Fusion with a laser beam

Researchers at the University of Rochester’s Laboratory for Laser Energetics (LLE) have successfully completed the first full-scale test of a technique to improve laser-driven fusion.

An implosion using this technique, dubbed polarisation smoothing, generated 70 percent more fusion neutrons than without and moves researchers closer to self-sustaining fusion.

The Laboratory of Laser Energetics houses what they believe is the world’s most powerful laser: the 60-beam Omega which is often used as a testing ground for technologies that are planned for the National Ignition Facility (NIF) under construction in California.

Omega fires its energy on a millimetre-sized pellet, causing the pellet to implode, crushing in on itself and triggering nuclear fusion. Initiating this implosion with 60 beams, however, is likened to 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.

Scientists look to polarisation – the tendency of light waves to vibrate in only one plane – to find a way to smooth the beams even more.

‘The idea of polarisation smoothing has been around for some time, but this is the first time it’s been used on a laser of this scale,’ said 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 smoothes the beam because the polarisation’s of the individual half-beams cancel out some of each other’s irregularities.

The 60 recombined beams then strike the pellet from all sides, crushing it with far few ‘gaps between fingers.’ Though the entire polarisation smoothing system was first fully implemented in August 2000, 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 said to be extremely rare, and can only be manufactured at two plants in the world.

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.