Experimental determination of first Townsend ionization coefficient in mixtures of He and N₂

Abstract

First Townsend ionization coefficient α in gas mixture of He with N2 is relevant for modeling of the development and chemical activity of atmospheric pressure plasma jets. This study determined α from measurements of current versus electrode separation in a steady-state non-self-sustaining Townsend discharge set-up. The measurements were carried out in absolute pressure range of 10–800 Torr and reduced electric field E/N range of 20–1000 Td. The effect of the N2 concentration in the gas mixture on the density normalized ionization coefficient α/N depended on the reduced electric field strength E/N. At E/N values below 200 Td, increased N2 content reduced the α/N while at E/N values above 200 Td, increased N2 content increased the plateau value of α/N. Reasonably good coincidence was observed between calculation performed with Bolsig+ and measured α/N. The discrepancy between experimental results and calculations at N2 percentage in the mixture below 5% and E/N values below 200 Td can be attributed to the Penning effect.

Experimental determination of the first Townsend ionization coefficient in mixtures of Ar and N2

“…Figure 6 depicts the comparison of α/N in gas mixtures of Ar with N 2 studied in this work and He with N 2 studied in our previous work [50]. The solid lines on figure 6(b) show the change calculated with Bolsig+.…”

“…The issue with these approaches is that processes other than electron impact excitation/ionization are not taken into account in the calculation. In our previous work with mixtures of He and N 2 , measurements of α/N deviated from Bolsig+ calculations due to Penning ionization at reduced electric field values between 20 and 100 Td [50]. In addition, the Wieland approximation is accurate only when the electron energy distribution function (EEDF ) of the mixture components does not change with mixture composition.…”

“…The Townsend discharge set-up used in this work and depicted as a sketch in figure 1 was similar to the one already used in several previous works conducted by our group [50][51][52][53][54]. The non-self-sustaining Townsend discharge reactor was made of aluminum and had quartz windows on the sides.…”

“…The methodology used for determining the Townsend ionization coefficient α was identical to the one used in earlier publications concerning C5 perfluorinated ketone mixtures with air and He/N 2 mixtures [50,51]. In a non-self-sustaining discharge, in a homogenous electric field and at a set E/N value, the theoretical dependence of current I on electrode gap distance d is given by equation [58]: where α is the Townsend ionization coefficient, γ is the secondary electron emission coefficient from the cathode caused by ion bombardment and I 0 is the initial current at the cathode.…”

“…In works by Kruithof et al [32] and Lakshiminarasimha et al [34] great emphasis was put on gas purity. While we used gases with purity of 99.999%, impurities can still originate from air backflow estimated to be at most 250 ppm [50] and from the gas tubing (e.g. trace amounts of organics with ionization potential below 10 eV).…”