Neutral atoms play important roles in non-fully-ionized plasmas. In helicon sources, neutral pumping and neutral damping of waves are poorly understood. Measurement of the spatial distribution of neutral atoms is possible with spectroscopic diagnostics such as laser-induced fluorescence (LIF) and passive emission spectroscopy. However, these measurements typically apply to an excited neutral atom state, rather than the entire neutral population. With a collisional-radiative (CR) model employing Langmuir probe measured electron parameters in argon helicon source plasmas, we have reproduced LIF and emission spectroscopy measured radial profiles for three excited neutral states. The CR model indicates a neutral depletion on axis of at least 60%. Simple calculations based on measured edge neutral pressures and peak plasma densities significantly underestimate the degree of ionization in the core of the helicon plasma.
