nan Dipti

L-shell electron excitations of Mg- through O-like tungsten ions

Electron impact excitations from ground state L shell viz. n = 2 → n = 3 transitions in Mg-like W62+, Na-like W63+, Ne-like W64+, F-like W65+ and O-like W66+ ions have been considered. A fully relativistic distorted wave (RDW) theory has been used to calculate the excitation cross-sections for an electric and magnetic dipole, as well as for the quadrupole transitions for these ions, which were observed recently in the measurements taken from an electron-beam-ion trap (EBIT-I) at the Livermore laboratory. We have performed calculations in the electron impact energy range from a threshold excitation energy of up to 60 keV. Our results, where possible, have been compared with the few previous theoretical calculations that are available. Analytic fits to our calculated excitation cross-sections have also been done for plasma modeling purposes. Our calculated cross-sections have been further employed to obtain the linear polarization of the photon emissions for dipole-allowed transitions due to the decay of the electron impact excited anisotropic states to the ground state for all five of the tungsten ions that are considered in the present work.

Spectroscopic diagnostics of low-pressure inductively coupled Kr plasma using a collisional–radiative model with fully relativistic cross sections

A collisional–radiative (C–R) model for krypton plasma using fully relativistic distorted-wave cross sections for electron excitations was developed. The model was applied to the characterization of inductively coupled Kr plasma with cylindrical geometry over the pressure regime 1–50 mTorr. Radially averaged emission intensities from transitions of Kr (4p55p → 4p55s) in the range 500–900 nm were recorded at 17 cm from the planar RF-driven coil, with the plasma operated in the inductive regime (H mode). The measured emission intensities were then fitted by varying the electron density, n e, and electron temperature, T e, in the C–R model. At both low and high pressures, variations of the electron density by over two orders of magnitude had only a minor role on the relative emission intensities. On the other hand, T e values deduced from the comparison between experiment and model decreased from 6.7 to 2.6 eV as pressure increased from 1 to 50 mTorr. These results are found to be in good agreement with the effective electron temperature determined from Langmuir probe measurements and the predictions of a model based on the particle balance equation of charged particles.

Polarization correlations for electron-impact excitation of neon at 50 eV

We present measurements and calculations of linear and circular polarization correlations for angle-differential electron-impact excitation of the neon resonance transition at an impact energy of 50 eV, most importantly extended to large scattering angles up to 105°. The measured linear polarization parameters of the vacuum ultra-violet transition are in very good agreement with a number of theoretical predictions at all angles. At intermediate scattering angles, however, the circular polarization parameter measurements are only found to be in excellent agreement with theoretical results obtained with the B-spline R-matrix theory, but not with those from other models.

Dynamics of Surface Streamer Plasmas at Atmospheric Pressure: Mixtures of Argon and Methane

A nonequilibrium atmospheric streamer discharge was investigated as a means to seed a large-gap arc breakdown. The dynamics of the streamer were analyzed with high speed imaging, photodiode light intensity, and current–voltage measurements. The temporal evolution of the discharge included a localized surface corona and a positive surface streamer. With the addition of an impurity gas (methane), the ionization was suppressed, which inhibited surface streamer propagation. The electron temperature was determined from time and spatially averaged spectra, coupled with a collisional–radiative model. The electron temperature in argon was measured at 1.25 eV for an electron density range of 10¹⁹ – 10²⁰<italic>m</italic>^–3. Partial local thermodynamic equilibrium calculations showed that the Ar II 4p states followed a Boltzmann distribution with an excitation temperature of 0.7 eV. The gas temperature was estimated at 815 K from a black-body distribution. The velocity of the surface streamer in argon was estimated at ~100 km/s with a diameter of <inline-formula> <tex-math notation=LaTeX>

Collisional-radiative model for the diagnostics of low pressure inductively coupled krypton plasma

sim 500~mu text{m}

Electron impact excitation and polarization studies of Fe-like W48+ to Al-like W61+ ions

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Electron-impact excitation rate-coefficients and polarization of subsequent emission for Ar+ ion

Summary form only given. The physics driving the low and atmospheric pressure discharges have been extensively studied using plasma diagnostics based on optical emission spectroscopy (OES). A popular approach is based on the simulation of emission spectrum, obtained from a suitable population kinetic model accounting the various population and depopulation mechanisms, using specific plasma characteristics as the adjustable parameters1. The accuracy of such approach highly depends upon the cross sections used in the kinetic-model. We have recently showed the effectiveness of using fully relativistic fine-structure cross sections for the analysis of low pressure argon plasmas2. In the present work, we have developed a collisional- radiative (CR) model using our fine-structure relativistic-distorted wave (RDW) cross sections. This model is applied to the study a low pressure inductively coupled (ICP) Kr plasma for which the literature is scarce.Being a heavier element the relativistic effects such as j-j coupling and spin-orbit interactions are expected to play very crucial role in the modelling of Kr plasmas. In the model, we have incorporated 40 fine structure levels in addition to atomic as well as ion ground state. The various processes considered in the model are electron-impact excitation, ionization and their reverse processes through detailed balance principle. The detailed fine-structure electron-impact excitation cross sections have been calculated by using our reliable RDW method3. The required rate coefficients have been calculated from these cross-sections assuming Maxwellian energy distribution. Radially-averaged Kr emission lines from the 2pi →1sj (Paschen notation) were recorded as a function of pressure from 1 to 50mTorr. The electron temperature obtained by the best fit between the measurements and CR model was found to decrease from 6.7 to 2.6eV with the pressure varying from 1 to 50mTorr. These data are in very good agreement with Langmuir probe measurements.