A View from Kevin Bullis
Scientists Overcome Obstacle to Fusion
The world’s largest laser system has uniformly compressed and superheated a fuel capsule.
One of the key outstanding questions about whether it’s possible to use lasers to ignite fusion has been answered. A huge, stadium-sized laser facility at the Lawrence Livermore National Laboratory in Livermore, CA, uniformly compressed and heated a tiny capsule to very high temperatures. The experiments confirmed a theory the scientists there had about how to control the energy from 192 high-power lasers to compress the spherical capsule evenly from all sides.
Siegfried Glenzer, the Plasma Physics Group Leader at LLNL, says that the experiments clear away a major hurdle on the way to igniting fusion, a self-sustaining reaction of the sort that powers the sun. He says there’s a good chance the researchers will achieve this goal by the end of the year. If they’re successful, the facility will allow scientists to study the inner workings of stars and nuclear weapons in a controlled lab setting. It could also lead to a new type of power plant that runs on abundant hydrogen isotopes.
Igniting fusion requires extremely high temperatures and pressures, achieved by applied energy evenly to the entire surface of a spherical fuel capsule. To do this, the researchers plan to put the sphere–which measures a couple of millimeters across, inside a small gold can called a hohlraum. The lasers would enter the can from the ends and hit its interior walls. Each of the 192 lasers would come in at a different angle. When the lasers collide with the walls of the hohlraum, they produce X-rays which are supposed to bath the sphere uniformly.
But the researchers knew that the energy likely wouldn’t be distributed perfectly. To correct for this, they proposed the following solution:
As the lasers enter the hohlraum, they interact with each other, producing an interference pattern, which in turn creates a plasma with regularly spaced dense areas alternating with less dense areas. This produces a sort of “grating” which acts as a prism. This prism that diffracts different colors of light to different degrees. The researchers hypothesized they could fine tune the distribution of the laser energy by very slightly altering the wavelength of the laser light. The recent experiments, reported in the journal Science, confirmed that this works. After a series of laser shots, in which they gradually altered the color of the laser, they compressed the spherical capsule evenly, and were able to heat it up to 3.3 million degrees Kelvin.
By extrapolating from these results, they scientist say they should be able to achieve fusion using the laser system at Livermore, which was officially opened last year. Glenzer says this could happen by the end of the year.
Obstacles remain, however. To make it work, they’ll have to crank up the lasers, doubling their output compared to these initial experiments. In so doing, they’ll be trying to approximately double the amount the sphere gets compressed, which will require very precisely timed laser pulses. What’s more, so far they haven’t included the deuterium and tritium fuel in the capsule for the tests. They’ve demonstrated they can create and maintain the precise fuel layers needed in the lab, but not within the laser system, he says.
What’s more, even if the researchers ignite fusion, it won’t be in a form useful for generating electricity. Although the reactions will be self-sustaining, the amount of fuel in each capsule will be small, and so the duration of the burn will be brief. Generating electricity will mean developing a system that can ignite many capsules each second, and then capture the heat released to produce steam to power a turbine.