Understanding Plasmas through Ion Velocity Distribution Function measurements

Understanding Plasmas through Ion Velocity Distribution Function measurements This work concerns measurements of ion velocity distribution functions (IVDF) using three different diagnostic techniques in expanding argon plasmas generated by two different helicon plasma sources. A theoretical prediction by Lieberman et al., of enhanced upstream ionization by electrons accelerating upstream, due to the formation of a stable electrostatic current-free double layer (DL) in an expanding plasma, was verified through laser induced fluorescence (LIF) and retarding field energy analyzer (RFEA) measurements of the IVDF and Langmuir probe measurements of the plasma density. Keeping all other external source parameters constant and varying only the operating frequency of the helicon source HELIX, it was possible to initiate a transition from an unstable to a stable DL. As predicted, the upstream plasma density increased significantly when the stable DL appeared. The instabilities that prevented the formation of a stable DL at low helicon source operating frequency were studied with electrostatic double probes while simultaneously measuring the IVDF. Because the two methods typically used to measure IVDFs in low temperature plasmas require a large population of the target excited state (as in LIF) or are invasive and can be used only for ions in very low density plasmas (as in RFEA), another portion of this work focused on the development of a new method for the non-invasive measurement of IVDFs that is considerably more sensitive than LIF. Continuous wave cavity ring down spectroscopy (CW-CRDS) is a proven, ultrasensitive, cavity enhanced spectroscopic technique that has, in the past, been used to measure the absorption line shapes of particular atomic, ionic, and molecular transitions in test cells. Here we report the first CW-CRDS measurements of the Ar II ion velocity distribution function in a plasma along with conventional LIF measurements of the same ion state in the same plasma. Saikat Chakraborty Thakur