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A distinctive way of quantitatively imaging inertial fusion implosions has resulted in the characterization of two different types of electromagnetic configurations and in the measurement of the temporal evolution of capsule size and areal density. Radiography with a pulsed, monoenergetic, isotropic proton source reveals field structures through deflection of proton trajectories, and areal densities are quantified through the energy lost by protons while traversing the plasma. The two field structures consist of (i) many radial filaments with complex striations and bifurcations, permeating the entire field of view, of magnetic field magnitude 60 tesla and (ii) a coherent, centrally directed electric field of order 10⁹ volts per meter, seen in proximity to the capsule surface. Although the mechanism for generating these fields is unclear, their effect on implosion dynamics is potentially consequential.

Measurements of x-ray-driven implosions with charged particles have resulted in the quantitative characterization of critical aspects of indirect-drive inertial fusion. Three types of spontaneous electric fields differing in strength by two orders of magnitude, the largest being nearly one-tenth of the Bohr field, were discovered with time-gated proton radiographie imaging and spectrally resolved proton self-emission. The views of the spatial structure and temporal evolution of both the laser drive in a hohlraum and implosion properties provide essential insight into, and modeling validation of, x-ray-driven implosions.

Focusing laser light onto a very small target can produce the conditions for laboratory-scale nuclear fusion of hydrogen isotopes. The lack of accurate predictive models, which are essential for the design of high-performance laser-fusion experiments, is a major obstacle to achieving thermonuclear ignition. Here we report a statistical approach that was used to design and quantitatively predict the results of implosions of solid deuterium–tritium targets carried out with the 30-kilojoule OMEGA laser system, leading to tripling of the fusion yield to its highest value so far for direct-drive laser fusion. When scaled to the laser energies of the National Ignition Facility (1.9 megajoules), these targets are predicted to produce a fusion energy output of about 500 kilojoules—several times larger than the fusion yields currently achieved at that facility. This approach could guide the exploration of the vast parameter space of thermonuclear ignition conditions and enhance our understanding of laser-fusion physics. A statistical approach to designing and predicting the fusion yield of cryogenic deuterium–tritium implosions leads to tripled yield in direct-drive laser fusion of deuterium–tritium layered targets.

Irradiating a small capsule containing deuterium and tritium fuel directly with intense laser light causes it to implode, which creates a plasma hot enough to initiate fusion reactions between the fuel nuclei. Here we report on such laser direct-drive experiments and observe that the fusion reactions produce more energy than the amount of energy in the central so-called hot-spot plasma. This condition is identified as having a hot-spot fuel gain greater than unity. A hot-spot fuel gain of around four was previously accomplished at the National Ignition Facility in indirect-drive inertial confinement fusion experiments where the capsule is irradiated by X-rays. In that case, up to 1.9 MJ of laser energy was used, but in contrast, our experiments on the OMEGA laser system require as little as 28 kJ. As the hot-spot fuel gain is predicted to grow with laser energy and target size, our work establishes the direct-drive approach to inertial fusion as a promising path towards burning and ignited plasmas in the laboratory. Additionally, we report a record (direct-drive) fusion yield of 0.9 kJ on OMEGA, which we achieved with thin-ice deuterium–tritium liner targets. Inertial confinement fusion experiments in a direct-drive configuration report more energy produced in deuterium–tritium fusion reactions than the amount of energy in the central part of the plasma created by laser irradiation of the fuel capsule.

Focussing laser light onto the surface of a small target filled with deuterium and tritium implodes it and leads to the creation of a hot and dense plasma, in which thermonuclear fusion reactions occur. In order for the plasma to become self-sustaining, the heating of the plasma must be dominated by the energy provided by the fusion reactions—a condition known as a burning plasma. A metric for this is the generalized Lawson parameter, where values above around 0.8 imply a burning plasma. Here, we report on hydro-equivalent scaling of experimental results on the OMEGA laser system and show that these have achieved core conditions that reach a burning plasma when the central part of the plasma, the hotspot, is scaled in size by at least a factor of 3.9 ± 0.10, which would require a driver laser energy of at least 1.7 ± 0.13 MJ. In addition, we hydro-equivalently scale the results to the 2.15 MJ of laser energy available at the National Ignition Facility and find that these implosions reach 86% of the Lawson parameter required for ignition. Our results support direct-drive inertial confinement fusion as a credible approach for achieving thermonuclear ignition and net energy in laser fusion. Hydro-equivalent scaling of recent direct-drive inertial confinement fusion implosions shows that a burning plasma can be achieved with a higher laser energy.

Laser-direct drive (LDD), along with laser indirect (X-ray) drive (LID) and magnetic drive with pulsed power, is one of the three viable inertial confinement fusion approaches to achieving fusion ignition and gain in the laboratory. The LDD programme is primarily being executed at both the Omega Laser Facility at the Laboratory for Laser Energetics and at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. LDD research at Omega includes cryogenic implosions, fundamental physics including material properties, hydrodynamics and laser–plasma interaction physics. LDD research on the NIF is focused on energy coupling and laser–plasma interactions physics at ignition-scale plasmas. Limited implosions on the NIF in the ‘polar-drive’ configuration, where the irradiation geometry is configured for LID, are also a feature of LDD research. The ability to conduct research over a large range of energy, power and scale size using both Omega and the NIF is a major positive aspect of LDD research that reduces the risk in scaling from OMEGA to megajoule-class lasers. The paper will summarize the present status of LDD research and plans for the future with the goal of ultimately achieving a burning plasma in the laboratory. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 2)’.

A magneto-inertial fusion (MIF) approach to inertial confinement fusion (ICF), based on laser-driven magnetic-flux compression (LDFC) is described. This approach benefits from both the high-energy-density characteristic to ICF and the thermal insulation of the fuel by magnetic fields, typical of MFE. The reduction in thermal-conduction losses in the hot spot of an imploding target that has trapped and amplified a pre-seeded magnetic flux leads to increased hot-spot temperatures at lower implosion velocities than required in conventional ICF. This can lead to ignition designs with larger energy gains. This work describes the main concept and the use of a compact magnetic-pulse system to seed a macroscopic magnetic field into cylindrical DD-filled targets, which are radially driven with the OMEGA laser. The compression of the internal magnetic flux is measured with proton deflectometry. Magnetohydrodynamic simulations predict compression of a 0.1-MG seed field to multi-megagauss values, at which levels the radial electron thermal conduction in the hot spot is significantly inhibited. Initial benchmark experiments are described.

Polar-direct-drive experiments at the National Ignition Facility (NIF) are being used to validate direct-drive-implosion models. Energy coupling and fast-electron preheat are the primary issues being studied in planar and imploding geometries on the NIF. Results from backlit images from implosions indicate that the overall drive is well modeled although some differences remain in the thickness of the imploding shell. Implosion experiments to mitigate cross-beam energy transfer and preheat from two-plasmon decay are planned for the next year.

An improved the set of nuclear diagnostics on NIF measures the properties of the stagnation plasma of implosions, including the drift velocity, areal density (ρr) anisotropy and carbon ρr of the compressed core. Two types of deuterium-tritium (DT) gas filled targets are imploded by shaped x-ray pulses, producing stagnated and burning DT cores of radial convergence (Cr) ∼ 5 or ∼20. Comparison with two-dimensional modeling with inner and outer surface mix shows good agreement with nuclear measurements.