A tripolar vortex, three aligned vortices with alternate signs of polarity of rotation, has been
observed in a plasma for the first time. The tripolar vortex always appears with a deep
density depression in the neutral particles, and the rotation direction of each vortex is
opposite to that of the E× B rotation due to the ambipolar electric field. It is shown that a net
momentum transfer during the charge-exchange interaction produces an effective force
acting on the ions. The present experiment shows that this effective force may dominate the …