Ignition, the process of releasing fusion energy equal to or greater than the amount of energy used to confine the fuel, has long been considered the “holy grail” of inertial confinement fusion science. A key step along the path to ignition is to have “fuel gains” greater than unity, where the energy generated through fusion reactions exceeds the amount of energy deposited into the fusion fuel. Though ignition remains the ultimate goal, the milestone of achieving fuel gains greater than 1 has been reached for the first time ever on any facility. In a paper published in the Feb. 12 online issue of the journal Nature, scientists at Lawrence Livermore National Laboratory (LLNL) detail a series of experiments on the National Ignition Facility (NIF), which show an order of magnitude improvement in yield performance over past experiments.
“What’s really exciting is that we are seeing a steadily increasing contribution to the yield coming from the boot-strapping process we call alpha-particle self-heating as we push the implosion a little harder each time,” said lead author Omar Hurricane.
Boot-strapping results when alpha particles, helium nuclei produced in the deuterium-tritium (DT) fusion process, deposit their energy in the DT fuel, rather than escaping. The alpha particles further heat the fuel, increasing the rate of fusion reactions, thus producing more alpha particles. This feedback process is the mechanism that leads to ignition. As reported in Nature, the boot-strapping process has been demonstrated in a series of experiments in which the fusion yield has been systematically increased by more than a factor of 10 over previous approaches.
The experimental series was carefully designed to avoid breakup of the plastic shell that surrounds and confines the DT fuel as it is compressed. It was hypothesized that the breakup was the source of degraded fusion yields observed in previous experiments. By modifying the laser pulse used to compress the fuel, the instability that causes break-up was suppressed. The higher yields that were obtained affirmed the hypothesis, and demonstrated the onset of boot-strapping.
The experimental results have matched computer simulations much better than previous experiments, providing an important benchmark for the models used to predict the behavior of matter under conditions similar to those generated during a nuclear explosion, a primary goal for the NIF.
The chief mission of NIF is to provide experimental insight and data for the National Nuclear Security Administration’s science-based Stockpile Stewardship Program. This experiment represents an important milestone in the continuing demonstration that the stockpile can be kept safe, secure and reliable without a return to nuclear testing. Ignition physics and performance also play a key role in fundamental science, and for potential energy applications.
“There is more work to do and physics problems that need to be addressed before we get to the end,” said Hurricane, “but our team is working to address all the challenges, and that’s what a scientific team thrives on.” Hurricane is joined by co-authors Debbie Callahan, Daniel Casey, Peter Celliers, Charlie Cerjan, Eduard Dewald, Thomas Dittrich, Tilo Doeppner, Denise Hinkel, Laura Berzak Hopkins, Sebastien Le Pape, Tammy Ma, Andrew MacPhee, Jose Milovich, Arthur Pak, Hye-Sook Park, Prav Patel, Bruce Remington, Jay Salmonson, Paul Springer and Riccardo Tommasini of LLNL, and John Kline of Los Alamos National Laboratory.
In the NIF, a weak laser pulse-about 1 billionth of a joule-is created, split, and carried on optical fibers to 48 preamplifiers that increase the pulse's energy by a factor of 10 billion, to a few joules. The 48 beams are then split into four beams each for injection into the 192 main laser amplifier beamlines. Each beam zooms through two systems of large glass amplifiers, first through the power amplifier and then into the main amplifier. In the main amplifier, a special optical switch traps the light, forcing it to travel back and forth four times, while special deformable mirrors and other devices ensure the beams are high quality, uniform, and smooth.
From the main amplifier, the beam makes a final pass through the power amplifier. By now, the beams' total energy has grown from 1 billionth of a joule to 4 million joules-all in a few millionths of a second. The 192 beams proceed to two ten-story switchyards on either side of the target chamber where they are split into quads of 2-2 arrays. Just before entering the target chamber, each quad passes through a final optics assembly, where the pulses are converted from infrared to ultraviolet light and focused onto the target.
For ignition experiments, the target consists of a tiny metal can called a hohlraum containing a capsule of frozen fusion fuel. Laser beams entering the top and bottom holes of the hohlraum strike its inside walls, creating x rays that compress the fuel capsule to extreme temperatures and densities. NIF's 192 laser beams travel about 1,500 meters from their birth to their destination at the center of the spherical target chamber. Yet the journey from start to finish takes only 1.5 microseconds.