Flux rope dynamics: experimental study of bouncing and merging.

We show experimentally for the first time that two mutually attracting flux ropes may bounce back instead of merging together, leading to a variety of dynamics not expected from a two-dimensional model. Attraction forces due to flux rope currents compete with repulsion from field line bending of in-plane and out-of-plane magnetic fields and elastic plasma compression. Bouncing dynamics occurs if the line-bending force due to an out-of-plane field dominates. Otherwise, the ropes merge. Further reduction in the field line-bending force results in violently erratic magnetic states.

My brother's keeper: simultaneous hospitalization of siblings.

Milieu therapists are confronted with a dilemma when more than one child in the same family requires hospitalization. Conventional wisdom discourages simultaneous hospitalization of siblings on the same unit. The sibling bond, however, is an important aspect of development and can be used therapeutically. This paper examines issues and opportunities presented to milieu therapists by the simultaneous hospitalization of siblings, and makes recommendations for maximizing therapeutic use of the sibling bond.

A three dimensional probe positioner.

In order to sort out the physics that is important in many plasma experiments, data in three dimensions (3D) are becoming necessary. Access to the usual cylindrical vacuum vessel is typically restricted to radially or axially insertable probes that can pivot. The space that can be explored usually has significant restrictions either because probe travel must be along a travel path, or a "wobbly" probe positioner requires one to map between a moveable coordinate system and a preferred laboratory coordinate system. This could for example introduce errors in measurements of vector quantities such as magnetic field or flow. We describe the design and implementation of a 3D probe positioner that slides in two dimensions on a double O-ring seal and radially inserts along the third dimension. The net result is that a 3D space can be explored in a laboratory Cartesian reference frame.

Transition of MHD kink-stability properties between line-tied and non-line-tied boundary conditions.

Magnetic flux tubes or flux ropes in plasmas are important in nature and the laboratory. Axial boundary conditions strongly affect flux rope behavior, but this has never been systematically investigated. We experimentally demonstrate for the first time axial boundary conditions that are continuously varied between ideal magnetohydrodynamic (MHD) line-tied (fixed) and non-line-tied (free). In contrast with the usual interpretation that mechanical plasma motion is MHD line-tied to a conducting boundary, we constrain boundary plasma motion to cause the line-tied condition.

Experimental demonstration of plasma-drag acceleration of a dust cloud to hypervelocities.

Simultaneous acceleration of hundreds of dust particles to hypervelocities by collimated plasma flows ejected from a coaxial gun is demonstrated. Graphite and diamond grains with radii between 5 and 30 microm, and flying at speeds up to 3.7 km/s, have been recorded with a high-speed camera. The observations agree well with a model for plasma-drag acceleration of microparticles much larger than the plasma screening length.

Current-driven rotating-kink mode in a plasma column with a non-line-tied free end.

First experimental measurements are presented for the kink instability in a linear plasma column which is insulated from an axial boundary by finite sheath resistivity. An instability threshold below the classical Kruskal-Shafranov threshold, axially asymmetric mode structure, and rotation are observed. These are accurately reproduced by a recent kink theory, which includes axial plasma flow and one end of the plasma column that is free to move due to a non-line-tied boundary condition.