D. Shvarts

Dimensionality dependence of the Rayleigh-Taylor and Richtmyer-Meshkov instability late-time scaling laws

The late-time nonlinear evolution of the three-dimensional (3D) Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities for random initial perturbations is investigated. Using full 3D numerical simulations, a statistical mechanics bubble-competition model, and a Layzer-type drag-buoyancy model, it is shown that the RT scaling parameters, αB and αS, are similar in two and three dimensions, but the RM exponents, θB and θS are lower by a factor of 2 in three dimensions. The similarity parameter hB/〈λ〉 is higher by a factor of 3 in the 3D case compared to the 2D case, in very good agreement with recent Linear Electric Motor (LEM) experiments. A simple drag-buoyancy model, similar to that proposed by Youngs [see J. C. V. Hanson et al., Laser Part. Beams 8, 51 (1990)], but using the coefficients from the A=1 Layzer model, rather than phenomenological ones, is introduced.

Potential flow models of Rayleigh–Taylor and Richtmyer–Meshkov bubble fronts

A potential flow model of Rayleigh–Taylor and Richtmyer–Meshkov bubbles on an interface between an incompressible fluid and a constant supporting pressure (Atwood number A=1) is presented. In the model, which extends the work of Layzer [Astrophys. J. 122, 1 (1955)], ordinary differential equations for the bubble heights and curvatures are obtained by considering the potential flow equations near the bubble tips. The model is applied to two‐dimensional single‐mode evolution as well as two‐bubble competition, for both the Rayleigh–Taylor (RT) and the Richtmyer–Meshkov (RM) instabilities, the latter treated in an impulse approximation. The model predicts that the asymptotic velocity of a single‐mode RM bubble of wavelength λ decays as λt−1, in contrast with the constant asymptotic velocity attained in the RT case. Bubble competition, which is believed to determine the multimode front evolution, is demonstrated for both the RT and RM instabilities. The capability of the model to predict bubble growth in a fin...

Two-dimensional simulations of plastic-shell, direct-drive implosions on OMEGA

Multidimensional hydrodynamic properties of high-adiabat direct-drive plastic-shell implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are investigated using the multidimensional hydrodynamic code, DRACO [D. Keller et al., Bull. Am. Phys. Soc. 44, 37 (1999)]. Multimode simulations including the effects of nonuniform illumination and target roughness indicate that shell stability during the acceleration phase plays a critical role in determining target performance. For thick shells that remain integral during the acceleration phase, target yields are significantly reduced by the combination of the long-wavelength (l<10) modes due to surface roughness and beam imbalance and the intermediate modes (20⩽l⩽50) due to single-beam nonuniformities. The neutron-production rate for these thick shells truncates relative to one-dimensional (1D) predictions. The yield degradation in the thin shells is mainly due to shell breakup at short wavelengths (λ∼Δ, where Δ is the in-flight s...

Experimental and numerical investigation of the Richtmyer–Meshkov instability under re-shock conditions

(Received 6 October 2008 and in revised form 26 December 2008) An experimental and numerical systematic study of the growth of the Richtmyer– Meshkov instability-induced mixing following a re-shock is made, where the initial shock moves from the light fluid to the heavy one, over an incident Mach number range of 1.15–1.45. The evolution of the mixing zone following the re-shock is found to be independent of its amplitude at the time of the re-shock and to depend directly on the strength of the re-shock. A linear growth of the mixing zone with time following the passage of the re-shock and before the arrival of the reflected rarefaction wave is found. Moreover, when the mixing zone width is plotted as a function of the distance travelled, the growth slope is found to be independent of the re-shock strength. A comparison of the experimental results with direct numerical simulation calculations reveals that the linear growth rate of the mixing zone is the result of a bubble competition process.

Multidimensional analysis of direct-drive, plastic-shell implosions on OMEGA

Direct-drive, plastic shells imploded on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] with a 1ns square pulse are simulated using the multidimensional hydrodynamic code DRACO in yield degradation in “thin” shells is primarily caused by shell breakup during the acceleration phase due to short-wavelength (l>50, where l is the Legendre mode number) perturbation growth, whereas “thick” shell performance is influenced primarily by long and intermediate modes (l⩽50). Simulation yields, temporal history of neutron production, areal densities, and x-ray images of the core compare well with experimental observations. In particular, the thin-shell neutron production history falls off less steeply than one-dimensional predictions due to shell-breakup-induced undercompression and delayed stagnation. Thicker, more-stable shells show burn truncation due to instability-induced mass flow into the colder bubbles. Estimates of small-scale mix indicate that turbulent mixing does not influence p...

