Fields, forces, and flows: What laboratory experiments reveal about the dynamics of arched plasma structures.

摘要

Magnetic flux tubes and, more generally, magnetic field structures that link a plasma volume to its boundary are prominent features in plasma systems of significant interest, such as the solar atmosphere and the interiors of magnetic fusion devices.;In order to study the fundamental physics of these systems, experiments were conducted in the laboratory using a magnetized plasma gun to produce individual arched, plasma-filled magnetic flux tubes. More complex plasma topologies were also explored. The absence of confining walls allowed plasmas to evolve freely—which they did, very dynamically, over the course of several microseconds. The experiment setup featured excellent reproducibility, extensive diagnostic accessibility, and several tunable parameters. In particular, a plasma "color coding" technique and magnetic measurements provided new and interesting results.;The single arches or "loops" of plasma exhibited sustained axial collimation, even during a dramatic evolution from a small, semicircular arch into a kinked structure up to seven times larger. The loops' magnetic structure was verified as consistent with that of a flux tube, and their evolution was found to be in quantitative agreement with two interrelated magnetohydrodynamic (MHD) theories: a simplified hoop force model for the axis expansion and a recently proposed MHD flow model for the collimation. More complex plasma structures were found to be similarly dominated by the effects of the magnetic field, exhibiting behavior that was highly repeatable but varied significantly from one magnetic structure to the next.;These findings suggest that MHD-driven flows are an important mechanism for the transport of plasma in arched flux tubes and other magnetic plasma structures. Because MHD has no inherent length scale, the forces driving the evolution of these experiments are expected to similarly affect other systems with low plasma beta and a high Lundquist number.