A path-dependent entropic Lagrangian formulation for exact thermoviscous evolution of sheared magnetic tori with anisotropic resistivity

Abstract

Resistive evolution of a three-dimensional magnetostatic equilibrium requires compatibility among induction, momentum balance and thermal transport. We construct exact thermoviscous evolutions of a non-axisymmetric magnetic torus with nonzero magnetic shear using a path-dependent entropic Lagrangian (PDEL) formulation. A symmetric positive-definite resistivity tensor is constructed so that the resistive differential expression preserves the spatial form of a known sheared toroidal equilibrium. Homogeneous expansion is coupled to an ideal gas and positive bulk viscosity in the absence of body forces and volumetric heat supply. For a one-temperature gas with heat-capacity ratio $5/3$, a $T^{-2/3}$ resistivity prefactor gives elementary time dependence for all fields. A two-temperature extension admits a $T_e^{-3/2}$ prefactor and positive interspecies heat exchange on a finite state interval. Material magnetic flux decays while the rotational-transform profile is preserved. Both branches satisfy local total-energy conservation, moving-boundary power balance and material transport of dissipation histories. A seed-specific obstruction excludes positive scalar resistivity along the same homologous trajectory; the two-temperature exchange law has a nonremovable endpoint pole. Symbolic identities, independent Cartesian differentiation and surface--volume quadrature verify these results. The solutions provide coupled MHD benchmarks with passive, inverse-designe