Soft x-ray power diagnostics are essential for measuring the total x-ray flux, radiation temperature, conversion efficiency, and albedo that define the energetics in indirect and direct drive, as well as other types of high temperature laser plasma experiments. A key diagnostic for absolute radiation flux and radiation temperature in hohlraum experiments is the Dante broadband soft x-ray spectrometer. For the extended range of x-ray fluxes predicted for National Ignition Facility (NIF) compared to Omega or Nova hohlraums, the Dante spectrometer for NIF will include more high energy (<2 keV) edge filter band-pass channels and access to an increased dynamic range using grids and signal division. This will allow measurements of radiation fluxes of between 0.01 to 100 TW/sr, for hohlraum radiation temperatures between 50 eV and 1 keV. The NIF Dante will include a central four-channel imaging line-of-sight to verify the source size, alignment as well as checking for any radiation contributions from unconverted laser light plasmas.
Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low-Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high-Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD).
Supersonic fluid flow and the interaction of strong shock waves to produce jets of material are ubiquitous features of inertial confinement fusion ͑ICF͒, astrophysics, and other fields of high energy-density science. The availability of large laser systems provides an opportunity to investigate such hydrodynamic systems in the laboratory, and to test their modeling by radiation hydrocodes. We describe experiments to investigate the propagation of a structured shock front within a radiation-driven target assembly, the formation of a supersonic jet of material, and the subsequent interaction of this jet with an ambient medium in which a second, ablatively driven shock wave is propagating. The density distribution within the jet, the Kelvin-Helmholz roll-up at the tip of the jet, and the jet's interaction with the counterpropagating shock are investigated by x-ray backlighting. The experiments were designed and modeled using radiation hydrocodes developed by Los Alamos National Laboratory, AWE, and Lawrence Livermore National Laboratory. The same hydrocodes are being used to model a large number of other ICF and high energy-density physics experiments. Excellent agreement between the different simulations and the experimental data is obtained, but only when the full geometry of the experiment, including both laser-heated hohlraum targets ͑driving the jet and counter-propagating shock͒, is included. The experiments were carried out at the University of Rochester's Omega laser ͓J. M. Soures et al., Phys. Plasmas 3, 2108 ͑1996͔͒.
Experiments have been conducted using laser-driven cylindrical hohlraums whose walls are machined from Ta2O5 foams of 100 mg/cc and 4 g/cc densities. Measurements of the radiation temperature demonstrate that the lower density walls produce higher radiation temperatures than the high density walls. This is the first experimental demonstration of the prediction that this would occur [M. D. Rosen and J. H. Hammer, Phys. Rev. E 72, 056403 (2005)10.1103/PhysRevE.72.056403]. For high density walls, the radiation front propagates subsonically, and part of the absorbed energy is wasted by the flow kinetic energy. For the lower wall density, the front velocity is supersonic and can devote almost all of the absorbed energy to heating the wall.
We present the first Thomson scattering measurements of local plasma conditions in ignition-relevant, gas-filled, inertial-confinement-fusion hohlraums. The experimental data provide a benchmark for two-dimensional hydrodynamic simulations using LASNEX, which is presently in use to predict the performance of future megajoule laser-driven hohlraums of the National Ignition Facility. The data are consistent with modeling using significantly inhibited heat transport at the peak of the drive. Further, we find that stagnating plasma regions on the hohlraum axis are well described by the calculations.[S0031-9007(97)03841-6] PACS numbers: 52.58. Ns, 52.40.Nk, 52.65.Kj, 52.70.Kz A key issue for the ignition of indirectly driven inertial confinement fusion (ICF) capsules is the controlled deposition of energy into a radiation cavity (hohlraum) and a symmetric high-convergence implosion of the capsule [1]. Using large glass laser drivers, as planned for the National Ignition Facility (NIF), present hydrodynamic simulations show that more than 1 MJ of laser energy has to be delivered with a shaped laser pulse into a cylindrical centimeter-size high-Z hohlraum [2]. A high-Z plasma is created at the hohlraum wall where the laser energy converts into soft x rays which heat and implode a deuteriumtritium filled spherical capsule. Present ignition designs [2,3] use hohlraums filled with a low-Z gas [4,5] to reduce inward motion of the high-Z wall plasma and to obtain high soft x-ray radiation symmetry which is necessary to achieve high capsule convergence.In this Letter, we present the first Thomson scattering measurements [6,7] within closed geometry, hohlraum targets. These experiments provide the first direct and accurate measurement of the electron temperature T e , ion temperature T i , and plasma flow v of the low-Z plasma in ignition-relevant, gas-filled, hohlraums [8-10], thus benchmarking hydrodynamic simulations. The experimental quantities are important parameters to understand the energetics of hohlraums and physical processes such as beam pointing [11] and stagnation processes which influence the capsule implosion in indirect drive experiments.Radiation hydrodynamic simulations of both NIF and Nova hohlraums are carried out with the LASNEX [12] computer code which uses a flux-limited diffusion model for heat transport. In this approximation heat flow per unit area in regions of large classical heat flow (2k=T e ) has an upper bound of f͑n e T e y e ͒, where n e T e y e is the so-called free streaming value of heat transport and f is the flux limit. Flux limiters of 0.01 , f , 0.1 have been found necessary to model laser-heated disk experiments [13] where steep temperature gradients near the disk surface cause classical heat flow to become unphysically large. In laser-heated hohlraums, however, the confined geometry results in very different temperature and density profiles so that the heat flow is flux limited over the volume of the hot plasma. For that reason, equivalent results can be obtained by merely reducin...
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