Evidence of noninductive current drive during tokamak sawtooth crashes using fast internal magnetic measurements

Characterization of internal magnetic fields using reactor-compatible diagnostics will be important for magnetic-confinement fusion power generation; exploration of internal magnetic fields and their implications during tokamak sawtooth crashes has been performed using Faraday-effect polarimetric measurements to constrain Grad-Shafranov equilibrium reconstruction during sawtooth crashes on DIII-D. Results show q₀ rising from 0.86±0.01 to 0.92±0.01 during the 0.2 ms sawtooth crash duration, providing evidence of incomplete relaxation, while J₀ decreases from 2.23 to 2.16 MA/m². A flux-surface-averaged E_∥ profile can be determined, revealing E_∥(0)≈40 V/m during the crash, exceeding the value of ηJ by more than a factor of 1000, and implying other terms in generalized Ohm’s law, such as the fluctuation-induced dynamo effect, may be significant. Internal magnetic and density fluctuation measurements observe m=n=1 mode activity persisting throughout the sawtooth cycle, providing further evidence that q is not everywhere raised above 1 during the sawtooth crash. Internally measured radial magnetic fluctuations increase ~20% in amplitude during crashes and may drive anti-dynamo action in the plasma core.