High-Powered Lasers Deliver Fusion Energy Breakthrough

A new experiment releases more energy than is pumped into fuel—a major milestone—but a long journey still remains for sustainable energy from fusion

Feb 12, 2014 |By David Biello, Scientific American Associate Editor

The power of the sun has edged a little closer to Earth. Under x-ray assault, the rapid implosion of a plastic shell onto icy isotopes of hydrogen has produced fusion and, for the first time, 170 micrograms of this superheated fusion fuel released more energy than it absorbed. Experimental shots of the 192 lasers at the National Ignition Facility at Lawrence Livermore National Laboratory in California have reproduced such fusion at least four times since September 2013. The advance offers hope that someday in the far future scientists might reliably replicate the power source of the sun and stars.
 
“This is closer than anyone’s gotten before, and it’s really unique to get out of the fuel as much energy as put in,” says Livermore physicist Omar Hurricane, lead author of the paper presenting the results published in Nature. “We got more fusion energy out of the DT fuel than we put in to the DT fuel.” (Scientific American is part of Nature Publishing Group.)
 
DT fuel stands for deuterium and tritium, the isotopes of hydrogen that encompass one proton and one neutron or one proton and two neutrons, respectively. One shot at Livermore’s National Ignition Facility (NIF) on November 19, 2013, that lasted less than 2 X 10^–8 seconds—less time than the blink of an eye—produced nearly twice as much energy as was applied, according to the new paper. Changing the timing of how the lasers put energy into the hohlraum, a tiny can that holds the fusion fuel pellet, proved key. The scientists concentrated more energy earlier in the shot to make conditions hotter earlier in the process, which seems to help hold the fuel pellet together longer as it implodes.
 
The fuel pellet itself is a perfectly spherical capsule of plastic, roughly two millimeters in diameter and precisely shaped (at a cost of roughly $1 million per pellet) to ensure the best performance. The deuterium and tritium are added as a gas to the hollow pellet. Then the sphere is cooled to 18.6 kelvins, or –254.55 degrees Celsius. That cooling causes the deuterium and tritium to form a layer of ice on the inside of the sphere roughly 70 micrometers thick—thinner than the width of a human hair. Roughly 500 megajoules of electricity feed lasers that then pump out 1.9 megajoules worth of energy. Those lasers take a long, power-boosting trip through amplifying optics and shoot into the hohlraum, which is made of gold and measures 5.75 millimeters in diameter and 9.425 millimeters long. “It’s a soup can but very small [and] made out of gold with two holes on the end where the lasers go in,” explains Livermore physicist Debbie Callahan, a member of the fusion team.
 
Employing 1.9 megajoules in slightly more than a nanosecond, the lasers deliver 500 terawatts of power inside the hohlraum (a terawatt is a trillion watts). A cloud of helium gas holds back the gold plasma that would otherwise intrude as the laser power is translated into x-rays by the hohlraum. These x-rays hit the plastic shell of the capsule, which absorbs roughly one tenth of the energy put into the lasers to begin with. That’s enough energy to obliterate the outside shell and drive the fuel together “like a rocket,” in the words of Hurricane, collapsing the sphere of fuel until it is one thirty-fifth its original size in almost no time at all, the equivalent of going from a sphere the size of a basketball to one the size of a pea almost instantly. The fuel absorbs roughly one tenth of the energy delivered by the x-rays onto the plastic capsule. That energy and implosion create a high pressure (150 gigabars) region of fusion that is even smaller than the layer of fuel itself—a hotspot that is 60 microns in diameter and shaped, depending on the qualities of the shot, like a doughnut without a hole, or an apple. “The conditions are quite ferocious,” Hurricane says, noting the key challenge of maintaining a roughly spherical shape. “Mother Nature doesn’t like putting a lot of energy in small volumes so she fights you on it.”

Hell of an electric bill.  Negligible, if it actually works…