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.