  In this work, we report a confocal laser induced fluorescence (LIF)
configuration, which allows for high spatial resolution measurements of plasma
properties in plasma setups and sources with a limited optical access. The
proposed LIF configuration utilizes a ring-shaped laser beam generated by a
pair of diffractive axicons. LIF signal is collected along the main optical
axis within the ring region. It is shown experimentally that at the focal
distance of 300 mm, the spatial resolution of approximately 5.3 mm can be
achieved. More than that, theoretical estimations predict the possibility of
achieving ~1 mm resolution at the same focal distance by adjusting laser beam
parameters. This is approaching the localization accuracy of conventional LIF
collection methods (with crossing laser beam injection and fluorescence
collection optical paths). Measurements of the ion velocity distribution
function in an argon plasma using both the confocal LIF with ring shaped laser
beam and conventional LIF demonstrate a satisfactory agreement. The proposed
LIF setup has potential applications for diagnostics in various plasma
processing equipment, and plasma sources such as hollow cathodes, microplasmas,
electric propulsion, etc.
