Review of the Department of Energy's Inertial Confinement Fusion Program: The National Ignition Facility (1997)

The two major scientific uncertainties ''in predicting ignition with the NIF are the laser-plasma interactions in the hohlraum and the hydrodynamic instabilities in the imploding capsule. The former reduce the uniformity and energy of the drive, while the latter limit the ultimate compression of the fuel that can be achieved.

The NOVA Technical Contract (NTC) of 1990 specifies seven experimental objectives in hohlraurn laser physics (HLP 1 to 7) and five in hydrodynamic equivalent physics (HEP 1 to 5) that were to be completed in preparation for proceeding to an ignition facility. Each NTC milestone was defined, and is limited by, the performance of the NOVA laser. Although the completion of the NTC cannot guarantee that ignition will be achieved with the NIF, the completion of each milestone, in the context of the understanding of the underlying physics gained in pursuing it, increases confidence in the extrapolation of the results to the performance of the NIF. DOE's assessment of the technical readiness to proceed to Critical Decision 2 (CD-2)¹ was based partly on sustained progress on the NTC and on the extrapolation of those results to the NIF.

The Target Physics Subcommittee of the ICFAC completed a detailed review of progress toward completion of the NTC on January 3, 1994, and endorsed proceeding to CD-2 of the NIF. The status of the NTC milestones at the completion of CD-2 is summarized in Table 1. Inspection of the differences between the NIF requirement and the NTC requirement for each category shows the degree of extrapolation necessary for reaching ignition on the NIF. Comparison of the NTC requirement and the value achieved at CD-2 in June 1994 shows that the milestones HEP 1, 2, 3, and 4 and HLP 1, 2, 6, and 7 were met to the satisfaction of the ICFAC subcommittee. There was also substantial progress in meeting HLP 4 and 5.

Four of the five NTC milestones in capsule physics (HEP 1 through 4 in Table 1) have been met and the fifth—the convergence ratio of a successful target in the NIF geometry—has reached a value of 10, which is short of the originally projected value of 20. However, in a different NOVA experiment, capsules in a different hohlraum geometry have achieved a convergence ratio of 24, suggesting that milestone HEP 5 would have been met if the laser symmetry on NOVA had been adequate.

The difference between the performance achieved and the performance expected in the NIF-like geometry deserves some examination. The original projections were based on two-dimensional calculations, which necessarily ignore the azimuthal asymmetry caused by the small number of beams on NOVA. More recent three-dimensional calculations are consistent with the observed convergence ratio of 10 on NOVA and the expected achievable value of 35 on the NIF. Fully integrated two-dimensional radiation-hydrodynamics calculations have addressed concerns about time-dependent low-mode number asymmetries. Although milestone HEP 5 itself has not been reached, understanding of the physics behind it has been advanced significantly. An experimental campaign toward meeting the HEP 5 milestone, involving reconfigured NOVA and OMEGA lasers, is in progress and would increase confidence in reaching ignition with the NIF. In the committee's judgment the additional technical

confidence that might be gained by completion of this milestone is not sufficient to justify delaying the NIF program.

Motion of the hohlraum wall during the laser pulse can impair the symmetry of the radiation on the capsule and spoil the implosion. Motion of the laser spots and the small number of beams on NOVA allowed three-dimensional effects to complicate the results of experiments addressing this issue. Considering all the experimental parameters, the data that are available on that phenomenon are consistent with the NIF design but are not adequate to resolve the wall-motion issue on the NIF.

The current generation of computer codes is not adequate to perform three-dimensional integrated calculations of the physics of the imploding capsule from first principles. However, the models that have been developed are in good agreement with the data and are consistent with the objectives of the NIF. Moreover, ICF target physics is sufficiently relevant to weapon physics to justify including the resolution of target-physics issues as worthwhile objectives for the operational phase of the NIF. On balance, while the current understanding of the science of the imploding capsule does not guarantee that ignition can be achieved with the NIF, it does gives reasonable expectation that ignition will be achieved.

Since completion of CD-2, progress has occurred in hohlraum laser-plasma physics. Prior to CD-2, most experiments utilized unlined gold hohlraums, with the short laser pulse on NOVA preventing significant wall motion from perturbing the capsule implosion. The long scale-length plasmas of the NIF hohlraum. were simulated by a gas bag. The data for the HLP milestones established as requirements for CD-2 were based on these two configurations. In addition, a NIF hohlraum. with a CH liner was proposed to retard wall motion on the longer time scales required on the NIF. Integrated modeling of the hohlraum and capsule and experiments showed that the plasma from the CH liner would stagnate on the axis and destroy the symmetry of the implosion. This realization led to the adoption of a gas-filled hohlraum as the baseline design. Elegant and detailed experiments with these gas-filled targets revealed lower temperatures, larger backscatter, evidence of filamentation, and larger beam bending than had been observed in the unlined hohlraums and gas bags. As shown in Table 1, the new experiments reduced the values achieved for the HLP 1, 2, 4, 5, and 6 milestones (in most respects, below the NTC requirements) but enhanced the understanding of the underlying physics and made the results more relevant to the NIF design.

