Spatially resolved emission using a geometry-dependent system function and its application to excitation temperature profile measurement

Abstract

As typical emission spectroscopy involves chord integration along the line of sight, a local measurement with high spatial resolution is attempted using simple lens optics in this work. In the experiment, chord integrated optical plasma emission profile was measured by moving a scanning lens located outside the plasma. The measured emission intensities were spatially reconstructed by employing a geometry-dependent system function, and the local (i.e., only from the lens focal point) emission intensities were obtained with all out-focused emissions subtracted. The 34 different Ar I emission lines spatially reconstructed in this way were used to determine excitation temperature ( T exc) of the argon plasma by the Boltzmann plot method. Being different from the plasma driven at 13.56 MHz where a rather uniform profile was obtained, the spatial profile of T exc from the plasma driven at 90 MHz showed a hollow profile, which is similar to that of the electron temperature ( T e) measured by a Langmuir probe. This hollow profile is attributed from the electromagnetic phenomena such as skin effect and standing wave effect. The similar spatial tendency of T exc and T e implies that T exc can be a representative of T e. This is particularly useful for the cases in which conventional Langmuir probe measurements are limited, such as in large size plasmas.