L. L. Alves

Generation and confinement of microwave gas-plasma in photonic dielectric microstructure.

We report on a self-guided microwave surface-wave induced generation of ~60 μm diameter and 6 cm-long column of argon-plasma confined in the core of a hollow-core photonic crystal fiber. At gas pressure of 1 mbar, the micro-confined plasma exhibits a stable transverse profile with a maximum gas-temperature as high as 1300 ± 200 K, and a wall-temperature as low as 500 K, and an electron density level of 10¹⁴ cm⁻³. The fiber guided fluorescence emission presents strong Ar⁺ spectral lines in the visible and near UV. Theory shows that the observed combination of relatively low wall-temperature and high ionisation rate in this strongly confined configuration is due to an unprecedentedly wide electrostatic space-charge field and the subsequent ion acceleration dominance in the plasma-to-gas power transfer.

Systematic characterization of low-pressure capacitively coupled hydrogen discharges

This paper presents a systematic characterization of pure hydrogen capacitively coupled discharges, produced in a parallel plate cylindrical setup. A two-dimensional, time-dependent fluid model is used to describe the production, transport, and destruction of electrons and positive ions H+, H2+, and H3+, at different frequencies (13.56–60 MHz), pressures (0.2–8 Torr), rf applied voltages (50–450 V) and geometric dimensions (1.6–12.8 cm radii and 1.6–6.4 cm interelectrode distances). A good agreement is found between calculation results and experimental measurements for the coupled electrical power, the plasma potential, and the self-bias potential, at various frequencies and rf applied voltages. However, the model generally underestimates the electron density with respect to its measured values. The paper discusses different space-time events, such as the development of double-ionization structures or the occurrence of field inversion and field reversal phenomena. The dependencies on pressure and frequenc...

Desulfurization of dibenzothiophene, benzothiophene, and other thiophene analogs by a newly isolated bacterium, Gordonia alkanivorans strain 1B

A novel bacterium, Gordonia alkanivorans strain 1B, was isolated from hydrocarbon-contaminated soil. Assessment of the biodegradation of distinct organic sulfur-compounds, such as dibenzothiophene (DBT), benzothiophene (BT), DBT sulfone, and alkylated tiophenic compounds, as the sole source of sulfure was investigated. G. alkanivorans strain 1B was able to remove selectively the sulfur from DBT while keeping intact the remaining carbon-carbon structure. Orthophenyl phenol (2-hydroxybiphenyl) was the only detected metabolic product. The bacterial desulfurization activity was repressed by sulfate. G. alkanivorans straini 1B consumed 310 μM DBT after 120 h of cultivation, corresponding to a specific desulfurization rate of 1.03 μmol/(g of dry cells·h). When an equimolar mixture of DBT/BT was used as a source of sulfur in the growth medium, G. alkanivorans strain 1B assimilated both compounds in a sequential manner, with BT as the preferred source of sulfur. Only when BT concentration was decreased to a very low level was DBT utilized as the source of sulfur for bacterial growth. Thespecific desulfurization overall rates of BT and DBT obtained were 0.954 and 0.813 μmol/(g of dry cells·h), respectively. The newly isolated G. alkanivorans strain 1B has good potential for application in the biodesulfurization of fossil fuels.

Two-dimensional fluid modelling of charged particle transport in radio-frequency capacitively coupled discharges

This paper reviews the formulation and updates some numerical procedures usually adopted in two-dimensional, time-dependent fluid models to study the transport of charged particles in radio-frequency capacitively coupled discharges. The description of charged particle transport is made by solving the continuity and momentum transfer equations for electrons and ions, coupled with Poissons equation and the electron mean energy transport equations. Inertia terms are considered in the ion momentum transfer equations, by generalizing the earlier definition of effective electric field. The electron mean energy equations are written using specific energy transport parameters, deduced from integration over the electron energy distribution function (EEDF). The model adopts the local mean energy approximation, i.e. it computes the electron transport parameters as a function of the electron mean energy, using either a homogeneous, two-term Boltzmann equation solver or a Maxwellian EEDF. More appropriate boundary conditions for the electron and ion fluxes are used successfully. The model is solved for a GEC Cell reactor type (with 6.4 cm radius and 3.2 cm interelectrode distance) operating at frequency 13.56 MHz, pressures between 10 mTorr and 10 Torr and applied voltages from 100 to 500 V, in electropositive (helium) and electronegative (silane–hydrogen) gases or gas mixtures. The ion kinetics in silane and hydrogen is updated with respect to previous works, by further considering SiH2+, H+ and H3+ ions. In general, simulation results for some typical electrical parameters are closer to experimental measurements available than calculations reported in previous works.

