Digital imaging technique for optical emission spectroscopy of a hydrogen arcjet plume

A digital imaging technique has been developed for optical emission spectroscopy measurements of a 1.6-kW hydrogen arcjet plume. Emissions from the Balmer α and β transitions of excited atomic hydrogen were measured with a computer-controlled red-green-blue color CCD detector with and without line-centered bandpass interference filters. A method for extending the effective dynamic range of the detector was developed, whereby images obtained with a wide range of exposure times are combined to form a single composite nonsaturated map of the plume emission structure. The line-of-sight measurements were deconvoluted to obtain the true radial intensity distribution with an inverse Abel transformation. Analysis of the inverted measurements indicates that the upper levels of the Balmer α and β transitions are not thermalized with the electrons in the plasma. The local thermodynamic equilibrium assumption fails for this plasma, and the electron temperature is not equivalent to the apparent excitation temperature obtained when a Boltzmann energy distribution is assumed for the atomic hydrogen excited states.