Styrene emulsion polymerization: Particle-size distributions

Data are presented on the time evolution of particle-size distributions (PSDs) in seeded and ab initio styrene emulsion polymerization systems. Initiation was by chemical reagent (potassium persulfate) or γ-radiation. The unswollen PSDs at various times during interval II of the polymerization were obtained by direct measurement of calibrated electron micrographs. Experimental results were fitted with the equations that describe the time evolution of an initial PSD. Analytic solutions to these equations that allow for entry, exit, and propagation of free radicals were obtained. The values of the rate coefficients for these processes used to fit the experimental data were in excellent agreement with those obtained from dilatometric kinetics experiments.     

Free radical entry in emulsion polymerizations

Measurements of the rate coefficients characterising the entry of free radicals into seed particles in styrene emulsion polymerizations has allowed the rate determining step for entry to be identified. This was found to be the rate of production of oligomeric species in the aqueous phase by monomer addition to the primary free radicals. Once formed the subsequent diffusion of these species to the latex particles (and their incorporation within these particles) is relatively fast, contrary to the assumptions of the previous diffusion controlled theories. The experimental results imply that the entering free radicals contain only two or three monomer units. Thermodynamic considerations show that such species should be both water soluble and surface active. Similar conclusions have been reached for other sparingly water soluble monomers, such as butyl acrylate and butyl methacrylate.     

The addition of methyl radicals to tetrafluoroethylene

The addition of methyl radicals to tetrafluoroethylene in the gas phase has been studied over the temperature range 80–180°C, using a material balance method. Arrhenius parameters of 10^11.95±0.23 (mole^?1 cm³ sec^?1) and 5.7 ± 0.4 (kcal/mole) have been measured for the addition reaction. Electrophilic reagents such as O or CF₃ appear to react almost equally readily with ethylene and tetrafluoroethylene but methyl radicals add much more rapidly to tetrafluoroethylene than to ethylene, the difference in reactivity being principally due to an activation energy difference of ～2 kcal/mole.     

Effects of nonuniform illumination on implosion asymmetry in direct-drive inertial confinement fusion

Target areal density (rhoR) asymmetries in OMEGA direct-drive spherical implosions are studied. The rms variation / for low-mode-number structure is approximately proportional to the rms variation of on-target laser intensity / with an amplification factor of approximately 1/2(C(r)-1), where C(r) is the capsule convergence ratio. This result has critical implications for future work on the National Ignition Facility as well as OMEGA.     

Progress in direct-drive inertial confinement fusion research at the laboratory for laser energetics

Direct-drive inertial confinement fusion (ICF) is expected to demonstrate high gain on the National Ignition Facility (NIF) in the next decade and is a leading candidate for inertial fusion energy production. The demonstration of high areal densities in hydrodynamically scaled cryogenic DT or D₂ implosions with neutron yields that are a significant fraction of the “clean” 1-D predictions will validate the ignition-equivalent direct-drive target performance on the OMEGA laser at the Laboratory for Laser Energetics (LLE). This paper highlights the recent experimental and theoretical progress leading toward achieving this validation in the next few years. The NIF will initially be configured for X-ray drive and with no beams placed at the target equator to provide a symmetric irradiation of a direct-drive capsule. LLE is developing the “polar-direct-drive” (PDD) approach that repoints beams toward the target equator. Initial 2-D simulations have shown ignition. A unique “Saturn-like” plastic ring around the equator refracts the laser light incident near the equator toward the target, improving the drive uniformity. LLE is currently constructing the multibeam, 2.6-kJ/beam, petawatt laser system OMEGA EP. Integrated fast-ignition experiments, combining the OMEGA EP and OMEGA Laser Systems, will begin in FY08.     

High-power, kilojoule class laser channeling in millimeter-scale underdense plasma

Experiments were performed using the Omega EP laser, operating at 740 J of energy in 8 ps (90 TW), which provides extreme conditions relevant to fast ignition studies. A carbon and hydrogen plasma plume was used as the underdense target and the interaction of the laser pulse propagating and channeling through the plasma was imaged using proton radiography. The early time expansion, channel evolution, filamentation, and self-correction of the channel was measured on a single shot via this method. A channel wall modulation was observed and attributed to surface waves. After around 50 ps, the channel had evolved to show bubblelike structures, which may be due to postsoliton remnants.