Identifying Beam-Driven Instabilities Responsible for Current Drive on Pegasus-III

Local Helicity Injection (LHI) is a non-solenoidal startup technique that requires an Alfvénic instability to generate turbulence that drives toroidal current through magnetic relaxation. Significant broadband magnetic turbulence has been measured in LHI discharges on Pegasus and is responsible for current amplification of injected current streams through dynamo-like effects. The turbulence arises from instabilities driven by super-Alfvénic electrons. Several candidates are kinetic Alfvén wave, electron beam ion cyclotron, and electron beam Alfvén wave instabilities. Dependence on relative electron beam density, Alfvén Mach number, and guide field can distinguish these instabilities. Identifying the instability responsible for initiating the turbulence is important to predicting the requirements for a helicity injection system for larger tokamaks and ultimately a fusion power plant. Upcoming experiments leverage insertable probe arrays, the improved Pegasus-III diagnostic set, the expanded operating space of Pegasus-III, and multi-dimensional correlation analysis to identify the instability or instabilities driving the turbulence and explain the observed current drive scaling.