Investigating the Nature of a Hollow Radial Profile of Runaway Electrons During Tokamak Disruptions

Understanding the creation and transport of runaway electrons in tokamak plasmas is vital for the safety and preventing machine damage during plasma disruptions. We use a Multi-energy Soft X-Ray (ME-SXR) camera on the Madison Symmetric Torus (MST) to probe the radial distribution of low-energy photon emission between 2-3 and 40keV (using a 450 μm silicon detector) which is characteristic of fast electrons up to 100 keV. Low-density plasmas are tailored using a density ramp-down to ~10●10¹⁸ m^-3, and disruptions occur due to a programmed ramp-down of the toroidal field. The exposure of the ME-SXR camera is gated to allow for finer temporal resolution of the runaway electron behavior during the rapid plasma termination. Preliminary evidence of a hollow radial profile of x-ray emission during the termination is discussed, including the possible role of a pre-existing fast electron population. Ongoing work is described in which runaway acceleration in MST is modeled using the Fokker-Planck code CQL3D, and ME-SXR signals are predicted using a synthetic diagnostic.