Capsules with beryllium ablators have long been considered as alternatives to plastic for the National Ignition Facility laser ; now the superior performance of beryllium is becoming well substantiated . Beryllium capsules have the advantages of relative insensitivity to instability growth, low opacity, high tensile strength, and high thermal Zimmerman and W. L. h e r , Comments Plasmas Phys. Controlled Thermonucl. Fusion, 2 , 5 1 (2975)l results that particular beryllium capsule designs are several times less sensitive than the CH point design to instability growth from DT ice roughness. These capsule designs contain more ablator mass and leave some beryllium unablated at ignition. By adjusting the level of copper dopant, the unablated mass can increase or decrease, with a corresponding decrease or increase in sensitivity to perturbations . A plastic capsule with the same ablator mass as the beryllium and leaving the same unablated mass also shows this reduced perturbation sensitivity. Beryllium's low opacity permits the creation of 250 eV capsule designs. Its high tensile strength allows it to contain DT fuel at room temperature. Its high thermal conductivity simplifies cryogenic fielding.

Ionospheric plasma instabilities are usually discussed in terms of local parameters. However, because electric fields of scale size λ ≳ 1 km map along magnetic field lines, plasma populations far away from a locally unstable region may be affected by the instability process and vice versa. We present observations of electron density variations in the F1 region of the ionosphere at two locations near the magnetic equator. Oscillations in electron number density that were confined to a narrow wavelength regime were observed in a region of the ionosphere with a very weak vertical density gradient. Since magnetic flux tube interchange instabilities cannot create structure in such an environment we suggest that these are “images” of instabilities occurring elsewhere along the magnetic field line. A simple steady state theory of image formation is developed that is in good agreement with the observations. Moreover, this theory predicts a scale size dependent “effective diffusion” process in the F region that may dominate over classical cross‐field diffusion at kilometer scale sizes. Such a scale size dependent diffusion process is required to explain recent scintillation observations of decaying equatorial plumes.

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Recent experiments have shown that low density foam layers can significantly mitigate the perturbing effects of beam nonuniformities affecting the acceleration of thin shells. This problem is studied parametrically with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targets are employed, consisting typically of 250 Å of gold, and 50 μm of 50 mg/cm3 C10H8O4 foam attached to a 10 μm foil. In simulation these were characteristically exposed to 1.2 ns, flat-topped green light pulses at 1.4×1014 W/cm2 intensity, bearing 30 μm lateral perturbations of up to 60% variation in intensity. Without the buffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown to derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding 5×1013 W/cm2 are needed for assured stability beyond 2 ns; (3) longer foams at lower densities are needed for effective mitigation with shorter wavelength light; (4) the gold layer hastens conversion of the structured foam to a uniform plasma.

In order to obtain striation power spectra and to understand the nonlinear saturated state of the gradient drift instability, which is believed to be responsible for the striations in plasma clouds coupled to the background ionosphere, a two-level (one for the plasma cloud and one for the background ionosphere), two-dimensional numerical simulation has been performed. The plasma cloud is taken to be in the F region, the background ionospheric level is in the lower F or the E region, and the temperature is taken to be zero at both levels. The plasma cloud is initially assumed to be Gaussian in one direction (y) and uniform in the direction (x) of the ambient electric field, and the ambient magnetic field is uniform in the z direction. The cloud is initialized with random perturbations, and a high-resolution numerical simulation is carried out over the entire two-dimensional (x, y) mesh. In the linear phase of the simttlation, striations grow on the back side of the cloud and image striations appear in the background ionosphere. The nonlinear phase is characterized by pinching of the original perturbations within the cloud, production of secondary perturbations, a bubbling through of the back side striations to the front side of the cloud, and the appearance of large image striations in the background ionosphere. The modal development shows that at late times in the nonlinear phase the power spectrt/m for the density fluctuations obeys a power law depende.nce in k space. Specifically,Ahe x one-dimensional power spectrum • kx -nx where n•, • 2-3 (for wav.elengths between 0.6 and 12 km), and the y one-dimensional power spectrum • ky-ny where ny • 2 (for wavelengths between 2 and 100 km). This nonlinear modal evolution favors long wavelengths, whereas the linear.t. heory for our model system shows that the growth rate maximizes at short wavqlengths. Such nonlinear inversion may be responsible for explaining the longwavelength preferred striation scale size seen in barium plasma clouds. ß ß eß.ß ß ß ,,,ß. ß ß .ß. 10-6 10-7 _ t:960 SEC ß ß ß ß ß ß ß ß ß ß

The nonlinear motion of an F region barium release electrostatically coupled to the E region ionosphere is studied by numerical simulation. The effect of varying the ratio of the height‐integrated Pedersen conductivity of the barium cloud (Σpb) to that of the background ionosphere (Σpi) and the formation of striations are discussed.

The nonlinear behavior of a barium ion cloud whose integrated Pedersen conductivity is small (of order e) in comparison with the background ionosphericPedersen conductivity is studied in terms of fluid equations governing the motion of the barium plasma and the ionospheric plasma, which are both approximated by uniform layers. The barium cloud drives images in the ionospheric conductivity that are of order e•/2 relative to the undisturbed ionosphere. These then produce motion of the barium cloud and the ionospheric image cloud on a time scale of LB/(cEoe•/"), where L is a linear dimension of the cloud perpendicular to the earth's magnetic field B and E0 is the ambient transverse electric field in the ionosphere. In the limit of zero diffusion, numerical solution of the equations shows that at long times the contours of constant barium ion density for relatively high density values are thin sheets making a definite angle 0 --tan -1 (Pl/•Ci) with the direction of the E X B cloud velocity (pt and o•,, are the ionneutral collision frequency and the ion cyclotron frequency, respectively, for E region ions). These results are consistent with observations of an F region release of a small barium cloud. Also at long times, large image cloud densities coincide, in the plane transverse to B. with regions of the barium cloud that are relatively far from the initial position of the barium cloud. Modifications for small diffusion, which include back side steepening, are also given. of Defense Nuclear Agency. Also the work of one of us (S. R. Goldman) was partially supported by a grant from the Faculty Research Assistance Program of the City University of New York. We are grateful to J.P. Boris for supplying the fast Fourier transform Poisson solver. Helpful conversations with A. Scannapieco are also acknowledged. The authors would like to thank the referees for several valuable comments. The Editor thanks L. M. Linson and F. W. Perkins for their assistance in evaluating this paper.