Instability in water vapor discharges*

In this study a moderate pressure glow discharge in 100% water vapor is characterized. Experiments are conducted at 8 Torr, an interelectrode spacing of ~ 5 cm, and a DC power supply. Voltage-current characteristic is measured over a broad current range. Measurements show that a normal glow is established at ~ 4.0 mA and continues up to 15 mA (the limit of the power supply). However, beyond ~ 6.0 mA several instabilities start appearing in the plasma system. At 6.0 mA a striated positive column appears with 7 strata which then decreases as the discharge current is increased. With the onset of a striated structure, oscillations in the discharge voltage and current are also observed. The striations are found to appear and disappear in sync with the current oscillations. As the discharge current is increased a distinct ionization wave is observed through high-speed imaging. The ionization wave travels from the cathode to the anode having a velocity of approximately 7 km/s. Estimates of drift velocities of negative ions differ by a factor ~ 1.5 from measured wave velocity. Analysis of the voltage and current wave forms reveal multiple frequencies with the dominant being in the 100 - 160 kHz range and the secondary and the tertiary frequencies appearing as harmonics of the primary.