Impact of Injected Current Streams on Magnetic Field Structure During Local Helicity Injection

Local helicity injection (LHI) is a solenoid-free plasma startup method that uses direct current injection (I_inj < 16 kA) to supply magnetic helicity and drive plasma current (I_p > 200 kA). In LHI-initiated plasmas there exists up to 16 kA of open field line current that may perturb the plasma edge due to this current injection. This current creates a large (^~b/B_T ~ 10^-3–10^-2), 3D magnetic perturbation that is expected to deform magnetic surfaces and induce stochasticity in the plasma edge region. Understanding this effect on Pegasus-III is important because of its potential impact on realized transport and confinement and, therefore, the scalability of LHI to higher B_T devices. A larger 3D perturbation is expected for such devices, as they will likely require higher I_inj to achieve relaxation. Internal B(R,t) and external dB/dt measurements have been obtained for plasmas with 0.15 < B_T < 0.3 T and 3 < I_inj < 6 kA to constrain models of LHI current stream and magnetic field structure. Magnetic topology visualization via field line tracing with the FLARE code shows that the stream perturbation induces stochasticity. This result is being extended by including the 3D plasma response to the stream perturbation via calculations with M3D-C1.