  We have observed spontaneous formation of a stationary vortex structure in a
rotating magnetized plasma produced in a linear ECR plasma device named HYPER-I
at National Institute for Fusion Science. The vortex appears with a deep
density-cavity, plasma hole, in its center, and is identified as a dissipative
vortex in viscous fluids (Burgers vortex) [1]. The kinematic viscosity
estimated from the vorticity profile and the ion flow field is 4 orders of
magnitude higher than the classical value, which may be attributable to
insufficiency of the Debye shielding or the breaking of quasi-neutrality. Here
we present the results of potential measurements and the evaluation of
quasi-neutrality breaking. It is revealed that the potential has a bell-shaped
axisymmetric structure localized inside the core region, in which its maximum
value exceeds 5 Te. Using Poisson's equation, degree of quasi-neutrality
breaking is evaluated, which shows 3 orders of magnitude larger than that of
usual laboratory plasma. The strong electric field produced by this
quasi-neutrality breaking drives the supersonic rotation of the plasma hole. It
should be mentioned that the degree of quasi-neutrality breaking in the ambient
region of the plasma hole is comparable to that of usual laboratory plasma.
Hence the plasma hole can be considered as the structure of a nonneutral core
surrounded by a quasi-neutral plasma. It is of great interest to study the
fluctuations associated with the connected structure of the nonneutral and the
quasi-neutral plasma, especially the interfacial layer in between. Our
preliminary observation shows that the pronounced fluctuations exist in the
interfacial layer. [1] K. Nagaoka, A. Okamoto, S. Yoshimura, M. Kono, and M. Y.
Tanaka, Phys. Rev. Lett. 89 (2002) 075001
