Plasma-assisted ignition and combustion

We review the data and models describing the production of the electrons, termed secondary electrons, that initiate the secondary and subsequent feedback avalanches required for the growth of current during breakdown and for the maintenance of low-current, cold-cathode discharges in argon First we correlate measurements of the production of secondary electrons at metallic cathodes, ie the yields of electrons induced by Ar+ ions, fast Ar atoms, metastable atoms and vuv photons The yields of electrons per ion, fast atom and photon vary greatly with particle energy and surface condition Then models of electron, ion, fast atom, excited atom and photon transport and kinetics are fitted to electrical-breakdown and low-current, discharge-maintenance data to determine the contributions of various cathode-directed species to the secondary electron production Our model explains measured breakdown and low-current discharge voltages for Ar over a very wide range of electric field to gas density ratios E/n, ie 15 Td to 100 kTd We review corrections for nonequilibrium electron motion near the cathode that apply to our local-field model of these discharges Analytic expressions for the cross sections and reaction coefficients used by this and related models are summarized

https://iopscience.iop.org/article/10.1088/0963-0252/8/3/201/pdf

Cold-cathode discharges and breakdown in argon: surface and gas phase production of secondary electrons

Electron energy distribution functions (EEDFs) have been measured in a cylindrical inductively coupled plasma (ICP) with a planar coil over a wide range of external parameters (argon pressure, discharge power and driving frequency). The measurements were performed under well-defined discharge conditions (discharge geometry, rf power absorbed by plasma, external electrical characteristics and electromagnetic field and rf current density profiles). Problems found in many probe measurements in ICPs were analysed and a rationale for designing probe diagnostics that addresses these problems is presented in this paper. A variety of plasma parameters, such as, plasma density, effective and screen electron temperatures, electron-atom transport collision frequency, effective rf frequency and rates of inelastic processes, have been found as appropriate integrals of the measured EEDFs. The dependence of these ICP parameters over a wide range of argon pressure, rf power and frequency results in experimental scaling laws that are suitable for comparison with ICP models and helpful in ICP design for many applications.

https://iopscience.iop.org/article/10.1088/0963-0252/11/4/320/pdf

Electron energy distribution function measurements and plasma parameters in inductively coupled argon plasma

This review covers the results obtained in the period 2006–2014 in the field of plasma-assisted combustion, and in particular the results on ignition and combustion triggered or sustained by pulsed nanosecond discharges in different geometries. Some benefits of pulsed high voltage discharges for kinetic study and for applications are demonstrated. The necessity of and the possibility of building a particular kinetic mechanism of plasma-assisted ignition and combustion are discussed. The most sensitive regions of parameters for plasma–combustion kinetic mechanisms are selected. A map of the pressure and temperature parameters (P–T diagram) is suggested, to unify the available data on ignition delay times, ignition lengths and densities of intermediate species reported by different authors.

https://iopscience.iop.org/article/10.1088/0022-3727/47/35/353001/pdf

Plasma-assisted ignition and combustion: nanosecond discharges and development of kinetic mechanisms

We use a global (volume averaged) model to study the dissociation processes and the presence of negative ions and metastable species in a low pressure high density O2/Ar discharge in the pressure range 1?100?mTorr. The electron density and the fractional dissociation of the oxygen molecule increases with increased argon content in the discharge. We relate this increase in fractional dissociation to an increase in the reaction rate for electron impact dissociation of the oxygen molecule which is due to the increased electron temperature with increased argon content in the discharge. The electron temperature increases due to higher ionization potential of argon than for molecular and atomic oxygen. We find the contribution of dissociation by quenching of the argon metastable Arm by molecular oxygen (Penning dissociation) to the creation of atomic oxygen to be negligible. The negative oxygen ion O? is found to be the dominant negative ion in the discharge. Dissociative attachment of the oxygen molecule in the ground state and in particular the metastable oxygen molecule O2(a?1?g) are the dominating channels for creation of the negative oxygen ion O?.

/pdf/oxygen-discharges-diluted-with-argon-dissociation-processes-4qg2kdm010.pdf

Oxygen discharges diluted with argon: dissociation processes

Excitation rate coefficients for the 1${s}_{5}$, 1${s}_{4}$, 1${s}_{3}$, and 1${s}_{2}$ levels of argon by collisions with low-energy electrons have been measured using a drift-tube technique. Time dependences of the absolute population densities of the excited levels were measured by an absorption method with a tunable diode laser as a light source. The absorption data were analyzed according to the rate equations for these levels and the excitation rate coefficient per unit length of electron drift and per argon-atom density was obtained for each level as a function of the electric field to gas density ratio E/N. The values for the 1${s}_{5}$ level vary from 2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ to 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$ as E/N increases from 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ V ${\mathrm{cm}}^{2}$. In comparison with these values, those for the 1${s}_{3}$ level are about one-fifth, those for the 1${s}_{4}$ level are about the same, and those for the 1${s}_{2}$ level are slightly larger in respective measured E/N ranges. In order to estimate the cascading effects from the higher-lying levels, excitation rate coefficients for the 2p and 3p levels have also been measured from absolute intensities of the line emissions. From a comparison of all the measured values of the excitation rate coefficients with those calculated from the Boltzmann analysis, a recommended set of cross sections for these levels has been deduced.

K. Tachibana

Papers

Excitation of the 1s5,1s4, 1s3, and 1s2 levels of argon by low-energy electrons.