A spatially resolved plasma diagnostic in an expanding microwave discharge containing an Ar- gas mixture: uniformity and stability of the discharge

Abstract

Microwave expansion of a plasma is certainly a promising process by which to obtain a large-diameter plasma profile; however, it generates instability regions and local inhomogeneities. The purpose of this work is to study the wave-propagation conditions and plasma-expansion profile in a pure argon discharge and in argon-methane mixture. Spatially resolved plasma parameters are measured using an array of electrostatic probes (simple and double) moving along the discharge axis. This device gives radial and axial measurements which are correlated to the spatially resolved Ar(420 nm) emission line intensity. We show that, as expected, the wave-propagation conditions are satisfied within the luminous part of the discharge. In this region the electron energy distribution function (EEDF) can be roughly approximated by using a Maxwell distribution function of low temperature (about 1000 K). The absorbed microwave power is mainly transferred to electrons as potential energy. Instabilities appear at the edge of this luminous region, where the wave-propagation conditions are not satisfied. In this region the EEDF is strongly disturbed and cannot be approximated using a Maxwell distribution function. The microwave incident power is mainly reflected, so the potential energy of electrons decreases strongly. Nevertheless, the kinetic energy of electrons increases because of stochastic electron heating due to strong inhomogeneities in the charged particle density producing local electrical fields. When methane is mixed with argon, the energy necessary to maintain an electron free in the plasma increases with increasing frequency of inelastic collisions. Consequently, the plasma expansion length decreases with increasing percentage of methane added to argon.

Study of Non-Thermal DC Arc Plasma of CH4/Ar at Atmospheric Pressure Using Optical Emission Spectroscopy and Mass Spectrometry

“…As presented in Fig. 4, T exc varies within the temperature range from 0.5 eV to 1 eV in our experiments, which is of the same order of magnitude as other atmospheric pressure non-thermal plasma [16,28,29] . Moreover, as can be seen from Fig.…”

“…Moreover, as can be seen from Fig. 4, T exc decreases with both discharge current and CH 4 /Ar percentage, which could be ascribed to the increase in the number of inelastic collisions between electrons and molecules [29,30] .…”

Spectroscopic investigation of an expanding surface wave produced plasma

“…Such kinds of discharges are not only interesting from a theoretical point of view, but also important for applications. 11 Especially in the field of thin film deposition, expanding microwave discharges have found an increasing interest. Borges et al, 12 for example, have used an expanding surface wave plasma for the deposition of diamond films.…”

Dielectric properties in microwave remote plasma sustained in argon: Expanding plasma conditions

“…In this case, charged species produced on the expanding plasma edge, move from the side to the center of the plasma expansion probably because of diffusion and/or because of local radial electric fields. In the previous works, 16 we show that expanding plasma in the vessel occurs when the electron density is larger than a critical value. We observe an increase of the electron density and a decrease of the electron energy simultaneously to the plasma expansion.…”