Highly ionized atoms in tokamak discharges

Abstract

Tokamak discharges are characterized by electron densities usually ∼(0.3-1.0)×${10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and temperatures from a few hundred eV to several keV. In addition to the working gas (H or He), the plasma normally contains some light impurities (∼${10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$ O or C) that are completely stripped except at the outer periphery, and heavier elements from the vacuum wall and current-aperture limiter (Fe, Cr, Ni, W, Mo, and others, ∼${10}^{10}$-${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$) that remain partly stripped, hence relatively strongly radiating, throughout the discharge. Other elements, especially noble gases, may be deliberately added for diagnostic purposes. Resonance lines of Fe and Ar in the beryllium and lithium sequences, of Fe, Kr, and Mo in the magnesium and sodium sequences, and of Mo and Xe in the zinc and copper sequences have been used for rough determination of plasma composition. Since crucial plasma characteristics such as temperature and confinement time are sensitively affected by the local composition, it is essential to improve the available atomic data necessary for more accurate analysis: wavelengths, transition probabilities, excitation, ionization, and recombination rates, especially for the heavier elements.