Gas-filled hohlraums have shown two problematic effects: (1) a temporally dependent shift from the calculated symmetry of irradiation, equivalent to a beam position shift, and (2) a difference between measurements and modeling predictions (most importantly in radiation drive temperature). With respect to (1), the effect is qualitatively understood and is consistent with the high transverse flow velocity of the hohlraum plasma near the laser entrance hole. The shift can be compensated by optics on the NIF and/or by laser beam smoothing using random phase plates (RPPs) to suppress nonlinear filamentation (small-scale self-focusing) in the gas.

With respect to (2), propagation of the laser light through relatively long, subcritical-density plasmas in the hohlraum. results in loss of laser energy backscattered through stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). The dynamics of these instabilities, and the resulting smaller wall radiation temperature in the gas-filled hohlraum, are understood qualitatively, but their saturation behavior is currently under intensive experimental and theoretical study. Again, the use of RPPs on one beam of NOVA has largely eliminated hot-spot-induced effects. An experimental solution to reducing total SBS and SRS backscatter was the introduction of smoothing by spectral dispersion (SSD) to broaden the laser bandwidth, in combination with an RPP. The result was 9% total measured backscatter with 0.05-nm SSD at the required laser intensity of 2 × 10¹⁵ W/cm², the intensity required for the 300-eV NIF target design temperature. This bandwidth for SSD is within the NIF baseline plan. Evaluation of consistency with the LASNEX two-dimensional modeling code awaits experiments following the implementation (in FY97) of 10-beam smoothing on NOVA.

Current computer codes play key roles in target design, laser design, the modeling of laser-target interactions, the interpretation of experiments, and other ICF-related research projects. Codes in wide use today include one-and two-dimensional codes based on Lagrangian, Eulerian, mixed Eulerian-Lagrangian, and particle methods. For example, the LASNEX code is used in various forms at LLNL and LANL for a broad variety of ICF applications.

ICF target design models are routinely performed in one dimension, where the maximum range of physical processes can be modeled and parameter studies can be carried out economically.

Applications include the replication of implosion experiments, often on a shot-by-shot basis. A limited number of the most promising designs and representative experimental shots are simulated in two dimensions to assess the departure from the expected one-dimensional behavior.

Other codes are used to model the target corona, where temperature and density profiles must be calculated with high precision. Additional applications involve such problems as the effects of non-Spitzer thermal transport on thermal smoothing and cell focusing in the corona, as well as preheating in the ablation region.

The following kinds of physics are now reasonably well treated in one-and two-dimensional simulations:

• One fluid, two-temperature descriptions for electrons and ions;

• Thermal conductivity: electron and ion with flux-limited diffusion;

• Radiation transport: multigroup flux-limited diffusion using opacity tables based on local thermal equilibrium and an appropriate method of transport calculation;

• Complex equations of state;

• Laser deposition: inverse bremsstrahlung;

• Suprathermal electron transport: multigroup flux-limited diffusion effects;

• Thermonuclear burn and transport of reaction products: multigroup flux-limited diffusion; and

• Models of early-stage mixing, driven by hydrodynamic instability.

Other computer codes are routinely used now to model propagation of laser beams through the system and to compute critical properties such as opacities and other aspects of complex ICF-related physics.

Despite the significant simulation capabilities now available, the ICF program would benefit from improved, more comprehensive simulations of ignition and burn. The DOE has created the Accelerated Strategic Computing Initiative (ASCI) to enhance computational resources and to stimulate the development of new and/or improved numerical methods and computational physics, as well as new classes of computer codes, especially three-dimensional codes. Three new machines have already been contracted for, delivery is in process, and the machines will be phased in over the next 2 years. While all of these computers exploit parallel processing, they have very different architectural features that will require attention in developing applications aimed at achieving high performance. The three machines are described in Table 2.

Even with these ASCI machines, the demands of three-dimensional calculations will still be severe, and good physical models coupled with good algorithms and code implementations will be required. Advanced developments enabled by the overall ASCI program in algorithms, models, and codes will be essential to realizing a true three-dimensional simulation capability.

