Constraining $Z_{\mathrm{eff}}$ and Particle Diffusion using X-ray Spectroscopy on MST*

The X-ray spectroscopy diagnostic on MST consists of six SXR detectors and six HXR detectors capable of measuring photons in the energy range $3-25$\ keV and $10-60$\ keV, respectively. The detectors can be installed on any of 17 ports viewing a poloidal cross-section, with tangency radii from $r/a=0.87$ inboard to $r/a=0.84$ outboard. Measurements have been made in enhanced confinement plasmas with plasma current of $\sim400$\ kA, electron density of $\sim0.6\times10^{19}$\ m$^{-3}$, and electron temperature of $\sim1200$\ eV . Measured spectra are used to constrain radial profiles of $Z_{\mathrm{eff}}$ and $D_{\mathrm{r}}$ by comparison with spectra calculated from CQL3D, a Fokker-Planck solver [R.W. Harvey and M.G. McCoy, ``The CQL3D Fokker-Planck Code,'' General Atomics (2011)]. The plasma equilibria required for CQL3D are produced by the reconstruction code MSTfit. Minimization is performed using a custom parallel simplex algorithm on a 248 core cluster.