Detection of fast electrons in pulsed argon inductively-coupled plasmas using the 420.1–419.8 nm emission line pair

Abstract

Pulsed rf plasmas exhibit many differences as compared to continuous wave plasmas with the same average power levels, including large temporal variations in the electron temperature, with a sharp spike when the power is applied and falling dramatically in the afterglow. We present a comparison of time-resolved measurements of the effective electron temperature in pulsed inductively-coupled plasmas by means of (i) optical emission spectroscopy (OES) using different sets of argon emission lines and (ii) Langmuir probe measurements. One OES diagnostic used six strong Ar( → 2p 1sx y ) emission lines in the 700-800 nm wavelength range, the second used only the Ar 420.1-419.8 nm line pair. For pulsed plasmas with long afterglow periods, the line pair method reveals the presence of a significant number of hot electrons ( ⩾ E 22 eV) at the start of the pulse. Under these conditions, the metastable atom density is very low, and the diagnostic using the Ar( → 2p 1sx y ) emission lines is ineffective for determining the electron temperature. For later parts of the pulse and pulsed plasmas with short periods (i.e. 10 μs), the metastable density is high and the two OES methods yield similar results which are also in agreement with probe measurements.

A Computationally Assisted Ar I Emission Line Ratio Technique to Infer Electron Energy Distribution and Determine Other Plasma Parameters in Pulsed Low-Temperature Plasma

“…The excitation rates used in previous experiments [2,6] were calculated [3] using optical emission cross sections that accounted for the branching ratio as well as cascades into the excited state. The cascade contribution may be accounted for in the high-energy tail of the optical emission cross sections, as a function of pressure, in a procedure outlined in [7]. It is also worth noting that, similar to the dipole selection rules, electron impact excitations follow specific excitation patterns, namely ΔJ = 1, 0; however, J = 0→0 is forbidden.…”

“…The 420.1/419.8 nm emission line ratio is sensitive to stepwise excitation via the 420.1 nm emission and the presence of metastable atoms: initially R = 1, and the ratio will increase when a sufficient number of metastable atoms are present such that the 3p 9 state is populated by both direct and stepwise excitation. The ratio should be less than unity only in the presence of a significant population of high-energy (>30 eV) electrons, as argued in [2,3,6] and shown in [7]. Alternatively, the emission line ratio can be used in combination with the measured metastable atom density (e.g., by tunable diode laser absorption) to determine the EEDF by comparing the measured emission with the predicted emission for different EEDFs.…”

“…Passive optical diagnostics that are non-invasive and non-perturbative, like the 420.1/419.8 nm argon-emission line-height-ratio technique [2][3][4], are desirable because they can be implemented in real time and can be used to determine metastable-atom density [2,5] and effective electron temperature [4]. Emission spectra have been shown to correlate with high energy electrons [2,3,6,7]. The technique described here represents a computationally assisted argon emission line ratio technique to infer electron energy distribution and determine other plasma parameters in pulsed low-temperature plasma.…”

Correlating Metastable-Atom Density, Reduced Electric Field, and Electron Energy Distribution in the Initiation, Transient, and Post-Transient Stages of a Pulsed Argon Discharge

“…While the use of the 420.1-419.8nm emission-line ratio proved successful in past WVU collaborations [Fox-Lyon et al, 2013;Franek et al, 2015;Franek et al, 2016], and independently at University of Wisconsin [Boffard et al, 2012;Boffard et al, 2015], the proximity of the emission-lines requires a high-resolution spectrometer to resolve the emission-line profiles, thus limiting the technique's availability and affordability. Furthermore, the 419.8nm emission depends sensitively on pressure [Boffard et al, 2007], which may hamper the extrapolation of the technique to higher-pressure systems.…”

“…While stepwise excitation into the optically emitting 3px states forms the crux of this dissertation, the cross-sections presented in figure (5.7) electric-field that in turn ionizes the gas and creates the metastable atoms. This stage is characterized by a static (i.e., zero slope) emission ratio that is often less that unity, which suggests a non-negligible population of supra-thermal electrons [Boffard et al, 2015]; therefore, the resulting EEPF cannot be understood by the logic expressed in Adams et al, [2012] and Hagelaar & Pitchford, [2005]. The disagreement between the observed emission ratio and the expected emission ratio in the Initiation stage shows the modeling of [Adams et al, 2012] is not valid when considering a non-thermal (non-Druyvesteyn) distribution of electrons.…”