Spectrally filtered fast imaging of internal magnetohydrodynamic activity in the DIII-D tokamak

The detailed poloidal structure of internal magnetohydrodynamic (MHD) modes is imaged using broadband visible bremsstrahlung emission from the core of the DIII-D tokamak. Spectral analysis of individual pixel time series recorded by a fast framing camera (up to 26,000 frames/s at 256x256 spatial resolution) is used to reconstruct two-dimensional images of mode amplitude. Application of this spectrally filtered fast imaging (SFFI) technique reveals spatially extended coherent structures that correspond to a (m,n)=(1,1) kink mode and a (2,1) neoclassical tearing mode rotating in the laboratory frame. The SFFI technique produces images with significantly less noise than images produced with the commonly used background subtraction method. Extension of SFFI to other core MHD events and coherent fluctuations in general is straightforward and could lead to further understanding of core MHD activity in fusion devices.