Reduced magnetohydrodynamic modeling of tearing mode interactions in tokamaks

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Abstract
A series of three‐dimensional reduced magnetohydrodynamic (MHD) simulations has been undertaken to investigate the nonlinear tearing mode interaction mechanism widely believed to explain hard disruptions in tokamaks. Validation of the computations has taken into account both linear and nonlinear numerical stability. Linear stability and energy conservation are shown to be necessary for convergence. Many features of ‘‘hard disruption’’ simulations bear out our theory of a spurious finite time singularity. Adjusting numerical and/or physical parameters suppresses the nonlinear instability as predicted, thus allowing calculations to be pursued indefinitely. The sensitivity of converged calculations to variations in the number of modes and to the initial and boundary conditions has been studied. Mode interactions are insensitive to parameter variations; large amplitude tearing modes lead to transient Alfvén wave bursts, with the persistence of the bursts being greater for fixed boundary voltage calculations.