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Accurate models of X-ray absorption and re-emission in partly stripped ions are necessary to calculate the structure of stars, the performance of hohlraums for inertial confinement fusion and many other systems in high-energy-density plasma physics. Despite theoretical progress, a persistent discrepancy exists with recent experiments at the Sandia Z facility studying iron in conditions characteristic of the solar radiative–convective transition region. The increased iron opacity measured at Z could help resolve a longstanding issue with the standard solar model, but requires a radical departure for opacity theory. To replicate the Z measurements, an opacity experiment has been designed for the National Facility (NIF). The design uses established techniques scaled to NIF. A laser-heated hohlraum will produce X-ray-heated uniform iron plasmas in local thermodynamic equilibrium (LTE) at temperatures
eV and electron densities
. The iron will be probed using continuum X-rays emitted in a
diameter source from a 2 mm diameter polystyrene (CH) capsule implosion. In this design,
of the NIF beams deliver 500 kJ to the
mm diameter hohlraum, and the remaining
directly drive the CH capsule with 200 kJ. Calculations indicate this capsule backlighter should outshine the iron sample, delivering a point-projection transmission opacity measurement to a time-integrated X-ray spectrometer viewing down the hohlraum axis. Preliminary experiments to develop the backlighter and hohlraum are underway, informing simulated measurements to guide the final design.
Experiments to demonstrate the effects of various
beam-smoothing techniques have been performed on the 60-beam,
30-kJ UV OMEGA laser system. These include direct measurements
of the effect beam-smoothing techniques have on laser beam
nonuniformity and on both planar and spherical targets.
Demonstrated techniques include polarization smoothing
and “dual-tripler” third-harmonic generation
required for future broad bandwidth (∼1 THz) smoothing
by spectral dispersion (SSD). The effects of improvements
in single-beam uniformity are clearly seen in the target-physics
experiments, which also show the effect of the laser pulse
shape on the efficacy of SSD smoothing. Saturation of the
Rayleigh-Taylor (RT) growth of the broad-bandwidth features,
in agreement with the Haan model (Haan, 1989), produced
by laser imprinting has also been observed.
Inertial confinement fusion (ICF) requires high compression of fusion fuel to densities approaching 1000 times liquid density of deuterium-tritium (D–T) at central temperatures in excess of 5 keV. The goal of ICF is to achieve high gain (of the order of 100 or greater) in the laboratory. To meet this objective with minimum driver energy, a number of central issues must be addressed. Research in ICF with laser drivers has shown the importance of using short wavelength (λ < 0.5 µm). To achieve conditions for high gain at driver energies of a few megajoules or less, high intensities (>1014W/cm2) are required. The directdrive approach to ICF is more energy efficient than indirect drive if the stringent drive symmetry and hydrodynamic stability requirements can be met by a suitable laser irradiation scheme and target design. Experiments carried out at 351 nm on the 2-kJ, 24-beam OMEGA laser system at the Laboratory for Laser Energetics (LLE) at the University of Rochester, and future experiments to be performed on a 30-kJ upgrade of this laser, can resolve the remaining physics issues for direct drive: (1) energy coupling and transport scaling; (2) irradiation-uniformity requirements for high gain; (3) hydrodynamic stability constraints; and (4) hot-spot and main-fuel-layer physics. We review progress made on achieving uniform drive conditions with the OMEGA system and present results for direct-drive cryogenic-fuel-capsule and CD-shell, “surrogate” cryogenic-capsule implosion experiments that illustrate the constraints imposed by hydrodynamic instabilities and drive uniformity on the design of high-performance direct-drive targets. Target designs have been identified that will explore the ignition-scaling regime using the OMEGA Upgrade. Experiments on the OMEGA Upgrade will signal whether or not there is a high probability of achieving modest to high gain using direct drive on an upgrade of the NOVA facility.
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