Simulations of laser imprint for Nova experiments and for ignition capsules Page: 8 of 31
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where 6I(k)/I is the fractional modulation of laser intensity at wavenumber, k. The imprint
efficiency may be calculated at some 6I(k)/I < 1, so that the response remains linear.
The result is multiplied by the intensity modulation for a case of interest to determine the
III. SIMULATIONS OF NOVA CH2 EXPERIMENTS
Experiments have been carried out on the Nova laser in which CH2 foils were accel-
erated by one smoothed beam in the two beam chamber and modulation was diagnosed
by face-on x-ray radiography using gated pinhole imaging.13,6,16,17 One recent experiment13
compared imprint of static speckle, using a random phase plate1 (RPP) but no laser band-
width, between 0.35 pm and 0.53 pm illumination at ~ 1013 W/cm2. The f/ number of the
illumination was adjusted, f/16.5 for 0.35 pm and f/li for 0.53 pm to obtain the same size
speckles for both cases. A surface perturbation of 50 pm wavelength and 2 pm amplitude
was imposed upon the foils to allow extraction of the equivalent surface finish. This does
not quite correspond to Eqn. 2 because modulation for the same surface finish wavelength
was used to divide imprint at different wavelengths. The drive laser intensity pattern was
measured and used to deduce the imprint efficiency at the predicted shock breakout time, 2
ns for 30 pm thick foils.
Figure 1 shows the modal amplitude spectrum of imprint efficiency for the two illumi-
nation wavelengths. Note that the efficiency is higher for 0.35 pm. The smaller standoff
distance gives less thermal smoothing than for 0.53 pm illumination. We modeled this ex-
periment by performing 2-D single mode imprint simulations and post-processing to obtain
the optical depth modulation (including instrumental resolution). Then we divided by the
simulated evolution of the surface perturbation to get the equivalent surface finish, then
by the applied SI/I to obtain the imprint efficiency. The results are shown as symbols in
Figure 1, connected by lines to guide the eye. Agreement is generally quite good. There is
greater disagreement at lowest mode numbers. However, experimental uncertainly is largest
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Weber, S.V.; Glendinning, S.G.; Kalantar, D.H.; Key, M.H.; Remington, B.A.; Rothenberg, J.E. et al. Simulations of laser imprint for Nova experiments and for ignition capsules, article, November 8, 1996; California. (https://digital.library.unt.edu/ark:/67531/metadc692918/m1/8/: accessed June 16, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.