26 research outputs found
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Role of Hydrodynamics Simulations in Laser-Plasma Interaction Predictive Capability
Efforts to predict and control laser-plasma interactions (LPI) in ignition hohlraum targets for the National Ignition Facility [G. H. Miller et al., Optical Eng. 43, 2841 (2004)] are based on plasma conditions provided by radiation hydrodynamic simulations. Recent experiments provide compelling evidence that codes such as hydra [M. M. Marinak et al., Phys. Plasmas 8, 2275 (2001)] can accurately predict the plasma conditions in laser heated targets such as gas-filled balloon (gasbag) and hohlraum platforms for studying LPI. Initially puzzling experimental observations are found to be caused by bulk hydrodynamic phenomena. Features in backscatter spectra and transmitted light spectra are reproduced from the simulated plasma conditions. Simulations also agree well with Thomson scattering measurements of the electron temperature. The calculated plasma conditions are used to explore a linear-gain based phenomenological model of backscatter. For long plasmas at ignition-relevant electron temperatures, the measured backscatter increases monotonically with gain and is consistent with linear growth for low reflectivities. These results suggest a role for linear gain postprocessing as a metric for assessing LPI risk
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Quasi-isentropic material property studies at extreme pressures: from Omega to NIF
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RAYLEIGH-TAYLOR STRENGTH EXPERIMENTS OF THE PRESSURE-INDUCED alpha->epsilon->alpha' PHASE TRANSITION IN IRON
We present here the first dynamic Rayleigh-Taylor (RT) strength measurement of a material undergoing solid-solid phase transition. Iron is quasi-isentropically driven across the pressure-induced bcc ({alpha}-Fe) {yields} hcp ({var_epsilon}-Fe) phase transition and the dynamic strength of the {alpha}, {var_epsilon} and reverted {alpha}{prime} phases have been determined via proton radiography of the resulting Rayleigh-Taylor unstable interface between the iron target and high-explosive products. Simultaneous velocimetry measurements of the iron free surface yield the phase transition dynamics and, in conjunction with detailed hydrodynamic simulations, allow for determination of the strength of the distinct phases of iron. Forward analysis of the experiment via hydrodynamic simulations reveals significant strength enhancement of the dynamically-generated {var_epsilon}-Fe and reverted {alpha}{prime}-Fe, comparable in magnitude to the strength of austenitic stainless steels
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Experimental Studies of ICF Indirect-Drive Be and High Density C Candidate Ablators
To validate our modeling of the macroscopic and microscopic hydrodynamic and equation of state response of these candidate ablators to NIC-relevant x-ray drive, a multi-lab experimental program has been verifying the behavior of these new ablators. First, the pressures for onset and termination of melt for both Be and HDC under single or double shock drive has been measured at the Z and Omega facilities. Second, the level and effect of hard x-ray preheat has been quantified in scaled experiments at the Omega facility. Third, a long planar x-ray drive has been developed to check 2D and 3D perturbation growth at the ablation front upon acceleration. The concept has been extended to study growth at and near the ablator-ice interface upon deceleration. In addition, experimental designs for validating the expected low level of perturbation seeding due to possible residual microstructure after melt during first and second shock transit in Be and HDC have been completed. Results so far suggest both Be and HDC can remain ablator choices and have guided pulse shaping designs
Lawson criterion for ignition exceeded in an inertial fusion experiment
For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37Â MJ of fusion for 1.92Â MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion
Skier et apprendre la montagne
Apprentissage du ski à l'école primaire : connaître l'activité, déterminer les objectifs et les compétences, enseigner et faire apprendre. En document d'accompagnement, une fiche pédagogique cycle 2 : 'Atout ski alpin', avec des situations d'apprentissage axées sur la maîtrise de la glisse et du pilotage des skis par l'enfant