Direct drive with the argon fluoride laser as a path to high fusion gain with sub-megajoule laser energy.

Argon fluoride (ArF) is currently the shortest wavelength laser that can credibly scale to the energy and power required for high gain inertial fusion. ArF's deep ultraviolet light and capability to provide much wider bandwidth than other contemporary inertial confinement fusion (ICF) laser drivers would drastically improve the laser target coupling efficiency and enable substantially higher pressures to drive an implosion. Our radiation hydrodynamics simulations indicate gains greater than 100 are feasible with a sub-megajoule ArF driver. Our laser kinetics simulations indicate that the electron beam-pumped ArF laser can have intrinsic efficiencies of more than 16%, versus about 12% for the next most efficient krypton fluoride excimer laser. We expect at least 10% 'wall plug' efficiency for delivering ArF light to target should be achievable using solid-state pulsed power and efficient electron beam transport to the laser gas that was demonstrated with the U.S. Naval Research Laboratory's Electra facility. These advantages could enable the development of modest size and lower cost fusion power plant modules. This would drastically change the present view on inertial fusion energy as being too expensive and the power plant size too large. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 1)'.

Mitigation of cross-beam energy transfer in inertial-confinement-fusion plasmas with enhanced laser bandwidth.

Cross-beam energy transfer (CBET) is a significant energy-loss mechanism in directly driven inertial-confinement-fusion (ICF) targets. One strategy for mitigating CBET is to increase the bandwidth of the laser light, thereby disrupting the resonant three-wave interactions that underlie this nonlinear scattering process. Here, we report on numerical simulations performed with the wave-based code lpse that show a significant reduction in CBET for bandwidths of 2-5 THz (corresponding to a normalized bandwidth of 0.2%-0.6% at a laser wavelength of 351nm) under realistic plasma conditions. Such bandwidths are beyond those available with current high-energy lasers used for ICF, but could be achieved using stimulated rotation Raman scattering in diatomic gases like nitrogen.

Origins and Scaling of Hot-Electron Preheat in Ignition-Scale Direct-Drive Inertial Confinement Fusion Experiments.

Planar laser-plasma interaction (LPI) experiments at the National Ignition Facility (NIF) have allowed access for the first time to regimes of electron density scale length (∼500 to 700 µm), electron temperature (∼3 to 5 keV), and laser intensity (6 to 16×10^{14} W/cm^{2}) that are relevant to direct-drive inertial confinement fusion ignition. Unlike in shorter-scale-length plasmas on OMEGA, scattered-light data on the NIF show that the near-quarter-critical LPI physics is dominated by stimulated Raman scattering (SRS) rather than by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. SRS sidescatter tangential to density contours and other SRS mechanisms are observed. The fraction of laser energy converted to hot electrons is ∼0.7% to 2.9%, consistent with observed levels of SRS. The intensity threshold for hot-electron production is assessed, and the use of a Si ablator slightly increases this threshold from ∼4×10^{14} to ∼6×10^{14} W/cm^{2}. These results have significant implications for mitigation of LPI hot-electron preheat in direct-drive ignition designs.

Theory of the corrugation instability of a piston-driven shock wave.

We analyze the two-dimensional stability of a shock wave driven by a steadily moving corrugated piston in an inviscid fluid with an arbitrary equation of state. For h≤-1 or h>h(c), where h is the D'yakov parameter and h(c) is the Kontorovich limit, we find that small perturbations on the shock front are unstable and grow--at first quadratically and later linearly--with time. Such instabilities are associated with nonequilibrium fluid states and imply a nonunique solution to the hydrodynamic equations. The above criteria are consistent with instability limits observed in shock-tube experiments involving ionizing and dissociating gases and may have important implications for driven shocks in laser-fusion, astrophysical, and/or detonation studies.

Experimental evidence of impact ignition: 100-fold increase of neutron yield by impactor collision.

We performed integrated experiments on impact ignition, in which a portion of a deuterated polystyrene (CD) shell was accelerated to about 600 km/s and was collided with precompressed CD fuel. The kinetic energy of the impactor was efficiently converted into thermal energy generating a temperature of about 1.6 keV. We achieved a two-order-of-magnitude increase in the neutron yield by optimizing the timing of the impact collision, demonstrating the high potential of impact ignition for fusion energy production.

Diversity and sensitivity of epiphytes to oxides of nitrogen in London.

This study investigated the distribution and diversity of epiphytes in London in relation to NO(x) using fine-scale atmospheric dispersion modelling. The survey recorded over 3000 epiphytes from 334 trees (Fraxinus excelsior) representing 74 lichen, 14 moss, 7 fungal and 3 algal species. There was a significant inverse relationship between diversity and NO(x). Diversity declined where NO(x) exceeded 70 microg m(-3) and NO2 exceeded 40 microg m(-3), suggesting a phytotoxic effect. However, there was a significant positive relationship between NO(x) and lichen abundance due to the ubiquitous distribution of pollution tolerant species, mainly associated with eutrophication. A scale of lichen sensitivity to NO(x) has been derived.

