Beschreibung:
<jats:p>Three-dimensional electrostatic particle-in-cell simulations of a laboratory experiment with an elongated plasma cloud entering a curved magnetic field are presented. A moving grid is used to follow the plasma motion from a region with longitudinal magnetic field, through a “transition region” where the field curves, and into a region where the magnetic field has a constant angle of 45° to the flow direction. In order to isolate the physics from disturbing boundary effects a method to create open boundary conditions has been implemented. As a result the boundaries are essentially moved to infinity. The simulation reproduces and gives physical insight into several experimental results concerning the plasma’s macroscopic behavior in the transition region, which have earlier been only partly understood. First, the deformation of the plasma from a cylinder to a slab; second, the formation of strong currents along the sides of the plasma cloud in the transition region, which continue into field-aligned currents in the (upstream) flow-parallel field region, and close across the magnetic field both in the front and in the back of the penetrating cloud; and, third, the formation of a potential structure including (in the transition region) magnetic-field-aligned electric fields, and (both in, and downstream of, the transition region) a potential trough structure in the plasma’s rest frame. It is found that all these macroscopic phenomena are intimately linked and can be understood within one consistent physical picture. The basic driving mechanism is the azimuthal electric field that is induced when, in the plasma’s rest frame, the transverse magnetic field grows in time. The plasma’s response is complicated by the fact that penetrating plasma clouds are in a parameter range where currents are not related to electric fields by a local conductivity: the ion motion is instead determined by the macroscopic potential structure.</jats:p>