Nonthermal ultrasoft x-ray spectra on ZT-40M

Thin-foil differential transmission measurements of ultrasoft x rays have been performed on the ZT-40M device at Los Alamos National Laboratory. Time histories of the ratio of intensities seen through 1-mil and 2-mil Be foils are in reasonable (+-50 eV) agreement with Thomson scattering measurements up to approximately 3 ms into the discharge, but examination (at any particular time) of the intensity-vs-foil thickness for all four available foils shows evidence of pronounced nonthermal characteristics. In an effort to explain the observation, the detailed theory of emission for a runaway electron distribution function will be examined, and predictions based on this theory will be shown capable of reproducing the observed behavior. Comments on the ability of oxygen impurity radiation in the 600- to 800-eV range to also fit the observation will be made