Final Report LDRD 04-ERD-019 Development of absolute spectroscopic diagnostics for non-LTE plasmas

Abstract

Overview:This project sought to further our understanding of non-Local Thermodynamic Equilibrium (NLTE) processes by providing benchmark data to validate computational models. This has been a difficult regime to study in the laboratory, where experimental scales produce strong gradients while interpretation requires well-characterized uniform plasmas. It has also been a difficult regime to simulate, as evidenced by the large discrepancies in predictions of NLTE spectra for fixed plasma properties. Not surprisingly, discrepancies between data and calculations of recombining laser-produced plasmas have been in evidence since the 1980's. The goal here was to obtain data of sufficient accuracy to help resolve these discrepancies and enable better modeling of the NLTE processes that are integral to high-energy density experiments.Advances in target fabrication, diagnostic development and simulation capabilities provided the foundations for this project. Uniform plasmas were to be achieved by using aerogel foams of low enough density (~mg/cm 3 ) and thickness (~mm) to be volumetrically heated by a laser. The foams were doped with Ti to provide K-and L-shell emission and recombination spectra during the experiments. A new absolutely calibrated transmission grating spectrometer provided absolute temporal measurements at 18 frequencies, in addition to a CCD image of the timeintegrated spectrum. Finally, atomic models of varying degrees of sophistication and detail, combined with NLTE radiation transport and hydrodynamics, were used to simulate the experiments and understand the observed spectra.We give here a summary of the main achievements, challenges and results of this project. Details are available in the documents included as appendices. Summary:The first set of experiments was performed on the NIKE laser at NRL in March, 2004, with the goals of evaluating the performance of the diagnostics and the achieved plasma uniformity. By varying the laser parameters, we determined the required parameters for creating L-shell emission and were able to obtain K-shell (He-like) Ti. Pinhole x-ray images of the K-shell emission showed transverse plasma uniformity depended upon the target quality. Not all targets had acceptable quality, as it proved difficult to fabricate targets of the desired thickness (½ mm).Using thicker targets also adversely affected the production of uniform conditions through the plasma, as the plasma was expected to have a moderate optical thickness (~few) to the laser radiation at early times during the laser pulse. Large differences in predictions of target 1 performance by different codes were traced to the differences in the calculation of laser absorption, and this is discussed at length in the reports from UCSD.The first absolutely calibrated, time resolved L-shell emission spectra (from 4 to 26 Å) were also obtained in this series of experiments. The spectral resolution was not sufficient to match any individual spectral features. However, combined with the time resolution, it was sufficient to de...