Measurement of $Z_{eff}$ and Particle Diffusion via X-ray Spectroscopy in MST

Measured x-ray spectra constrain Fokker-Planck modeling of MST discharges and provide a means to determine the effective ionic charge $Z_{eff}$ and the radial particle diffusion coefficient $D_{r}$. A new radial array of Si photodiode detectors measures 2-10 keV x rays from thermal and runaway electrons in most plasma conditions. An array of CdZnTe detectors measures 10-150 keV x rays from high energy runaway electrons present only in plasmas with reduced stochasticity and improved particle confinement. The Fokker-Planck code CQL3D models the electron distribution function and predicts the resulting bremsstrahlung emission expected along each detector's line of sight. The code is run iteratively to find the $Z_{eff}(r)$ and $D_{r}(r)$ that produce the best fit to the data. Tests of this measurement technique with various plasma conditions will be presented, including largely stochastic plasmas with locally improved confinement within a magnetic island, and plasmas with reduced tearing modes and globally improved confinement. In the core of a typical high-temperture, low-density, improved-confinement plasma, $Z_{eff}$ = 4-6 and $D_{r}$ = 1 m$^{2}$/s. Work supported by the USDOE.