With direct symmetric laser illumination of a spherical target, there is the potential for high enough target gains to produce economically viable fusion power. Proposed solutions to the various laser-target physics problems have mostly relied upon modifications to the laser, while keeping the target relatively simple. Necessary but not sufficient laser constraints include a short laser wavelength, very uniform illumination of the target in both high and low spherical perturbation modes, a wide bandwidth, temporal pulse shaping, and a reduction of the focal spot size during the implosion. An electron-beam-pumped argon-fluoride gas laser best satisfies all of these constraints and has the potential for fusion energy gains that far exceed requirements for an economically viable power plant. Various diode-pumped solid-state laser concepts are reviewed. None can simultaneously satisfy most if not all of the constraints. For example, the recently proposed PolyKrōm design produces a set of narrowband discrete wavelengths. The intensity coherence time of this discrete set is too long, and it could not produce the required uniform laser illumination. It is also shown that solid state lasers cannot produce with sufficient efficiency the subnanosecond pulse that is required for the shock-ignition target design.

Direct-drive laser fusion has one potential advantage over all other approaches to fusion energy. The hot plasma can be kept near or below the various plasma instability thresholds, if one uses purely spherical targets, with a short wavelength,large bandwidth and optically smoothed excimer laser. Instead of trying to manage laser–plasma instabilities, one avoids them. There is a path to complete the evaluation and development of this energy option, with moderate costs and a moderate time scale. Glass lasers, with their longer wavelength and narrower bandwidth, are no longer useful to evaluate fusion targets.

We will review some of the requirements for a laser that would be used with a laser fusion energy power plant, including frequency, spatial beam smoothing, bandwidth, temporal pulse shaping, efficiency, repetition rate, and reliability. The lowest risk and optimum approach uses a krypton fluoride gas laser. A diode-pumped solid-state laser is a possible contender.

A brief review is given of the requirements of inertial fusion energy drivers and their status and prospects in the U.S. Drivers based on lasers (diode-pumped solid-state and KrF) and heavy ions are discussed.

Nike is a multi-kilojoule KrF laser with very high beam uniformity (ΔI/I<0.2% with all 36 overlapped beams), and the capability to accelerate relatively thick targets on a low adiabat under conditions scalable to direct drive ICF. In a first set of experiments we determined the effect of the imprinting by varying the uniformity of the foot of the laser pulse and measuring the growth of the subsequent Rayleigh-Taylor instability. We found that the lower the imprint, the longer the mass modulations take to reach a given level. This is in quantitative agreement with our 2-D hydrodynamics simulations. The results are promising for direct drive with a very uniform laser.

Nike is a multi-kilojoule KrF laser with very high beam uniformity (ΔI/I<0.2% with all 36 overlapped beams), and the capability to accelerate relatively thick targets on a low adiabat under conditions scalable to direct drive ICF. In a first set of experiments we determined the effect of the imprinting by varying the uniformity of the foot of the laser pulse and measuring the growth of the subsequent Rayleigh-Taylor instability. We found that the lower the imprint, the longer the mass modulations take to reach a given level. This is in quantitative agreement with our 2-D hydrodynamics simulations. The results are promising for direct drive with a very uniform laser.

Thin plastic (CH) foils were irradiated by the Naval Research Laboratory Nike [Obenschain et al., Phys. Plasmas 3, 2098 (1996)] KrF laser and were imaged in the x-ray and extreme ultraviolet regions with two-dimensional spatial resolution in the 3–10 μm range. The CH foils were backlit by a silicon plasma. A spherically curved quartz crystal produced monochromatic images of the Si+12 resonance line radiation with energy 1865 eV that was transmitted by the CH foils. Instabilities that were seeded by linear ripple patterns on the irradiated sides of CH foils were observed. The ripple patterns had periods in the 31–125 μm range and amplitudes in the 0.25–5.0 μm range. The silicon backlighter emission was recorded by an x-ray spectrometer, and the 1865 eV resonance line emission was recorded by a fast x-ray diode. The multilayer mirror telescope recorded images of the C+3 1550 Å emission (energy 8.0 eV) from the backside of the CH foils.

A new X-ray imaging system based on spherically curved crystals has been developed. It is designed and used for diagnostics of targets ablatively accelerated by the Nike KrF laser. Applications of this instrument include spectroscopy with one-dimension of spatial resolution and two-dimensional monochromatic self-imaging and backlighting. The imaging system with various crystals of mica and quartz is used for plasma diagnostics of the main target and for characterization of potential backlighters. A spherically curved quartz crystal (2d=6.687 Angstrom, R=200 mm) is used to produce monochromatic backlit images with the He-like Si resonance line (1865 eV) as the source of radiation. The spatial resolution of the X-ray optical system is 1.7 mu m in selected places and 2-3 mu m over large area. Time resolved backlit monochromatic images of CH planar targets driven by the Nike facility have been obtained with 6-7 mu m spatial resolution.

Foil targets irradiated by the Naval Research Laboratory Nike KrF laser were imaged in the x-ray region with two-dimensional spatial resolution in the 2–10 μm range. The images revealed the smoothness of the emission from target and backlighter foils, the acceleration of the target foils, and the growth of Rayleigh–Taylor instabilities that were seeded by patterns on the irradiated sides of CH foils.

Plastic foils were irradiated by the NRL Nike KrF laser and were imaged in the x-ray and extreme ultraviolet regions with 2D spatial resolution in the 3–10 μm range. The CH foils were backlit by a silicon plasma. The silicon backlighter emission was recorded by an x-ray spectrometer, and the Si+12 1,865 eV resonance line emission was recorded by a fast x-ray diode. A spherically curved quartz crystal produced monochromatic images of the 1,865 eV radiation that was transmitted by the CH foils. Instabilities that were seeded by linear ripple patterns on the irradiated sides of CH foils were observed. The ripple patterns had periods in the 31–125 μm range and amplitudes in the 0.25–5.0 μm range.

We have developed an improved x-ray imaging system based on spherically curved crystals. It is designed and used for diagnostics of targets ablatively accelerated by the Nike KrF laser. A spherically curved quartz crystal (d = .?, R = mm) has been used to produce monochromatic backlit images with the He-like Si resonance line (1865 eV) as the source of radiation. The spatial resolution of the x-ray optical system is 1.7 mum in selected places and 2-3 mum over a larger area. Time-resolved backlit monochromatic images of polystyrene planar targets driven by the Nike facility have been obtained with a spatial resolution of 2.5 mum in selected places and 5 mum over the focal spot of the Nike laser.