The Lawrence Livermore National Laboratory (LLNL) Inertial Confinement Fusion (ICF) Program is addressing the critical physics and technology issues directed toward demonstrating and exploiting ignition and propagating burn to high gain with ICF targets for both defense and civilian applications. Nova is the primary U.S. facility employed in the study of the X-ray-driven (indirect drive) approach to ICF. Nova's principal objective is to demonstrate that laser-driven hohlraums can achieve the conditions of driver-target coupling efficiency, driver irradiation symmetry, driver pulseshaping, target preheat, and hydrodynamic stability required by hot-spot ignition and fuel compression to realize a fusion gain.

The LLNL ICF Program believes that the next major step in the national ICF Program is the demonstration of ignition and moderate gain (G ≤ 10). Recent theoretical and experimental results show that the ignition drive energy threshold can be reduced significantly by operating indirectly driven targets at radiation temperatures ∼ 1.3–1.6 times higher (thereby achieving higher implosion velocity) than originally proposed for the Laboratory Microfusion Facility (LMF). (Temperatures of ∼ 1.3 times higher have already been demonstrated on Nova.) Specifically, it should be possible to demonstrate ignition and propagating with burn about 1–2 MJ of laser energy as against the 5–10 MJ necessary for the high-yield LMF. LLNL proposes to upgrade the existing Nova facility to 1–2 MJ (2- to 4-ns pulses) and demonstrate ignition and propagating burn to moderate gain with appropriately scaled hydrodynamic equivalents of high-yield targets.

Once moderate gain has been demonstrated at 1–2.0 MJ on the Nova Upgrade, investigations into improving, by about 50%, the coupling efficiency between the driver and the capsule could provide gains >20 at about 1 MJ or less. A database for gain below 1-MJ driver energies could lead to a low-capital-cost Engineering Test Facility (ETF) for a first inertial fusion energy engineering reactor. Because the capital cost for both the target chamber and the driver scale with size, there is the opportunity to realize large savings by lowering the required driver energy necessary to demonstrate the technology for a first demonstration power plant. A target gain, G ∼ 25, at a driver energy, ED ∼ 0.75 MJ, would be self-sustaining for a driver efficiency of ∼10% and a thermal-to-electric conversion efficiency of ∼33% and at 12 Hz would generate ∼10 MW of gross electric power. Although the cost of electricity would be high, the combination of low capital cost and early demonstration of reactor technology would be an attractive step in the development of inertial fusion energy.

Inertial confinement fusion (ICF) requires high compression of fusion fuel to densities approaching 1000 times liquid density of deuterium-tritium (D–T) at central temperatures in excess of 5 keV. The goal of ICF is to achieve high gain (of the order of 100 or greater) in the laboratory. To meet this objective with minimum driver energy, a number of central issues must be addressed. Research in ICF with laser drivers has shown the importance of using short wavelength (λ < 0.5 µm). To achieve conditions for high gain at driver energies of a few megajoules or less, high intensities (>1014W/cm2) are required. The directdrive approach to ICF is more energy efficient than indirect drive if the stringent drive symmetry and hydrodynamic stability requirements can be met by a suitable laser irradiation scheme and target design. Experiments carried out at 351 nm on the 2-kJ, 24-beam OMEGA laser system at the Laboratory for Laser Energetics (LLE) at the University of Rochester, and future experiments to be performed on a 30-kJ upgrade of this laser, can resolve the remaining physics issues for direct drive: (1) energy coupling and transport scaling; (2) irradiation-uniformity requirements for high gain; (3) hydrodynamic stability constraints; and (4) hot-spot and main-fuel-layer physics. We review progress made on achieving uniform drive conditions with the OMEGA system and present results for direct-drive cryogenic-fuel-capsule and CD-shell, “surrogate” cryogenic-capsule implosion experiments that illustrate the constraints imposed by hydrodynamic instabilities and drive uniformity on the design of high-performance direct-drive targets. Target designs have been identified that will explore the ignition-scaling regime using the OMEGA Upgrade. Experiments on the OMEGA Upgrade will signal whether or not there is a high probability of achieving modest to high gain using direct drive on an upgrade of the NOVA facility.

