Robert VanDervort

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

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.

LOW-frequency wave propogation in partially ionized plasmas and a strong density gradient in the Hot hELicon eXperiment (HELIX)

Damping of Alfvén waves is one of the most likely mechanisms for ion heating in the solar corona. Ion-neutral collisions have significant but poorly-understood effects on energy transfer and Alfvén wave propagation in partially ionized plasmas, such as those found in the solar chromosphere. Alfvén waves are thought to play a major role in the formation of auroral arcs as well as the generation of parallel electric fields in the Earths ionosphere. The high densities created in helicon plasmas make them ideal for investigating the propagation and damping of Alfvén waves, which have very long wavelengths and low frequencies (ω/ωci ≤ 1). In the Hot hELicon eXperiment (HELIX), the neutral density varies strongly with radius, giving access to a wide range of Alfvén dynamics across the plasma column. The ratio of ion-cyclotron collision frequency in the solar atmosphere varies from 10-6 to 10, while in HELIX the ratio varies from about 0.02 to 0.5. A new low-frequency internal wave-launching antenna has been used by the West Virginia University helicon source group to induce azimuthal (sheer) magnetic field perturbations close to the high-density core and across the strong density gradient region. A small-scale, translatable magnetic sense coil is used to measure the propagation behavior of these low-frequency waves. Cross-correlation measurements are used to understand how wave phase speed changes across the plasma column and wavelet analysis is applied to characterize how the density affects the wave power and coupling. This work builds on previous low-frequency wave launching studies conducted in HELIX and extends them with more precise analysis techniques. For the first time, Laser Induced Fluorescence (LIF) measurements of the perturbed and unperturbed plasma column are compared the show the heating effect of the Alfvén waves.