ELECTRON SCATTERING FROM ATOMS (IONS) AND PLASMA MODELING

Abstract

Study of electron collisions with atoms, ions and molecules is fundamentally important subject and has numerous applications related to the different branches of science such as plasma physics, astrophysics, laser physics, fusion research, material sciences and medical science etc. Electron collision processes enhances our knowledge about the structure and collisional dynamics of atomic systems. Currently, it is one of the most active research areas due to the growing demand of the electron collision atomic data in various fields and in particular in the modeling and characterization of verity of plasmas. For the diagnostics of any plasma, optical emission spectroscopy (OES) is one of the most straightforward and non-invasive techniques. In such experiments, the intensities of the emitted lines from the plasma are recorded which give the information about the local environment of the plasma. Therefore, combining the emission intensities to an appropriate collisional radiative (C-R) model provides the plasma characterization parameters such as electron density and temperature. However, in the C-R model, all the collisional and radiative process should be included in proper manner for reliable diagnostics. Since electron impact excitation is one of the dominant collision processes, the availability of its reliable and detailed cross section data for all various fine structure transitions in the wide range of energy is necessary to get the correct plasma parameters. In the recent years, due to the advancement of technology, many experimental techniques have emerged which can provide atomic data with a very high precision. However, the electron collision experiments are quite sophisticated and have thus provided very limited set of e--atom collision data for only selected electron impact energies. Also the available experimental studies have mainly focused on the electron excitation from ground level to few excited levels. While for plasma modeling, there is requirement of large set of collision data for several fine structure transitions in the wide range of electron energies. Consequently, available reliable theoretical methods have to meet such requirements. On the theoretical front, largely available theories are the non-relativistic approaches which have been found totally in adequate to describe the fine structure transitions. Therefore, one needs to adopt only fully relativistic perturbative and non- ii perturbative approaches. The relativistic versions of the non-perturbative approaches viz. R-matrix and CCC methods are supposed to be very accurate but their applications are limited to the low impact energies due to computationally complexities. However, among the perturbative methods viz. the relativistic distorted wave approximation has proved to be very reliable and practical to adopt considering the need of large-scale production of atomic data for modeling of plasmas. In the light of the present context, the work of the thesis focuses on the calculations of the electron impact excitation cross sections required for the various fine structure atomic transitions important to plasma and utilizing them in developing the suitable collisional radiative models to characterize it. Thus the thesis has mainly two objectives. First is to obtain electron impact excitation cross section of various transitions from ground as well as from the exited states in wide range of electron incident energy for neutral atoms. Thereafter, to incorporate these cross sections to development of C-R model to diagnose the variety of low temperature plasma. Second objective is to study the electron impact excitation of highly charged tungsten ions and to study the polarization of their photon emissions which is needed for the diagnostics of high temperature plasma such as of the ITER tokomak. The entire work of the thesis is presented through eight Chapters as briefly described below: Chapter 1 introduces the subject of the thesis and gives the current status of the work related to the thesis work. This chapter also provides briefly the different available theoretical methods to describe electron atom collisions as well as an overview of C-R plasma model for non-local thermodynamic equilibrium (non-LTE) plasma. Finally, outlines the chapter wise thesis work. In Chapter 2, a C-R model developed to characterize the hydrogen-cesium plasma is given. Such a study is relevant to the negative ion based neutral beam injectors for the ITER project. A complete set of data for electron impact excitation cross-sections and rate coefficients for several fine-structure transitions from the ground as well as excited states of cesium atom in the wide range of incident electron energy has been calculated using fully relativistic distorted wave theory. These cross sections are then incorporated in the C-R model. The calculated cross-sections and the extracted plasma parameters from the present model are compared with the available experimental and theoretical results. iii Chapter 3 describes a C-R model developed for Ar-O2 mixture plasma. The model has been applied to diagnose the rf generated Ar-O2 (0-5%) mixture plasma at low temperature. The detailed cross sections for the fine structure transitions involving ground and excited levels of argon obtained from fully relativistic distorted wave (RDW) theory have been used. Processes which account for the coupling of argon with the oxygen molecules have been further added in the model. Through coupling of C-R model to the optical spectroscopic measurements reported by Jogi et. al. [J. Phys. D: Appl. Phys. 47 335206 (2014)], the plasma parameters viz. electron density (ne) and electron temperature (Te) as a function of O2 concentration have been obtained using thirteen intense emission lines out of 3p54p → 3p54s transitions observed in their spectroscopic measurements. The Ar-3p54s (1si) fine-structure level populations at our extracted plasma parameters were found to be in very good agreement with those obtained from the measurements. Furthermore, the estimation of individual contributions coming from the ground state, 1si manifolds and cascade contributions to the population of the radiating Ar-3p54p (2pi) states as a function of a trace amount of O2 has been reported and discussed. Chapter 4 presents detailed electron impact excitation cross section results for xenon in the wide range of incident energy from threshold to 1000eV are calculated using relativistic distorted wave (RDW) theory. Various transitions from the ground 5p6 state to the excited 5p56s, 5p56p, 5p55d, 5p57s and 5p57p as well as among these excited states are considered. Where available the calculated cross section results are compared with previously reported measurements and other calculations. The fitting of the obtained cross section to suitable analytical expressions is also provided for the plasma modeling applications. As an application a collisional Radiative (C-R) model has been developed using our calculated cross sections to characterize a inductively coupled Xe plasma. The plasma parameters obtained from model are reported and discussed. Chapter 5 reports a systematic study of the N-shell electron impact excitation of highly charged tungsten ions viz. Rb-like W37+ through Br-like W39+ in the framework of a fully relativistic distorted wave approach. The cross sections are calculated for various transitions in the electron impact energy range from the excitation threshold to 20 keV. Analytic fitting of the calculated cross sections are also provided so that these can be directly used in any plasma model. Linear polarization of the emitted photons, due to decay of the different electron excited states of the tungsten ions has also been obtained and reported. The present calculations are useful for the diagnostics of the ITER plasmas. iv Chapter 6 presents the RDW calculations for electron impact excitation cross-sections of the M- and L-shell transitions in the tungsten ions viz. Fe-like W48+, K-like through Ne-like (W55+-W64+). This calculation are carried out in the light of wavelength measurements performed at Super EBIT facility at Livermore for the n = 3→3 transitions in 19–25 Å soft X-ray range for these ions. The fitting of the obtained cross section are also provided for the modeling purposes. The polarizations of the decay of photons from the excited tungsten ions are calculated and reported. Chapter 7 finally aids the concluding remarks about the present thesis work.