Implosion Physics

Schematic depictions of nuclear weapons are found in many documents. Figures likely to be encountered will resemble either a single circle (have one center) for single stage weapons, or two adjacent circles (i.e., two centers of symmetry) for a staged or thermonuclear weapon. This is only a rough characterization.

Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instabilities, and the ensuing mixing that usually follows their development, are of interest because of their occurrence in inertial fusion, astrophysics, and the implosion of both a fision primary and a fusion secondary. Another area of unstable flow which has become of significant interest is the study of compressible turbulence and mixing in high-speed jets and shear layers. Turbulence may be qualitatively different at high Mach numbers and/or high compressibilities.

The study of the physical mechanisms involved in the coupling of energy deposited by a strong radiative source above, on, or below the surface of dense matter and fluids is of special interest. This energy coupling varies from zero for sources at large heights above the surface to 100% for deeply buried sources. The shape of the coupling curve as a function of source position, of course, has to do with questions of range-to-effect and cratering, but is also of scientific interest for the study of the exact mechanisms determining the coupling. It has been proposed that the coupling of energy proceeds through three stages: first, the stripped-sphere stage of complete ionization, then thermal wave propagation, and finally a momentum-conserving transition to shock propagation and cratering.

These instabilities refer to the growth of perturbations at a fluid interface. The type of instability in the flow is determined by the origin of the perturbation. If it is related to the action of shear or constant acceleration, the instability is generally referred to as Kelvin-Helmholtz or Rayleigh-Taylor, respectively, and the growth is exponential in time. An impulsive acceleration caused by a shock normal to the interface causes the Richtmyer-Meshkov instability, which, in the absence of shear or acceleration, then grows linearly in time.

These phenomena have been extensively studied in the past to understand the resulting interface motion and material mix, which are important effects in inertial confinement fusion, for clumping in supernova explosions, and the implosion of both a fision primary and a fusion secondary. Nevertheless, important questions remain, because often experiments do not agree with the theory or with the numerical simulations in certain flow domains, especially at high compression. Experiments on the NIF with long drive times and large spatial scales are expected to be especially important for the understanding of these instabilities.

Clear-air turbulence (CAT) is generally believed to be caused by a subtle mechanism referred to as the Kelvin-Helmholtz instability. The change in wind vector with height (vertical wind shear) may approach a critical strength, beyond which it overcomes atmospheric stability and begins to turn over the layer. The Kelvin-Helmholtz instability, in particular, has been credited with waves and ripples propagating along the magnetopause.