This March, researchers at the National Ignition Facility demonstrated a 1.1 megajoule laser designed to ignite nuclear fusion reactions by 2010. But the facility's technology, which is housed at the Lawrence Livermore National Laboratory in California, cannot yet generate enough energy to drive a practical power plant. So, even as physicists look forward to next year's demonstration, they're working on even more powerful lasers that could make possible a method for a kind of laser-induced fusion called fast ignition.
This week, at the annual meeting of the Optics Society of America in San Jose, CA, researchers from the University of Texas presented plans to build an exawatt laser that would be three orders of magnitude more powerful than anything that exists today. Today's most powerful lasers operate on the order of about a petawatt, or 10 to the power 15 (one quadrillion) watts. An exawatt is 10 to a power of 18 watts. Exawatt lasers will be able to concentrate that power in areas measuring micrometers, creating tremendous intensities.
One way to increase the power of a laser is to decrease the duration of the laser pulse. But working with laser pulses on the order of picoseconds or even femtoseconds is difficult because such pulses are made up of a wide bandwidth of light frequencies that damage optical glass, including the phosphate glass often used to amplify laser light, for example at the National Ignition Facility.
Todd Ditmire, director of the High Intensity Laser Science Group at the University of Texas at Austin, reported at this week's meeting that a new type of glass should be able to handle the intense pulses of light needed to create an exawatt laser. The glass would be doped and used to create devices called amplifiers--when light from a laser shines on the glass amplifier, ions in the glass absorb the light and re-emit it at higher energy. "The glass is just a host--it's a transparent material that holds the ions," says Ditmire.
The advantage of sticking with glass instead of another material is that manufacturers can readily make it into large devices, which increases the power of the resulting beam. In contrast, titanium sapphire can act as an amplifier for high-power lasers, but it's difficult to make in big pieces, says Ditmire. Working with German manufacturer Schott, the Texas group has begun characterizing the properties of their new type of glass, which combines silicate, the material that makes up everyday glass objects, with the metal element tantalum. Ditmire says his group is now working with Schott to create larger pieces of the material that will be assembled to make a prototype laser.
Ditmire expects that the first application of exawatt lasers will be as energy sources for medical particle accelerators. Bombarding tumors with protons causes fewer side effects than x-ray therapy because the protons release their energy all at once, sparing surrounding tissues. However, proton therapy hasn't come into wide use because it requires large particle accelerators. Compact exawatt lasers should be powerful enough to accelerate protons for medical therapy.
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.
By Katherine Bourzac