One- and two-dimensional laser induced fluorescence at oblique incidence

The diagnostic technique of laser induced fluorescence (LIF), generalized to the case of oblique laser injection angle relative to the local magnetic field direction, is employed for studies of the ion velocity distribution function (IVDF) in the magnetic expansion region of a helicon plasma source. One-dimensional LIF measurements reveal key characteristics of the acceleration mechanism responsible for creation of an ion beam in the expansion regions: a bimodal IVDF comprising a slowly drifting (similar to 150 m s(-1)) ion population and a fast ion beam (similar to 10.7 km s(-1)). Two-dimensional LIF, LIF tomography, provides additional insight regarding the origins of the two ion populations: the nearly isotropic slow population is a locally created background population whereas the distorted velocity distribution of the fast population is consistent with an origin upstream of the measurement location.