NIF falls short of ignition

The National Ignition Facility (NIF) will not meet its goal of igniting a fusion plasma before the end of September, the Lawrence Livermore National Laboratory (Livermore, CA) said on Friday. A spokeswoman says Livermore "will continue working toward achieving ignition." The laser is delivering the desired energy, but the target shots are not yielding the expected fusion energy.

NIF was declared complete on March 31, 2009, after it had delivered 1.1 MJ pulses at 355 nm. The 192-beam system was designed to deliver 1.8 MJ pulses, which simulations indicated would be sufficient to ignite a pellet of deuterium-tritium fusion fuel, producing fusion reactions that yielded more energy than the input pulse. The Department of Energy set a target of reaching ignition by September 30, 2012--the end of the fiscal year.

Wary of optical damage, Livermore ramped pulse power and energy slowly. The first 1.8 MJ pulse was not fired until March of this year. On July 5, NIF delivered peak power of 500 tW to a target for the first time in a 1.85 MJ pulse. From outside, it looked like NIF should be closing in on ignition.

But now NIF has become the latest in a long list of fusion lasers that yielded experimental results well short of predictions. A news story in the September 21 issue of Science magazine reports that although computer models predict NIF shots should achieve ignition, the yield of fusion energy from NIF experiments has so far reached only 0.1 of the ignition level.

The National Nuclear Security Administration (NNSA) has already begun studying its options. The first draft of a report is due October 1, with a final report due to Congress on November 30.

Meanwhile, NIF continues firing shots that can produce temperatures and pressures far beyond anything previously possible on the surface of the Earth. Livermore fusion researchers will keep pressing for ignition, and NNSA weapon scientists will get additional shots for their simulations of nuclear explosions as part of the agency's Stockpile Stewardship program.

NIF's laser bay, showing 96 of the 192 beamlines.

Light guides light up 3M solid-state bulbs

3M has added a new twist to solid-state lighting--embedding light guides in the outer shell of the bulb to redistribute light emission evenly across its surface like the venerable frosted-glass incandescent bulb.

Solid-state lighting has been widely touted for its outstanding energy efficiency. LED bulbs now in hardware stores draw 13 W of electric power, emit as much visible light as 60 W incandescents, and have lifetimes of 25,000 hours, far beyond 1000-hour incandescents. But high prices and some subtle but significant problems are slowing their adoption.

The 3M bulb is aimed at one of those subtle problems. LEDs emit directionally from a small area. Hot filaments and fluorescent tubes are omnidirectional, and although filaments are small, frosted incandescent bulbs scatter the light so it seems to radiate from entire surface. Directionality is good news for applications that want light concentrated in one direction, such as street lighting outdoors and downlighting in homes and offices. But it can be a problem in light fixtures in the line of sight, especially when the light comes from a small area. An example is a non-name solid-state lamp I bought earlier this year from a big-box hardware store. Light comes from a small zone where blue LEDs and yellow phosphor are mounted, not from the bulb's frosted surface, producing an unpleasant glare.

Deep inside, the 3M bulb contains similar blue LEDs with yellow phosphors to generate directional white light. But instead of shining directly into the room, the light is coupled into light guides embedded in the bulb. Total internal reflection guides the light around the bulb to areas where the light is scattered out the surface and into the room, as shown in the figure. That reduces brightness to an acceptable level, making the bulb much more presentable in a light fixture.

The light guide in the 3M LED bulb carries light from the LED source to diffusing areas on the bulb surface. (Courtesy of 3M)

The bulb, shown in the photo below, can't be mistaken for an incandescent. It needs slits to dissipate heat, a cooling problem that it shares with other LED bulbs, and requires heat sinks that add to its environmental impact. But the design is an innovative step in the right direction, making LED lamps an attractive piece of decor rather than an efficient eyesore.

3M's Advanced LED light distributes light like an incandescent bulb. (Courtesy of 3M)

DARPA PULSE program

Ultrafast laser research has produced some elegant science, from slicing time into incredibly thin slivers to generating combs of frequencies uniformly spaced across a wide band of the spectrum. These capabilities, in turn, have led to a similarly wide range of applications, including transferring time and frequency standards, measuring short intervals of time, and producing pulses so short that they generate extremely high peak powers with only modest amounts of energy.

However, ultrafast lasers traditionally have been bulky and complex things, custom-assembled on optical tables and delicately aligned in a laboratory. That complexity makes it hard to realize many potential practical applications such as putting frequency combs in space to boost the precision of GPS systems or to measure stellar spectra with extreme precision. Now the Defense  Advanced Research Projects Agency (Arlington, VA) is trying to do something about the problem by creating the Program in Ultrafast Laser Science and Engineering.

DARPA is not the first to think of making smaller and more durable ultrafast lasers. I mentioned the need for "robust frequency combs" for telecommunications systems or space-based instruments in the January Photonic Frontiers. A web search four pages which include the phrase "rugged femtosecond laser," but all of them cite an Army contract awarded last year to Arbor Photonics. However, such references are few and far between, and Google could not find a single page using the phrase "rugged frequency comb" (or combs) when I was writing this blog.

Shrinking the size and improving the robustness of ultrafast lasers is a big challenge, but success could pay off in important ways. DARPA cites some potential military applications that require rugged sources. One is using the time stability of the microwave-band repetition rate of a femtosecond laser to greatly reduce the close-to-carrier phase noise in a microwave oscillator. Others include transferring time or frequency measurements across the spectrum, and generating high-flux isolated attosecond pulses. Civilian science and technology also would benefit from compact  sources of ultrashort pulses.

As is normal with DARPA, success is not guaranteed, but the payoff could be high. In fact, somebody at DARPA surely should have already earned credit in the Pentagon bureaucracy for exceptional skill in acronym creation. Program in Ultrafast Laser Science and Engineering neatly translates into an entirely appropriate acronym -- PULSE.