Fast imaging of transients and coherent magnetohydrodynamic modes in DIII-D

A fast framing camera is used to image plasma waves and instabilities in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] in unprecedented detail including tearing modes (TMs) and sawtooth crashes. To image core magnetohydrodynamic (MHD) activity, the fast camera detects visible bremsstrahlung emission εB in moderate to high density plasmas. For coherent MHD activity such as TMs, high-resolution two-dimensional images of mode amplitude and phase are obtained by Fourier filtering each pixel’s time series at the mode frequency. Images of m/n=2/1 TMs show that inside the q=2 surface, the camera measurements are in excellent agreement with an analytic model of a 2/1 island superimposed on the equilibrium εB profile. Direct comparison of the measurements to a NIMROD simulation shows significant discrepancies, most likely due to artificially high-density diffusion used in the code for numerical stability. The first visible-light images of transient sawtooth crashes show the structure and location of the perturbed emission from an m=1 precursor oscillation and show that during the nonlinear crash phase the instability extends to more than half of the plasma minor radius.