| Vortex as a 2-dimensional (2D) coherent structure is of common interest in both self-organization and turbulent transport in plasmas. Much attention has been paid to inviscid vortices such as a drift vortex so far. We have observed the plasma hole in a cylindrical plasma with magnetic field, and identified it as a Burgers vortex, which is inherent to viscous fluids. The observation of viscous-vortex suggests that the viscosity of a plasma is not negligibly small and bears a key role in vortex formation. The essential difference between an inviscid vortex and a viscous-vortex is the existence of radial flow, by which the viscous vortices can interact with a different manner from that of inviscid vortices. Starting from the fluid equation, we derived the equation of motion for ''point viscous-vortices'' and numerically examined the dynamical behavior to compare with that of point vortices. For a system of two viscous-vortices with the same sign of vorticity, they attract each other and coalesce into one as time elapses, while two point-vortices rotate each other and never coalesce into one. For a systems of N vortices, we obtained vortex crystals (or vortex lattices), which have much longer lifetime compared with the decay time due to viscosity. |
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