Ion beam acceleration in a divergent magnetic field

Two-dimensional argon ion velocity distribution functions (IVDFs) in the expansion region of a helicon plasma source have been measured by laser-induced-fluorescence tomography. Below a threshold value of the magnetic field in the expansion region, the IVDFs show a bimodal structure comprised of a supersonic ion population axially moving away from the source and an isotropic, slow, background, ion population. Increasing the magnetic field divergence leads to an increase in the axial speed of the supersonic component. A maximum axial speed of ∼2.9cs was obtained for a source/expansion magnetic field ratio of 43.

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