Time-resolved spectroscopy of nonequilibrium ionization in laser-produced plasmas
The highly transient ionization characteristic of laser-produced plasmas at high energy densities has been investigated experimentally, using x-ray spectroscopy with time resolution of less than 20 ps. Spectroscopic diagnostics of plasma density and temperature were used, including line ratios, line profile broadening and continuum emission, to characterize the plasma conditions without relying immediately on ionization modeling. The experimentally measured plasma parameters were used as independent variables, driving an ionization code, as a test of ionization modelling, divorced from hydrodynamic calculations. Several state-of-the-art streak spectrographs, each recording a fiducial of the laser peak along with the time-resolved spectrum, characterized the laser heating of thin signature layers of different atomic numbers imbedded in plastic targets. Spherical targets were illuminated uniformly with the OMEGA 351 nm laser system, to approximate a one-dimensional homogeneous plasma. A novel design of crystal spectrograph, with a conically curved crystal, was developed. Coupled with a streak camera, it provided high resolution and a collection efficiency roughly 20--50 times that of planar crystal spectrographs, affording improved spectra for quantitative reduction and greater sensitivity for the diagnosis of weak emitters. A novel temperature diagnostic was developed, using special targets with a known proportion of elements: analogous emission lines from isoelectronic charge-states were used to provide a temperature-sensitive diagnostic ratio. Experimental results were compared to hydrocode and ionization code simulations, with poor agreement. The conclusions question the appropriateness of describing electron velocity distributions by a temperature parameter during the time of laser illumination and emphasize the importance of characterizing the distribution more generally.
- Research Organization:
- Rochester Univ., NY (USA). Lab. for Laser Energetics
- Sponsoring Organization:
- DOE/DP
- DOE Contract Number:
- FC03-85DP40200
- OSTI ID:
- 7164582
- Report Number(s):
- DOE/DP/40200-120; ON: DE90008663
- Resource Relation:
- Other Information: Thesis (Ph.D.)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
LASER-PRODUCED PLASMA
X-RAY SPECTROSCOPY
IONIZATION
NON-EQUILIBRIUM PLASMA
OMEGA FACILITY
PLASMA DENSITY
PLASMA DIAGNOSTICS
STREAK CAMERAS
TEMPERATURE MEASUREMENT
THERMAL EQUILIBRIUM
TIME RESOLUTION
CAMERAS
EQUILIBRIUM
MEASURING INSTRUMENTS
PLASMA
RADIATION DETECTORS
RESOLUTION
SPECTROSCOPY
TIMING PROPERTIES
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