Neutral flow field structure of partially ionized plasma in the HYPER-I and HYPER-II devices*

It has been widely recognized that plasma-neutral coupling plays an important role in atmosphere-ionosphere coupling, transport phenomena in laboratory plasmas, and plasma acceleration in plasma propulsion systems. In weakly ionized plasmas, a change in the plasma fluid induces a change in the neutral fluid. However, the neutral gas flow has been usually treated as too much simplified because of the difficulty of spatial profile measurement of neutral gas flow. We have experimentally studied the effect of neutral gas flow on plasma structure formation in the HYPER-I device (NIFS) and the HYPER-II device (Kyushu Univ.). An ECR plasma was produced with a 2.45 GHz microwave and the high-density plasma (typically 10% ionization degree) was used. The local velocity distribution function measurement for the neutral particles in plasma carried out with a high accuracy LIF system composed with a tunable diode laser. We have observed the generation of vortex pattern and axial flow reversal of neutral gas flow associated with neutral depletion. These results indicate that plasma and neutral fluids affect each other to make the characteristic spatiotemporal structures.