Lawrence Livermore National Laboratory
Target Physics
Researchers use Livermore´s LASNEX code to model the target physics. The code is used to produce simulated data, such as this neutron image, which will be compared with data from NIF experimentsExecuting a target shot at the National Ignition Facility requires the collaborative expertise of target designers, experimental physicists, laser and materials scientists and engineers.
The designers have to work with the experimentalists to set the goals for each experiment and to establish target, target diagnostic and laser specifications accordingly. Livermore researchers are guided by increasingly detailed modeling that uses the latest generation of supercomputers. The modeling must account for a variety of physical phenomena that occur during an implosion and resulting ignition.
The simulations and experiments study the physics of both laser-driven hohlraums and capsule implosions. The study of hohlraums includes the refraction, reflection and absorption of laser light passing through the hohlraum's laser entrance holes, the interaction of the laser light with low-density plasma, the conversion of absorbed laser light into X-rays, the flow of those X-rays within the hohlraum and their absorption onto the ablator layer. Capsule physics encompasses the time-dependence of shocks compressing the capsule, the capsule ablation and ensuing implosion, the hydrodynamic instability growth and mixing within the capsule and the thermonuclear burn of the deuterium-tritium fuel.
Livermore's radiation hydrodynamics code HYDRA was used to simulate a two-millimeter-diameter ignition target for NIF. This cutaway view shows representative laser rays from two beams (green, red) as they illuminate the hohlraum wall (gold). Their energy is converted to thermal X-rays, which heat the capsule ablator.
The simulations reflect certain experimental realities: Implosion is an inherently unstable process, and ignition experiments on NIF will involve neither perfectly smooth and spherical capsules nor a perfectly uniform field of X-rays to ablate the outer layer and compress the fuel inside (see How to Make a Star). Several Livermore-developed codes are used because no single code can simultaneously model all ignition phenomena.
LASNEX is a venerable two-dimensional radiation hydrodynamics code with very complete modeling of most relevant physical processes. Researchers use LASNEX to model the full geometry of the hohlraum and capsule as well as the incident laser beam. In these simulations, called integrated modeling, the capsule, hohlraum and laser light are modeled simultaneously.
These density isosurfaces from a HYDRA simulation of a NIF ignition capsule show the irregularities that result from Rayleigh-Taylor instabilities. The outer surface is near the ablator-fuel interface, and the inner surface is in the deuterium-tritium fuel. At 140 picoseconds before ignition time, the ignition capsule has a density of 60 grams per cubic centimeter and a diameter of 100 micrometers at maximum implosion velocity.
HYDRA is a three-dimensional radiation hydrodynamics code that contains all the physics necessary for imploding NIF ignition capsules. HYDRA can simulate the entire ignition target in 3D, including the hohlraum, capsule and all relevant features. The code is flexible enough to model intrinsic asymmetries that result from the ideal laser illumination pattern and those that result from effects of irregularities in laser pointing and power balance. It also simulates the hydrodynamic instabilities that occur when the capsule implodes. HYDRA calculates all of the radiation, electron, ion and charged-particle transport and the hydrodynamics from first principles – that is, no adjustments are made to the modeling parameters.
These simulations allow scientists to evaluate the robustness of a target design. For example, a designer can place realistic roughness on the capsule surfaces and calculate how these features evolve into irregularities – bubble and spike patterns – as a result of hydrodynamic instabilities. Three-dimensional simulations indicate that the ultimate amplitudes of the bubbles and spikes are greater than are shown in the 2D simulations. Thus, the 3D calculations provide more accurate information on peak amplitudes of these irregularities and how they affect target performance.
Designers are also using HYDRA to evaluate alternative target designs, including one with two concentric spherical shells and direct-drive targets that eliminate the need for a hohlraum (See How ICF Works). The HYDRA development team continues to enhance the code's capabilities in response to user requests. One new physics package will treat magnetic fields in 3D, further improving understanding of the target physics.
Other codes model in detail the laser-plasma instabilities (see Plasma Physics and ICF). A principal code for this application is pF3D, which simulates the interactions of the laser light with the electrons and ions in the plasma. NIF Early Light experiments have been modeled with extraordinary high fidelity using the pF3D code on Livermore's massively parallel supercomputers. The results, showing the effects of four NIF lasers pointed at high energy on a full-scale target for the first time, were a forerunner to experiments in 2008 using 96 beams (see "Simulations of early experiments show laser project is on track" (PDF) and full-scale experiments since 2009 using all 192 beams.
The success at creating a computerized view of the fusion experiment might be compared to looking "in the back of the book" since the computations were carried out on LLNL's supercomputers using the pF3D laser-plasma interaction code after the actual experiment was done. The calculations helped understand and explain what occurred. The agreement bodes well for doing more predictive modeling in the future.
More Information
"Experiments and multiscale simulations of laser propagation through ignition-scale plasmas," (abstract) Nature Physics, Sept. 2, 2007 (Subscription required)
"Meeting the Target Challenge," Science & Technology Review, July/August 2007
"On Target: Designing for Ignition," Science & Technology Review, July/August 1999
