The first full-scale test of a technique to improve laser-driven fusion has been successfully completed by researchers at the University of Rochester's Laboratory for Laser Energetics (LLE). Implosions using this technique, called polarization smoothing, generated 70 percent more fusion neutrons than without and moves researchers closer to self-sustaining fusion. These test results are being presented at the American Physical Society Division of Plasma Physics annual meeting in Quebec City this week.
The Laboratory of Laser Energetics houses the world's most powerful laser: the 60-beam Omega. Omega is often used as a testing ground for technologies that are planned for the National Ignition Facility (NIF) under construction in California. Since the process shows such promise, it will be incorporated into each of the NIF's 192 laser beams for direct-drive implosions.
Omega fires its energy on a millimeter-sized pellet, causing the pellet to implode, crushing in on itself and triggering nuclear fusion. Initiating this implosion with 60 beams, however, is like trying to crush a balloon in your hands: though you exert force with your fingers, the balloon bulges out between them. Likewise, if the energy from each beam of the laser pushes harder in one spot than another, the pellet will implode unevenly, losing some potential to trigger fusion.
To combat this uneven pressure, scientists develop ways of "smoothing" the beams so they strike the pellet evenly. Imagine the shadow rings a flashlight shines on a wall smoothing out to a perfectly even circle of light. The more smoothing, the more wallop the laser can throw at the target. Scientists look to polarization-the tendency of light waves to vibrate in only one plane-to find a way to smooth the beams even more.
"The idea of polarization smoothing has been around for some time, but this is the first time it's been used on a laser of this scale," says scientist David Meyerhofer, professor of mechanical engineering at the University of Rochester.
The technique requires that each of Omega's 60 beams (which in sum release more than 100 times the total power output of the nation in a billionth of a second) be split into two. Each beam shines on a prism-like crystal wedge that refracts about half of the beam, while letting the other half pass straight through. The "half-beams" are then recombined and focused on the target. This recombination smooths the beam because the polarizations of the individual half-beams cancel some of each other's irregularities, somewhat like the way merging cars on a freeway fill in the free space between each other and create a more solid stream of cars. The 60 recombined beams then strike the pellet from all sides, crushing it with far few "gaps between fingers."
Though the entire polarization smoothing system was first fully implemented in August, the process for its creation began in 1996. The scientists at the laboratory had to install 240 of the refracting crystals, each more than a foot in diameter, onto the laser one beam at a time. Flawless crystals of such a size are extremely rare, and can only be manufactured at two plants in the world.
"We saw a significant increase in performance," says Meyerhofer. "As we fine tune the system, we should be able to get even more than the 70 percent increase in neutron yield."
Scientists at LLE hope to improve the system's performance even more by cooling the target pellets below minus 400 degrees Fahrenheit to pack more material into the tiny space. Tests have begun on the freezing method and scientists are already experimenting with a combination of the two techniques.
The research was funded by the U.S. Department of Energy.