Fluctuation-induced dynamo electromotive forces in current-driven tokamak sawtooth relaxation

We show how a fluctuation-induced dynamo electromotive force (EMF) is responsible for a Kadomtsev-like, current-driven tokamak sawtooth cycle in zero-thermal-pressure, visco-resistive magnetohydrodynamics (MHD), via nonlinear simulations performed with the NIMROD code. We observe a pulsing fluctuation-induced dynamo EMF that, synchronous with time-evolving fluid velocity and magnetic fluctuation energies, arises naturally in the relaxation process, repeatedly driving the safety factor q profile within the q=1 surface from below to above unity during the quasiperiodic crash-like relaxation events, each with a change in on-axis q of roughly Δq0≈0.1. We show how the structures of the velocity and magnetic fluctuations lead to the form of the dynamo EMF and how its structure changes during the sawtooth cycle, consistent with its current profile flattening effect in the sawtooth crash. The process demonstrates the importance of the inductive plasma response in balancing the dynamo in sawtooth activity. We also discuss higher-dissipation cases exhibiting steady helical cores, whose quiescent dynamo EMF modifies the equilibrium profile in a way similar to sawtooth crash relaxation. This work expands the description of the MHD dynamo's importance beyond its role in maintaining quiescent core q profiles in sawtoothing or sawtooth-free tokamak states to its role in modulating the q profile during current-driven sawtooth activity.