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This paper deals with an experimental investigation of the conditions for local thermodynamic equilibrium (LTE) to exist in an atmospheric argon plasma jet. We have measured the emission coefficient of seven Ar I transition and the line shape of the hydrogen H β transition. After transforming the side-on data into radial space, the population densities of the excited neutral argon atoms and the electron density are determined as functions of the radius of the plasma jet. It is found that LTE does not exist below an electron density of 6 x 10 22 m -3 . The two-temperature Saha equation is shown to give non-physical results for gas temperatures in our electron-density range of 2–6 x 10 22 m -3 . We have also shown that diffusion effects are important under our experimental conditions.

New tables are presented for fitting experimental data to the Voigt function. We used a 16,000-point line profile to generate new tables and have found up to 28% difference with existing tables.

The Stark widths of seven Ar I transitions are reported. Axial line shape data from an atmospheric d.c. argon plasma jet were Abel-inverted to obtain radial line shapes. The electron density was determined from Stark-width measurements of the hydrogen H β transition. In the electron-density region of ⩽6 × 10 22 m -3 , the experimental Ar I Stark widths are fitted to a linear dependence on the electron density. Values of Stark widths extrapolated to other electron densities are compared with measurements reported in the literature on the 4 s -4 p array. Experimental values are up to 45% smaller than those predicted by Griem's theory of Stark broadening. We report experimental values of the electron-impact parameter W . Uncertainties in our results are ⩽ 10%.

The effect of sinusoidal power variations on emissivity and line-shape measurements of Ar I transitions and the Hβ transition are examined, both by a computer simulation and with experiments. The average over substantial power variation (20–50%) for both total emissivity and transition half-width data is shown to agree, within data-processing errors, with results obtained from pure d.c. operation.

Measurements are presented which allow determination of temperature fluctuations in the plasma column of a non-transferred d.c. argon plasma. Temperatures, computed from spectral line emission data, show short-term fluctuations of up to 10%.