Soft X-ray Emission from Post-Pulse Expanding Laser-Produced Plasmas

A new diagnostic spectrometer has been developed at the Naval Research Laboratory (NRL) to measure the time resolved absolute intensity of radiation emitted by the Nike laser irradiated targets. The spectrometer consists of a dispersive transmission grating of 2500 lines/mm or 5000 lines/mm and a detection system consisting of an absolutely calibrated Si photodiode array and a CCD camera. In this paper, this spectrometer was used to study the spatial distribution of soft x-ray radiation from low Z elements (primarily carbon) that lasted tens of nanoseconds after the main laser illumination was over. Using the newly developed spectrometer, we recorded soft x-ray emission as a function of the target material and target orientation with respect to the incoming laser beam and the spectrometer line of sight. While a number of spectral features have been identified in the data, the instrument’s combined temporal and spatial resolution led to the observation of the plasma expansion from CH targets for up to ~ 25 ns after the cessation of the main laser pulse. The inferred plasma expansion velocities are slightly higher than those previously reported.