Laser pulse delivers ignition-sized punch
Located at the Lawrence Livermore National Laboratory (LLNL; Livermore, CA), the National Ignition Facility (NIF) contains a collection of behemoth laser amplifiers and optics up to a meter in size, all designed and built in hopes of achieving...
Located at the Lawrence Livermore National Laboratory (LLNL; Livermore, CA), the National Ignition Facility (NIF) contains a collection of behemoth laser amplifiers and optics up to a meter in size, all designed and built in hopes of achieving nuclear-fusion "ignition" in a 2- to 3-mm-diameter target pellet containing a mix of deuterium and tritium. The 192 beamlines at NIF contain the largest commercially produced flashlamps, the highest energy capacitor arrays, and the greatest mass of nonlinear optical crystals in one laser system (see Fig. 1). Now, NIF researchers can add one more superlative to their list: the most energetic ultraviolet (UV) laser pulse.
Previously, one of the beamlines produced infrared (1064-nm) pulses of 21 kJ and visible (532-nm) pulses of 11.4 kJ. The latest result of 10.4 kJ for a single beamline is at the 355-nm wavelength deemed important for achieving ignition. In ignition, a self-sustaining fusion burn front propagates outward from the center of the pellet, according to Craig Wuest, one of the researchers. "The energy liberated in the propagating fusion reactions helps to burn the entire fuel mass, similar to the combustion of a fuel/air mixture in a gasoline engine," he explains.
The predicted pulse energy required from all 192 beamlines to achieve ignition is 1.8 MJ. If replicated in all other beamlines, the 10.4-kJ performance of the single beamline would result in delivery of 2 MJ of energy to the target, exceeding specifications.
In addition, the beams must be uniform across their profile. The contrast—defined as the root-mean-square variation of the intensity over the central 80% of the beam profile—was 0.17 for the UV beam, 0.13 for the visible beam, and 0.06 for the infrared beam.
Ideal pulse shapes and durations have been modeled for pulses of different wavelengths. "We have models for ignition pulse shapes based on sophisticated computer calculations of the laser beams in an indirect-drive inertial-confinement fusion configuration," says Wuest. The modeled UV pulse is referred to as a Haan pulse, named after LLNL physicist Steve Haan. A related pulse known as a Suter pulse—optimized for 532 nm and named after Larry Suter, another physicist at LLNL—has also been modeled (see Fig. 2).
Fusion experiments will begin at NIF this year and will grow in scale as more beamlines are completed. All 192 laser beams should be firing by 2008. Data from the experiments will be used in basic science, the development of fusion as a practical energy source, and the maintenance of the U.S. nuclear stockpile.