Increasing signal-to-noise ratio in over-determined Mueller matrices

This work investigates how the signal-to-noise ratio (SNR) of an over-determined Mueller matrix can be improved by changing the method of calculation. Specifically, our investigation focused on comparing SNRs achieved using the vector methodology from the field of partial Mueller polarimetry, and the matrix methodology. We use experimentally derived measurements from an investigation into the time-varying signal produced by the Mueller matrix of an electro-optic Bismuth Silicon Oxide (BSO) crystal undergoing cyclical impact of a Helium plasma ionisation wave. Our findings show that the vector methodology is superior to the matrix methodology, with a maximum SNR of 7.54 versus 4.97. We put forth that the superiority of the vector methodology is due to its greater flexibility, which results in the Mueller matrix being calculated with better condition matrices, and higher levels of SNR in the intensity measurements used for calculation.</p

Validation of non-equilibrium kinetics in CO2-N2 plasmas

This work explores the effect of N2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent kinetic model, previously validated for pure CO2 and CO2-O2 discharges, is further extended by adding the kinetics of N2. The vibrational kinetics considered include levels up to v = 10 for CO, v = 59 for N2 and up to v1 = 2 and v2 = v3 = 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and account for electron-impact excitation and de-excitation (e-V), vibration-to-translation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge operating in pure CO2 and in CO2-N2 mixtures, at pressures in the range 0.6 - 4 Torr (80.00 - 533.33 Pa) and a current of 50 mA. The experimental results show a higher vibrational temperature of the different modes of CO2 and CO and an increased dissociation fraction of CO2, that can reach values as high as 70%, when N2 is added to the plasma. On the one hand, the simulations suggest that the former effect is the result of the CO2-N2 and CO-N2 V-V transfers and the reduction of quenching due to the decrease of atomic oxygen concentration; on the other hand, the dilution of CO2 and dissociation products, CO and O2, reduces the importance of back reactions and contributes to the higher CO2 dissociation fraction with increased N2 content in the mixture, while the N2(B3Pg) electronically excited state further enhances the CO2 dissociation

Study of vibrational kinetics of CO2 and CO in CO2-O2 plasmas under non-equilibrium conditions

This work explores the effect of O2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent model, previously validated for pure CO2 discharges, is further extended by adding the vibrational kinetics of CO, including electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The vibrational kinetics considered include levels up to v = 10 for CO and up to v1=2 and v2=v3=5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and accounts for e-V, V-T in collisions between CO, CO2 and O2 molecules and O atoms and V-V processes involving all possible transfers involving CO2 and CO molecules. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge, operating at pressures in the range 0.4 - 5 Torr (53.33 - 666.66 Pa). The experimental results show a lower vibrational temperature of the different modes of CO2 and a decreased dissociation fraction of CO2 when O2 is added to the plasma but an increase of the vibrational temperature of CO. On the one hand, the simulations suggest that the former effect is the result of the stronger V-T energy-transfer collisions with O atoms which leads to an increase of the relaxation of the CO2 vibrational modes; On the other hand, the back reactions with O2 contribute to the lower CO2 dissociation fraction with increased O2 content in the mixture.Comment: 34 pages 15 figure

DISSOCIATION OF O2 BY ELECTRON IMPACT IN PLASMA

Константа скорости диссоциации О2 была измерена в широком диапазоне E/N в тлеющем разряде постоянного тока. Поле Е определялось зондовым методом, а плотность газа N - из измерений газовой температуры по спектру излучения Р-ветви полосы O2(b1g+) O2(X3g-). Плотность атомов О(3P) измерялась методом TALIF, а отношение O(3P)/N, кроме этого, определялось методом актинометрии по атомам Ar. Время разрешенная актинометрия в модулированном разряде использовалась для определения скорости гибели атомов О(3Р). Из данных измерений была получена константа скорости диссоциации О2 как функция E/N. Анализ полученных данных позволил детально рассмотреть механизмы диссоциации и возбуждения высоких электронных состояний О2 и сформировать новый самосогласованный набор сечений для молекулы О2.DC glow discharges in pure O2 in a Pyrex tube were studied to measure dissociation rate constant over a wide range of E/N. E was found from probe measurements while gas density N - from the measurements of the gas temperature from the O2(b1g+) O2(X3g-) emission spectrum. O(3P) atom density was measured by TALIF while O(3P)/N ratio was also determined by Ar actinometry. Time-resolved actinometry on partially-modulated discharges was used to measure the loss rate of O atoms. From these measurements the O2 dissociation rate constant was determined as a function of E/N. The obtained data allowed analyzing both dissociation and excitation mechanisms of the high electronic states of O2 as well as forming a new self-consistent cross section set for O2 molecule.119-11

Fluorescence (TALIF) measurement of atomic hydrogen concentration in a coplanar surface dielectric barrier discharge

