Observation of Axial Neutral-Gas Flow Reversal in an ECR Plasma

Axial neutral gas transport has been experimentally investigated in a partially ionized cylindrical plasma. The high-accuracy laser induced fluorescence spectroscopy is embraced to evaluate the absolute flow velocity of ions and neutral particles, and the axial flow velocity associated with the mass transport parallel to the magnetic field has been measured by changing the filling gas pressure. It has been found that the neutral gas flow is spatially non-uniform, and the axial flow reversal takes place under a certain circumstance. The experimental results indicate that non-uniformity of neutral gas flow field is generated as a consequence of the plasma-neutral coupling and the axial neutral transport plays an important role in the plasma structure formation.