Modeling of low-pressure microwave discharges in Ar, He, and O/sub 2/: similarity laws for the maintenance field and mean power transfer

A kinetic study of low-pressure microwave discharges in Ar, He, and O/sub 2/ is carried out using electron-transport parameters and rate coefficients derived from solutions to the Boltzmann equation, together with the continuity and transport equations for the charged particles, taking into account stepwise-ionization processes. The Boltzmann equation is solved over a wide range of the applied frequency, omega /2 pi , but assuming that the angular frequency omega > tau /sub e//sup -1/, with tau /sub e/, denoting the characteristic time for electron-energy relaxation by collisions. The formulation provides discharge characteristics for the maintenance field and for mean absorbed power per electron in the three gases, which are shown to agree satisfactorily with experimental data obtained from surface-wave discharges. It is shown that such an agreement would not always be obtained without consideration of the role played by stepwise-ionization processes in sustaining the discharge. >

Two-dimensional modelling of - radio-frequency discharges for a-Si:H deposition

A two-dimensional numerical code, including three fluid modules to account for the description of electrical, thermal and chemical phenomena, has been developed for the modelling of hydrogenated amorphous silicon deposition from radio-frequency glow discharges in a cylindrical PECVD reactor. The results of the model are compared to experimental data, obtained by different diagnostic techniques. The calculated radical densities are compared to those measured by threshold ionization mass spectrometry, at the centre of the substrate; the calculated SiH density profile between the electrodes is compared to those measured by laser-induced fluorescence and the radial distribution of the deposition rate on the substrate is compared to profilometry measurements. Globally, the model correctly predicts the main discharge characteristics for experimental conditions normally used for amorphous silicon deposition in the dust-free regime. The moderate agreement between model and experiment occurring for the hydrogen-dominated condition can be attributed to the simplified surface kinetics adopted in the model.

A collisional-radiative model for microwave discharges in helium at low and intermediate pressures

A stationary collisional-radiative model for helium microwave discharges in cylindrical geometry is developed by coupling the rate balance equations for the n<or=6 excited states of helium to the continuity and transport equations for the electrons, He+ atomic ions and He2+ molecular ions, and to the homogeneous Boltzmann equation. The latter is solved using the DC effective field approximation but taking into account stepwise inelastic and superelastic processes from the 23S, 21S and 23P states, as well as electron-electron collisions. A coherent set of electron cross sections is deduced in order to solve the Boltzmann equation. Special attention is paid to the atomic collisions considered (by taking into account /-change reactions and associative ionization reactions), and to the effects of radiation imprisonment. This, together with the inclusion of the kinetics of the molecular ions, allows the range of validity of the model to extend up to atmospheric pressure. The theoretical populations for the excited states, characteristics for the steady-state reduced maintenance electric field and mean absorbed power per electron at unit gas density agree very well with experimental data from surface wave discharges.

Electron kinetics in weakly ionized helium under DC and HF applied electric fields

The electron kinetics in weakly ionized helium under the action of DC and HF fields of angular frequency omega is investigated by solving the homogeneous electron Boltzmann equation using the classical two-term expansion approximation. The analysis is based on a consistent set of electron cross sections which is derived by adjusting experimental cross section data in such a way that calculated and measured electron swarm parameters are in agreement. In the case of HF fields the analysis is based on the DC effective field approximation which is valid for omega > tau e-1, where tau e is the characteristic time for electron energy relaxation by collisions with the atoms. The influence of omega on the electron energy distribution function, transport parameters, rate coefficients and fractional power transfer is investigated and a detailed comparison of the DC and HF situations is made. It is shown that for reduced effective fields in the range 10-16-10-15 V cm2, as typically found in low-pressure discharges, the mean electron kinetic properties are nearly the same in the whole range of omega > tau e-1 as for the DC case.

Global model and diagnostic of a low-pressure SF6/Ar inductively coupled plasma

A global model has been developed for low-pressure (3–20 mTorr), radio-frequency (rf) (13.56 MHz) inductively coupled plasmas (ICPs), produced in SF6/Ar mixtures. The model is based on a set of mass balance equations for all the species considered, coupled to the discharge power balance equation and the charge neutrality condition. Simulations are used to show the impact of operating conditions, such as the rf power, the pressure and the percentage of argon in the mixture, on the evolution of charged and neutral species. Langmuir probe and optical emission spectroscopy measurements are used to determine the electron temperature and the densities of electrons, ions and atomic fluorine in the SF6/Ar ICPs under study. These data are compared with simulation results obtained from the global model. A satisfactory agreement is found between the simulation results and the measured values of the electron density and temperature, for rf powers in the range 900–1700 W, regardless of the percentage of argon in the mixture. Predictions for the atomic fluorine density (~1014 cm−3) are in good agreement with experiment, for various rf powers.

Capacitively coupled radio-frequency hydrogen discharges: The role of kinetics

This paper presents a systematic characterization of capacitively coupled radio-frequency hydrogen discharges, produced within a parallel plate cylindrical setup at different rf applied voltages (Vrf=50–600V), frequencies (f=13.56–40.68MHz), and pressures (p=0.2–1torr). A two-dimensional, time-dependent fluid model for charged particle transport is self-consistently solved coupled to a homogeneous kinetic model for hydrogen, including vibrationally excited molecular species and electronically excited atomic species. Numerical simulations are compared with experimental measurements of various plasma parameters. A good quantitative agreement is found between simulations and experiment for the coupled electrical power and the plasma potential. The model underestimates the values of the electron density, the self-bias potential, and the H(n=1) atom density with respect to measurements, but agrees with experiment when predicting that all these parameters increase with either Vrf, f, or p. The dissociation degr...