Potential structure of a plasma hole in a rotating magnetized plasma

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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