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

Magnetic flux ropes produced during non-inductive startup of the Pegasus Toroidal Experiment (Eidietis 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.