Spectroscopic Investigations of the Helium Short Arc

Abstract

A short (5 mm) helium arc between a ⅜-in.-diam thoriated tungsten cathode and a 1-in.-diam copper anode was examined spectroscopically. Electron densities were determined from the Inglis-Teller relation for a range of gas pressures and arc currents from 500 Torr, 50 A to 5000 Torr, 250 A. At 850 Torr, 100 A, the electron density was 1.2(+2.1/−0.2)×1016 per cm3. At 850 Torr, 100 A (2.4-kW power level) the atomic excitation temperature was measured using a line intensity ratio technique. Self-absorption corrections were determined experimentally using a tandem arc arrangement and were found to be significant. A numerical computational technique was solved for the spatial dependence of emission and absorption of the HeI 23S-N3P series. The state populations followed a Boltzmann distribution with an excitation temperature of 4800°K. A best straight line fit to the data was obtained with spontaneous decay coefficients computed by Bates and Damgaard and by Schiff and Perkeris. This excitation temperature is too low to account for the observed electron density assuming equilibrium so that local thermodynamic equilibrium (LTE) does not exist in the arc at this power level.