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

Scott, H. A.

Lawrence Livermore National Laboratory

关键词: Thomson Scattering;    Thickness;    L Shell;    Spectra;    K Shell;   

DOI  :  10.2172/1012432
RP-ID  :  LLNL-TR-429029
RP-ID  :  W-7405-ENG-48
RP-ID  :  1012432

美国|英语

来源: UNT Digital Library

【 摘 要 】

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 ({approx}mg/cm{sup 3}) and thickness ({approx}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 time-integrated 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. 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 (1/2 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 ({approx}few) to the laser radiation at early times during the laser pulse. Large differences in predictions of target 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 {angstrom}) 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 demonstrate that the emission shifted to lower photon energies later in time, consistent with the plasma recombining as it cooled. Simulations of these targets produced time-integrated emission in 3 different wavelength bands that matched the experimental measurements within a factor of three and provided a constraint on the average plasma temperature. The simulations also found rise times for the spectral bands similar to those actually observed. However, the highly non-uniform plasma conditions along with the poor spectral resolution did not place any detailed constraints on the NLTE modeling. The first set of experiments succeeded in mapping out the achievable plasma parameters and in identifying the major constraints and deficiencies due to target fabrication and experimental design. The second set of experiments was designed to provide increased spectral resolution, through modifications to the spectrometer, and increased plasma uniformity. Simulations predicted that illuminating the targets from two sides would provide good uniformity and still achieve the desired plasma temperatures with the decreased laser intensity achievable with this configuration. This also allowed the use of layered targets, with undoped foam regions sandwiching the doped foam to provide hydrodynamic tamping. Thomson scattering would provide an independent measurement of electron temperature in the plasma. The second set of experiments, originally planned for the summer of 2005, was repeatedly delayed. Due to personnel and budget constraints, the additional temperature diagnostics were to be provided by thin layers of spectroscopic dopants rather than Thomson scattering. A set of experiments was finally performed in September 2006, just before the termination of this LDRD. No analysis has been done on the data obtained in these experiments. This project did produce some significant achievements. The absolutely-calibrated spectrometer was successfully fielded, tested and improved.