Electron Temperatures and Densities from Transient Helium-Like Ion Line Emission

Abstract

Measurement of plasma electron temperature is based upon the observation of the decay rate, through ionization, of lines from helium-like ions. Ionization from the ground state is supplemented at higher densities by stepwise ionization through the triplet metastable levels. In an experiment on C v ions in a 15-kJ theta-pinch plasma, this method gives a temperature of 230 eV ±18%, compared with 240 eV measured by the Thomson scattering of laser light. In addition, measurement of the intensity ratio of resonance to intercombination lines in C v as a function of density gives the rate for triplet-singlet transfer collisions, together with the ratio of singlet to triplet excitation, thereby yielding the high-density corrections for the above ionization rate. With this theory, measurements of the intensity ratio can now be used for the determination of electron density in the range 2 × 1014 to 1019cm−3, by selecting the appropriate helium-like ion from B iv to Ne ix, with accuracies of the order 30%. An alternative method for measuring the electron temperature from the absolute intensity of the 23S – 23P multiplet of helium-like ions, based upon the above model together with the Seaton semi-empirical formula for the singlet excitation, gave a value of 265 eV ±25% when the temperature measured by scattering was 210 eV.