Characterization of Current Stream Structure in Local Helicity Injection on the Pegasus ST

Local Helicity Injection (LHI) uses small, high-power electron current injectors at the plasma edge to provide non-solenoidal tokamak startup. LHI dynamics on Pegasus are consistent with a discrete injected current stream structure that persists in the plasma edge region following relaxation. Scaling LHI to larger devices requires an understanding of how this current stream structure evolves in time and its potential effect on equilibrium properties. Strong (^~b_Z/B_f ~ 10^-2), low-frequency (~20–50 kHz) n = 1 activity is observed on the low field side (LFS) during LHI on Pegasus and is well-characterized by a singly line-tied kink instability of the injected current streams. A simple model of an oscillating helical current stream just outside the plasma edge closely reproduces LFS dB_Z/dt and B(R,t) measurements. Accounting for the effects of this 3D current stream structure is important because prior work on other tokamaks has shown that even small, nonaxisymmetric perturbations (dB/B₀ ~10^-4) can greatly modify plasma performance. The helical current stream model can be used to inform future 3D equilibrium studies of LHI using codes like the Generalized Perturbed Equilibrium Code (GPEC).