Spatially and temporally resolved measurements of atomic hydrogen concentration above the dielectric of coplanar barrier discharge are presented for atmospheric pressure in 2.2% H2/Ar. The measurements were carried out in the afterglow phase by means of two-photon absorption laser-induced fluorescence (TALIF). The difficulties of employing the TALIF technique in close proximity to the dielectric surface wall were successfully addressed by taking measurements on a suitable convexly curved dielectric barrier, and by proper mathematical treatment of parasitic signals from laser–surface interactions. It was found that the maximum atomic hydrogen concentration is situated closest to the dielectric wall from which it gradually decays. The maximum absolute concentration was more than 10^22 m-3. In the afterglow phase, the concentration of atomic hydrogen above the dielectric surface stays constant for a considerable time (10 us - 1 ms), with longer times for areas situated farther from the dielectric surface. The existence of such a temporal plateau was explained by the presented 1D model: the recombination losses of atomic hydrogen farther from the dielectric surface are compensated by the diffusion of atomic hydrogen from regions close to the dielectric surface. The fact that a temporal plateau exists even closest to the dielectric surface suggests that the dielectric surface acts as a source of atomic hydrogen in the afterglow phase

A rotational Raman study under non-thermal conditions in a pulsed CO2 glow discharge

The implementation of \u27in situ\u27 rotational Raman spectroscopy is realized for a pulsed glow discharge in CO2 in the mbar range and is used to study the rotational temperature and molecular number densities of CO2, CO, and O2. The polarizability anisotropy of these molecules is required for extracting number densities from the recorded spectra and is determined for incident photons of 532 nm. The spatiotemporally-resolved measurements are performed in the same reactor and at equal discharge conditions (5-10 ms on-off cycle, 50 mA plasma current, 6.7 mbar pressure) as in recently published work employing \u27in situ\u27 Fourier transform infrared (FTIR) spectroscopy. The rotational temperature ranges from 394 K to 809 K from start to end of the discharge pulse and is constant over the length of the reactor. The discharge is demonstrated to be spatially uniform in gas composition, with a CO2 conversion factor of 0.15 ± 0.02. Rotational temperatures and molecular composition agree well with the FTIR results, while the spatial uniformity confirms the assumption made for the FTIR analysis of a homogeneous medium over the line-of-sight of absorption. Furthermore, the rotational Raman spectra of CO2 are related to vibrational temperatures through the vibrationally averaged nuclear spin degeneracy, which is expressed in the intensity ratio between even and odd numbered Raman peaks. The elevation of the odd averaged degeneracy above thermal conditions agrees well with the elevation of vibrational temperatures of CO2, acquired in the FTIR study

Simulation of the discharge propagation in a capillary tube in air at atmospheric pressure

International audienceThis paper presents simulations of an air plasma discharge at atmospheric pressure initiated by a needle anode set inside a dielectric capillary tube. We have studied the influence of the tube inner radius and its relative permittivity ε r on the discharge structure and dynamics. As a reference, we have used a relative permittivity ε r = 1 to study only the influence of the cylindrical constraint of the tube on the discharge. For a tube radius of 100 µm and ε r = 1, we have shown that the discharge fills the tube during its propagation and is rather homogeneous behind the discharge front. When the radius of the tube is in the range 300 to 600 µm, the discharge structure is tubular with peak values of electric field and electron density close to the dielectric surface. When the radius of the tube is larger than 700 µm, the tube has no influence on the discharge which propagates axially. For a tube radius of 100 µm, when ε r increases from 1 to 10, the discharge structure becomes tubular. We have noted that the velocity of propagation of the discharge in the tube increases when the front is more homogeneous and then, the discharge velocity increases with the decrease of the tube radius and ε r. Then, we have compared the relative influence of the value of tube radius and ε r on the discharge characteristics. Our simulations indicate that the geometrical constraint of the cylindrical tube has more influence than the value of ε r on the discharge structure and dynamics. Finally, we have studied the influence of photoemission processes on the discharge structure by varying the photoemission coefficient. As expected, we have shown that photoemission, as it increases the number of secondary electrons close to the dielectric surface, promotes the tubular structure of the discharge

Atmospheric pressure plasma jets in contact with a dielectric surface:the electric field and the charge at the surface

Atmospheric pressure plasma jets are in the current research focus, as they are cheap and simple to make, yet have shown tremendous potential in material modification or biomedical applications. The common point is the presence of a target in the effluent of the jet. Targets can be metallic, dielectric or made of biological material, however in all cases they modify the plasma to a certain extent and it is therefore necessary to perform research on non-thermal atmospheric pressure plasma sources in the presence of a target.The focus of this paper is the interaction of a kHz helium plasma jet and a dielectric target, more precisely the electric field and the charge at the surface of the target. The Pockels’ method was used to measure the electric field on the surface between 3 mm and 10 mm away from the nozzle of the plasma jet, as a function of the gas flow and applied voltage. The obtained field is of the order of magnitude of 105 V/m.<br/