Identification of the different diffuse dielectric barrier discharges obtained between 50 kHz to 9 MHz in Ar/NH₃at atmospheric pressure

Abstract

The aim of this work was to identify the different diffuse dielectric barrier discharges (DBDs) obtained in the same electrode configuration and in the same gas for an excitation frequency ranging from 50 kHz to 9 MHz. The gas mixture was argon with 133 ppm of NH 3 . This Penning mixture is useful to obtain both low-frequency glow DBDs (GDBDs) and diffuse radio-frequency (RF) discharges. Electrical measurements and short exposure time photographs showed that whatever the frequency, a discharge free of micro-discharge was obtained. In the same configuration, the discharge was a GDBD up to 200 kHz. For frequencies higher than 250 kHz, the discharge behavior was that of a Townsend-like discharge associated with a maximum energy transfer close to the anode and a higher power (about twice that of the GDBD). The cathode fall formation was no longer observed during the discharge current increase because of ion trapping in the gas gap by the rapid electric field oscillations. In the same configuration, the alpha RF mode was observed from 1.3 MHz. Gamma secondary electron emission gave way to electron acceleration by the cathode sheath formation. Bulk ionization was important due to the high electron collision rate at atmospheric pressure. One consequence of the transition from low-frequency to high-frequency discharge was a significant increase in the power (factor ≈30), which reached 35 W cm −3 , while the breakdown voltage decreased from 900 V to less than 200 V.

Atmospheric-pressure diffuse dielectric barrier discharges in Ar/O2gas mixture using 200 kHz/13.56 MHz dual frequency excitation

“…According to literature, the bulk discharge in this study can be attributed to the sheath expansion and the sheath collapse structures [50,53]. The sheath expansion is induced by the excitation/ionization near the sheath edge caused by the energetic electrons which are accelerated by sheath oscillations toward the opposite electrode [44]. When the sheath collapses, the collision rate at atmospheric-pressure is so high that the generated electrons cannot instantaneously follow the retreating sheath merely by diffusion.…”

“…In this study, the first breakdown of the plasma cannot be obtained by the single RF voltage, an additional LF voltage must be supplied in order to produce a stable RF discharge. According to the literature, the amplitude of electron oscillation A can be estimated [44,52]:…”

“…In the α mode, the discharge is sustained by the volumetric ionization processes in the bulk [44]. In this case the secondary ion-electron emission on the electrodes is not essential for the existence of the self-sustained plasma.…”

“…In this case the secondary ion-electron emission on the electrodes is not essential for the existence of the self-sustained plasma. The bulk electrons usually have a relatively low mean energy because of the low local electric field value; nevertheless, this Ohmic heating is sufficient to sustain the plasma [44]. Due to the high excitation energy threshold of Ar 2p 1 and Ar 2p 5 states (13.5 eV and 13.3 eV, respectively), the phase-resolved (1) and (2) in figure 6.…”

“…Figure 8 shows the phaseresolved excitation of the α-γ coexisting discharge with an input power of 150 W. Unlike the α mode discharge which is sustained by volumetric ionization processes, the γ mode discharge is sustained by the sheath gas breakdown allowing substantial ionic flux to the instantaneous cathode and a strong secondary electron emission [44]. The electron avalanches are created in the high electric field of the sheath with a particularly high excitation rate near the substrate, see figure 8.…”