Gas temperature measurements in oxygen plasmas by high-resolution Two-Photon Absorption Laser-induced Fluorescence

Abstract

One of the most important, and difficult to measure, parameters of laboratory discharges in molecular gases is the gas translational temperature. We propose a novel technique to measure directly, with excellent spatial and temporal resolution, the velocity distribution of ground-state atoms (oxygen atoms in this case) in plasmas from the Doppler broadening of their laser excitation spectra. The method is based on the well-known Two-Photon Laser-induced Fluorescence (TALIF) technique, but uses a specially-built pulsed tunable ultraviolet laser with very narrow bandwidth which allows the Doppler profiles to be measured with high precision. This laser consists of a pulsed Nd:YAG-pumped Ti:Sapphire ring cavity which is injection-seeded by a singlemode cw Ti:sapphire laser. The single-mode infrared output pulses are frequency quadrupled by two non-linear crystals to reach the necessary UV wavelength (226 nm, 0.2 mJ) for TALIF excitation. This technique should be applicable to a wide range of discharges, ranging from low-pressure RF plasmas for surface processing to atmospheric pressure plasmas. Results of preliminary tests on low-pressure O 2 DC discharges are presented.

Oxygen (3P) atom recombination on a Pyrex surface in an O2 plasma

“…This causes  O to vary with discharge current and pressure, in addition to varying with surface temperature. Expressions (28) and (29) were used to fit the observed O( 3 P) surface loss probabilities,  O , (at pressures above 0.75 Torr) using the measured [O( 3 P)]/N ratio and the gas temperature near the wall. The results are shown in figure 11a and 11b, and the derived parameters are presented in Table 1.…”

“…The linear behavior of  O at pressures above ~2 Torr (left side of the graphs) is explained by the Arrhenius form of the recombination probability with an activation energy E act . Deviations from this behavior at lower pressures are caused by the terms in brackets in (28) and (29), which account for recombination with weakly bonded physisorbed atoms, whose concentration depends on both the O atom flux and the surface temperature. This model therefore explains the experimentally-observed deviations from the Arrhenius plots ("knees" of different amplitudes) at the conditions (pressure and current) where the O( 3 P) flux to the wall is maximal.…”

“…Sites distribution on E a Figure 13. Calculated  O as a function of reverse temperature of O atoms (300/T nw ) for 30 mA, for two surface temperatures (T w =+5C and +50C) and different activation energy distributions (E a ) defined by expression (29). The distributions are shown in the insert.…”

Oxygen atom kinetics in CO2 plasmas ignited in a DC glow discharge

“…The rotational temperature of CO 2 (T rot ) was obtained by fitting the FTIR absorption spectra [53]. The translational temperature of the O atoms was obtained by fitting the Doppler broadening of the TALIF profile [60]. Figure 4 a) shows the variation of both with current and pressure.…”

Helimak control system

“…A flexible data acquisition framework is designed based on NI-DAQmx and NI-Scope, which makes it very easy to add and remove data acquisition devices and channels; adjust the sampling rate from 2 MSPS (Mega-Samples Per Second) to 250 MSPS; modify each impedance and dynamic range of each channel. The flexibility provides upgrade possibilities for laser-aided diagnostics [16] and microwave diagnostics [17] that will be added in the future. Additionally, an image acquisition software based on NI-Vision can provide high transmission capability for the high-speed camera through Camera Link.…”