Polarization measurements of post-disruption runaway electron synchrotron emission in DIII-D*

Visible synchrotron emission by runaway electrons (REs) has been studied with spectra and images to gain insight on RE dynamics and energy in several devices. RE pitch angle in post-disruption RE plateaus is important for radial transport loss of REs, current dissipation, and deposition of RE kinetic energy into the wall during final loss. A slow (\textasciitilde 100 ms) decrease of synchrotron brightness is usually observed during RE plateau, which is interpreted as dropping pitch angle for relativistic (\textgreater MeV) REs due to slowly evolving impurity content (and decreasing pitch angle scattering). If massive D2 injection is done, there is a more rapid purge of Ar out of RE plateau and a much faster drop of synchrotron brightness (\textasciitilde 20 ms). The brightness depends on pitch angle and RE density, so investigating polarization ratios (vertical/horizontal $=$ Pz/Px) is a way of isolating pitch angle. Initial analysis indicates that the ratio depends on pitch angle. This is supported by drop in Pz/Px from \textasciitilde 10 to about 3 during D2 MGI. The observation Pz/Px \textgreater 1 is consistent with earlier observations [Tinguely 2019]. Comparisons of full orbit (KORC) and guiding center (SOFT) simulations of the polarization will be given.