Abstract
Measurements of electron densities and population densities of excited states of neutral helium are presented for afterglows in the C stellarator. Electron densities range from ${10}^{11}$ to 3\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and electron temperatures range from 0.04 to 1 eV. Semiempirical cross-section formulas for transitions among excited levels are deduced by comparing observed excited-state population densities with solutions of detailed transition-rate equations. The agreement between computed and measured values is good, when the assumed cross sections are comparable with impact-parameter approximation results; the agreement is poor - especially at low electron densities and temperatures - when classical cross sections are assumed.
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