Radiative shocks produced from spherical cryogenic implosions at the National Ignition Facility
Authors: Pak A. , Divol L. , Gregori G. , Weber S. , Atherton J. , Bennedetti R. , Bradley D. K. , Callahan D. , Casey D. T. , Dewald E. , Döppner T. , Edwards M. J. , Frenje J. A. , Glenn S. , Grim G. P. , Hicks D. , Hsing W. W. , Izumi N. , Jones O. S. , Johnson M. G. , Khan S. F. , Kilkenny J. D. , Kline J. L. , Kyrala G. A. , Lindl J. , Landen O. L. , Le Pape S. , Ma T. , MacPhee A. , MacGowan B. J. , MacKinnon A. J. , Masse L. , Meezan N. B. , Moody J. D. , Olson R. E. , Ralph J. E. , Robey H. F. , Park H.-S. , Remington B. A. , Ross J. S. , Tommasini R. , Town R. P. J. , Smalyuk V. , Glenzer S. H. , Moses E. I. 
Published: May 2013

Spherically expanding radiative shock waves have been observed from inertially confined implosion experiments at the National Ignition Facility. In these experiments, a spherical fusion target, initially 2 mm in diameter, is compressed via the pressure induced from the ablation of the outer target surface. At the peak compression of the capsule, x-ray and nuclear diagnostics indicate the formation of a central core, with a radius and ion temperature of ∼20 μm and ∼ 2 keV, respectively. This central core is surrounded by a cooler compressed shell of deuterium-tritium fuel that has an outer radius of ∼40 μm and a density of &gt;500 g/cm3. Using inputs from multiple diagnostics, the peak pressure of the compressed core has been inferred to be of order 100 Gbar for the implosions discussed here. The shock front, initially located at the interface between the high pressure compressed fuel shell and surrounding in-falling low pressure ablator plasma, begins to propagate outwards after peak compression has been reached. Approximately 200 ps after peak compression, a ring of x-ray emission created by the limb-brightening of a spherical shell of shock-heated matter is observed to appear at a radius of ∼100 μm. Hydrodynamic simulations, which model the experiment and include radiation transport, indicate that the sudden appearance of this emission occurs as the post-shock material temperature increases and upstream density decreases, over a scale length of ∼10 μm, as the shock propagates into the lower density (∼1 g/cc), hot (∼250 eV) plasma that exists at the ablation front. The expansion of the shock-heated matter is temporally and spatially resolved and indicates a shock expansion velocity of ∼300 km/s in the laboratory frame. The magnitude and temporal evolution of the luminosity produced from the shock-heated matter was measured at photon energies between 5.9 and 12.4 keV. The observed radial shock expansion, as well as the magnitude and temporal evolution of the luminosity from the shock-heated matter, is consistent with 1-D radiation hydrodynamic simulations. Analytic estimates indicate that the radiation energy flux from the shock-heated matter is of the same order as the in-flowing material energy flux, and suggests that this radiation energy flux modifies the shock front structure. Simulations support these estimates and show the formation of a radiative shock, with a precursor that raises the temperature ahead of the shock front, a sharp μm-scale thick spike in temperature at the shock front, followed by a post-shock cooling layer.