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

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 (Texc) 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 Texc from the plasma driven at 90 MHz showed a hollow profile, which is similar to that of the electron temperature (Te) 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 Texc and Te implies that Texc can be a representative of Te. This is particularly useful for the cases in which conventional Langmuir probe measurements are limited, such as in large size plasmas.