Anti-ExB flow field associated with a vortex formation in a partially ionized plasma
Résumé
A high-density magnetized plasma has been studied for understanding of plasma dynamics in partially ionized plasmas. While research of plasma dynamics on fully ionized plasma has been developed in the past century as well as on weakly ionized plasma, the understanding of partially ionized plasma is a current issue especially in considering divertor plasmas, industrial plasmas and ionosphere plasma. In this paper, the experimental result of ion flow field associated with a vortex formation in a partially ionized plasma is presented. The most remarkable result is that the direction of rotation is opposite to that of the ExB drift. The experiments have been performed in the high density plasma experiment (HYPER-I) device at the National Institute for Fusion Science. The HYPER-I device produces a cylindrical plasma (30 cm diameter and 200 cm axial length) by electron cyclotron resonance (ECR) heating with a microwave (2.45 GHz, 80 kW maximum) launched at an open end of the chamber. The plasma density and the electron temperature are about 10^{13} cm^{-3} and 5 eV, respectively, for the operation pressure 30 mTorr (Argon), which is several ten times higher than that of usual ECR plasma experiments. In the experimental condition described above, a tripolar vortex has spontaneously been formed. Ion flow velocity field, obtained with a directional Langmuir probe, shows that the rotational direction of each vortex is opposite to that of the ExB drift. Measurement of neutral density profile reveals that there is a steep density gradient of the neutrals around the vortex, thus inward momentum of the neutrals is generated due to the density gradient. The anti-ExB rotation is caused by the effective force attributable to radial momentum transfer from the directed neutrals to the ions with charge-exchange collision. The present experiment shows that this effective force may dominate the ambipolar-electric field and drive the anti-ExB vortical motion of ions.