Observation of Anti-E×B Tripole Vortex in a Plasma

Flow velocity field of ions has been measured in a magnetized cylindrical plasma, and the vorticity distribution on a plane perpendicular to the magnetic field has been determined to identify vortex formation. A tripole vortex, which consists of three aligned vortices with alternate signs of polarity of rotation, has been observed for the first time. The most important fact on the tripole vortex is that each vortex rotates in the direction opposite to the E×B drift. This means the existence of another force acting on ions, which overcomes the radial electric field. We have confirmed that there always presents a deep density hole in the background neutrals when the tripole vortex is generated, and that the scale of tripole vortex is closely related to the scale of neutral density hole. It is also found that the inward flow of neutrals induced by its steep density gradient is converted into ions through charge exchange interaction. In this process, a net momentum transport takes place, resulting in generation of an effective force acting on ion fluid. Since the direction of effective force is opposite to the radial electric field, it may drive the plasma into anti-E×B rotation, dominating the radial electric field. The vorticity distribution of ion flow field is determined by∇_⊥² logn_n in this case, where n_n is the density profile of background neutrals. The quantity log n_n becomes stream function instead of electrostatic potential. We have experimentally confirmed that the two-dimensional distribution of ∇_⊥² log n_n well agrees with that of ω = rot u. The observed tripole vortex is considered to be a vortex structure generated in the coupled system of plasma and flow of neutrals.