Influence of voltage duty ratio on current asymmetry and mode of a helium dielectric-barrier discharge excited by a modulated voltage

Excited by a modulated sinusoidal voltage, dielectric barrier discharge in atmospheric pressure helium has been investigated through a one-dimensional fluid model. Under a lower voltage amplitude, the discharge presents one current pulse per half sinusoidal cycle. The positive discharge is weaker than the negative one if the duty ratio of the modulated voltage is lower. When increasing the duty ratio, the intensity of the positive discharge increases and that of the negative discharge keeps approximately constant, which implies that the discharge symmetry is improved. A similar phenomenon regarding the discharge symmetry is also observed under a higher voltage amplitude despite the fact that a secondary discharge following a primary one is presented per half sinusoidal cycle. Besides the duty ratio, the discharge symmetry can be improved by increasing the voltage amplitude, especially under a lower duty ratio. These phenomena are qualitatively explained by analyzing the variation of the electron density when increasing the duty ratio. Results also indicate that the positive primary discharge at the peak moment belongs to a Townsend mode or a glow one, which is determined by the duty ratio and the voltage amplitude. Finally, the critical voltage for the Townsend mode transiting to the glow one is investigated as a function of the duty ratio.