The Nova Upgrade Facility is being designed to achieve inertial confinement fusion (ICF) ignition and gain. A baseline design of the laser and experimental facilities has been completed, and a development plan, which includes a fully integrated laser demonstration, has been formulated. With adequate funding, the activities in this plan could be completed by late 1993. Construction of the Nova Upgrade beginning at that time would make possible the achievement of ICF ignition before the end of the decade.

Validation of the direct-drive approach to inertial confinement fusion requires the development of a 351-nm wavelength, 30-kJ, 50-TW laser system with flexible pulse shaping and irradiation uniformity approaching 1%. An upgrade of the existing OMEGA direct-drive facility at Rochester is planned to meet these objectives. In this article, we review the design rationale and specifications of the OMEGA Upgrade laser with particular emphasis on techniques planned to achieve the required degree of beam smoothing, temporal pulse shape, and beam-to-beam power balance.

The AURORA KrF laser at Los Alamos became operational in August 1989. AURORA is the first integrated system for demonstrating the capability of a KrF laser to perform target physics experiments for inertial confinement fusion (ICF) and is currently configured as a 5-kJ, 5-ns, 96-beam device. Both laser physics and ICF target physics experiments have been performed over the last year. Of the four major amplifiers in the AURORA laser system, one performed better than expected, one performed about as expected, and two performed below expectations. The causes of the variability in the amplifier performance are now well enough understood that this information can be used to improve the detailed design of the NIKE laser currently under construction at the Naval Research Laboratory. design of the NIKE laser currently under construction at the Naval Research Laboratory. High-dynamic-range pulse shapes have been propagated with minimal distortion through the AURORA amplifier chain, verifying theoretical predictions. Target physics experiments have been performed with intensities greater than 100 TW/cm2, pulse lengths ranging from 2–7 ns, and spot-size diameters from 500–1100 µm. The analysis of this first-generation kJ-class KrF laser target physics facility identified the strengths and weaknesses of KrF lasers for ICF applications. Detailed measurements of amplifier performance led to a better understanding of issues for KrF laser-fusion systems, and design studies for future KrF lasers for ICF applications incorporate improvements based in part on AURORA experience.

This article describes the current status of the KrF development programme based on the Sprite laser system at the Rutherford Appleton Laboratory. High reliability and high shot rate have been demonstrated. Using a unique KrF-pumped Raman laser architecture, beam brightness exceeding 2×1019 Wcm-2 sterad-1 giving a focussed intensity >5 ×1017 Wcm-2 has been achieved. The development of transform-limited short-pulse oscillators is shown to be of importance in avoiding spectral broadening in air propagation of high-intensity beams. Beam smoothing of KrF beams in a multiplexed configuration has been demonstrated for the first time. The technique of echelon-free induced spatial incoherence has been shown to produce smooth intensity distributions in the far field, which remain essentially unchanged on amplification. The development of pulsed-power systems capable of exciting multikilojoule laser amplifiers for the next phase of development, the Supersprite system, is briefly discussed.

Achieved performance of and the future plan for the ASHURA, a 700-J KrF laser, are described. Nanosecond pulses (3–20 ns) with a bandwidth of 50 cm-1 have been produced by gain switching of a preamplifier. The pulses have been six-time angularly multiplexed and double-pass amplified by two electron-beam-pumped amplifiers. The output from the main amplifier is strongly saturated, and maximum energy of 660 J has been extracted in 90 ns with a stage gain of 40. Focused by an f = 1m spherical lens, maximum power density of 6 × 1014 W/cm2 can be obtained on target. The energy density ratio of prepulse on target has been suppressed below 1/107. The ASHURA is being upgraded by adding a next-stage amplifier that aims at 10-kJ output energy in 270 ns.

