Spectroscopy Study of the ASTRAL helicon plasma source

A spectroscopy study of the ASTRAL (\textbf{A}uburn \textbf{S}teady s\textbf{T}ate \textbf{R}esearch f\textbf{A}ci\textbf{L}ity) helicon plasma source is presented. A spectrometer which features a 0.33 m Criss-Cross Scanning monochromator and a CCD camera is used for this study. ASTRAL produces Ar plasmas with the following parameters: n$_{e}$ = 10$^{12}$ to 10$^{13}$ cm$^{-3}$ and T$_{e}$ = 2 to 15 eV. Ar I , Ar II and Ar III species are monitored as a function of rf power. In the 250 to 450 nm range, Ar II transitions dominate the spectrum and very few Ar I transitions are present. In the 300 to 400 nm range Ar III transitions are barely visible at low power and become intense at high power. In the 700-1000 nm range, Ar I transitions dominate the spectrum while very few Ar II transitions are observed. Ar II and Ar III intensity increases with rf power while Ar I intensity are independent of power. This constant Ar I intensity strongly suggests that neutral depletion occurs within the core as the power is raised. A discussion relative to the different observations is presented with links to theoretical excitation rate coefficients.