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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.

AbstractThe goal of the National Direct-Drive Program is to demonstrate and understand the physics of laser direct drive (LDD). Efforts are underway on OMEGA for the 100-Gbar Campaign to demonstrate and understand the physics for hot-spot conditions and formation relevant for ignition at the 1-MJ scale, and on the National Ignition Facility to develop an understanding of the direct-drive physics at long scale lengths for the MJ Direct-Drive Campaign. The strategy of the National Direct-Drive Program is described; the requirements for the deuterium-tritium cryogenic fill-tube target being developed for OMEGA are presented; and preliminary LDD implosion measurements of hydrodynamic mixing seeded by laser imprint, the target-mounting stalk, and microscopic surface debris are reported.

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 D2implosions 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 some of the recent experimental and theoretical progress toward this validation.
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. 
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.

Recent progress in kilojoule-scale high-intensity lasers has opened up new areas of research in radiography, laboratory astrophysics, high-energy-density physics, and fast-ignition (FI) laser fusion. FI requires efficient heating of pre-compressed high-density fuel by an intense relativistic electron beam produced from laser–matter interaction. Understanding the details of electron beam generation and transport is crucial for FI. Here we report on the first visualization of fast electron spatial energy deposition in a laser-compressed cone-in-shell FI target, facilitated by doping the shell with copper and imaging the K-shell radiation. Multi-scale simulations accompanying the experiments clearly show the location of fast electrons and reveal key parameters affecting energy coupling. The approach provides a more direct way to infer energy coupling and guide experimental designs that significantly improve the laser-to-core coupling to 7%. Our findings lay the groundwork for further improving efficiency, with 15% energy coupling predicted in FI experiments using an existing megajoule-scale laser driver.

Recent advances in hydrodynamics theory and experiments at the Laboratory for Laser Energetics are described. Particular emphasis is laid on improvements in the implosion stability achieved by shaping the ablator adiabat and on the newly developed designs for fast ignition fuel assembly. The results of two-dimensional simulations and a recent set of implosion experiments on OMEGA are presented to verify the role of adiabat shaping on the hydrodynamic stability of direct-drive implosions. Adiabat shaping laser pulses are also used to implode massive capsules on a low adiabat and low implosion velocity in order to assemble high density plasmas for fast ignition. The areal densities measured in implosion experiments of such targets on OMEGA are among the highest ever recorded in a laser-driven compression experiment. Slow low-adiabat implosions of massive wetted-foam DT capsules are used in the simulations to generate the fuel assemblies for different driver energies. Such dense cores are then ignited by a fast electron beam and the resulting thermonuclear yield is used to compute the target gain. It is shown that a 200?kJ UV laser can assemble fuel yielding about 18?MJ of energy when ignited by 15?kJ of 1-2?MeV electrons.

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 some of the recent experimental and theoretical progress toward this validation.?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. ?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.

AbstractA high-energy petawatt laser, OMEGA EP, is currently under construction at the University of Rochester's Laboratory for Laser Energetics. Integrated into the existing OMEGA laser, it will support three major areas of research: (a) backlighting of high-energy-density plasmas, (b) integrated fast ignition experiments, and (c) high-intensity physics. The laser will provide two beams combined collinearly and coaxially with short pulses (~1 to 100 ps) and high energy (2.6 kJ at 10 ps). Cone-in-shell fuel-assembly experiments and simulations of short-pulse heated cryogenic targets are being performed in preparation for cryogenic integrated fast ignitor experiments on OMEGA EP.

Integrated fast-ignition experiments on the combined OMEGA/OMEGA EP laser systems have been simulated with the multidimensional hydrodynamic code DRACO. The OMEGA laser system provides up to 30?kJ of compression energy, and OMEGA EP will provide two short-pulse beams, each with energies up to 2.6?kJ. In the electron transport model included in DRACO, the relativistic electrons are introduced at the pole of a two-dimensional (2D) simulation and transported in a straight line toward the target core. The electron's energy is calculated from the laser irradiance using a semi-empirical formula. An OMEGA cryogenic DT target designed to reach a one-dimensional fuel ?Rof 0.5?g?cm?2has been simulated in 2D, with and without non-uniformities, to assess the sensitivity to energy, timing, and irradiance of the Gaussian fast-ignitor beam. For the uniform case, the neutron yield is predicted to be in excess of 1015(compared to ~1014without an ignitor beam) over a synchronization range of ~80?ps. Implosions with the ignitor beam show little decrease in the neutron yield with increasing inner-ice non-uniformity, in contrast to implosions without the ignitor beam, which show a significant decrease in neutron yield with increasing non-uniformity.

The fuel assembly of gas-filled, cone-in-shell, fast-ignitor targets is being studied experimentally at the OMEGA Laser Facility. The spherical plastic (CH) shells with a ~24?µm wall thickness had a 70° or 35° opening-angle gold cone inserted. The targets, filled with ~10?atm of D3He or D2, were imploded by direct illumination with up to 21?kJ of 351?nm laser light. Some experiments used a backlighter to study the fuel assembly and mixing of cone material into the dense core. No significant mixing of the cone and core material was observed. Using both fusion products and backlit images, an areal density of ~60 to 70?mg?cm?2was inferred for the dense core assembly. The filling of the inside of the cone, where the ultrafast laser propagates in integrated fast-ignitor experiments, was studied using a streaked optical pyrometer. No plasma was seen inside the cone before the assembled core reached peak compression. These results are promising for successful integrated fast-ignitor experiments on the OMEGA EP Facility, scheduled to be completed in 2007.

A large number of diagnostics have been used to study the performance of gas-filled, plastic-shell direct-drive implosions on the 60-beam OMEGA laser system (Boehly T R et al 1997 Opt. Commun. 133 495). These diagnostics allow an estimate of the amount of fuel-pusher mixing in the compressed core. Implosions have primary neutron yields and fuel areal densities that are ~35% and ~100% of the predicted one-dimensional values, respectively. A highly constrained model of the core conditions and fuel-shell mix has been developed. It suggests that there is a `clean' fuel region surrounded by a mixed region, with the clean region accounting for ~70% of the fuel areal density.

Inertial fusion requires the compression of fusion fuel to densities approaching 1,000 times the density of liquid DT. Compression of DT to densities in the range of 100 to 200 times its liquid density has been achieved using cryogenically cooled solid-fuel-layer DT-filled capsules irradiated directly with high-intensity symmetric short-wavelength laser radiation.