The single-beam Asterix IV high-power iodine laser, presently operated at a pulse length of 0.3 ns, has achieved its projected output power of 4 TW. It is designed to deliver output pulses with lengths ranging from 0.1–3 ns with a maximum power of 5 TW and an energy of up to 2 kJ. This laser has been developed on the basis of 10 years of experience with the 1 TW Asterix III laser and with the support of a 1-D and 3-D pulse propagation code. Special emphasis has been put on increasing the overall efficiency of the laser system and obtaining a laser beam intensity profile as homogeneous as possible. In this article, the measures for improving the laser performance and the results obtained hitherto are presented.

Los Alamos National Laboratory has assembled an array of experimental and theoretical tools to optimize amplifier design for future single-pulse KrF lasers. The next opportunity to exercise these tools is with the design of the second-generation NIKE system under construction at the Naval Research Laboratory with the collaboration of Los Alamos National Laboratory. Major issues include laser physics (energy extraction in large modules with amplified spontaneous emission) and diode performance and efficiency. Low cost is increasingly important for larger future KrF single-pulse systems (low cost and high efficiency is important for larger repetitively pulsed applications such as electric power production). In this article, we present our approach to amplifier scaling and discuss the more important design considerations for large single-pulse KrF amplifiers. We point out where improvements in the fundamental database for KrF amplifiers could lead to increased confidence in performance predictions for large amplifiers and address the currently unresolved issues of anomalous absorption near line center and the possibility of diode instabilities for lowimpedance designs. Los Alamos has applied these amplifier design tools to the conceptual design of a 100-kJ Laser Target Test Facility and a 3-MJ Laboratory Microfusion Facility.

A new technique to achieve smooth laser irradiation profiles on laser fusion targets is evaluated numerically and experimentally. In this technique, smoothing is obtained by using a spectral dispersing optic and a random-phase plate placed in front of a focusing lens employing the broad-bandwidth of KrF lasers. Experimental results and numerical calculations agree well, verifying that this simple optical system is effective for smooth irradiation on targets.

Two concepts have been applied to the classical problem of accelerators for the ignition of indirectly driven inertial fusion. The first is the use of non-Liouvillian stacking based on photoionisation of a singly charged ion beam. A special FEL appears the most suited device to generate the appropriate light beam intensity at the required wavelength. The second is based on the use of a large number of (>1000) beamlets–or “beam straws”–all focussed by an appropriate magnetic structure and concentrated on the same spot on the pellet. The use of a large number of beams–each with a relatively low-current density–elegantly circumvents the problems of space charge, making use of the non-Liouvillian nature of the stopping power of the material of the pellet. The present conceptual design is based on a low-current (〈i〉 ≈ 50 mA) heavy-ion beam accelerated with a standard LINAC structure and accumulated in a stack of rings with the help of photoionisation. Beams are then extracted simultaneously from all the rings and further subdivided with the help of a switchyard of alternate paths separating and synchronising the many bunches from each ring before they hit the pellet. Single beam straws carry a reasonable number of ions: Beams and technology are directly relatable to the ones presently employed, for instance, at the CERN-PS. Space-charge-dominated conditions arise only during the last few turns before extraction and in the beam transport channel to the reaction chamber. In a practical example, we aim at a peak power of 500 TW delivered to the pellet for a duration of 10–15 ns. High-energy (10 GeV) beam straws of Ba doubly ionised ions are concentrated on several (four) focal spots of a radius of about 1 mm. The power density deposited on these tiny cylindrical absorbers inside a hermetic “hohlraum” is about 2.5 × 1016 w/g. These conditions are believed to be optimal for X-ray conversion, i.e., with an estimated conversion efficiency of about 90%.

