Transition of MHD Kink-Stability Properties between Line-Tied and Non-Line-Tied Boundary Conditions

Abstract

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 verification of the Kruskal-Shafranov stability limit in line-tied partial-toroidal plasmas

“…This direct matching condition does not exist in any of the prior linear flux rope experiments. [15][16][17][18][19] The results present here also represent the first experimental identification of q a = 1 stability in a partial toroidal system. Though this lack of dependence on toroidicity is not particularly surprising given the success of the KS theory in explaining tokamak stability, it does reinforce the applicability of these results to the kink stability properties of other partial toroidal plasmas such as those found in the solar corona.…”

Equilibrium and Stability of Partial Toroidal Plasma Discharges

“…On one hand, clear stability conditions consistent with line-tied boundary conditions were observed as a part of spheromak formation [14] or in screw pinch discharges [15]. On the other hand, stability conditions consistent with one end line-tied and the other end free or partially free were also reported from another linear screw pinch experiment [16,17]. In order to apply these results to astrophysical plasmas such as coronal flux loops, however, we must establish the conditions under which line-tied stability can be studied in the laboratory.…”

A phenomenological model on the kink mode threshold varying with the inclination of sheath boundary

“…3 This theoretical result has been verified in a series of experiments conducted in the Reconnection Scaling Experiment (RSX), which features a long thin linear plasma column and adjustable BCs. [11][12][13][14][15] In these dedicated kink mode experiments, we found a continuum of BCs between NLT and LT, accessible by varying the inclination of sheath boundaries with respect to the axial magnetic field. 11 To explain the continuous changing of kink thresholds with tilted sheath boundaries, the phenomenological model of Ryutov et al can be extended.…”

“…[11][12][13][14][15] In these dedicated kink mode experiments, we found a continuum of BCs between NLT and LT, accessible by varying the inclination of sheath boundaries with respect to the axial magnetic field. 11 To explain the continuous changing of kink thresholds with tilted sheath boundaries, the phenomenological model of Ryutov et al can be extended. This theory had only considered the perfect line tied and perfect non-line tied BCs.…”