Core inductive electric field during sawtooth crashes on DIII-D

Abstract

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 ( <1 µs) can be computed, allowing analysis of how equilibrium parameters such as safety factor q, current density J, and parallel electric field E∥ , particularly on-axis, evolve. At the sawtooth crash, on-axis safety factor q 0 is observed to rise by 5% but remain below 1 throughout the cycle, and on-axis current density J 0 is observed to drop by 5%. On-axis parallel electric field E∥(0) is found to be balanced by ηJ0 (resistivity times on-axis current density) except during the 200 µs crash period, where E∥(0) reaches 22 V m−1, exceeding ηJ0 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.