But the most exciting potential application for exawatt lasers is in fusion power plants that rely on a process called fast ignition. In the early stages, the National Ignition Facility will use petawatt lasers to compress a pellet of gold fuel until it heats up to 100 million °C, triggering fusion. Also at the conference this week, researchers from the facility reported that they’ve completed another step along the way to controlled fusion reactions, describing preliminary tests of their system using a 500,000-joule pulse to implode a fusion fuel pellet.
Fast ignition works differently. Instead of a single pulse, the technique would use lower-power lasers to “compress the fuel without worrying about heating it, and then a short-pulse [exawatt] laser that acts as a spark plug,” igniting the fusion reaction, says Ditmire.
“Whether this will work is controversial,” Ditmire admits. Aiming such a short pulse might be problematic. In theory, though, the fast-ignition process should take less energy to operate. The most important measure of the performance of a fusion reactor is its gain, or the ratio of the energy required to operate the lasers to the amount of energy produced by the reaction. The Livermore facility’s goal is a gain of 15 to 20. “You need a gain of 100 to make a fusion power plant, and calculations show that exawatt lasers could get it,” says Ditmire.
But the new glass material isn’t the only key to building an exawatt laser. Ditmire’s group has also had success with new amplification techniques for making very short-duration pulses using the university’s Texas Petawatt Laser. According to Ditmire, the trick to making very high power is a technique called chirping, in which different frequencies of light are separated, run through glass amplifiers, and then run through a compressor to put them back together into a single, higher-power pulse. The Texas group’s method combines different types of glass amplifiers for this process, allowing for more compression of the light and therefore increasing the power output further. At the meeting, Ditmire reported using this technique to create 100-femtosecond pulses.
Ditmire isn’t the only researcher pushing for the development of exawatt lasers. The inventor of chirping, Gérard Mourou of the Ecole Polytechnique in France, is spearheading a European exawatt laser project called ELI, or Extreme Light Infrastructure. The European group plans to use titanium sapphire amplifiers instead of conventional glass.