Laser induced florescence and continuous wave ring down spectroscopy: Measurements of argon ion velocity distribution functions in a helicon plasma

Abstract

Summary form only given. In this work we compare measurements of argon ion velocity distribution functions (IVDFs) in a helicon plasma using two spectroscopic techniques: laser induced florescence (LIF) and continuous wave cavity ring down spectroscopy (CW-CRDS). LIF is an established and powerful technique, but suffers from the requirement that the initial state of the LIF sequence have a substantial density. This usually limits LIF to ions and atoms with large metastable state densities for the given plasma conditions. CRDS is a proven, ultra-sensitive, cavity enhanced absorption spectroscopy technique and when combined with a CW diode laser that has a sufficiently narrow line width, the Doppler broadened absorption line, i.e., the VDFs, can be measured. CW-CRDS is more sensitive than LIF and can potentially be applied to much lower density populations of ion and atom states. However, being a line integrated technique CW-CRDS lacks the spatial resolution of LIF. We present CW-CRDS and LIF measurements of the IVDFs in an argon plasma using the 668.614 nm (in vacuum) line of Ar II.