Analysis of Lithum-Heated Hohlraum Experiments on PBFA II
Analysis of lithium-ion-driven target experiments at Sandia
National Laboratories reveals important features of ion-driven
hohlraum performance. The peak lithium beam intensity in these
experiments was 1 to 2 TW/cm2, with a 15-ns full width at
half maximum for the beam. The truncated conical hohlraum (6 mm tall,
6 mm average diameter) contained low-density foam. For these beam
and hohlraum characterization experiments, a fuel capsule was not
present. A conical shape was selected so that diagnostics of the
incident beam and the x-ray emission would have an adequate
cross-sectional view of the target. The conical shape also allowed
us to compare the azimuthal beam symmetry with variations in the
soft x-ray spectra and to observe hydrodynamic features not
accessible with more closed geometries. The x-ray spectrum from
the foam is nearly Planckian (i.e., the foam is "optically thin,"
with the entire volume participating in radiation cooling) and has
a peak brightness temperature of 58 eV. Examination of the soft
x-ray emission indicates that the gold shell acts as a radiation
case and partially confines and redistributes the energy deposited
in the foam: time-integrated images illustrate differences in
radiation emitted from the inside and the outside of the hohlraum.
The contrast between the foam core and the outer wall shows that the
foam is nearly transparent to x-rays, while the gold wall is
optically thick. Comparison of azimuthal variations in the ion beam
and the x-ray spectra indicates that the gold radiation case and the
low-density foam will have a smoothing effect on an imploding fuel
capsule. Moreover, a time-resolved soft x-ray imaging diagnostic
reveals that the gold creates a static hohlraum for the duration of
the power pulse, since its velocity is less than the instrument
resolution of 3 cm/microsecond. Subsequent experiments will
concentrate on further characterization of hohlraum response and on
optimizing the radiation temperature within the hohlraum.
Other Reports on High Energy Density and Inertial Confinement Fusion
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