The basis of the correctness of these codes and, ultimately, the determining factor in the confidence with which they can be used, is the validity not only of the algorithms but also of the physics data sets and models used to calibrate and validate the codes. Some of these data sets are used internally in the codes to provide opacities, nuclear and atomic reaction rates, and equations of state, while others are used to compare computational with experimental data. The level of detail at which such experimental data can be used is clearly greater when the codes are three dimensional than when they are two or one dimensional. That is, comparison between experiment and computation in one dimension and two dimensions necessarily involves ''coarse-graining" of data at a level that is not required in three dimensions. Obtaining useful data for three-dimensional modeling requires better diagnostic instrumentation and, in general, more advanced experimental facilities than are required for comparison with one-and two-dimensional code results. The NIF, with its advanced instrumentation and diagnostic facilities, is required to provide the inputs to yield real progress on three-dimensional computations. It is essential that future simulation capabilities be up to the task.

The committee has considered two approaches to assessing the likelihood of achieving the scientific objectives of ignition and propagating burn on the NIF.

The first approach takes a historical perspective, looking at technical progress since the 1990 NRC review of the ICF program.² In the ensuing years of intense scrutiny of the NTC and the technical objectives that have evolved, the primary change to the laser functional requirements has been the addition of improved beam-smoothing techniques. Although the NTC anticipated the need for beam smoothing, the choice of smoothing technique awaited the results of laser-plasma interaction experiments. The main change in the baseline target design has been to replace a low-Z lined hohlraum with a gas-filled hohlraum. The lined hohlraum design was initially adopted because of its perceived greater ease of fabrication.

Although the change from a lined to a gas-filled hohlraum design had a minimal impact on the laser functional requirements, the program has nevertheless faced significant challenges during the past 6 years. To cite some specific examples: (1) the lined hohlraums had a calculated asymmetry originating from a jet caused by the liner; (2) the gas-filled hohlraums were found to have a measured symmetry shift with unsmoothed beams, which had not been predicted but could be tuned out; and (3) under some conditions, large scattering levels were measured with unsmoothed laser beams. These latter two effects resulted in the specification of the beam-smoothing requirements for the NIF. In addition, in 1990 there were far fewer experimental data and a more rudimentary modeling capability for the NIF targets than now exist. Developing an integrated modeling capability with full radiation transport, developing the experimental and diagnostic techniques required to quantitatively evaluate target performance, and developing a variety of capsule materials along with demonstrating smooth cryogenic layers, have all represented significant challenges to the NOVA team, the national ICF program, and the NIF project. The fact that the level of technical sophistication has increased so dramatically during the past 6 years, with only a modest impact on the laser and target requirements for the NIF, increases confidence that the NIF will achieve its ignition goals.

A second approach to assessing the likelihood of achieving ignition and propagating burn is to examine the NIF's flexibility should the physics turn out to be different from what is currently expected. For example, target designs are being developed that would, according to integrated two-dimensional calculations, ignite with incident laser light with an energy around 1 MJ. Since the NIF is designed to

routinely deliver 1.8 MJ, these designs accommodate variations in laser entrance hole or hohlraum size that might be desirable for easier control of symmetry, or for compensation of the observed variation in stimulated scattering or the uncertainties in x-ray conversion and wall losses. Most plausible variations in the physics database, such as the capsule equation of state or opacity, are performance-neutral. The surface finish on the capsules might also be improved relative to present NOVA capsules if this is required to help suppress hydrodynamic instabilities. In addition, other capsule materials under development, such as Be and B₄C, can reduce the growth of hydrodynamic instabilities and are less sensitive to roughness of the cryogenic fuel layer. Operation with somewhat lower-temperature hohlraums, while requiring a laser energy at the upper end of the NIF operating range, would reduce the effects of laser-plasma instabilities.

The major differences between the present NIF design and the concept envisaged in 1990 have resulted from modifications to the target chamber, the target area, and the front end of the NIF to incorporate direct-drive capability. The likelihood of ignition and burn propagation through direct drive has increased significantly since 1990 owing to better understanding of both laser-beam imprinting and the growth of hydrodynamic instabilities. By an optimal choice of port locations for indirect drive, the direct-drive geometric requirements can be met by moving one-half of the NIF beams to 24 new beam ports that have been added to the chamber. The detailed requirements for beam smoothing await the results of experiments over the next several years on OMEGA and NIKE. Space has been provided in the NIF front end to implement one or more of the wide variety of beam-smoothing techniques now being evaluated. A further illustration of the NIF's flexibility is that the direct-drive beam configuration also allows a more nearly spherical (tetrahedral) hohlraum to be used.

The campaign to achieve ignition with the NIF will certainly be the most challenging ever undertaken by the national ICF program. However, given the success in the indirect-drive target physics campaign on NOVA over the past 6 years and the flexibility of the NIF design, the ignition and burn propagation objectives of the NIF will likely be achieved, but cannot be guaranteed.