Numerical Simulation of Local and Coaxial Helicity Injection in Pegasus-III*

Non-inductive plasma startup techniques based on local helicity injection (LHI) and coaxial helicity injection (CHI), which drive DC current along open field-lines, subject to magnetic relaxation, are being explored in Pegasus-III. Accurately simulating the nonlinear MHD behavior of these techniques is important for understanding their viability and scaling. Previous LHI calculations with NIMROD found that relaxation proceeds through multiple magnetic reconnection events that release rings of current and build poloidal magnetic flux. Here, we report new LHI simulation results with increased numerical resolution, and we discuss developments for modeling Pegasus-III with greater fidelity. In particular, simulations of outboard LHI feature resistive wall modeling and vertical magnetic field programming for comparison with experiments. The setup and results of predictive CHI computations for Pegasus-III are also presented and are being used to inform the design of discharge scenarios. Recent CHI computations for the narrow footprint configuration in Pegasus-III include density and temperature evolution, subject to Ohmic heating, for increased fidelity. Whether closed flux surfaces form following injector shutoff is being investigated.