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INERTIAL CONFINEMENT FUSION 62 quantitative understanding of the Rayleigh-Taylor instability in hot, ablating plasmas. Future work will examine the transition to turbulence and address Rickmyer-Meshkov and Kelvin-Helmholtz-like instabilities. Two-dimensional hydrodynamic simulations have modeled successfully the linear and early nonlinear evolution of the Rayleigh-Taylor instability in ablating plasmas with a range of initial sources, Attwood numbers, and accelerations. Photon and electron energy deposition lead to finite density gradients and mass removal, which can substantially reduce the Rayleigh-Taylor growth rate from its classical value. Fokker-Planck codes embedded in hydrodynamic simulations have been developed to model better the nonlocal electron energy transport. These simulations describe phenomena such as thermal filamentation and thermal conduction, and influence our basic understanding of the details of the Rayleigh-Taylor instability. Using the Nova laser facility, successful experiments were conducted that addressed the physics of interpenetrating materials at accelerated interfaces (an area of hydrodynamics critical to both ICF and weapons research). Several of the ''ignition physics milestones" described in the NRC's 1990 review of the ICF program were achieved, including experimental confirmation of the LASNEX simulation code predictions for Rayleigh-Taylor instability growth rates in the presence of ablation and density gradients for both radiation-driven and electron-conduction-driven planar foils. New diagnostic techniques were demonstrated, including large neutron scintillator arrays; a single-hit scintillator array neutron spectrometer; and a high-energy, ring-aperture x-ray microscope. The Nova target chamber is shown in Figure 3.2. There has been significant progress in the ability to measure and calculate the radiation properties of complex, partially stripped ions over a wide range of plasma conditions. The recent measurement of iron opacity in dense (ne > 1020 cm-3), warm (Te &70 eV) plasmas in local thermodynamic equilibrium illustrates these advances. These conditions are also relevant for astrophysical plasmas. The demonstration of nickel-like gold plasma x-ray lasers operating at a wavelength of 33 Ã is an example of the present capability to model non-LTE plasmas. Sophisticated opacity codes for dense plasmas composed of multi- electron ions have been developed. These codes describe the complex absorption and emission features of these ions in terms of unresolved transition arrays and super-transition arrays, and have led to improvements in modeling radiant energy flow in high-density plasma of interest in both inertial fusion and astrophysical applications. Laboratory experiments have been performed that validate these codes. Ion-Beam Fusion An equally robust and successful track record of accomplishments exists for the light- and heavy-ion ICF efforts, which represent alternative "driver" ap
INERTIAL CONFINEMENT FUSION 63 proaches, and they are being pursued concurrently with the laser program. A principal aim of heavy-ion fusion accelerator research is to gain an understanding of the dynamics of intense, space-charge-dominated beams in accelerator structures. These beams, which are effectively nonneutral plasmas, exhibit collective behaviors in addition to those bulk motions driven by the externally applied fields. The beam plasma frequency is comparable to the frequency associated with motion in the applied fields. Analytic theory originally predicted a multitude of instabilities; however, most of these were not observed. Computer simulations resolved the disagreement for transverse modes by elucidating the nonlinear behavior and early saturation of the instabilities. Resolution of this disagreement represents a major step in understanding these nonneutral plasmas. FIGURE 3.2 Photograph of the inside of the Nova 10-beam target chamber at Lawrence Livermore National Laboratory. An essential part of Nova is its diagnostic capability, which includes optical, x-ray, and neutron measurement techniques to study the performance of the ICF targets. These diagnostics surround the target, which is positioned in the center of the chamber on the end of a rod descending from the top of the picture. (Courtesy of Lawrence Livermore National Laboratory.)