Fast and slow ion populations in expanding helicon plasma observed by laser induced fluorescence tomography

Summary form only given. Since first reports, the current-free electric double layer (EDL) that spontaneously forms at the end of a helicon plasma source, in a region of divergent magnetic field and for pressures below a threshold value, has attracted considerable of interest due to its potential application as a plasma thruster. The measurable parameter of interest in studies of EDL physics is the ion velocity (energy) distribution function (ivdf). One method for obtaining ivdf in plasma is laser induced fluorescence (LIF). However, classical LIF can provide only one-dimensional information since the obtained ivdf is the projection of the 3D ivdf on the laser propagation direction kL. By rotating kL and employing filtered backprojection, the complete 2D ion velocity space distribution function can be acquired via optical tomography. Two dimensional ivdfs were obtained with an internal, scanning LIF probe employing the 3 level Aril LIF scheme 3d´²G_9/2rarr4p´²F_7/2rarr4s´2D_5/2. To detect the fast ion population accelerated by passing through the potential drop of the EDL, measurements were taken in the divergent magnetic field region, 19 cm downstream from the helicon source-diffusion chamber junction. It has been found that below a certain value of the magnetic field in the expansion region, the ivdfs change their character: from unimodal to bimodal, showing an almost isotropic slow ion population and a fast ion population. The shape of the fast ion distribution is skewed and distorted. The distortion is a clear signature of the magnetic field divergence. Furthermore, as the helicon source/diffusion chamber magnetic field ratio is increased, the location of the fast group ion population shifts toward higher axial speeds values. A maximum axial speed of ~10.7 km/s, ~2.8 c_s (ion sound speed), has been obtained for a magnetic field ratio of 43. The slow ion population location in velocity space is not - - affected by the change in the magnetic field ratio.