The effects of radiation transport on nonlocal electron heat flow in high Z laser-produced plasmas is studied. Using a Fokker-Planck model for the electron heat flow, which is coupled to a radiation transport model, it is found that radiation transport strongly modifies electron heat transport at the critical surface and in the overdense regions for an aluminum plasma. It is concluded that, without radiation transport effects, the plasma temperature, as computed from Fokker-Planck models, is overestimated in the critical region and underestimated in the overdense region, for high-Z plasmas.
Targets have been designed that produce moderate to high gain when directly driven by lasers. The intrinsic sensitivity of these targets to hydro instabilities is found using the FAST(2D) multidimensional radiation hydrocode [J. H. Gardner, A. J. Schmitt, J. P. Dahlburg et al., Phys. Plasmas 5, 1935 (1998)], which simulates the simultaneous behavior of a large bandwidth (e.g., l=2–256) of perturbations from compression to acceleration, and then to stagnation and burn. The development of the structure in these multimode simulations is benchmarked to theoretical analysis and single-mode calculations, which reveals the need to “renormalize” the simulation after compression. The simulations predict that a direct drive point design is expected to degrade significantly from its one-dimensional clean yield, yet still ignite and give appreciable gain. Simulations of high-gain pellets using a spike prepulse to inhibit Richtmyer–Meshkov growth show a considerable robustness, with high (>100) gains possible even with nominal surface finishes and laser imprint.
We report the first calculations of the 2ωpe instability in an IFE-scale corona of a direct drive target being driven by a StarDriver-class laser. Using realistic plasma profiles (density, temperature, flow velocity and Z) taken from full hydrodynamic simulations of the capsule implosion, we propagate the StarDriver beamlets to the ¼ critical surface using ray-tracing. We then use that 'k-space' of laser modes in a model of the absolute 2ωpe instability in the inhomogeneous plasma at ¼ critical density. Our calculations indicate that 15–20 THz of bandwidth at 351 nm wavelength (about 2% bandwidth) significantly reduces or suppresses the absolute instability at most times in the drive pulse. A bandwidth of 35 THz suppresses it at all times in the drive pulse. We note that 2% bandwidth ~20 THz is achievable using laser gain media available today.
In inertial confinement fusion (ICF) and high-energy density physics (HEDP), the most important manifestations of the hydrodynamic instabilities and other mixing processes involve lateral motion of the accelerated plasmas. In order to understand the experimental observations and to advance the numerical simulation codes to the point of predictive capability, it is critically important to accurately diagnose the motion of the dense plasma mass. The most advanced diagnostic technique recently developed for this purpose is the monochromatic x-ray imaging that combines large field of view with high contrast, high spatial resolution and large throughput, ensuring high temporal resolution at large magnification. Its application made it possible for the experimentalists to observe for the first time important hydrodynamic effects that trigger compressible turbulent mixing in laser targets, such as ablative Richtmyer–Meshkov (RM) instability, feedout, interaction of an RM-unstable interface with shock and rarefaction waves. It also helped to substantially improve the accuracy of diagnosing many other important plasma flows, ranging from laser-produced jets to electromagnetically driven wires in a Z-pinch, and to test various methods suggested for mitigation of the Rayleigh–Taylor instability. We will review the results obtained with the aid of this technique in ICF-HEDP studies at the Naval Research Laboratory and the prospects of its future applications.
The first calculations of time-dependent laser–plasma filamentation in three dimensions are reported. These calculations are done with a three-dimensional laser propagation code based on a previous two-dimensional code [Phys. Fluids 31, 3079 (1988)]. The effect of incident beam structure, and in particular optical smoothing techniques, on the behavior of filamentation is studied. Both ponderomotive and thermal conduction dominated nonlinearities are considered, and calculations are done simulating both homogeneous nonabsorbing plasmas and inhomogeneous laboratory plasmas. Random phase screen (RPS) and induced spatial incoherence (ISI) optical smoothing techniques are investigated and compared to generic unsmoothed laser beams. Qualitative examples are presented and scaling studies are done and compared to a simple theoretical analysis. In typical laser–plasma interactions without optical smoothing, three-dimensional effects lead to greatly increased filament intensities, as expected. Peak filament intensities of order 100–500 times the average intensity are routinely observed (without optical smoothing), as compared to earlier two-dimensional calculations where peak intensities were of order 10–50 times average. In spite of this tendency to create stronger filaments, three-dimensional filamentation (when measured on a time-averaged basis) can be suppressed by using ISI smoothing. Under the same conditions, instantaneous ISI intensities can show considerable enhancement, although much less than the unsmoothed beams. RPS smoothing exhibits less filamentation suppression. Under laser-fusion reactor conditions, calculations indicate that ISI suppression can completely eliminate filamentation.