The slide-sealing compound "glyceel".

"Glyceel" has been considered for many years to be the best sealant for whole mounts of soil, plant-parasitic, freshwater, and marine nematodes. However, "Glyceel" has not been available since the mid 1980s when its production was halted. Currently available substitutes are inadequate. The original formula for "Glyceel" has been found in the literature and is given here with a method of preparation. "Glyceel" prepared in this way by the author has been used and appears to function well.

Effects of sulphur dioxide and ozone on lichen colonization of conifers in the Liphook Forest Fumigation Project.

In the Liphook Forest Fumigation Project the conifers Picea abies(L.) Karst., P, sitchensis (Bong-) Carr. and Pinus sylvestris L. were grown from the seedling stage and subjected, by open-air fumigation, to three regimes of SO2 (ambient, 4 ppb (parts in 109 by volume); low, 12 ppb; high, 20 ppb) and two regimes of O3 (ambient, 25 ppb; high, 30 ppb) in factorial combination, between 1987 and 1990. We determined the abundance of three lichens, Evernia prunastri (L.) Ach., Hypogymnia physodes (L.) Nyl. and Lecanora canizaeoides Nyl. ex Crombie, which colonized the trunks and branches of the saplings, at the end of the experiment between May-July 1991. All three species showed a marked preference for P. abies and were scarcest on P, sylvestris which had the most acidic hark. E. prunastri was the rarest of the three lichens and only colonized plots exposed to ambient SO2 (4 ppb). Diminutive thalli of H. physodes were relatively numerous in the ambient SO2 , plots, much scarcer in those receiving low SO2 (12 ppb) and rare in those with high SO2 (20 ppbl. Coverage by L. conizaeoides was very low in the ambient SO2 , plots but appreciable in the low and high SO2 treatment plots. The results indicate directly beneficial effect of SO2 on L. conizaeoides colonization rather than an indirect benefit through competitor release. The O3 treatment (1.3 times ambient during Spring-Autumn) did not influence the abundance of the lichens. All three lichen species appeared to be more sensitive (positive and negative effects) to SO2 , than mapping studies have suggested. Possible reasons for the high SO2 -sensitivity of the lichens are discussed. These include the high acidity of conifer bark and the fact that growth from diaspores and development of immature thalli is involved rather than survival of established populations.

Epiphyte recolonization of oaks along a gradient of air pollution in South-East England, 1979-1990.

The abundances of epiphytes on free-standing oaks (Quercus robur) along a transect extending approximately 70 km SSW from Central London were redetermined annually from 1979 to 1990. During this period SO(2) levels in inner London fell to between one-quarter and one-seventh of levels occurring in the 1960s. The sample trees were also scored annually using Hawksworth and Rose's (1970) scale for predicting SO(2) levels from the epiphytes present. Little evidence was obtained for epiphyte recolinization of oaks at sites within the built-up area. At Epsom Common, on the urban fringe, cover of the pollution-tolerant lichen Lecanora conizaeoides fell, and in the inner suburbs, at Putney Heath, L. conizaeoides and Lepraria incana increased, while the alga Desmococcus viridis declined at this site. These changes might be due to a contraction inwards of zones of peak abundance of L. conizaeoides and D. viridis with decreasing SO(2) levels. However, it is also possible that the changes at Putney Heath resulted from scrub clearance activities and alteration to the microclimate. The cover of the moderately SO(2) tolerant foliose lichen Hypogymnia physodes remained low and static at Putney Heath during the 12-year period. Possible reasons for the poor recolinization of London's oaks by epiphytes are discussed. Particular emphasis is given to the highly acidic bark of oak in the London area (pH2.9-4.0) in comparison with other tree species and with oaks in unpolluted regions.

The effect of seawater on the metabolism of some seashore and inland mosses.

Photosynthesis and protein synthesis were determined, by measuring uptake and incorporation of radioactive bicarbonate and L-valine, in some seashore and inland mosses after treatment with artificial seawater. In the inland species there was a progressive decline in both processes with increasing seawater concentration. Photosynthesis in the seashore mosses Grimmia maritima and Tortella flavovirens was unaffected by increasing seawater concentration, and supralittoral Ulota phyllantha was less seriously affected than epiphytic material from a subcoastal location. There was a marked decline in chlorophyll content and cessation of photosynthesis in the inland moss G. pulvinata after 3 days' seawater treatment whereas material treated with deionised water showed increased photosynthesis and chlorophyll levels over the same period. In G. maritima both chlorophyll levels and photosynthesis remained relatively unaffected by treatment with seawater for 4 days. Treatment of G. pulvinata with seawater and NaCl led to increased leakage of photosynthetic products to the external solution but this was insufficient to explain the reduction in carbon retained by the plant. The seawater-induced disruption of metabolism in inland bryophytes is believed to be primarily due to the uncontrolled entry of toxic ions into the moss cells whereas seawater tolerance in the seashore species probably reflects the possession of an efficient intracellular cation control mechanism.