voltage are nearly in phase, which means that the plasma is purely resistive. The instantaneous plasma power is always active, and the average output power is the same as the plasma power. Thus, almost all the active energy flows instantaneously into the plasma. The large fraction of active energy that flows into the plasma is used for heating neutral particles in collisions. The number and polarity of the charged particles at the inner surface of the dielectric tube in the plasma electrode were estimated by integrating the plasma current. When the plasma discharge is maintained between the plasma electrode and target plate, the polarity is always negative with respect to the target plate. When the plasma is exposed to the ambient air without the target plate, the polarity is bipolar. We attribute this difference to the loss mechanism of plasma.
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