Reactive species density measurements from Two-Photon Laser Induced Fluorescence in an ICP source

Reactive neutral atomic species such as O and N play an important role in several plasma-based applications. However, accurate measurement of these neutral densities is challenging. Commonly-deployed in-situ plasma probe methods (e.g. Langmuir probes or emissive probes) are not sensitive to neutrals. Nonintrusive measurement using optical emission spectroscopy (OES) is possible, but making localized measurements is exceedingly difficult and calculating parameters of interest from plasma emission often requires complex collisional-radiative models. In this work we use two-photon laser induced fluorescence (TALIF) to make accurate and localized measurements of neutral densities in nitrogen, and we focus on the restrictive diagnostic access requirement for distinct injection and collection lines of sight. We implement a confocal apparatus that uses injection and collection optics on the same axis, requiring only a single line of sight and allowing access to otherwise difficult or impossible regions of interest. We compare the performance of confocal to perpendicular collection in a nitrogen inductively coupled plasma (ICP) source over a range of internal pressures (10 - 500 mTorr) and operating powers (300 – 1000 W), and find general agreement within accepted measurement uncertainty between the methods over this range. Calibration mechanisms and opportunities for improvement are also discussed.