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Author(s):
A. Iwamae, T. Fujimoto, H. Zhang, D. P. Kilcrease, G. Csanak and K.A. Berrington
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Title:
Population Alignment Collisional Radiative Model for Helium-like Carbon: Polarization of Emission Lines and Anisotropy of the Electron Velocity Distribution Function in Plasmas
Date of publication:
Aug. 2003
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Key words:
CV ion, line emission, polarization, longitudinal alignment, anisotropic electron velocity distribution, population alignment collisional radiative model
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Abstract:
The polarization of emission lines from a plasma carries information about the anisotropic velocity distribution of electrons in the plasma, and thus polarization spectroscopy can give information that is inaccessible by other methods. We have developed a comprehensive population-alignment collisional-radiative (PACR) model code for helium-like carbon CV ions. This code is intended to correlate quantitatively the observed polarization of emission lines from the ions in a plasma with the anisotropy of the electron velocity distribution function. Specifically, the longitudinal alignment of CV triplet emission lines for the (1s2s ^3S_l - 1s2p ^3P_l,2) transitions are studied by this PACR model. The predominant process which produces alignment in the 1s2p ~3P_l,2 levels is the alignment production from the ground state, 1s2 ISI and from the metastable level, 1s2s ^3S_1. The alignment-production fluxes from these levels are in the opposite directions in the temperature range of practical interest, depending on the electron density n_e. When ne > 10^16 m^-3, the alignment-production flux from the metastable level is larger than that from the ground state. An anisotropic electron velocity distribution function that has higher values in the axial (toroidal) direction than in the radial (poloidal) direction produces negative longitudinal alignment of the emission lines, i.e., higher intensity of the linear polarized component in the radial direction than that in the axial direction.
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