Progress in direct-drive inertial confinement fusion

Significant progress in direct-drive inertial confinement fusion (ICF) research has been made since the completion of the 60-beam, 30-kJUV OMEGA Laser System [Boehly, Opt. Commun. 133, 495 (1997)] in 1995. A theory of ignition requirements, applicable to any ICF concept, has been developed. Detailed understanding of laser-plasma coupling, electron thermal transport, and hot-electron preheating has lead to the measurement of neutron-averaged areal densities of ∼200mg∕cm2 in cryogenic target implosions. These correspond to an estimated peak fuel density in excess of 100g∕cm3 and are in good agreement with hydrodynamic simulations. The implosions were performed using an 18-kJ drive pulse designed to put the converging fuel on an adiabat of two. The polar-drive concept will allow direct-drive-ignition research on the National Ignition Facility while it is configured for indirect drive. Advanced ICF ignition concepts—fast ignition [Tabak et al., Phys. Plasmas 1, 1626 (1994)] and shock ignition [Betti et al., P...

Thermonuclear ignition in inertial confinement fusion and comparison with magnetic confinement

The physics of thermonuclear ignition in inertial confinement fusion (ICF) is presented in the familiar frame of a Lawson-type criterion. The product of the plasma pressure and confinement time Pτ for ICF is cast in terms of measurable parameters and its value is estimated for cryogenic implosions. An overall ignition parameter χ including pressure, confinement time, and temperature is derived to complement the product Pτ. A metric for performance assessment should include both χ and Pτ. The ignition parameter and the product Pτ are compared between inertial and magnetic-confinement fusion. It is found that cryogenic implosions on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have achieved Pτ∼1.5 atm s comparable to large tokamaks such as the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] where Pτ∼1 atm s. Since OMEGA implosions are relatively cold (T∼2 keV), their overall ignition parameter χ∼0.02–0.03 is ∼5× lower than in JET (χ∼0.13), where the average temp...

Performance of direct-drive cryogenic targets on OMEGA

The success of direct-drive-ignition target designs depends on two issues: the ability to maintain the main fuel adiabat at a low level and the control of the nonuniformity growth during the implosion. A series of experiments was performed on the OMEGA Laser System [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to study the physics of low-adiabat, high-compression cryogenic fuel assembly. Modeling these experiments requires an accurate account for all sources of shell heating, including shock heating and suprathermal electron preheat. To increase calculation accuracy, a nonlocal heat-transport model was implemented in the 1D hydrocode. High-areal-density cryogenic fuel assembly with ρR>200mg∕cm2 [T. C. Sangster, V. N. Goncharov, P. B. Radha et al., “High-areal-density fuel assembly in direct-drive cryogenic implosions,” Phys. Rev. Lett. (submitted)] has been achieved on OMEGA in designs where the shock timing was optimized using the nonlocal treatment of the heat conductio...

Nonlinear evolution of multimode Rayleigh--Taylor instability in two and three dimensions

The nonlinear evolution of the Rayleigh–Taylor instability from multimode initial perturbations is studied by two complementary approaches. First, a statistical‐mechanics bubble‐merger model is presented, that enables determination of the late‐time scaling properties based on single‐mode and two‐bubble interaction physics. The results for Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) bubble and spike front penetrations are given, as well as scaling laws for mixing under a time‐dependent driving acceleration. The second approach is a modal model, in which nonlinear mode coupling and saturation are included in an equation for effective modes that describe the mixed region. The importance of mode coupling in the generation of large structure that dominates the late stage evolution, and the relative importance of long‐wavelength components in the initial perturbation spectra on the late‐stage evolution are studied. Finally, multimode RT instability in three dimensions is studied by both full‐scale simulatio...

Time-Resolved Absorption in Cryogenic and Room-Temperature Direct-Drive Implosions

Time-dependent and time-integrated absorption fractions are inferred from scattered-light measurements in room-temperature and cryogenic direct-drive-implosion experiments on OMEGA. The measurements agree reasonably well with hydrodynamic simulations that include nonlocal electron-heat transport. Discrepancies in the time-resolved scattered-light spectra between simulations and experiments remain for complex laser pulse shapes, indicating beam-to-beam energy transfer and commensurate coupling losses. Time-resolved scattered-light spectra near ω∕2 and 3ω∕2 as well as time-resolved hard-x-ray measurements indicate the presence of a strongly driven two-plasmon-decay (TPD) instability at high intensities that may influence the observed laser light absorption. Experiments indicate that energetic electron production due to the TPD instability can be mitigated with high-Z-doped plastic shells.