Indirect-drive hohlraum experiments at the National Ignition Facility have demonstrated symmetric capsule implosions at unprecedented laser drive energies of 0.7 MJ. 192 simultaneously fired laser beams heat ignition emulate hohlraums to radiation temperatures of 3.3 million Kelvin compressing 1.8-millimeter capsules by the soft x rays produced by the hohlraum. Self-generated plasma-optics gratings on either end of the hohlraum tune the laser power distribution in the hohlraum producing symmetric x-ray drive as inferred from the shape of the capsule self-emission. These experiments indicate conditions suitable for compressing deuterium-tritium filled capsules with the goal to achieve burning fusion plasmas and energy gain in the laboratory.With completion (1) and commissioning (2) of the National Ignition Facility (NIF) the quest for producing a burning fusion plasma has begun (3, 4). The goal of these experiments is to compress matter to densities and temperatures higher than the interior of the sun (5-7) which will initiate nuclear fusion and burn of hydrogen isotopes (8-10). This technique holds promise to demonstrate a highly efficient carbon-free process that will burn milligram quantities of nuclear fuel on each laser shot for producing energy gain in the laboratory.The NIF (11) consists of 192 laser beams that have been arranged into cones of beams to irradiate a target from the top and bottom hemispheres. This "indirect-drive" laser geometry has been chosen for the first experiments to heat the interior of centimeter-scale cylindrical gold hohlraums (8,(12)(13)(14)(15) through laser entrance holes (LEH) on the top and bottom end of the cylinder (Fig. 1). Hohlraums act as radiation enclosures that convert the optical laser light into soft x-rays that are characterized by the radiation temperature T RAD . Present ignition designs operate at temperatures of 270 to 305 eV or 3.1 to 3.5 million K. The radiation field compresses a spherical fusion capsule mounted in the center of the hohlraum by x-ray ablation of the outer shell. The ablation process compresses the cryogenically prepared solid deuterium-tritium fuel layer in a spherical rocket implosion. In the final stages, the fuel reaches densities 1000-times solid and the central hot spot temperatures will approach 100 million K to initiate the nuclear burn process.We have symmetrically imploded 1.8-mm diameter fusion capsules in cryogenically fielded centimeter-scale hohlraums at 20 K. These experiments show efficient hohlraum heating to radiation temperatures of 3.3 million K. In addition, the large scale-length plasmas encountered in these experiments have allowed us to use self-generated plasma optics gratings (16) to control the radiation symmetry (17) and to achieve symmetric fusion capsule implosions.Figure 2 A shows the laser power at the frequency-tripled wavelength of 351 nm versus time for two different pulse shapes. These 11-ns and 16-ns long pulses heat 8.4-mm long, 4.6-mm diameter hohlraums with 20% helium, 80% hydrogen (atomic) mixtures and ...

Along with laser-indirect (X-ray)-drive and magnetic-drive target concepts, laser direct drive is a viable approach to achieving ignition and gain with inertial confinement fusion. In the United States, a national program has been established to demonstrate and understand the physics of laser direct drive. The program utilizes the Omega Laser Facility to conduct implosion and coupling physics at the nominally 30-kJ scale and laser–plasma interaction and coupling physics at the MJ scale at the National Ignition Facility. This article will discuss the motivation and challenges for laser direct drive and the broad-based program presently underway in the United States.

Plasmas were generated by 400 fs KrF laser pulses at intensities of ~~10 17 Wcm -2 on aluminum targets. Reflectivity and x-ray emission were measured as a function of laser polarization, angle of incidence, and intensity. For the same absorbed intensity, p-polarized laser light is up to a factor of 5 more efficient in generating x rays ( > 0.5 keV) than s-polarized light. These results show the importance of an additional absorption process, besides collisional absorption, for short scale length plasmas that is effective for /^-polarized light only and has the characteristics of resonance absorption.

The first inertial confinement fusion implosion experiments with equimolar deuterium-tritium thermonuclear fuel have been performed on the National Ignition Facility. These experiments use 0.17 mg of fuel with the potential for ignition and significant fusion yield conditions. The thermonuclear fuel has been fielded as a cryogenic layer on the inside of a spherical plastic capsule that is mounted in the center of a cylindrical gold hohlraum. Heating the hohlraum with 192 laser beams for a total laser energy of 1.6 mega joules produces a soft x-ray field with 300 eV temperature. The ablation pressure produced by the radiation field compresses the initially 2.2-mm diameter capsule by a factor of 30 to a spherical dense fuel shell that surrounds a central hot-spot plasma of 50 µm diameter. While an extensive set of x-ray and neutron diagnostics has been applied to characterize hot spot formation from the x-ray emission and 14.1 MeV deuterium-tritium primary fusion neutrons, thermonuclear fuel assembly is studied by measuring the down-scattered neutrons with energies in the range of 10 to 12 MeV. X-ray and neutron imaging of the compressed core and fuel indicate a fuel thickness of (14 ± 3) µm, which combined with magnetic recoil spectrometer measurements of the fuel areal density of (1 ± 0.09) g cm −2 result in fuel densities approaching 600 g cm −3 . The fuel surrounds a hot-spot plasma with average ion temperatures of (3.5 ± 0.1) keV that is measured with neutron time of flight spectra. Absolute neutron yields of (7.5 ± 0.1) × 10 14 have been recorded from the magnetic recoil spectrometer and nuclear activation diagnostics while gamma ray measurements provide the duration of nuclear activity of (170 ± 30) ps. These indirect-drive implosions result in the highest areal densities and neutron yields achieved on laser facilities to date. This achievement is the result of the first hohlraum and capsule tuning experiments where the stagnation pressures have been systematically increased by more than a factor of 10 by fielding low-entropy implosions through the control of radiation symmetry, small hot electron production, and proper shock timing. The stagnation pressure is above 100 Gbar resulting in high Lawson confinement parameters of P τ 10 atm s. Comparisons with radiation-hydrodynamic simulations indicate that the pressure is within a factor of three required for reaching ignition and high yield. This will be the focus of future higher-velocity implosions that will employ additional optimizations of hohlraum, capsule and laser pulse shape conditions.

The original ignition ''point designs'' ͑circa 1992͒ for the National Ignition Facility ͑NIF͒ ͓J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 ͑1994͔͒ were made energetically conservative to provide margin for uncertainties in laser absorption, x-ray conversion efficiency and hohlraum-capsule coupling. Since that time, extensive experiments on Nova ͓J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 ͑1989͔͒ and Omega ͓J. M. Soures et al., Phys. Plasmas 3, 2108 ͑1996͔͒ and their related analysis indicate that NIF coupling efficiency may be almost ''as good as we could hope for.'' Given close agreement between experiment and theory/modeling, one can credibly explore target enhancements which couple more of NIFs energy to an ignition capsule. These include using optimized mixtures of materials to reduce x-ray wall losses, slightly reduced laser entrance holes, and laser operation strategies which increase the amount of energy one can extract from NIF. It is found that 3-4ϫ increases in absorbed capsule energy appear possible, providing a potentially more robust target and ϳ10ϫ increase in capsule yield.