?A STUDY OF DEVIATION FROM LOCAL THERMODYNAMIC EQUILIBRIUM IN THER- MAL PLASMAS BY OPTICAL EM~SSIONSPECTROSCOPY

Abstract

A fast, accurate and comprehensive optical emission spectroscopic set-up has been employed to investigate the devia-
tion from local thermodynamic equilibrium in a 75-ampere argon plasma arc. The HIIand H" lines of hydrogen were
monitored with a spectrograph while an Ar Iline and its neighbouring continuum were simultaneously monitored
with a monochromator. The electron number densities calculated from Stark-broadened H~and H" lines of hydrogen were compared with those derived from absolute intensity measurements of the 7635 A Ar l line. The Ha line showed
an order-of-magnitude higher number density than LTE measurements; the Ha results are generally known to be
inaccurate because this line is self-absorbing and its refined Stark-broadening parameters have not been calculated.
The H~line is usually the first choice for Stark-broadening calculations; its theory is well developed, and results are
expected to be accurate within 10%. Results from the H~line turned out to be about 4 times higher than LTE results.
This difference is attributed to two possible reasons: first, the strong Ar Iline also suffers from self-absorption and
second, there may be a considerable deviation from LTE.