Electric field effects during disruptions

During tokamak disruptions, the magnetic surfaces are broken creating large regions of chaotic magnetic field lines. The physics associated with post-disruption chaotic magnetic fields needs to be understood to address force, heat, and runaway electron loading on the walls. Simulations alone are too computationally demanding and have uncertain physics. As surfaces break, regions of chaos are created. When followed long enough, a single magnetic field line will come arbitrarily close to every point in a single chaotic region. The annuli of magnetic surfaces between chaotic regions break by forming cantori, which are toroidal surfaces punctured by pairs of inward and outward tubes of magnetic flux called turnstiles. In each chaotic region, the parallel current density divided by B relaxes toward a spatial constant by shear Alfvén waves. The electric field can be uniquely separated into a divergence-free part that gives the magnetic evolution and an electric potential Φ_q that is required for quasi-neutrality. This potential produces both a diffusion coefficient that is Bohm-like, D_q ≈ T_e/eB, and a largescale flow ≈T_e/eBa_T across the magnetic field lines, where a_T is the scale of the large scale difference in the electron temperature T_e. This diffusion and flow are important for sweeping impurities into the core of a disrupting tokamak plasma. For more details, see https://arxiv.org/pdf/2404.09744.