J. P. Knauer

Two-dimensional simulations of plastic-shell, direct-drive implosions on OMEGA

Multidimensional hydrodynamic properties of high-adiabat direct-drive plastic-shell implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are investigated using the multidimensional hydrodynamic code, DRACO [D. Keller et al., Bull. Am. Phys. Soc. 44, 37 (1999)]. Multimode simulations including the effects of nonuniform illumination and target roughness indicate that shell stability during the acceleration phase plays a critical role in determining target performance. For thick shells that remain integral during the acceleration phase, target yields are significantly reduced by the combination of the long-wavelength (l<10) modes due to surface roughness and beam imbalance and the intermediate modes (20⩽l⩽50) due to single-beam nonuniformities. The neutron-production rate for these thick shells truncates relative to one-dimensional (1D) predictions. The yield degradation in the thin shells is mainly due to shell breakup at short wavelengths (λ∼Δ, where Δ is the in-flight s...

Polar-direct-drive simulations and experiments

Polar direct drive (PDD) [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)] will allow direct-drive ignition experiments on the National Ignition Facility (NIF) [J. Paisner et al., Laser Focus World 30, 75 (1994)] as it is configured for x-ray drive. Optimal drive uniformity is obtained via a combination of beam repointing, pulse shapes, spot shapes, and∕or target design. This article describes progress in the development of standard and “Saturn” [R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 0952002 (2005)] PDD target designs. Initial evaluation of experiments on the OMEGA Laser System [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)] and simulations were carried out with the two-dimensional hydrodynamics code SAGE [R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005)]. This article adds to this body of work by including fusion particle production and transport as well as radiation transport within the two-dimensional DRACO [P. B. Radha et al., Phys. Plasmas 12, 032702 (2005)] hydr...

Theory of laser-induced adiabat shaping in inertial fusion implosions: The relaxation method

The theory of the adiabat shaping induced by a strong shock propagating through a relaxed density profile is carried out for inertial confinement fusion (ICF) capsules. The relaxed profile is produced through a laser prepulse, while the adiabat-shaping shock is driven by the foot of the main laser pulse. The theoretical adiabat profiles accurately reproduce the simulation results. ICF capsules with a shaped adiabat are expected to benefit from improved hydrodynamic stability while maintaining the same one-dimensional performances as flat-adiabat shells.

Response model for Kodak Biomax-MS film to x rays

X-ray-sensitive film is used for a variety of imaging and spectroscopic diagnostics for high-temperature plasmas. Replacement film must be found as older films are phased out of production. Biomax-MS is a “T-grain” class of film that is proposed as a replacement for Kodak DEF and a model of its response to x rays is presented. Data from dimensional measurements of the film, x-ray transmission measurements, scanning electron microscopy micrograph images, and x-ray calibration are used to develop this sensitivity model of Biomax-MS film as a function of x-ray energy and angle of incidence. Relative response data provide a check of the applicability of this model to determine the x-ray flux from spectrum data. This detailed film characterization starts with simple mathematical models and extends them to T-grain–type film.

Measured dependence of nuclear burn region size on implosion parameters in inertial confinement fusion experiments

Radial profiles of nuclear burn in directly driven, inertial-confinement-fusion implosions have been systematically studied for the first time using a proton emission imaging system sensitive to energetic 14.7MeV protons from the fusion of deuterium (D) and 3-helium (He3) at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Experimental parameters that were varied include capsule size, shell composition and thickness, gas fill pressure, and laser energy. Clear relationships have been identified between changes in a number of these parameters and changes in the size of the burn region, which we characterize here by the median “burn radius” Rburn containing half of the total DHe3 reactions. Different laser and capsule parameters resulted in burn radii varying from 20to80μm. For example, reducing the DHe3 fill pressure from 18to3.6atm in capsules with 20μm thick CH shells resulted in Rburn changing from 31to25μm; this reduction is attributed to increased fuel-shell mix for the mor...

Absolute calibration of Kodak Biomax-MS film response to x rays in the 1.5- to 8-keV energy range

The absolute response of Kodak Biomax-MS film to x rays in the range from 1.5- to 8-keV has been measured using a laboratory electron-beam generated x-ray source. The measurements were taken at specific line energies by using Bragg diffraction to produce monochromatic beams of x rays. Multiple exposures were taken on Biomax MS film up to levels exceeding optical densities of 2 as measured by a microdensitometer. The absolute beam intensity for each exposure was measured with a Si(Li) detector. Additional response measurements were taken with Kodak direct exposure film (DEF) so as to compare the results of this technique to previously published calibrations. The Biomax-MS results have been fitted to a semiempirical mathematical model (Knauer et al., these proceedings). Users of the model can infer absolute fluences from observed exposure levels at either interpolated or extrapolated energies. To summarize the results: Biomax MS has comparable sensitivity to DEF film below 3keV but has reduced sensitivity a...