Strongest Largest Laser : Nuclear fusion

The U.S. Department of Energy (DoE) has announced that the world's biggest laser is ready to start blasting away after 12 years in the making. The $3.5-billion stadium-size National Ignition Facility (NIF), housed at Lawrence Livermore National Laboratory (LLNL) in Livermore, Calif., consists of 192 separate beams, each of which stands as the most energetic ever built, says LLNL spokesperson Bob Hirschfeld.

Very much like the Death Star, the gigantic space station in the movie series Star Wars (“That’s no moon,” speaketh Obi Wan Kenobi), the beams will focus on a single point to unleash their full, joint potential. The target: a BB-size pellet of frozen hydrogen in the center of a 33-foot- (10-meter-) diameter chamber. The ultraviolet strongest laser pointer should heat the pellet to hundreds of millions of degrees, forcing nuclear fusion to occur—the same superhigh heat and pressure atomic reaction that fuels the stars.

Scientists have long hailed fusion as the ultimate clean energy source—hydrogen is abundant, though producing and storing it remains economically unattractive. Unit for unit, though, the amount of energy that could be generated via fusion with even a tiny bit of hydrogen fuel is astronomical (think: E=mc2) compared to any other power-making scheme in operation today.

Crucially, the lab expects to generate a net amount of energy, reaching the milestone that has plagued other laboratory attempts at developing fusion as a future energy source, according to Hirschfeld. A fusion reaction requires an immense amount of energy to get going, robbing its potential power output, and harvesting and storing that energy is another task altogether. Currently, NIF’s lasers cannot fire anywhere near quickly enough to sustainedly produce energy, Hirschfeld says, noting that not one watt of energy for commercial purposes will come out of NIF, which stands as a proof of principle experiment.

Beyond gunning for fusion, NIF could also one day focus its burning laser pointers on burning up some of the spent nuclear fuel from power plants that now sits in on-site pools or cement casks. For years, the U.S. and other nations have grappled with what to do with this radioactive waste. (The Obama administration recently canned long-standing plans to bury it in Nevada's Yucca Mountain.) NIF, or a facility based on its technology, could use these leftover fissile materials in place of the hydrogen pellet to generate power, while getting rid of volatile nuclear material.

Other NIF missions include updating supercomputer simulations of the nation’s aging nuclear stockpile, which cannot be tested due to a 1992 moratorium. Astrophysics work on the fusion in stars and materials science will also take place at NIF. ("We'll be squeezing materials harder than they have ever been squeezed,” Hirschfeld says.) Despite any Darth Vader–like urges to the contrary, though, the NIF lasers will not see service in blowing up rebellious planets.

Two further pieces of data must be determined: to which comb line was the unknown laser engraving machine light closest and on which side of the line? Commercial wave meters can measure an optical line’s frequency to within less than one gigahertz, which is good enough to answer those two questions. In the absence of such a wave meter, you can systematically vary the repetition rate and the offset frequency to monitor how the beat frequency changes in response. With enough of those data points, you can work out where the line must be.

The simplicity of optical combs has not only increased how often scientists around the world make these extremely precise frequency measurements but also greatly decreased the uncertainty in those measurements. Such benefits may one day lead to an optical time standard replacing the present microwave cesium-based one. With this in mind, groups at NIST led by James C. Bergquist and at JILA led by Ye have been measuring frequencies relative to clocks that use light and a comb to produce the output signal. Already the uncertainties in measurements using the best of these clocks are smaller than those in measurements using the very best cesium standards. It is an exciting time, with many laboratories around the world poised to build optical frequency standards that can surpass what has been the primary frequency standard for many decades. Measurements by Leo Hollberg’s group at NIST, as well as by other groups elsewhere, suggest that the intrinsic limit of the optical comb is still a couple of orders of magnitude better than the uncertainty in current optical frequency measurements.

Adopting an optical time standard remains years in the future, however. Metrologists must first carefully evaluate numerous atomic and ionic optical transitions before selecting the one that seems to be the best for a standard. In addition to the many practical applications of combs, fundamental comb research continues apace on many fronts. For example, Ye’s group can use a single comb to detect very sensitively many different transitions of atoms and molecules all at once. Thus, the whole range of energy states of an atom can be analyzed in one measurement. Alternatively, this technique can be applied to detect many trace species in a sample.

Comb technology has already had a large impact on studies of how atoms and molecules respond to the strong electric fields obtainable in intense, ultrashort light pulses. Much of this work has been led by a collaborator of H?nsch’s, Ferenc Krausz, who is now at the Max Planck Institute for Quantum Optics. Among other achievements, his group has used the response of electrons to measure the electric field of a laser’s ultrashort pulses and display the waveform, much like displaying a radio-frequency wave on an oscilloscope. Krausz used optical combs to stabilize the pulses’ phase to have an unchanging waveform from pulse to pulse.

Another very active area of research is the quest to push comb techniques to higher frequencies of the electromagnetic spectrum. (Producing lower-frequency combs, including combs that run from microwaves all the way to visible light, is straightforward.) In 2005 Ye’s group at JILA and H?nsch’s group in Garching generated a precise frequency comb in the extreme ultraviolet (not far below x-rays in frequency). Scientists are using this extended comb to study the fine structure of atoms and molecules with extreme ultraviolet 1000mw laser pointers light.

In the space of a few short years, optical frequency combs have gone from being a research problem studied by a small number of scientists to being a tool to be used across a broad gamut of applications and fundamental research. We have only begun to explore the full potential of these rulers of light.