Jovo Vranjes

Velocity shear driven electron skin size vortices

Nonlinear electromagnetic perturbations in a nonuniform plasma are studied in the presence of nonuniform flows that are aligned either perpendicular or parallel to the ambient magnetic field lines. Coherent structures with typical spatial scales of the order of the electron skin depth are found in the form of a stationary tripolar vortex, and for spatial scales of the order of the electron Larmor radius at the ion temperature in the form of a vortex chain. In both cases, the structures are driven and carried by the plasma flow. In the case of a parallel flow, the dipolar vortex propagates perpendicular to the field lines. For plasma in the Earth’s magnetotail, the solutions presented here should be expected at spatial scales of the order of about 10–20 km.

Linear and nonlinear electrostatic modes in a nonuniform magnetized electron plasma

Linear and nonlinear low-frequency modes in a magnetized electron plasma are studied, taking into account a proper description of the equilibrium plasma state that is inhomogeneous. Assuming a homogeneous magnetic field and sheared plasma flows, flute-like perturbations are studied in the presence of density and potential gradients. Linear analysis reveals the presence of a streaming instability and depicts conditions for global linear spiral mode. In the nonlinear domain, a tripolar vortex, which is driven and carried by the flow, is found. Also investigated are the consequences of a magnetic shear as well as nonuniformities along the magnetic field lines, which are shown to be responsible for the possible annulment of the magnetic shear effects. Streaming along the lines of the sheared magnetic field is also studied. A variety of nonlinear structures (viz. global multipolar vortices, local vortex chains, and tripolar vortices) is shown to be the consequence of the simultaneous action of the parallel and perpendicular flows.

Comment on "On quantum plasma: A plea for a common sense" by Vranjes J. et al. Reply

Shukla and Akbari-Moghanjoughi have {it corrected} their Comment (see their version 1 on `arXiv:1207.7029v1) to EPL on our work [1] after receiving our Response from the Editors of EPL. We have a pleasant duty at hand to present our second Response to their second version of the Comment. It is hoped that this response adds strength to our plea {it for a common sense} [1] on quantum description of plasmas.

Description of an experimental tripolar vortex

Summary form only given, as follows. 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. 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/spl times/B drift. We present an analytical description of the observed tripole Standard equations describing electrostatic perturbations in a spatially nonuniform, rotating electron-ion 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.

Observation of anti-E/spl times/B tripole vortex in a plasma

Summary form only given, as follows. Flow velocity field of ions has been measured in a magnetized cylindrical plasma, and the vorticity distribution on a plane perpendicular to the magnetic field has been determined to identify vortex formation. A tripole vortex, which consists of three aligned vortices with alternate signs of polarity of rotation, has been observed for the first time. The most important fact on the tripole vortex is that each vortex rotates in the direction opposite to the E/spl times/B drift. This means the existence of another force acting on ions, which overcomes the radial electric field. We have confirmed that there always presents a deep density hole in the background neutrals when the tripole vortex is generated, and that the scale of tripole vortex is closely related to the scale of neutral density hole. It is also found that the inward flow of neutrals induced by its steep density gradient is converted into ions through charge exchange interaction. In this process, a net momentum transport takes place, resulting in generation of an effective force acting on ion fluid. Since the direction of effective force is opposite to the radial electric field, it may drive the plasma into anti-E/spl times/B rotation, dominating the radial electric field. The vorticity distribution of ion flow field is determined. The observed tripole vortex is considered to be a vortex structure generated in the coupled system of plasma and flow of neutrals.