Application of Wall Current Modeling on the Pegasus-III Experiment

The Pegasus-III vacuum vessel was constructed with a continuous, resistive wall, and thus experiences wall currents induced by poloidal field coils. These wall currents can be on the same order as current driven in the poloidal field coils. Pegasus-III shots are on the order of 20 ms and the wall has a time constant of a few milliseconds, causing wall currents to significantly impact the poloidal field throughout the discharge duration. These impacts on the poloidal field must be included in MHD equilibrium analysis, necessitating a method for accounting for these induced wall currents. To this end, the vessel wall has been modeled as a discrete set of axisymmetric, inductively-coupled conducting filaments. Using the driven coil currents as a forcing term, the time evolving currents are calculated in all wall filaments. These currents are then used as an input in the equilibrium reconstruction process and will allow for the fast determination of the time-evolving plasma shape by fitting a current distribution in a set of axisymmetric plasma current filaments, which is a necessary input for power balance modeling.