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

Measured x-ray spectra and Fokker-Planck modeling are used to constrain the effective ionic charge $Z_{eff}$ and the radial particle diffusion coefficient $D_{r}$ in MST. A new single-photon counting Si detector measures 2-10 keV x rays while a multichord array of CdZnTe detectors measures the 10-150 keV range. Absolute calibration of the measured x-ray flux is required to find $Z_{eff}$ and $D_{r}$. The Fokker-Planck code CQL3D models the electron distribution function and predicts the x-ray spectrum resulting from bremsstrahlung. The code is run iteratively to find the $Z_{eff}$ and $D_{r}$ for which the predicted x-ray flux best matches the measurement. $Z_{eff}$ is then used to calculate quantities such as resistivity, ohmic power, and the energy confinement time. Results from standard RFP plasmas, with $D_{r}$ dependent on electron velocity, and improved confinement, pulsed parallel current drive plasmas, with $D_{r}$ independent of velocity, will be presented. Work supported by the USDOE.