Exploring dynamical accessibility in MHD relaxation models

A multi-region relaxed magnetohydrodynamic (MRxMHD) model is used to study the nonlinear evolution of internal modes in a cylindrical tokamak. In particular, we examine whether the Stepped Pressure Equilibrium Code (SPEC) can correctly recover the magnetic island structures, predicted by the implicit nonlinear plasma evolution in an MRxMHD model, for different discretizations of the q and pressure profiles. We compare the SPEC results to linear and nonlinear simulations performed using the initial-value extended MHD code, M3D-C1. These comparisons allow us to understand when a relaxation model can reach a lower energy state that is consistent with more complete physics models. Contrary to ideal MHD models, an MRxMHD model permits changes in the plasma topology caused by magnetic reconnection and can result in the formation of magnetic islands. Finally, we compare values like the total plasma current and volume-averaged beta calculated in SPEC to analytical results from an ideal MHD model, developing scalings to quantify the impact of different discretization schemes on the predicted plasma properties.

This work was made possible by funding from the Department of Energy for the Summer Undergraduate Laboratory Internship (SULI) program. This work is supported by the US DOE Contract No. DE-AC02-09CH11466.