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

Abstract

Direct-and stepwise-excitation rates as functions of reduced electric field used in this dissertation. 3.4 Optical emission, electron-collision-induced cross sections for direct excitation resulting in the argon 419.8, 420.1 and 425.9nm emission-lines. 4.1 Experimental Setup of the pulsed argon discharge. 4.2 Schematic of the Pulse-Conditioning Circuit (PCC). v 4.3 A detailed schematic of the experimental setup. 4.4 Digital Delay Generator output voltage waveforms for single-pulse experiments. 4.5 Digital Delay Generator output voltage waveforms for double-pulse experiments. 4.6 Observed transmittance function of the spectroscopic setup. 4.7 Expected transmittance function of a spectroscopic setup. 4.8 Detailed experimental setup of the Tunable-Diode Laser Absorption Spectroscopy (TDLAS) diagnostic. 5.1 Observed spectra from the pulsed argon positive column when a nitrogen impurity is present. 5.2 Look-up tables used for determining electron density and collision frequency. 5.3 Typical voltage waveform response from the Microwave Cavity Resonance Spectroscopy (MCRS) diagnostic. 5.4 Typical "Color Plot" of the MCRS diagnostic. 5.5 Typical collision frequency and electron density traces of the pulsed positive column. 5.6 Typical metastable-atom density and emission-line ratio measurements of the pulsed positive column. 5.7 Stepwise excitation rates out of the 1s5 metastableatom level. 5.8 Excited-state densities in a plasma-conditioned pulse in the pulsed argon positive column. 6.1 Typical emission-line ratio as a function of time. 6.2 Observed emission-line ratio as a function of reduced vi electric field (E/N) for several discharge pressures. 6.3 Measured Vs. model-predicted metastable-atom density for the Initiation, Transient, and Post-Transient Stages of the pulsed discharge. 6.4 Measured electron collision frequency for the Initiation, Transient, and Post-Transient Stages of the pulsed discharge. 6.5 Total, excitation, and ionization electron-collision cross-sections in argon. 6.6 Optical-emission cross-sections for 419.8nm emission for four discharge pressures. 6.7 Optical-emission cross-sections for 425.9nm emission for four discharge pressures. 6.8 Optical-emission cross-sections for 420.1nm emission for four discharge pressures. 6.9 Calculated 425.9-419.8nm emission-line ratio. 6.10 Calculated Electron Energy Distribution Functions (EEDFs) for increasing values of reduced electric field (E/N) for a given plasma density. 6.11 Calculated electron population in the 15-30eV range as functions of reduced electric field (E/N) for various plasma densities. 6.12 Calculated electron population above 30eV as functions of reduced electric field (E/N) for various plasma densities. 6.13 Typical argon spectrum using a low-resolution (1200 groove/mm) diffraction grating. 6.14 Empirical fit of the argon 425.9nm emission line. 6.15 Empirical fit of the argon 420.1/419.8nm emission-line convolution. vii 6.16 Empirically-corrected 420.1nm emission-line profiles A.1 Energy-level diagram for neon A.2 Energy-level diagram for ...

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 bounds of integration must include all electrons with sufficient energy to excite the transition to state i from the ground state; the lower bound is defined by the minimum energy required for an electron to excite the transition in question and the upper bound is defined by the most-energetic electron in the electron energy distribution function (EEDF). The EEDFs can be measured experimentally as in [5] or evaluated numerically as in [8]. Emission at 419.8 nm, 420.1 nm, and 425.9 nm [11] contains information about plasma parameters and atomic kinetics that can be extracted via emission line ratio techniques [8].…”

“…The EEDFs can be measured experimentally as in [5] or evaluated numerically as in [8]. Emission at 419.8 nm, 420.1 nm, and 425.9 nm [11] contains information about plasma parameters and atomic kinetics that can be extracted via emission line ratio techniques [8]. 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.…”

“…Examination of the direct and step-wise excitation cross sections and some knowledge of the electron energy distribution can help distinguish the valid regimes for application of the coronal model versus an extended coronal model. Generally, the coronal model is applicable to low-pressure discharges, while the extended coronal model provides a better description for higher pressure discharges [8]. Direct excitation cross sections decrease to zero quickly as the incident electron energy decreases below the onset energy of the excited state of interest, while the stepwise excitation cross sections remain non-zero as the incident electron energy decreases below the excited state energy because metastable atoms are in an excited state (11.5 eV above ground state).…”

“…Note the difference in resolution between the top and bottom spectra; in the bottom spectrum, the 419.8 nm emission line overlaps with the 420.1 nm emission line, which complicates the implementation of the 420.1/419. 8…”

“…where R is the observed 420.1/419.8 nm emission line intensity ratio, I 420.1 and I 419.8 are the observed intensity (W/m 2 ) of 420.1 nm and 419.8 nm emission, n M is the metastable atom density, n 0 is the neutral argon density, k 419.8 is the direct excitation rate from the ground state into the 3p 5 state (the upper state for 419.8 nm emission), which is equal in magnitude to the direct excitation rate from the ground state into the 3p 9 state (the upper state for 420.1 nm emission) [2,3], and k 420.1 is the stepwise excitation rate from the 1s 5 metastable state into the 3p 9 state. While the approximations made to obtain the final form expressed by Equation (1) are particular to the neutral argon transitions of interest, emission line pairs for neon, krypton, and xenon with similar sensitivity to the presence of metastable atoms are presented elsewhere [3] and may be used to expand the emission line ratio technique to other noble gases [8]. The utility of the 420.1/419.8 nm emission line ratio has been demonstrated in previous work [2,6].…”