Photo by Eduard Dewald/LLNL
This experiment was about ten times more effective at producing self-heating by helium nuclei in the target than the previous best effort. The improvement was achieved by reducing the amount of energy in the first billionths of a second of the laser pulse so that instabilities wouldn't develop in the target causing it to break apart too soon. The results were in agreement with the numerical models of the experiment, giving the scientific and engineering team hope that they understand the processes well enough to step up to even higher energies in future experiments.
The enormous National Ignition Facility at Livermore is far larger than a practical power facility would be because it uses "vacuum tube technology" instead of modern solid state lasers. Livermore's chief scientist working on developing fusion power believes that solid state technology will greatly improve the efficiency of the laser system while greatly reducing its size. The huge vacuum technology system generates far too much waste heat to ever produce power practically. And it's very costly. Solid state systems are anticipated to produce far less waste heat, wasting far less energy, reducing the demands for cooling the laser system. Costs should come way down with solid state technology, so there's hope that this technology could eventually become cost effective at producing power.
The National Ignition Facility has the world's most powerful lasers.
Seen from above, each of NIF's two identical laser bays has two clusters of 48 beamlines, one on either side of the utility spine running down the middle of the bay.
A NIF target contains a polished capsule about two millimeters in diameter, filled with cryogenic (super-cooled) hydrogen fuel.
LIVERMORE, Calif. - 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.