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

The development of non-inductive plasma startup techniques is important for simplifying the design of future tokamak power plants. Alternative startup methods 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 [IEEE Transactions on Plasma Science 50, 4009 (2022)]. Accurately simulating the nonlinear MHD behavior of these techniques is important for understanding their viability and scaling. The NIMROD code has been employed to model the complex dynamics of helicity injected startup. Previous LHI calculations found that relaxation proceeds through multiple magnetic reconnection events that release rings of current and build poloidal magnetic flux [Phys. Plasmas 19, 080701 (2012)]. While magnetic properties of early Pegasus LHI experiments were reproduced [J.B. O'Bryan, PhD Dissertation, UW-Madison, 2014], previous simulations did not reproduce the experiment's electron temperature. Here, we report new LHI simulation results with increased numerical resolution, and we discuss developments for modeling Pegasus-III with greater fidelity. The setup and results of predictive CHI computations for Pegasus-III are also presented and are being used to inform the design of future experimental scenarios.