Characterization of Magnetic Turbulence on the Pegasus-III Experiment

Local helicity injection (LHI) is a non-solenoidal startup technique that involves Taylor relaxation of helicity from edge-driven helical current streams into a tokamak-like state with high current multiplication (I_p >> I_inj). Broadband magnetic turbulence, arising from instabilities driven by super-Alfvénic electrons, has been measured in LHI discharges on Pegasus and may have a relationship with relaxation and current drive [Richner et al., Phys. Rev. Lett. 128, 105001 (2022)]. The upgraded Pegasus-III experiment will enable a similar turbulence characterization at a toroidal field of up to 0.6 T. Understanding the scaling of instabilities, turbulence, and relaxation behavior at higher field will facilitate the projection of LHI performance to larger-scale fusion energy systems. Upcoming experiments leverage magnetic probe arrays that can be inserted into the plasma edge and scrape-off layer, the increased toroidal field limit of Pegasus-III, and spectral analysis to expand previous characterization of turbulence in this new regime. In future work, the nature and scaling of the turbulence-generating beam instabilities will be investigated, as well as the possible presence of the fluctuation-generated Hall dynamo.