Ballistic propagation and focusing of intense light-ion beams requires (1) close limits on allowable beam divergence (∼5 mrad) and (2) gas-filled magnetic lenses that must function effectively even if deleterious self-fields are generated during the passage of the beam through the lens. An alternative to ballistic focusing was suggested some years ago in which magnetically compressed light-ion rings capable of delivering 3–4 MJ in a pulse of ∼1 ns are transported to the target in a tube. This concept is reexamined in light of recent work on the creation of ion rings and magnetic compression of stabilized liners. The physics issues of (1) magnetic compression, (2) transport of ion rings in a tube, and (3) the interaction of the ring with the target will be explored to evaluate the feasibility of this scheme.

Intense light-ion beams are being developed for investigations of inertial confinement fusion (ICF). This effort has concentrated on developing the Particle Beam Fusion Accelerator II (PBFA II) at Sandia as a driver for ICF target experiments, on design concepts for a high-yield, high-gain Laboratory Microfusion Facility (LMF), and on a comprehensive system study of a light-ion beam-driven commercial fusion reactor (LIBRA). This article reports on the status of design concepts and research in these areas.

In pulse-power technology, a transformer-type accelerator is a good candidate for a highvoltage pulse generator suitable for repetitive operations. In this article, a reverse-conducting thyristor has been used as a switching element in a primary circuit and good operations have been obtained. A dual-resonant mode and the mode proposed by Reed, preferable to the former in the sense of voltage gain, have been investigated experimentally. A voltage gain of 25% increase compared to that of the dual-resonant mode operation has been obtained. Two different types of transformers, a spiral and a Tesla type with various turn ratios, were constructed and the characteristics of each type when applied to the same circuit have also been investigated in detail. As a result, it has been made clear that the stray capacitance of the secondary winding restricts the upper limit of the turn ratio to get the large voltage multiplication.

Even though general conclusions cannot be derived for all the protection schemes in inertial confinement fusion (ICF) reactors, the feasibility of the ferritic alloy HT-9 as the main component of the first structural wall (FSW) in ICF facilities using thin-film Li17Pb83 liquid protection, flowing through porous tubes (INPORT), can be demonstrated as a solution in terms of radiation damage. Swelling and shift in the ductile-brittle transition temperature (DBTT) can be analyzed using the results of experimental fast-fission reactors, which are demonstrated to be good experimental tools in that ICF range. The good performance of HT-9 is remarkable. The generation of new solid transmutants and the depletion of initial constituents need also be considered. Further, a reduced-activation HT-9 (niobium-free) has been studied using recycling and shallow land burial (SLB) criteria. The recycling using that HT-9 is shown to be not feasible, as is SLB waste disposal. The unexpected critical role of some short-lived isotopes is remarkable, and more research on their nuclear data must be performed.

Recent developments in research on drivers for inertial confinement fusion identified the need to develop laser amplifiers with efficiencies of 80% or more. This article discusses a novel free electron laser amplifier that is based on vacuum interaction of laser beams with electrons, clusters of condensed matter, or neutral atoms. The scheme is based on the inversion of the ponderomotive expulsion of plasma from a focussed laser beam. The problem of Liouville conservation of phase space is overcome by employing the laser pulse transient processes. The optical energy for amplification is provided by the transfer of the translative kinetic energy of electrons, clusters, or neutral atoms that are injected nearly laterally into the laser pulse. The gains predicted to be achievable with this process are of interest for clusters, although the cluster densities presently available are too low to achieve the desired level of amplification. Neutral beams are shown to have the greatest potential for achieving large amplifications.

In inertial confinement fusion experiments, the symmetrical implosion calls for a new scheme: a nonsupported pellet. In this article, the first experiment is conducted using a plastic pellet covered with thin ferromagnetic material. In the system, the digital-phase-lead-compensator takes the position-related signal and processes it to provide a driving signal for the magnetic suspender. The suspended pellet is irradiated by the Gekko XII laser, having its total energy 2700 J with pulse width 800 ps at the wavelength 0.53 μm. The X-ray pinhole photograph confirms the spherically symmetrical implosion at the pellet core. At irradiation, the neutron yield is also observed.