Runaway Electron Plateau Current Profile Reconstruction from Fastcam and MDS measurements in DIII-D

The current profile of the post-disruption runaway electron (RE) beam plateau is important for understanding the RE beam stability and the wall damage resulting from the RE beam-wall impact. In DIII-D, standard magnetic equilibrium reconstruction methods (such as EFIT) are not reliable, as the RE beams have low elongation, the Motional Stark Effect (MSE) diagnostic does not work, and the assumptions used for equilibrium reconstruction in thermal plasmas are not completely valid. In this work, a new fast visible camera (Fastcam) diagnostic and the high-resolution, visible Multi-chord Divertor Spectrometer (MDS) were used to constrain the RE current profile and obtain current profile reconstructions during the RE plateaus. The high resolution Fastcam images show features in the RE synchrotron emission (SE) that are associated with island structures, which are then used to locally constrain the safety factor (q) profile. From MDS measurements of the Zeeman-split Ar-II lines that were corrected for reflections, we obtain a line-integrated magnetic field pitch angle, which can be used to further constrain the current profile. The high current (500 kA) RE plateaus studied here are found to have a centrally peaked and non-hollow current profile, consistent with test particle and Monte-Carlo simulations. The central q value is found to be low, even moving below the classical stability limit q < 1 in some cases.