A study of deviation from local thermodynamic equilibrium in thermal plasmas by optical emission spectroscopy

A fast, accurate and comprehensive optical emission spectroscopic set-up has been employed to investigate the deviation from local thermodynamic equilibrium in a 75-ampere argon plasma arc. The H(β) and H(α) lines of hydrogen were monitored with a spectrograph while an Ar I line and its neighbouring continuum were simultaneously monitored with a monochromator. The electron number densities calculated from Stark-broadened H(β) and H(α) lines of hydrogen were compared with those derived from absolute intensity measurements of the 7635 Å Ar I line. The H(α) line showed an order-of- magnitude higher number density than LTE measurements; the H(α) 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(β) 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(β) line turned out to be about 4 times higher than LTE results. This difference is attributed to two possible reasons: first, the strong Ar I line also suffers from self-absorption and second, there may be a considerable deviation from LTE.