Characterization of Injected Current Stream Structure and its Impact on Magnetic Topology in Local Helicity Injection on the Pegasus-III Experiment*

Local helicity injection (LHI) is a method that uses electron current injectors at the plasma edge to provide non-solenoidal tokamak startup. Observations of LHI on previous Pegasus operations show that injected current stream dynamics are consistent with a discrete, 3D injected stream structure that persists in the plasma edge region following relaxation. The relative size of the magnetic perturbation created by this stream (^~b/B_φ ~ 10^-2) is two orders of magnitude larger than that shown to greatly modify plasma performance on other tokamaks. Understanding the impact of this perturbation on the plasma edge is important for coupling LHI to other nonsolenoidal current drive methods and scaling these techniques to larger devices. A simple model of an oscillating, 3D, current-carrying flux tube surrounding an axisymmetric plasma closely reproduces low field side (LFS) dB/dt and B(R,t) measurements on Pegasus. Magnetic topology visualization via field line tracing shows that the stream perturbation induces stochastization of the edge region. Initial studies on the Pegasus-III Experiment will focus on characterizing the injected current stream structure during LFS LHI and further quantifying its impact on magnetic topology.