Investigation of Instabilities During Nonstandard Sawtooth Periods in the DIII-D Tokamak*

Sawtooth oscillations are one of the most commonly observed macroscopic instabilities in tokamaks. Standard sawtooth cycles are characterized by rearrangement of the confining magnetic field, leading to periodic peaking and rapid flattening of the central plasma temperature and density. In compound-sawtoothing plasmas, additional relaxation events are observed during the sawtooth period where the temperature peaking briefly stops and then restarts. These relaxation events are associated with m/n = 1/1 magnetohydrodynamic (MHD) instabilities in the plasma core. In this work, multiple diagnostics including Electron Cyclotron Emission and the Radial Interferometer Polarimeter on the DIII-D tokamak are used to characterize the dynamics of these MHD instabilities. We investigate the amplitude and phase relation between temperature, density and magnetic fluctuations. This will help test and further develop theoretical models and contribute to the goal of achieving a comprehensive understanding of sawteeth.