A comparison of laser induced florescence and continuous wave ring down spectroscopy IVDF measurements in an argon helicon plasma

In this work, we compare two spectroscopic methods for measuring the ion velocity distribution functions (IVDF) in an argon helicon plasma: laser induced florescence (LIF) and continuous wave cavity ring down spectroscopy (CW-CRDS). An established and powerful technique, LIF suffers from a requirement that the initial state of the LIF sequence have a substantial density. In most cases, this requirement limits LIF to ions and atoms with large metastable state densities for the given plasma conditions. CW-CRDS is considerably more sensitive than LIF and can potentially be applied to much lower density populations of ion and atom states. CRDS is a line integrated technique without the spatial resolution of LIF. CRDS is a proven, ultra-sensitive, cavity enhanced absorption spectroscopy technique and when combined with a CW diode laser that has a sufficiently narrow linewidth, the Doppler broadened absorption line, i.e., the IVDF, can be measured. We will present CW-CRDS and LIF measurements of the IVDF in argon using the 668.614 nm (in vacuum) line of Ar II.