Lalita Sharma

Near-infrared collisional radiative model for Xe plasma electrostatic thrusters: the role of metastable atoms

Mestastable Xe atoms play an important role in the collisional radiative processes of dense xenon plasmas, including those of electric thrusters for space vehicles. Recent measurements and calculations of electron-excitation processes out of the 5p56s J = 2 metastable state (1s5 state in Paschen notation) have allowed for the development of a collisional radiative model for Xe near-infrared (NIR) emissions based on the population of the metastable level through 2p?1s5 radiative transitions, and based on depopulation through electron-impact excitation. A modified plasma radiative model incorporating newly computed electron-impact excitation cross sections using both relativistic distorted wave and semi-relativistic Breit?Pauli B-Spline R-matrix methods is presented. The model applies to optically thin, low-density regions of the thruster plasma and is most accurate at electron temperatures below 10?eV. The model is tested on laboratory spectral measurements of the D55 TAL and BHT-200 Hall thruster plasma NIR radiation. The metastable neutral fraction is determined to rise from 0.1 to slightly above 1% as the electron temperature increases from ~2 to 10?eV, reaching a maximum around 15?eV. Electron temperatures derived with the modified model are approximately 20% lower than a previous version of the model that used an approximate approach to account for metastable population and line intensity enhancement.

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.

Electron-impact excitation of krypton: Cross sections of interest in plasma modeling

We have performed relativistic distorted-wave calculations to study the excitation of Kr from its ground 4p{sup 6} configuration to the higher lying fine-structure levels of the 4p{sup 5}4d, 4p{sup 5}5p, and 4p{sup 5}6s manifolds. We have obtained relativistic Dirac-Fock multiconfiguration wave functions for the ground and the excited states. We present results for differential cross section and compare these with the available experimental measurements for energies up to 100 eV. We also report integrated cross sections for incident electron energies up to 300 eV and provide analytic fits for plasma modeling applications.

Elastic scattering of electrons from Rb, Cs and Fr atoms

Differential, integrated elastic, momentum-transfer and total cross sections as well as differential S, T and U spin parameters for scattering of electrons from rubidium, caesium and francium atoms in the incident energy range up to 300 eV are calculated using a relativistic Dirac equation. The projectile electron–target atom interaction is represented by both real and complex parameter-free optical potentials for obtaining the solution of a Dirac equation for scattered electrons. The Dirac–Fock wavefunctions have been used to represent the Rb, Cs and Fr target atoms. The results of differential cross sections and spin asymmetry parameter S for e-Rb and e-Cs have been compared with the available experimental and theoretical results. Detailed results are reported for the elastic scattering of electrons from the ground states of a francium atom for the first time in the wide range of incident electron energies. The results of electron-Fr elastic scattering show the similar features to those obtained in the case of e-Rb and e-Cs elastic scattering.

Application of relativistic coupled-cluster theory to electron impact excitation of Mg+ in the plasma environment

Abstract A relativistic coupled-cluster theory is implemented to study electron impact excitations of atomic species. As a test case, the electron impact excitations of the 3s2S1∕2–3p2P1∕2;3∕2 resonance transitions are investigated in the singly charged magnesium (Mg+) ion using this theory. Accuracies of wave functions of Mg+ are justified by evaluating its attachment energies of the relevant states and compared with the experimental values. The continuum wave function of the projectile electron are obtained by solving Dirac equations assuming distortion potential as static potential of the ground state of Mg+. Comparison of the calculated electron impact excitation differential and total cross-sections with the available measurements are found to be in very good agreements at various incident electron energies. Further, calculations are carried out in the plasma environment in the Debye-Hückel model framework, which could be useful in the astrophysics. Influence of plasma strength on the cross-sections as well as linear polarization of the photon emission in the 3p2P3∕2–3s2S1∕2 transition is investigated for different incident electron energies. Graphical abstract

Excitation of the metastable states of argon

We have carried out calculations for the excitation from the lowest metastable states of argon (the J = 0, 2 levels of the 3p54s configuration) to the ten higher-lying fine-structure levels of the 3p55p configuration, using the relativistic distorted-wave approximation. We report our results for the integrated cross section at energies up to 300 eV.

Total polarization of the 185 nm emission line of mercury excited by electron impact

Results for the three Stokes parameters (polarization components) P1 ,P 2 and P3 of the VUV Hg transition 6s6p 1 P1 → 6s 2 1 S0 (185 nm) obtained from electron–photon (e, eγ ) coincidence measurements at electron impact energies of 15 eV, 50 eV and 100 eV (Aussendorf et al 2006 J. Phys. B: At. Mol. Opt. Phys. 39 2403) were combined to obtain the total degree of polarization Ptot ≡ √ P 2 1 + P 2 2 + P 2 3 . In addition, the Stokes parameter P4 was measured at an electron impact energy of 15 eV. The measured data are compared with predictions from a fullrelativistic distorted-wave calculation, a five-state semirelativistic Breit–Pauli R-matrix (close-coupling) model, and a convergent close-coupling ansatz, where relativistic effects are accounted for by recoupling of nonrelativistic results. In agreement with the theoretical predictions, no influence of the electron spin was observed for scattering angles θ 30 ◦ .A t 15 eV excitation energy and scattering angles θ 40 ◦ , however, an increasing importance of spin effects is predicted.

(e, eγ)-coincidence studies to determine spin-resolved Stokes parameters of the 185 nm emission line in mercury

Direct measurements of the three Stokes parameters (polarization components) P1, P2 and P3 of the VUV Hg transition 6s6p1P1 → 6s21S 0 (185 nm) have been carried out at electron impact energies of 15 eV, 50 eV and 100 eV. Within the experimental uncertainty, no influence of the electron spin was discovered for scattering angles ≤ 30°. At 15 eV excitation energy and scattering angles ≥ 80°, increasing spin effects become apparent. The experimental data are compared to theoretical predictions from a first-order full-relativistic distorted-wave model, a five-state Breit-Pauli R-matrix (close-coupling) approach, and a convergent close-coupling model, in which relativistic effects are accounted for by adding non-relativistic amplitudes using known intermediate-coupling coefficients. At scattering angles ≤ 15°, all of the theories reproduce the experimental data well, whereas the CCC model exhibits the best overall agreement with experiment at large scattering angles.

Electron impact excitation of the 5p57p levels of xenon from 5p56s metastable states

Relativistic distorted-wave (RDW) calculations have been carried out for the electron impact excitation from the lowest metastable states of xenon (the J = 0, 2 levels of the 5p56s configuration) to the ten higher-lying fine-structure levels of the 5p57p configuration. Integrated cross section results are reported for incident electron energies up to 300 eV and their analytic fits are also provided for obtaining results at higher energies.

Excitation of atomic oxygen by electron impact

We have carried out relativistic distorted-wave calculations for the excitation of atomic oxygen from its ground 2p4 3P state to the excited 2p33s 3S, 3P and 3D states and to the 2p33d 3D state in the energy range from 15 to 100 eV. We compare our results for the differential cross sections with both experimental measurements and other theoretical calculations for these transitions and find our calculations agree very well with them. We have also compared our integrated cross sections for the excitation of the 2p33s 3S state for which extensive theoretical and experimental data have been reported.