Core inductive electric field during sawtooth crashes on DIII-D

Sawtooth crashes on tokamak plasmas exhibit relaxation much faster than resistive time scales via a mechanism not fully understood. Using core magnetic measurements from the Radial Interferometer-Polarimeter (RIP) diagnostic on the DIII-D tokamak, Grad–Shafranov equilibria constrained by internal magnetic measurements that have high time resolution (µs) can be computed, allowing analysis of how equilibrium parameters such as safety factor q, current density J, and parallel electric field , particularly on-axis, evolve. At the sawtooth crash, on-axis safety factor q0 is observed to rise by 5% but remain below 1 throughout the cycle, and on-axis current density J0 is observed to drop by 5%. On-axis parallel electric field is found to be balanced by (resistivity times on-axis current density) except during the 200 µs crash period, where reaches 22 V m−1, exceeding by a factor of more than 2000. These first measurements in tokamak plasmas verify that generalized Ohm's law is not balanced during the crash by resistive effects alone; this is a finding expected due to the relaxation being much faster than resistive timescales. Measurement of the electric field during the tokamak sawtooth serves to illuminate the physical mechanisms at work.