A series of cryogenic, layered deuterium-tritium (DT) implosions have produced, for the first time, fusion energy output twice the peak kinetic energy of the imploding shell. These experiments at the National Ignition Facility utilized high density carbon ablators with a three-shock laser pulse (1.5 MJ in 7.5 ns) to irradiate low gas-filled (0.3  mg/cc of helium) bare depleted uranium hohlraums, resulting in a peak hohlraum radiative temperature ∼290  eV. The imploding shell, composed of the nonablated high density carbon and the DT cryogenic layer, is, thus, driven to velocity on the order of 380  km/s resulting in a peak kinetic energy of ∼21  kJ, which once stagnated produced a total DT neutron yield of 1.9×10^{16} (shot N170827) corresponding to an output fusion energy of 54 kJ. Time dependent low mode asymmetries that limited further progress of implosions have now been controlled, leading to an increased compression of the hot spot. It resulted in hot spot areal density (ρr∼0.3  g/cm^{2}) and stagnation pressure (∼360  Gbar) never before achieved in a laboratory experiment.

Obtaining a burning plasma is a critical step towards self-sustaining fusion energy1. A burning plasma is one in which the fusion reactions themselves are the primary source of heating in the plasma, which is necessary to sustain and propagate the burn, enabling high energy gain. After decades of fusion research, here we achieve a burning-plasma state in the laboratory. These experiments were conducted at the US National Ignition Facility, a laser facility delivering up to 1.9 megajoules of energy in pulses with peak powers up to 500 terawatts. We use the lasers to generate X-rays in a radiation cavity to indirectly drive a fuel-containing capsule via the X-ray ablation pressure, which results in the implosion process compressing and heating the fuel via mechanical work. The burning-plasma state was created using a strategy to increase the spatial scale of the capsule2,3 through two different implosion concepts4–7. These experiments show fusion self-heating in excess of the mechanical work injected into the implosions, satisfying several burning-plasma metrics3,8. Additionally, we describe a subset of experiments that appear to have crossed the static self-heating boundary, where fusion heating surpasses the energy losses from radiation and conduction. These results provide an opportunity to study α-particle-dominated plasmas and burning-plasma physics in the laboratory.

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The National Ignition Facility has been used to compress deuterium-tritium to an average areal density of ~1.0±0.1 g cm(-2), which is 67% of the ignition requirement. These conditions were obtained using 192 laser beams with total energy of 1-1.6 MJ and peak power up to 420 TW to create a hohlraum drive with a shaped power profile, peaking at a soft x-ray radiation temperature of 275-300 eV. This pulse delivered a series of shocks that compressed a capsule containing cryogenic deuterium-tritium to a radius of 25-35 μm. Neutron images of the implosion were used to estimate a fuel density of 500-800 g cm(-3).

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In a burning plasma state1–7, alpha particles from deuterium–tritium fusion reactions redeposit their energy and are the dominant source of heating. This state has recently been achieved at the US National Ignition Facility8 using indirect-drive inertial-confinement fusion. Our experiments use a laser-generated radiation-filled cavity (a hohlraum) to spherically implode capsules containing deuterium and tritium fuel in a central hot spot where the fusion reactions occur. We have developed more efficient hohlraums to implode larger fusion targets compared with previous experiments9,10. This delivered more energy to the hot spot, whereas other parameters were optimized to maintain the high pressures required for inertial-confinement fusion. We also report improvements in implosion symmetry control by moving energy between the laser beams11–16 and designing advanced hohlraum geometry17 that allows for these larger implosions to be driven at the present laser energy and power capability of the National Ignition Facility. These design changes resulted in fusion powers of 1.5 petawatts, greater than the input power of the laser, and 170 kJ of fusion energy18,19. Radiation hydrodynamics simulations20,21 show energy deposition by alpha particles as the dominant term in the hot-spot energy balance, indicative of a burning plasma state.

The classical theory of nucleation has been acceptably well developed, particularly as it pertains to vapor-liquid transformation in pure systems. When liquid solution-solid crystal transformations are considered the theory is still applied, typically, but with less confidence. Perhaps the best measure of the extent to which one can accept classical theory for these latter systems is provided by a study of the experimental results of Nielsen (1969Nielsen ( , 1971. He carried out homogeneous nucleation studies for a variety of ionic crystal-aqueous solution systems. H e was able to show reasonable agreement between classical theory and experiment for some salts (e.g., 2,2 electrolytes), while success was mixed for others. The major goal of this work is the characterization of the crystals which are produced through homogeneous nucleation by the Nielsen process. In order to do this, a preliminary experimental study of the process is required to identify the mechanisms by which the observed product is formed. This paper describes work serving to standardize the experimental procedure. ExperimentalPrecipitation experiments were carried out using a modified Nielsen apparatus, Figure 1. Nielsen's original apparatus was activated by two syringes operating in parallel, each of capacity 5 mL. These operated simultaneously to force the two reactant streams to intimately mix in a downstream tee. The device provided a sample of the precipitate for subsequent study-primarily the counting of the product crystalline entities; it also permitted a measure of the reaction or induction time t i , i.e., the period elapsed between the time of mixing and the visually determined onset of precipitation. The general idea behind this was to mix reactant solutions rapidly so that the precipitation mechanisms occurred from a well-mixed homogeneous solution. The mixing occurred in a specially designed mixing tee whose characteristic mixing time was estimated to be about one millisecond. Obtaining a particular relationship between the number of particles, Nvs. initial supersaturation, So, ensured that homogeneous nucleation had occurred.The apparatus modification involved in this work included a larger volume of reagents, say a maximum of 525 mL, and a mixing tee of simpler configuration. The motivating idea behind this latter change was to enable easy characterization of the tee in the event that future scale-up and modifications were to be considered. The tubing sizes given in Figure 1 are about the same as those used by Nielsen (1961). The larger quantities of reactants were desirable as the goals of the experiments included providing product crystals in sufficient quantities for more elaborate characterization and further processing. Plexiglass vessels D served to contain the reactants under air pressure as they were fed to the mixing tee. In Figure I , C is the pressure gauge, B, the pressure regulator and A, an in-line filter; vessels D were fed from vessels I whose contents were transported through filters G (0.22 wm) by the diaphragm ...