Effects of Injected Current Streams on MHD Equilibrium Reconstruction of Local Helicity Injection Plasmas in Pegasus-III

Local helicity injection (LHI) is a solenoid-free startup technique that produces tokamak plasmas by injecting force-free current along helical field lines. Magnetic measurements during LHI indicate the injected current streams remain coherent 3D structures throughout the discharge. These current streams may pose a challenge when inferring the plasma equilibrium with standard reconstruction codes that rely on the assumption of toroidal symmetry. This effect is considered here for the case of reconstructing LHI plasmas in the Pegasus-III spherical tokamak. Forward modeling quantifies the impact LHI current streams have on the magnetic diagnostic array and reconstructed equilibria using the KFIT free-boundary equilibrium code by treating the system as an axisymmetric plasma perturbed by helical current streams following open magnetic field lines originating from the injectors. It indicates the streams add a measurable perturbation to most of the magnetic diagnostic suite for typical operational parameters. These perturbations lead to statistically significant systematic errors in the reconstructed plasma equilibrium. An iterative approach for reducing these systematic errors is described.