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

Abstract

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 modelling 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 αγ-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.

Role of excimer formation and induced photoemission on the Ar metastable kinetics in atmospheric pressure Ar–NH3 dielectric barrier discharges

“…Clearly, the model [25] predicts much slower kinetics than the one seen in the experiment, both on the rising and falling side. A similar departure between the model and the experiment can be seen in figure 5(b) for the dual-frequency DBD (LF 50 kHz, RF 5 MHz) in the α-γ mode [25,47]. Since reactions (R1) to (R10) presented in table 1 are well-established in literature, such disagreement most likely results from the fictive reactions (R11) and (R12) representing the Penning effect, which are both involving Ar(1s) atoms.…”

“…Revised values of the normalized metastable number densities are presented in figure 5(a) for the LF discharge (50 kHz) and dual-frequency DBD (LF 50 kHz, RF 5 MHz) operated in the α-γ mode [25,47]. Clearly, much better agreement is observed with the revised version of the model than with the one originally developed by Magnan et al [25].…”

“…As for the electrons, table 3 shows that while Penning and direct ionization were the dominant ionization pathways in the initial model [25,47], about three times more electrons are now created by photoemission at the walls with the addition of the Ar 2 * kinetics and photoemission (R15)). However, the contribution of Penning ionization remains larger than the one of Ar direct ionization.…”

“…In this work, tunable diode laser absorption spectroscopy (TDLAS) is used to record the population of metastable argon 1s 3 atoms in plane-to-plane DBDs operated in a Penning Ar-NH 3 mixture at atmospheric pressure as a function of time at two locations in the inter-electrode gap for both LF and dual-frequency LF-radiofrequency (RF) excitations. Through a comparison of TDLAS measurements with the predictions of a 1D fluid model [25,47], it is found that VUV photons emitted by Ar 2 * dimers, created by three-body reactions involving Ar(1s) species and two ground state Ar atoms, can induce photoelectron emission at the dielectrics surface of the cathode and can thus play a very significant role on the physics driving Ar-based DBDs.…”

Electron dynamics and metastable species generation in atmospheric pressure non-equilibrium plasmas controlled by dual LF–RF frequency discharges

“…On the other hand, F. Massines et al investigated the influences of amplitude and frequency of applied voltages on DF-excited DBD plasmas in a wide voltage range [23,24]. Particularly, the influence of LF voltage amplitude on the RF discharge process and the mode transition from α to α-γ were studied [25]. Recently, our results also show that DF and tailored voltage waveform excitation are valid approaches to achieve independent control and optimization over both electron density and gas temperature in AP-DBD systems [26][27][28][29][30].…”