The feed-out process: Rayleigh-Taylor and Richtmyer-Meshkov instabilities in thin, laser-driven foils Page: 32 of 193

                                
17
photographed with an X-ray camera. A beryllium washer was glued to the hole in
the hohlraum on the backlighter side. Without the washer, gold vapor from the
hohlraum would fill in the small hole. Gold was not transparent to the high
energy X-rays from the backlighter, but beryllium was. A small gold wire was
placed in the trough of the machined perturbations as a fiducial for phase
changes. The line of sight of the camera was perpendicular to the plane of the
package.
In face-on radiography, X-rays from the backlighter were attenuated
more by passing through peaks in the perturbations of the package than by pass-
ing through the troughs. The difference in attenuation produced dark areas on the
film from the valleys of the perturbations, and lighter areas from the perturbation
peaks. The data could then be Fourier-analyzed to reveal the growth of the first,
second, and third harmonics with time.
Face-on radiography quantitatively revealed perturbation growth, but
did not show the location of the perturbations. They could be on the hot surface,
the cold surface, or even in the center of the foil. Side-on radiography, also shown
in Figure 1-5, revealed the location of the perturbations by placing the line of sight
of the camera in the same plane as the package, and parallel to the initial pertur-
bations. The backlighter likewise was repositioned to illuminate the package from
the side. Package surfaces adjacent to and away from the hohlraum were thus
simultaneously visible. Side-on radiography data was much more difficult to
compare to calculations than face-on and only provided qualitative information.