On Feb. 12, researchers at Lawrence Livermore National Laboratory announced that they have achieved fuel gain exceeding unity in a nuclear fusion reaction, meaning that the energy generated in the fusion reaction is greater than the amount of energy deposited into the fuel to begin the reaction.
In nuclear fusion reactions, smaller atomic nuclei are combined into larger nuclei, which for isotopes with less binding energy than iron-56, releases energy. In the experiment, study leader Omar Hurricane and his colleagues used the fusion reaction that combines a deuterium atom and a tritium atom to create an atom of helium-4, a spare neutron, and several times more energy than a nuclear fission reaction. This has the capability to cause a chain reaction, and this is essentially what a star is. But a star has very high temperatures and pressures, which are difficult to create and sustain in the laboratory.
Hurricane's team attempted to do this by using 192 lasers to apply energy to a small amount of deuterium-tritium fuel inside a plastic sphere to increase its temperature and pressure. These lasers were aimed at a small gold container called a hohlraum, which housed the sphere. The laser beams entered through two tiny holes on each end of the hohlraum. When the beams hit the walls inside the hohlraum, the surface released X-rays which penetrated the sphere. This created conditions more intense than the conditions inside the Sun for just over 100 picoseconds. With a manipulation of the laser pulse shape, the researchers were able to reduce instability in the implosion of the deuterium-tritium fuel. This technique is called a “high-foot implosion,” and was able to produce an order-of-magnitude improvement in performance over past experiments of this type.
“We had to compress the capsule by 35 times,” said D. A. Callahan, lead scientist on hotspot shape and hohlraum strategies. “This is like saying that if you started with a basketball it would be like compressing it down to the size of a pea, but keeping the perfect spherical shape, which is very challenging.”
The researchers measured the energy output of the reactions by measuring the energy levels of the spare neutrons produced in the reactions. The first spare neutron was measured at 14.4 kilojoules, 20 percent more than the 12 kilojoules of energy contained in the fuel mixture. The second spare neutron was measured at 17.3 kilojoules with a fuel mixture of 9.4 kilojoules, an 84 percent increase.
“We are closer than anyone has gotten before,” said Hurricane. “We are finally, by harnessing these reactions, getting more energy out of these reactions than we are putting into the deuterium-tritium fuel. We took a step back from what we tried before and in the process took a leap forward.”
Neither calculation includes the 1.8 megajoules of energy from the lasers that created the conditions for the reaction, however. But a fusion reaction that can at least provide more energy than the amount of energy deposited into the fuel is a start. The next task is to achieve the same results with less energy required from the lasers.
“We have waited 60 years to get close to controlled fusion, and we are now close in both magnetic and inertial-confinement research,” said Professor Steve Cowley, director of the Culham Centre for Fusion Energy. “We must keep at it.”