Growth of pellet imperfections and laser imprint in direct drive inertial confinement fusion targets

Abstract

Simple hydrodynamic models for describing the Richtmyer-Meshkov (RM) growth and the Rayleigh-Taylor (RT) instability are tested by simulation.The RM sharp boundary model predictions are compared with numerical simulations of targets with surface perturbations or stationary intensity perturbations. Agreement is found in the overall trends, but the specific behavior can be significantly different. RM growth of imprint from optically smoothed lasers is also simulated and quantified. The results are used to calculate surface perturbations, growth factors, and laser imprint efficiencies. These in turn are used with standard RT growth formulas to predict perturbation growth in multimode simulations of compression and acceleration of planar and spherical targets. The largest differences between prediction and theory occur during ramp-up of the laser intensity, where RT formulas predict more growth than seen in the simulations.

Direct Observation of Feedout-Related Mass Oscillations in Plastic Targets

“…Figure 1 presents the comparison as given by analytical small-amplitude theory applied to a planar layer of ideal gas with 3 / 5 = γ , rippled from either front or rear side [3,10,11]. Here the areal mass variation amplitude δm is expressed in units 0 0 a ρ (where 0 ρ is the post-shock density, 0 a is the initial ripple amplitude),…”

Large-scale high-resolution simulations of high gain direct-drive inertial confinement fusion targets

“…It is unknown at this time whether this is simply due to the simplifications involved in the RT formula, or is a reflection of the limits of either the current analysis or this more complex layered target (compared to all-DT). For instance, we can sometimes expect a smaller than expected perturbation due to the finite time it can take for an RT eigenmode to become established [9]. The study of these simulations are ongoing.…”

“…For the benchmarks of optical smoothing, five simulations at each perturbation wavelength were performed using well defined single-beam laser perturbations [12]) and averaged in order to reduce statistical fluctuations in the results. These single-mode benchmarks have been compared to the linear ablatively-stabilized RM theory and have been shown to produce very similar growth factors and oscillation behavior [9], and also exhibit linear behavior when the initial amplitudes are varied. These single-mode simulations were cross checked with small multimode simulations containing eight resolved modes of the same approximate amplitude (∼ 500Å), and the time behavior of each individual mode was verified to be very close to that of the single-mode simulation of the corresponding wavelength.…”

“…The resulting ablatively-driven RM instability is characterized by first growing and then oscillating and damped perturbations. We have used the recently developed ablatively-stabilized RM theory [2] to benchmark single-mode simulations of simple (all-Deuterium-Tritium (DT)) pellets [9]. These simulations follow either surface perturbations or imprint from optically smoothed lasers (e.g., induced-spatial-incoherence (ISI) [10] or smoothing by spectral dispersion (SSD) [11] methods).…”

“…An example is a simulation of the all-DT NIF pellet point design [9,13] where the outer surface is initialized with the National Ignition Facility (NIF)-standard specification spectrum [14] and the driving 0.35 µm laser is optically smoothed with 1 THz bandwidth. In these simulations, modes 1-256 are considered well resolved (there are 2048 points in the theta direction, and 16 points are believed to be needed to resolve a mode properly without numerical diffusion).…”