A Confocal Laser-Induced Fluorescence System for Measurements of the Velocity Distribution Function in ExB Plasma Sources

Laser induced fluorescence (LIF) is a nonintrusive diagnostic technique which can be used for obtaining velocity distribution functions (VDF) of ions and neutrals based on their Doppler shift [1]. Conventional LIF optical setups have the beam injection path perpendicular to the emission collection path. This perpendicular system inhibits the usage of LIF as a diagnostic in systems surrounded by solid walls, like many applied plasma devices such as plasma or ion sources (e.g. Hall thruster). Confocal LIF systems have been developed to overcome this issue, but a tradeoff has been lower spatial resolution. However, recent confocal systems have achieved a millimeter-scale spatial resolution, which is close to that of a conventional LIF system [2-3]. Here, we report experimental results from an alternative confocal LIF system that utilizes a pair of axicon lenses [4]. We characterized the optical system and found an ion VDF in argon plasma.