Metastable argon atoms under significant neutral depletion in helicon plasmas by laser-induced fluorescence

We study the spatial distribution of the metastable-state argon atoms in high density helicon plasmas by means of laser-induced fluorescence. It is observed that the neutral argon in metastable-state has an anomalous radial distribution in density; it has a caldera-like shape radially, which is rare in typical low-temperature plasmas such as inductively coupled plasmas wherein the density increases toward the discharge center, as previously reported. The formation of the distribution can be explained as it forms by the combined effects of significant neutral depletion in high plasma density, off-axis electron-density distribution, and increasing diffusive loss toward the wall. To establish the assertion with the underlying physics, we calculate a simple global model and obtain the neutral density distribution in metastable-state under various conditions. The calculated results qualitatively agree with the experimental results.