Description of an Experimental Tripolar Vortex

Recently, in experiments with an argon plasma, stationary coherent structures in the form of a global triple vortex were observed in a cylindrical plasma device at the National Institute for Fusion Science, Japan. The inner diameter of the chamber is about 30 cm and the axial length of about 200 cm. Density profile measurements in the axial direction reveal that the structures are very elongated along the magnetic field lines. The radial cross section of the structures reveals a clear triple structure in the density profile, with two explicit humps of the density, and a centrally situated trough. Such a density distribution is almost uniform along the magnetic field lines. Related to the density profile is the vorticity distribution as well, showing a set of two clockwise rotating lateral vortices, and a central counter-clockwise rotating vortex. The structures occupy practically the whole cross section of the system. An important feature of the system is a spatially nonuniform distribution of the neutral gas, which could be described by a r - dependent parabolic function. This neutral concentration results in an effective radial force directed towards the axial direction, causing a rotation of the plasma column in the direction which is opposite to the E×B drift. We present an analytical description of the observed tripole. Standard equations describing electrostatic perturbations in a spatially nonuniform, rotating electronion plasma with the presence of neutrals are used. A high order partial differential equation for the perturbed electrostatic potential is derived, and solved analytically for an equilibrium gaussian density distribution, and a rigid-body rotation of the plasma column. It is shown that in the process of the plasma self-organization stationary tripolar vortex can be obtained, similar to the structures observed in the experiment.