Punch Of World's Most Powerful Laser Ratchets Up A Notch

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