Measurements of ion velocity distribution function using a confocal laser induced fluorescence with annular laser beam

Laser Induced Fluorescence (LIF) is a widely used diagnostic, which allows for measurements of velocity distribution functions and temperature of ion or neutral species in plasma. Conventional LIF diagnostic requires access to plasma from two sides: for the laser beam injection and in perpendicular direction for the fluorescence emission collection [1]. It is not always possible to have such access to the plasma volume. Alignment of two optical branches is more complicated for the conventional LIF as well. Confocal configuration of laser induced diagnostics is widely used in biology and medicine and there are several works discussing its application for plasma diagnostic [2-4]. Its main advantage is that laser beam injection and fluorescence collection branches coincide. Axicon lenses were used to create an annular laser beam, which is focused into the argon plasma. In this work we present the results of initial measurements of argon ion velocity distribution function with the axicon based confocal LIF system.

[1] Bachet G et al 1998 Phys. Rev. Lett. 80 3260

[2] VanDervort R et al 2014 Rev. Sci. Instrum. 85 11E408

[3] Scime E Soderholm M 2015 APS Division of Plasma Physics Meeting Abstracts BP12-074

[4] Thompson D et al 2017 Rev. Sci. Instrum. 88 10350