Positive column dynamics of a low-current atmospheric pressure discharge in flowing argon

Abstract

We present the results of studies of a cold-plasma atmospheric pressure discharge in argon flow, in both DC and pulse-periodic modes. The burning voltage and the discharge current were 200 V and 15 mA in continuous mode; in pulse-periodic mode the voltage was 220 V and the current 15 mA for a pulse repetition rate of 100 kHz. Special attention was given to the positive column dynamics during the discharge stages of initiation, burning, and extinction. The main method of investigation employed optical diagnostics of the plasma in the visible range with high time resolution. The positive discharge column is shown to be a constricted current channel in the form of an elongated loop. In continuous mode, the length of the positive column, from anode attachment to inflection point outside the discharge system, is several millimeters. In pulsed mode, the positive column is an unsteady plasma formation, the length of which increases at about 4 m s−1, and reaches the same dimension over a time of order 1 ms. We find that the positive column plasma includes hydroxyl molecules OH, nitrogen N2, and atomic oxygen O I, along with atoms of the working gas argon. Observation of the time evolution of the optical emission from the plasma-forming gas shows that the plasma glow of the positive column is mainly determined by argon Ar I radiation, with maximum intensity at the trailing edge of the current pulse.