Atmospheric pressure dual RF-LF frequency discharge: transition from α to α-γ-mode

This paper investigates the transition from α to α-γ-mode of a dual frequency (5 MHz/50 kHz) dielectric barrier discharge (DBD) at atmospheric pressure. The study is based on both experiments and modeling of a plane/plane DBD in a Penning mixture (Ar-NH3). The discharge is in the α-RF mode with three different voltage amplitudes (250, 300 and 350 V) and biased by a low-frequency (LF) voltage with an amplitude varying from 0 to 1300 V. At a given threshold of LF voltage amplitude (of about 400 V for a 2 mm gap and 133 ppm of NH3), a transition from α to α-γ-mode occurs. It is characterized by a drastic increase of both the argon and NH emissions. Increasing the NH3 concentration leads to a decrease of the LF voltage amplitude required to reach the α-γ-mode (experiment). The transition from α to α-γ-mode is initiated when the ionization in the sheath increases and the α-γ-mode is established when this ionization becomes higher than the self-sustainment criterion (1/γ). The transition from α to α-γ-mode results in an increase of the particle densities and a stabilization of the gas voltage independently of the LF voltage amplitude. Without secondary electron emission there is no transition. In the model, increasing the secondary emission coefficient from 0.05 to 0.15 leads to a decrease of the LF voltage amplitude required to switch from α to α-γ-mode from 700 to 550 V.