Lawson criterion for ignition exceeded in an inertial fusion experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion

The aerodynamic behaviour of volcanic aggregates

A large proportion of solid material transported within the atmosphere during volcanic eruptions consists of particles less than 500 mum in diameter. The majority of these particles become incorporated into a wide range of aggregate types, the aerodynamic behaviour of which has not been determined by either direct observation or in the laboratory. In the absence of such data, theoretical models of fallout from volcanic plumes make necessarily crude assumptions about aggregate densities and fall velocities. Larger volcanic ejecta often consists of pumice of lower than bulk density. Experimental data are presented for the fall velocities of porous aggregates and single particles, determined in systems analogous to that of ejecta falling from a volcanic plume. It is demonstrated that the fall of aggregates may be modelled in identical fashion to single particles by using a reduced aggregate density dependent on the porosity, and a size corresponding to an enclosing sphere. Particles incorporated into aggregates attain a substantially higher fall velocity than single particles. This is due to the larger physical dimensions of the aggregate, which overcomes the effect of lower aggregate density. Additionally, the internal porosity of the aggregate allows some flow of fluid through the aggregate and this results in a small increase in fall velocity. The increase in fall velocity of particles incorporated into aggregates, rather than falling individually, results in the enhanced removal of fine material from volcanic plumes

Charge measurements on particle fallout from a volcanic plume

The aggregation of fine ash particles has an important role in controlling the deposition of widely dispersed volcanic ash. Here we report measurements of electrical charge on ash particles falling from the eruption columns of Sakurajima volcano in Japan. Absolute charge to mass (q/m) ratios ranged from +3 to +6x10-4 C kg-1 and from -2 to -5x10-4 C kg-1. The average q/m ratio ranged from +2 to +5x10-5 C kg-1. The generation of electrostatic charge may result from triboelectric effects in the plume, or from fracture-induced charging. Charge on ash particles provides attractive forces large enough to cause the aggregation of smaller particles and the adhesion of dust to larger particles. Particle aggregation may explain the polymodal grain-size distributions commonly found in ash-fall deposits, and the proximal deposition of fine ash, as well as the distal deposition of coarse particles in these deposits. Our data suggest that electrostatic effects greatly influence the dispersal and deposition of ash during explosive volcanic eruptions

The origin of accretionary lapilli.

Experimental investigations in a recirculating wind tunnel of the mechanisms of formation of accretionary lapilli have demonstrated that growth is controlled by collision of liquid-coated particles, due to differences in fall velocities, and binding as a result of surface tension forces and secondary mineral growth. The liquids present on particle surfaces in eruption plumes are acid solutions stable at 100% relative humidity, from which secondary minerals, e.g. calcium sulphate and sodium chloride, precipitate prior to impact of accretionary lapilli with the ground. Concentric grain-size zones within accretionary lapilli build up due to differences in the supply of particular particle sizes during aggregate growth. Accretionary lapilli do not evolve by scavenging of particles by liquid drops followed by evaporation — a process which, in wind tunnel experiments, generates horizontally layered hemispherical aggregates. Size analysis of particles in the wind tunnel air stream and particles adhering to growing aggregates demonstrate that the aggregation coefficient is highly grain-size dependent. Theoretical simulation of accretionary lapilli growth in eruption plumes predicts maximum sizes in the range 0.7–20 mm for ash cloud thicknesses of 0.5–10 km respectivel