Partial Equilibrium in an Arc Spectrum

Abstract

Time resolved spectra have been taken of a pulsed arc operating in 2 Torr of helium, C${\mathrm{O}}_{2}$, air, or argon. Wall confinement aided by close-fitting return conductors produce a 3.5-eV arc plasma in local thermodynamic equilibirum (LTE) during the first 10 \ensuremath{\mu}sec. The electron density is ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Thereafter, the plasma cools and the silicon impurity rises. The arc temperature is limited by radiation losses. LTE is found among eight species, as expected at these values of ${N}_{e}$; but neutral helium is found to be about 1.5 eV cooler than the rest of the plasma. Such a result could cause large temperature errors in plasma diagnostics using He II/I line ratios.