Simulated flux-rope evolution during non-inductive startup in Pegasus

Abstract

Magnetic flux ropes produced during non-inductive startup of the Pegasus Toroidal Experiment [Eidietis N W, et al., 2007 J. Fusion Energy 26 43] are simulated with nonlinear magnetohydrodynamic (MHD) and two-fluid plasma models. A single injector is represented by a localized source-density for magnetic helicity and thermal energy. Results show development of a hollow tokamak-like profile from a sequence of co-helicity merging events that release flux-rope rings from the driven flux rope. Accumulation of poloidal flux over many events redirects the driven flux rope so that its path traces a toroidal surface. Evidence of a quasi-separatrix layer is found from analysis of the squashing-degree parameter during a ring formation event in an MHD simulation. The layer bifurcates twice, once between non-reconnecting passes and again between reconnecting passes of the driven flux rope. Chaotic scattering during ring formation is also apparent from the distribution of field-line lengths. Correlation of flow-velocity and magnetic-field fluctuations, an MHD dynamo-like effect, concentrates symmetric poloidal flux during ring formation.

Objective magnetic vortex detection

“…III). The method can be used in conjunction with the squashing factor Q [35][36][37] to investigate magnetic reconnection in simulated and observational plasmas, since the IACD can identify the main structures involved in the reconnection while the Q operator detects the quasi-separatrix layers, which are the main magnetic reconnection sites between those structures [12]. A possible application is the reconnection of the solar emerging flux ropes with the coronal field [38].…”

“…For example, in a 3D data set assembled using the relaxation scaling experiment device, Sears et al [11] detected the signature of flux ropes transversal to a horizontal plane as regions where the contours of flux rope current density J z (x, y) fall below a certain threshold. Isosurfaces and contour plots of different quantities related to the parallel current and helical magnetic field components were used to locate flux ropes in MHD simulations of the Pegasus Toroidal Experiment in [12]. In [13] the thresholding is applied to the magnetic helicity mapping, a quantity defined as the magnetic helicity normalized by the magnitude of the magnetic field, integrated along the field lines.…”

Advancing local helicity injection for non-solenoidal tokamak startup

“…The physical mechanisms leading to LHI current drive generate strong MHD activity. Nonlinear, 3D resistive MHD NIMROD simulations of LHI startup have provided insight into dynamical processes observed in experiment and simulation [19], and spurred experimental investigations into MHD activity [13] and reconnection [20] during LHI.…”

Simulation of multi-pulse coaxial helicity injection in the Sustained Spheromak Physics Experiment

“…15 Like with current loops formed in the Pegasus ST, the dominantly n / ¼ 1 perturbations of the velocity and magnetic field beat together to produce a net toroidally averaged electric field transferring energy to the mean (n / ¼ 0) magnetic field. 16 Poloidal flux amplification in a spheromak is also phenomenologically equivalent to toroidal field reversal in a Reversed Field Pinch (RFP)-though typically more extreme-with the r-z plane in a spheromak corresponding to the r-/ plane in an RFP. 17 Linear, ideal MHD stability analysis predicts the onset of the column mode instability in terms of the core, edge, and the eigenvalue (Taylor state) k's, where k ¼ l 0 J k =B.…”